docs(attribute directives): copy edits squashed (#2508)
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:marked
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An **Attribute** directive changes the appearance or behavior of a DOM element.
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:marked
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In this chapter we will
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* [write an attribute directive to change the background color](#write-directive)
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* [apply the attribute directive to an element in a template](#apply-directive)
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* [respond to user-initiated events](#respond-to-user)
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* [pass values into the directive using data binding](#bindings)
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# Contents
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* [Directives overview](#directive-overview)
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* [Build a simple attribute directive](#write-directive)
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* [Apply the attribute directive to an element in a template](#apply-directive)
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* [Respond to user-initiated events](#respond-to-user)
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* [Pass values into the directive using data binding](#bindings)
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* [Bind to a second property](#second-property)
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Try the <live-example></live-example>.
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.l-main-section
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a#directive-overview
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:marked
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## Directives overview
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There are three kinds of directives in Angular:
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1. Components
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1. Structural directives
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1. Attribute directives
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1. Components—directives with a template.
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1. Structural directives—change the DOM layout by adding and removing DOM elements.
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1. Attribute directives—change the appearance or behavior of an element.
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A *Component* is really a directive with a template.
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It's the most common of the three directives and we tend to write lots of them as we build applications.
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*Components* are the most common of the three directives. Read more about creating them
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in step three of [QuickStart](../quickstart.html#root-component).
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[*Structural* directives](structural-directives.html) can change the DOM layout by adding and removing DOM elements.
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[NgFor](template-syntax.html#ngFor) and [NgIf](template-syntax.html#ngIf) are two familiar examples.
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An *Attribute* directive can change the appearance or behavior of an element.
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The built-in [NgStyle](template-syntax.html#ngStyle) directive, for example,
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*Structural Directives* change the structure of the view. Two examples are [NgFor](template-syntax.html#ngFor) and [NgIf](template-syntax.html#ngIf)
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in the [Template Syntax](template-syntax.html) page.
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*Attribute directives* are used as attributes of elements. The built-in [NgStyle](template-syntax.html#ngStyle) directive in the [Template Syntax](template-syntax.html) page, for example,
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can change several element styles at the same time.
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We are going to write our own attribute directive to set an element's background color
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when the user hovers over that element.
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.l-sub-section
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:marked
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We don't need *any* directive to simply set the background color.
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We can set it with the special [Style Binding](template-syntax.html#style-binding) like this:
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+makeExample('attribute-directives/ts/app/app.component.1.html','p-style-background')
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:marked
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That wouldn't be nearly as much fun as creating our own directive.
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Besides, we're not just *setting* the color; we'll be *changing* the color
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in response to a user action, a mouse hover.
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.l-main-section
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a#write-directive
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:marked
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## Build a simple attribute directive
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An attribute directive minimally requires building a controller class annotated with
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`@Directive`, which specifies the selector identifying
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the attribute associated with the directive.
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An attribute directive minimally requires building a controller class annotated with
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`@Directive`, which specifies the selector that identifies
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the attribute.
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The controller class implements the desired directive behavior.
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Let's build a small illustrative example together.
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This page demonstrates building a simple attribute
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directive to set an element's background color
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when the user hovers over that element.
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.l-sub-section
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:marked
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Technically, a directive isn't necessary to simply set the background color. Style binding can set styles as follows:
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+makeExample('attribute-directives/ts/app/app.component.1.html','p-style-background')
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:marked
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Read more about [style binding](template-syntax.html#style-binding) on the [Template Syntax](template-syntax.html) page.
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For a simple example, though, this will demonstrate how attribute directives work.
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:marked
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### Our first draft
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Create a new project folder (`attribute-directives`) and follow the steps in the [QuickStart](../quickstart.html).
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### Write the directive code
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Create a new project folder (`attribute-directives`) and follow the steps in [QuickStart](../quickstart.html).
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include ../_quickstart_repo
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:marked
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Create the following source file in the indicated folder with the given code:
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Create the following source file in the indicated folder with the following code:
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+makeExample('app/highlight.directive.1.ts')
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block highlight-directive-1
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:marked
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We begin by importing some symbols from the Angular `core`.
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We need the `Directive` symbol for the `@Directive` decorator.
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We need the `ElementRef` to [inject](dependency-injection.html) into the directive's constructor
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so we can access the DOM element.
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We also need `Renderer` so we can change the DOM element's style.
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We don't need `Input` immediately but we will need it later in the chapter.
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Then we define the directive metadata in a configuration object passed
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as an argument to the `@Directive` decorator function.
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The `import` statement specifies symbols from the Angular `core`:
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1. `Directive` provides the functionality of the `@Directive` decorator.
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1. `ElementRef` [injects](dependency-injection.html) into the directive's constructor
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so the code can access the DOM element.
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1. `Input` allows data to flow from the binding expression into the directive.
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1. `Renderer` allows the code to change the DOM element's style.
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Next, the `@Directive` decorator function contains the directive metadata in a configuration object
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as an argument.
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:marked
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`@Directive` requires a CSS selector to identify
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the HTML in the template that is associated with our directive.
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the HTML in the template that is associated with the directive.
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The [CSS selector for an attribute](https://developer.mozilla.org/en-US/docs/Web/CSS/Attribute_selectors)
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is the attribute name in square brackets.
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Our directive's selector is `[myHighlight]`.
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Angular will locate all elements in the template that have an attribute named `myHighlight`.
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Here, the directive's selector is `[myHighlight]`.
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Angular will locate all elements in the template that have an attribute named `myHighlight`.
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.l-sub-section
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:marked
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### Why not call it "highlight"?
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*highlight* is a nicer name than *myHighlight* and, technically, it would work if we called it that.
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However, we recommend picking a selector name with a prefix to ensure
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that it cannot conflict with any standard HTML attribute, now or in the future.
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There is also less risk of colliding with a third-party directive name when we give ours a prefix.
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Though *highlight* is a more concise name than *myHighlight* and would work,
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a best practice is to prefix selector names to ensure
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they don't conflict with standard HTML attributes.
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This also reduces the risk colliding with third-party directive names.
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We do **not** prefix our `highlight` directive name with **`ng`**.
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That prefix belongs to Angular.
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Make sure you do **not** prefix the `highlight` directive name with **`ng`** because
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that prefix is reserved for Angular and using it could cause bugs that are difficult to diagnose. For a simple demo, the short prefix, `my`, helps distinguish your custom directive.
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We need a prefix of our own, preferably short, and `my` will do for now.
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p
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| After the #[code @Directive] metadata comes the directive's controller class, which contains the logic for the directive.
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| After the #[code @Directive] metadata comes the directive's controller class, called #[code HighlightDirective], which contains the logic for the directive.
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+ifDocsFor('ts')
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| We export `HighlightDirective` to make it accessible to other components.
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| Exporting #[code HighlightDirective] makes it accessible to other components.
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:marked
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Angular creates a new instance of the directive's controller class for
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each matching element, injecting an Angular `ElementRef` and `Renderer`
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into the constructor.
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`ElementRef` is a service that grants us direct access to the DOM element
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through its `nativeElement` property and with `Renderer` we can set the element style.
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`ElementRef` is a service that grants direct access to the DOM element
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through its `nativeElement` property and `Renderer` allows the code to set the element style.
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.l-main-section
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a#apply-directive
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:marked
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## Apply the attribute directive
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The `AppComponent` in this sample is a test harness for our `HighlightDirective`.
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Let's give it a new template that
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To use the new `HighlightDirective`, create a template that
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applies the directive as an attribute to a paragraph (`p`) element.
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In Angular terms, the `<p>` element will be the attribute **host**.
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p
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| We'll put the template in its own
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| Put the template in its own
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code #[+adjExPath('app.component.html')]
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| file that looks like this:
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+makeExample('attribute-directives/ts/app/app.component.1.html',null,'app/app.component.html')(format=".")
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:marked
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A separate template file is clearly overkill for a 2-line template.
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Hang in there; we're going to expand it later.
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Meanwhile, we'll revise the `AppComponent` to reference this template.
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Now reference this template in the `AppComponent`:
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+makeExample('attribute-directives/ts/app/app.component.ts',null,'app/app.component.ts')
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:marked
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We'll add an `import` statement to fetch the 'Highlight' directive and,
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added that class to the `declarations` NgModule metadata so that Angular
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will recognize our directive when it encounters `myHighlight` in the template.
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Next, add an `import` statement to fetch the `Highlight` directive and
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add that class to the `declarations` NgModule metadata. This way Angular
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recognizes the directive when it encounters `myHighlight` in the template.
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+makeExample('attribute-directives/ts/app/app.module.ts',null,'app/app.module.ts')
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:marked
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We run the app and see that our directive highlights the paragraph text.
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Now when the app runs, the `myHighlight` directive highlights the paragraph text.
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figure.image-display
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img(src="/resources/images/devguide/attribute-directives/first-highlight.png" alt="First Highlight")
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.l-sub-section
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:marked
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### Your directive isn't working?
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### Your directive isn't working?
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Did you remember to add the directive to the the `declarations` attribute of `@NgModule`? It is easy to forget!
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Open the console in the browser tools and look for an error like this:
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code-example(format="nocode").
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EXCEPTION: Template parse errors:
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Can't bind to 'myHighlight' since it isn't a known property of 'p'.
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:marked
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Angular detects that we're trying to bind to *something* but it doesn't know what.
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We have to tell it by listing `HighlightDirective` in the `declarations` metadata array.
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Angular detects that you're trying to bind to *something* but it doesn't know what,
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so it looks to the `declarations` metadata array. By specifying `HighlightDirective`
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in the array, Angular knows to check the import statements and from there,
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to go to `highlight.directive.ts` to find out what `myHighlight` does.
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:marked
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Let's recap what happened.
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Angular found the `myHighlight` attribute on the `<p>` element. It created
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an instance of the `HighlightDirective` class,
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injecting a reference to the element into the constructor
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where we set the `<p>` element's background style to yellow.
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To summarize, Angular found the `myHighlight` attribute on the `<p>` element. It created
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an instance of the `HighlightDirective` class,
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injecting a reference to the element into the constructor
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where the `<p>` element's background style is set to yellow.
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.l-main-section
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a#respond-to-user
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:marked
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## Respond to user action
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## Respond to user-initiated events
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We are not satisfied to simply set an element color.
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Our directive should set the color in response to a user action.
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Specifically, we want to set the color when the user hovers over an element.
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We'll need to
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1. detect when the user hovers into and out of the element,
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2. respond to those actions by setting and clearing the highlight color, respectively.
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We apply the `@HostListener` !{_decorator} to methods which are called when an event is raised.
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Currently, `myHighlight` simply sets an element color.
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The directive should set the color when the user hovers over an element.
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This requires two things:
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1. detecting when the user hovers into and out of the element.
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2. responding to those actions by setting and clearing the highlight color.
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To do this, you can apply the `@HostListener` !{_decorator} to methods which are called when an event is raised.
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+makeExample('attribute-directives/ts/app/highlight.directive.2.ts','host')(format=".")
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.l-sub-section
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:marked
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The `@HostListener` !{_decorator} refers to the DOM element that hosts our attribute directive, the `<p>` in our case.
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We could have attached event listeners by manipulating the host DOM element directly, but
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there are at least three problems with such an approach:
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1. We have to write the listeners correctly.
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1. We must *detach* our listener when the directive is destroyed to avoid memory leaks.
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1. We'd be talking to DOM API directly which, we learned, is something to avoid.
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The `@HostListener` !{_decorator} refers to the DOM element that hosts an attribute directive, the `<p>` in this case.
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It is possible to attach event listeners by manipulating the host DOM element directly, but
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there are at least three problems with such an approach:
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1. You have to write the listeners correctly.
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1. The code must *detach* the listener when the directive is destroyed to avoid memory leaks.
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1. Talking to DOM API directly isn't a best practice.
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Let's roll with the `@HostListener` !{_decorator}.
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:marked
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Now we implement the two mouse event handlers:
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Now implement the two mouse event handlers:
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+makeExample('attribute-directives/ts/app/highlight.directive.2.ts','mouse-methods')(format=".")
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:marked
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Notice that they delegate to a helper method that sets the color via a private local variable, `#{_priv}el`.
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We revise the constructor to capture the `ElementRef.nativeElement` in this variable.
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Next, revise the constructor to capture the `ElementRef.nativeElement` in this variable.
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+makeExample('attribute-directives/ts/app/highlight.directive.2.ts','ctor')(format=".")
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:marked
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Here's the updated directive:
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+makeExample('app/highlight.directive.2.ts')
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:marked
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We run the app and confirm that the background color appears as we move the mouse over the `p` and
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disappears as we move out.
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Run the app and confirm that the background color appears when the mouse hovers over the `p` and
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disappears as it moves out.
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figure.image-display
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img(src="/resources/images/devguide/attribute-directives/highlight-directive-anim.gif" alt="Second Highlight")
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.l-main-section
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a#bindings
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:marked
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## Configure the directive with binding
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## Pass values into the directive using data binding
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Currently the highlight color is hard-coded within the directive. That's inflexible.
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We should set the color externally with a binding like this:
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A better practice is to set the color externally with a binding as follows:
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+makeExample('attribute-directives/ts/app/app.component.html','pHost')
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:marked
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We'll extend our directive class with a bindable **input** `highlightColor` property and use it when we highlight text.
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You can extend the directive class with a bindable **input** `highlightColor` property and use it to highlight text.
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Here is the final version of the class:
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+makeExcerpt('app/highlight.directive.ts', 'class')
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a#input
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:marked
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The new `highlightColor` property is called an *input* property because data flows from the binding expression into our directive.
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The new `highlightColor` property is called an *input* property because data flows from the binding expression into the directive.
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Notice the `@Input()` #{_decorator} applied to the property.
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+makeExcerpt('app/highlight.directive.ts', 'color')
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:marked
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`@Input` adds metadata to the class that makes the `highlightColor` property available for
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property binding under the `myHighlight` alias.
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We must add this input metadata or Angular will reject the binding.
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See the [appendix](#why-input) below to learn why.
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`@Input` adds metadata to the class that makes the `highlightColor` property available for
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property binding under the `myHighlight` alias.
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Without this input metadata Angular rejects the binding.
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See the [appendix](#why-input) below for more information.
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.l-sub-section
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:marked
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### @Input(_alias_)
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The developer who uses this directive expects to bind to the attribute name, `myHighlight`.
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The directive property name is `highlightColor`. That's a disconnect.
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We could resolve the discrepancy by renaming the property to `myHighlight` and define it as follows:
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+makeExcerpt('app/highlight.directive.ts', 'highlight', '')
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:marked
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Maybe we don't want that property name inside the directive perhaps because it
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doesn't express our intention well.
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We can **alias** the `highlightColor` property with the attribute name by
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### @Input(_alias_)
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Currently, the code **aliases** the `highlightColor` property with the attribute name by
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passing `myHighlight` into the `@Input` #{_decorator}:
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+makeExcerpt('app/highlight.directive.ts', 'color', '')
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:marked
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The code binds to the attribute name, `myHighlight`, but the
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the directive property name is `highlightColor`. That's a disconnect.
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You can resolve the discrepancy by renaming the property to `myHighlight` and define it as follows:
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+makeExcerpt('app/highlight.directive.ts', 'highlight', '')
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:marked
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Now that we're getting the highlight color as an input, we modify the `onMouseEnter()` method to use
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it instead of the hard-coded color name.
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We also define red as the default color to fallback on in case
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the user neglects to bind with a color.
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Now that you're getting the highlight color as an input, modify the `onMouseEnter()` method to use
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it instead of the hard-coded color name and define red as the default color.
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+makeExcerpt('attribute-directives/ts/app/highlight.directive.ts', 'mouse-enter', '')
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:marked
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Now we'll update our `AppComponent` template to let
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users pick the highlight color and bind their choice to our directive.
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Here is the updated template:
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To let users pick the highlight color and bind their choice to the directive,
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update `app.component.html` as follows:
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+makeExcerpt('attribute-directives/ts/app/app.component.html', 'v2', '')
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.l-sub-section
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:marked
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### Where is the templated *color* property?
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The eagle-eyed may notice that the radio button click handlers in the template set a `color` property
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and we are binding that `color` to the directive.
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We should expect to find a `color` on the host `AppComponent`.
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**We never defined a color property for the host *AppComponent***!
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And yet this code works. Where is the template `color` value going?
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Browser debugging reveals that Angular dynamically added a `color` property
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You may notice that the radio button click handlers in the template set a `color` property
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and the code is binding that `color` to the directive.
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However, you never defined a color property for the host `AppComponent`.
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Yet this code works. Where is the template `color` value going?
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Browser debugging reveals that Angular dynamically added a `color` property
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to the runtime instance of the `AppComponent`.
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This is *convenient* behavior but it is also *implicit* behavior that could be confusing.
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While it's cool that this technique works, we recommend adding the `color` property to the `AppComponent`.
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This is *convenient* behavior but it is also *implicit* behavior that could be confusing.
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For clarity, consider adding the `color` property to the `AppComponent`.
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:marked
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Here is our second version of the directive in action.
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||||
Here is the second version of the directive in action.
|
||||
figure.image-display
|
||||
img(src="/resources/images/devguide/attribute-directives/highlight-directive-v2-anim.gif" alt="Highlight v.2")
|
||||
|
||||
.l-main-section
|
||||
a#second-property
|
||||
:marked
|
||||
## Bind to a second property
|
||||
Our directive only has a single, customizable property. What if we had ***two properties***?
|
||||
|
||||
Let's allow the template developer to set the default color, the color that prevails until the user picks a highlight color.
|
||||
We'll add a second **input** property to `HighlightDirective` called `defaultColor`:
|
||||
This example directive only has a single customizable property. A real app often needs more.
|
||||
|
||||
Let's allow the template developer to set the default color—the color that prevails until the user picks a highlight color.
|
||||
To do this, first add a second **input** property to `HighlightDirective` called `defaultColor`:
|
||||
+makeExample('attribute-directives/ts/app/highlight.directive.ts', 'defaultColor')(format=".")
|
||||
:marked
|
||||
The `defaultColor` property has a setter that overrides the hard-coded default color, "red".
|
||||
We don't need a getter.
|
||||
|
||||
How do we bind to it? We already "burned" the `myHighlight` attribute name as a binding target.
|
||||
|
||||
Remember that a *component is a directive too*.
|
||||
We can add as many component property bindings as we need by stringing them along in the template
|
||||
as in this example that sets the `a`, `b`, `c` properties to the string literals 'a', 'b', and 'c'.
|
||||
You don't need a getter.
|
||||
|
||||
How do you bind to it? The app is already using `myHighlight` attribute name as a binding target.
|
||||
|
||||
Remember that a *component is a directive, too*.
|
||||
You can add as many component property bindings as you need by stringing them along in the template
|
||||
as in this example that sets the `a`, `b`, `c` properties to the string literals 'a', 'b', and 'c'.
|
||||
code-example(format="." ).
|
||||
<my-component [a]="'a'" [b]="'b'" [c]="'c'"><my-component>
|
||||
:marked
|
||||
We do the same thing with an attribute directive.
|
||||
The same holds true for an attribute directive.
|
||||
+makeExample('attribute-directives/ts/app/app.component.html', 'defaultColor')(format=".")
|
||||
:marked
|
||||
Here we're binding the user's color choice to the `myHighlight` attribute as we did before.
|
||||
We're *also* binding the literal string, 'violet', to the `defaultColor`.
|
||||
|
||||
Here the code is binding the user's color choice to the `myHighlight` attribute as before.
|
||||
It is *also* binding the literal string, 'violet', to the `defaultColor`.
|
||||
|
||||
|
||||
Here is the final version of the directive in action.
|
||||
figure.image-display
|
||||
img(src="/resources/images/devguide/attribute-directives/highlight-directive-final-anim.gif" alt="Final Highlight")
|
||||
|
@ -307,17 +308,17 @@ figure.image-display
|
|||
.l-main-section
|
||||
:marked
|
||||
## Summary
|
||||
We now know how to
|
||||
- [build a simple **attribute directive** to attach behavior to an HTML element](#write-directive),
|
||||
- [use that directive in a template](#apply-directive),
|
||||
- [respond to **events** to change behavior based on an event](#respond-to-user),
|
||||
- and [use **binding** to pass values to the attribute directive](#bindings).
|
||||
This page covered how to:
|
||||
- [Build a simple **attribute directive** to attach behavior to an HTML element](#write-directive).
|
||||
- [Use that directive in a template](#apply-directive).
|
||||
- [Respond to **events** to change behavior based on an event](#respond-to-user).
|
||||
- [Use **binding** to pass values to the attribute directive](#bindings).
|
||||
|
||||
The final source:
|
||||
|
||||
+makeTabs(
|
||||
`attribute-directives/ts/app/app.component.ts,
|
||||
attribute-directives/ts/app/app.component.html,
|
||||
attribute-directives/ts/app/app.component.html,
|
||||
attribute-directives/ts/app/highlight.directive.ts,
|
||||
attribute-directives/ts/app/app.module.ts,
|
||||
attribute-directives/ts/app/main.ts,
|
||||
|
@ -337,39 +338,38 @@ a#why-input
|
|||
.l-main-section
|
||||
:marked
|
||||
### Appendix: Input properties
|
||||
|
||||
Earlier we declared the `highlightColor` property to be an ***input*** property of our
|
||||
`HighlightDirective`
|
||||
|
||||
We've seen properties in bindings before.
|
||||
We never had to declare them as anything. Why now?
|
||||
|
||||
|
||||
In this demo, the `highlightColor` property is an ***input*** property of
|
||||
`HighlightDirective`.
|
||||
|
||||
You've seen properties in bindings before but never had to declare them as anything. Why now?
|
||||
|
||||
Angular makes a subtle but important distinction between binding **sources** and **targets**.
|
||||
|
||||
|
||||
In all previous bindings, the directive or component property was a binding ***source***.
|
||||
A property is a *source* if it appears in the template expression to the ***right*** of the equals (=).
|
||||
|
||||
A property is a *target* when it appears in **square brackets** ([ ]) to the **left** of the equals (=) ...
|
||||
as it is does when we bind to the `myHighlight` property of the `HighlightDirective`,
|
||||
|
||||
A property is a *target* when it appears in **square brackets** ([ ]) to the **left** of the equals (=)
|
||||
as it is does when binding to the `myHighlight` property of the `HighlightDirective`.
|
||||
+makeExample('attribute-directives/ts/app/app.component.html','pHost')(format=".")
|
||||
:marked
|
||||
The 'color' in `[myHighlight]="color"` is a binding ***source***.
|
||||
A source property doesn't require a declaration.
|
||||
|
||||
|
||||
The 'myHighlight' in `[myHighlight]="color"` *is* a binding ***target***.
|
||||
We must declare it as an *input* property.
|
||||
Angular rejects the binding with a clear error if we don't.
|
||||
|
||||
You must declare it as an *input* property or
|
||||
Angular rejects the binding with a clear error.
|
||||
|
||||
Angular treats a *target* property differently for a good reason.
|
||||
A component or directive in target position needs protection.
|
||||
|
||||
Imagine that our `HighlightDirective` did truly wonderous things.
|
||||
We graciously made a gift of it to the world.
|
||||
|
||||
To our surprise, some people — perhaps naively —
|
||||
started binding to *every* property of our directive.
|
||||
Not just the one or two properties we expected them to target. *Every* property.
|
||||
That could really mess up our directive in ways we didn't anticipate and have no desire to support.
|
||||
|
||||
The *input* declaration ensures that consumers of our directive can only bind to
|
||||
the properties of our public API ... nothing else.
|
||||
|
||||
Imagine that `HighlightDirective` did truly wonderous things in a
|
||||
popular open source project.
|
||||
|
||||
Surprisingly, some people — perhaps naively —
|
||||
start binding to *every* property of the directive.
|
||||
Not just the one or two properties you expected them to target. *Every* property.
|
||||
That could really mess up your directive in ways you didn't anticipate and have no desire to support.
|
||||
|
||||
The ***input*** declaration ensures that consumers of your directive can only bind to
|
||||
the properties of the public API but nothing else.
|
||||
|
|
Loading…
Reference in New Issue