377 lines
16 KiB
Plaintext
377 lines
16 KiB
Plaintext
include ../_util-fns
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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
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* apply the attribute directive to an element in a template
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* respond to user-initiated events
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* pass values into the directive using data binding
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[Live Example](/resources/live-examples/attribute-directives/ts/plnkr.html)
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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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The *Component* is really a directive with a template.
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It's the most common of the three directives and we write lots of them as we build our application.
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The [*Structural* directive](structural-directives.html) changes the DOM layout by adding and removing DOM elements.
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[NgFor](template-syntax.html#ng-for) and [NgIf](template-syntax.html#ng-if) are two familiar examples.
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The *Attribute* directive changes the appearance or behavior of an element.
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The built-in [NgStyle](template-syntax.html#ng-style) directive, 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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code-example.
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<p [style.background]="'lime'">I am green with envy!</p>
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<br>
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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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: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 a
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`Directive` decorator. The `Directive` decorator specifies the selector identifying
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the attribute associated with the directive.
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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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: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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include ../_quickstart_repo
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:marked
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Add a new file to the `app` folder called `highlight.directive.ts` and add the following code:
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+makeExample('attribute-directives/ts/app/highlight.directive.1.ts', null, 'app/highlight.directive.ts')
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:marked
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We begin by importing some symbols from the Angular library.
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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 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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A `@Directive` decorator for an attribute 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 [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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.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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We do **not** prefix our `highlight` directive name with **`ng`**.
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That prefix belongs to Angular and
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we don't want to confuse our directives with their directives.
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We need a prefix of our own, preferably short, and `my` will do for now.
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:marked
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After the `@Directive` metadata comes the directive's controller class which we are exporting
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to make it accessible to other components.
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The directive's controller class contains the logic for the directive.
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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`
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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.
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That's all we need to set the element's background color using the browser DOM API.
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.l-main-section
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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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applies the directive as an attribute to a `span` element.
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In Angular terms, the `<span>` element will be the attribute **host**.
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We'll put the template in its own `app.component.html` 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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+makeExample('attribute-directives/ts/app/app.component.ts',null,'app/app.component.ts')
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:marked
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We've added an `import` statement to fetch the 'Highlight' directive and
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added that class to a `directives` array in the component metadata so that Angular
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will recognize our directive when it encounters `myHighlight` in the template.
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We run the app and see that our directive highlights the span 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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#### Why isn't my directive working?
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Did you remember to set the `directives` array? 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("").
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EXCEPTION: Template parse errors:
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Can't bind to 'myHighlight' since it isn't a known native property
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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 `directives` metadata array.
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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 `<span>` 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 `<span>` element's background style to yellow.
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.l-main-section
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:marked
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## Respond to user action
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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 mouses over the element.
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We'll need to
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1. detect when the user mouses into and out of the element
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1. respond to those actions by setting and clearing the highlight color.
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Start with event detection.
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We add a `host` property to the directive metadata and give it a configuration object
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that specifies two mouse events and the directive methods to call when they are raised.
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+makeExample('attribute-directives/ts/app/highlight.directive.2.ts','host')(format=".")
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:marked
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.l-sub-section
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:marked
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The `host` property refers to the DOM element that hosts our attribute directive, the `<span>` in our case.
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We could have attached an event listener to the native element (`el.nativeElement`) with
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plain old JavaScript.
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There are at least three problems with that 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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Let's roll with the `host` property.
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:marked
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Now we implement those 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 to set the color.
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We no longer need the constructor body but
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we still want the injected `ElementRef`.
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We revise the constructor signature to capture the injected `ElementRef` in a private variable
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and clear the body.
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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('attribute-directives/ts/app/highlight.directive.2.ts',null, 'app/highlight.directive.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 `span` and
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disappears as we move 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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:marked
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.l-main-section
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:marked
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## Configure the directive with 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 highlight color externally with a binding like this:
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+makeExample('attribute-directives/ts/app/app.component.html','span')
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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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Here is the final version of the class:
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+makeExample('attribute-directives/ts/app/highlight.directive.ts', 'class-1', 'app/highlight.directive.ts (class only)')
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<a id="input"></a>
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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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Notice that we call the `@Input()` decorator function while defining the property.
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+makeExample('attribute-directives/ts/app/highlight.directive.ts', 'color')
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:marked
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This `@Input` decorator adds metadata to the class that makes the `highlightColor` property available for property binding
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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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.l-sub-section
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:marked
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### @Input(alias)
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The developer who uses our 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 can resolve the discrepancy by renaming the property to `myHighlight` and define it as follows:
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+makeExample('attribute-directives/ts/app/highlight.directive.ts', 'highlight')
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<br>
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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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passing `myHighlight` into the `@Input` decorator:
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+makeExample('attribute-directives/ts/app/highlight.directive.ts', 'color')
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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 a red default color as a fallback in case
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the user neglects to bind with a color.
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+makeExample('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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+makeExample('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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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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:marked
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Here is our second version of the directive in action.
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figure.image-display
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img(src="/resources/images/devguide/attribute-directives/highlight-directive-v2-anim.gif" alt="Highlight v.2")
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.l-main-section
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:marked
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## Bind to a second property
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Our directive only has a single, customizable property. What if we had ***two properties***?
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Let's let the template developer set the default color, the color that prevails until the user picks a highlight color.
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We'll add a second **input** property to `HighlightDirective` called `defaultColor`:
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+makeExample('attribute-directives/ts/app/highlight.directive.ts', 'defaultColor')(format=".")
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:marked
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The `defaultColor` property has a setter that overrides the hard-coded default color, "red".
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We don't need a getter.
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How do we bind to it? We already "burned" the `myHighlight` attribute name as a binding target.
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Remember that a *component is a directive too*.
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We can add as many component property bindings as we need by stringing them along in the template
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as in this example that sets the `a`, `b`, `c` properties to the string literals 'a', 'b', and 'c'.
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code-example(format="." ).
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<my-component [a]="'a'" [b]="'b'" [c]="'c'"><my-component>
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:marked
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We do the same thing with an attribute directive.
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+makeExample('attribute-directives/ts/app/app.component.html', 'defaultColor')(format=".")
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:marked
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Here we're binding the user's color choice to the `myHighlight` attribute as we did before.
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We're *also* binding the literal string, 'violet', to the `defaultColor`.
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Here is the final version of the directive in action.
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figure.image-display
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img(src="/resources/images/devguide/attribute-directives/highlight-directive-final-anim.gif" alt="Final Highlight")
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.l-main-section
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:marked
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## Summary
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Now we know how to
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- build a simple **attribute directive** to attach behavior to an HTML element,
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- use that directive in a template,
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- respond to **events** to change behavior based on an event,
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- and use **binding** to pass values to the attribute directive.
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The final source:
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+makeTabs(
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`attribute-directives/ts/app/app.component.ts,
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attribute-directives/ts/app/app.component.html,
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attribute-directives/ts/app/highlight.directive.ts,
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attribute-directives/ts/app/main.ts,
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attribute-directives/ts/index.html
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`,
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',,full',
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`app.component.ts,
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app.component.html,
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highlight.directive.ts,
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main.ts,
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index.html
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`)
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<a id="why-input"></a>
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.l-main-section
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:marked
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### Appendix: Input properties
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Earlier we declared the `highlightColor` property to be an ***input*** property of our
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`HighlightDirective`
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We've seen properties in bindings before.
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We never had to declare them as anything. Why now?
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Angular makes a subtle but important distinction between binding **sources** and **targets**.
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In all previous bindings, the directive or component property was a binding ***source***.
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A property is a *source* if it appears in the template expression to the ***right*** of the equals (=).
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A property is a *target* when it appears in **square brackets** ([ ]) to the **left** of the equals (=) ...
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as it is does when we bind to the `myHighlight` property of the `HighlightDirective`,
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+makeExample('attribute-directives/ts/app/app.component.html','span')(format=".")
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:marked
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The 'color' in `[myHighlight]="color"` is a binding ***source***.
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A source property doesn't require a declaration.
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The 'myHighlight' in `[myHighlight]="color"` *is* a binding ***target***.
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We must declare it as an *input* property.
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Angular rejects the binding with a clear error if we don't.
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Angular treats a *target* property differently for a good reason.
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A component or directive in target position needs protection.
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Imagine that our `HighlightDirective` did truly wonderous things.
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We graciously made a gift of it to the world.
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To our surprise, some people — perhaps naively —
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started binding to *every* property of our directive.
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Not just the one or two properties we expected them to target. *Every* property.
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That could really mess up our directive in ways we didn't anticipate and have no desire to support.
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The *input* declaration ensures that consumers of our directive can only bind to
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the properties of our public API ... nothing else.
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