"intro":"Displaying data is job number one for any good application. In Angular, you bind data to elements in HTML templates and Angular automatically updates the UI as data changes."
"intro":"In Angular, we display data by binding component properties to elements in HTML templates using interpolation and other forms of Property Binding."
Angular components use properties to identify the data associated with the component. For example, a hero component may have properties such as a hero name. Let's walk through how we'd display a property, a list of properties, and then conditionally show content based on state. We'll end up with a UI that looks like this:
:markdown
## Displaying Component Properties
figure.image-display
We typically display data in Angular by binding controls in an HTML template
to properties of an Angular Component.
In this chapter, we'll create a component with a list of heroes. Each hero has a name.
We'll display the list of hero names and
conditionally show a selected hero in a detail area below the list.
The simple technique for displaying the data from a component property is to bind the property name through interpolation. With interpolation, you put the property name in the view template enclosed in double curly braces: { { myHero } } .
To see this working, follow the steps in the Getting Started section. Then modify the app.ts file as follows:
import {Component, View, bootstrap} from 'angular2/angular2';
@Compo
nent({
selector: 'my-app'
})
@View({
template: '<h1>{{title}}</h1><h2>My favorite hero is: {{myHero}}</h2>'
})
class AppComponent {
title: string = 'Tour of Heroes';
myHero: string = 'Windstorm';
}
bootstrap(AppComponent);
The template is a multi-line string within ECMAScript 2015 back-tics (\`).
The back-tick (\`) is not the same character as a single quote (').
It has many nice features. The feature we're exploiting is
the ability to compose the string over several lines which
makes for much more readable HTML.
:markdown
Angular automatically pulls the value of the `title` and `myHero` properties from the component and
inserts those values into the browser. Angular will update the display
when these properties change.
.l-sub-section
:markdown
This code defines a component and associated view for the app. The component now has two properties: title and myHero. The view defines a template that displays those two properties using interpolation:
More precisely, the re-display occurs after some kind of asynchronous event related to
the view such as a keystroke, a timer completion, or an asynch `XHR` response.
We don't have those in this sample.
But then the properties aren't changing on their own either. For the moment we must operate on faith.
:markdown
Notice that we haven't called **new** to create an instance of the `AppComponent` class.
Angular automatically pulls the value of the title and myHero properties from the component and inserts those values into the browser. Angular automatically updates the display whenever the property value changes.
:markdown
When we bootstrap with the `AppComponent` class (see the bottom of `app.ts`), Angular looks for a `<my-app>`
in the `index.html`, finds it, instantiates an instance of `AppComponent`, and renders it
inside the `<my-app>` tag.
One thing to notice here is that we haven't called **new** to create an instance of the AppComponent class. When defining the component in app.ts, we identified a selector named ‘my-app’. As shown in the Getting Started section, we used an HTML element named 'my-app' in the index.html file. By associating the AppComponent with the element named 'my-app' in the DOM, Angular knows to automatically call new on AppComponent and bind its properties to that part of the template.
## Template inline or template file?
There are two techniques for defining a template for the view associated with a component. The template can be defined inline using the template property, as shown in the example above. Or the template can be defined in a separate HTML file and referenced using the templateUrl property.
We can store our component's template in one of two places.
We can define it "inline" using the template property as we do here.
Or we can define the template in a separate HTML file and link to it in
the component metadata using the `@Component` decorator's `templateUrl` property.
In either case, when building templates the data bindings have access to the same scope of properties as the component class. Here, the class AppComponent and has two properties: title and myHero. The template can bind to either or both of those properties.
We're using the *inline* style because the template is small and it makes for clearer demonstration.
The choice between them is a matter of taste, circumstances, and organization policy.
In either style, The template data bindings have the same access to the component's properties.
## Constructor or variable initialization?
We initialized our component properties using variable assignment.
This is a wonderfully concise and compact technique.
Some folks prefer to declare the properties and initialize them within a constructor like this:
That's fine too. The choice between them is a matter of taste and organization policy.
We'll adopt the more terse "variable assignment" style in this chapter simply because
there will be less code to read.
.l-main-section
:markdown
## Showing an array property with NgFor
Moving up from a single property, let’s create an array to display as a list. And let’s move the initialization of the properties to the class constructor.
We want to display a list of heroes. We begin by adding a mock heroes name array to the component,
just above `myHero` and redefine `myHero` to be the first name in the array.
We added a somewhat mysterious `*ng-for` to the `<li>` element.
That's the Angular "repeater" directive.
It's presence on the `<li>` tag marks that `<li>` element (and its children) as the "repeater template".
.alert.is-important
:markdown
We can use the NgFor directive in the template to display each item in this array as shown below. Add the NgFor directive to one of the DOM elements. Angular then creates a copy of that DOM element for each item in the array.
Don't forget the leading asterisk (\*) in front of `*ng-for`. It is an essential part of the syntax.
Learn more about this and `NgFor` in the [Template Syntax](./template-syntax.html#ng-for) chapter.
code-example(format="linenums" language="html").
template: `
<h1>{{title}}</h1>
<h2>My favorite hero is: {{myHero}}</h2>
<p>Heroes:</p>
<ul>
<li *ng-for="#hero of heroes">
{{ hero }}
</li>
</ul>
`
:markdown
Notice the `#hero` in the `NgFor` double-quoted instruction.
The `#hero` is a "[template local variable](./template-syntax.html#local-vars")" *declaration*.
The (#) prefix declares a local variable name named `hero`.
Angular will duplicate the `<li>` for each item in the list, setting the `hero` variable
to the item (the hero) in the current iteration. Angular uses that variable as the
context for the interpolation in the double curly braces.
.l-sub-section
:markdown
Notice that we are now using the backtick instead of the single quote to enclose the template. This allows us to define multiple lines of HTML for the template.
We happened to give `NgFor` an array to display.
In fact, `NgFor` can repeat items for any [iterable](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Iteration_protocols)
object.
:markdown
## Register the NgFor Directive
We added the NgFor to the li element, so Angular will define an li element for each item in the list. However, by default Angular does not automatically include the NgFor directive. We have to do that manually by making two changes to the app.ts file.
Angular doesn't know that this template uses the `NgFor` directive.
Our application will not run right now. Angular will complain that it doesn't know what `NgFor` is.
First, we need to add NgFor to the import statement as follows:
```
import {Component, View, bootstrap, NgFor} from 'angular2/angular2';
```
Second, we need to define NgFor as a directive accessible to the view as follows:
We have to register the `NgFor` directive with the component metadata by making two changes to the app.ts file.
code-example(format="linenums" language="html").
@View({
template: `
<h1>{{title}}</h1>
<h2>My favorite hero is: {{myHero}}</h2>
<p>Heroes:</p>
<ul>
<li *ng-for="#hero of heroes">
{{ hero }}
</li>
</ul>
`,
directives: [NgFor]
})
First, we import the `NgFor` symbol from the Angular library by extending the existing `import` statement.
The heroes then appear in the view as an unordered list. Angular will automatically update the display any time that the list changes. Add a new item and it appears in the list. Delete an item and Angular deletes the item from the list. Reorder items and Angular makes the corresponding reorder of the DOM list.
:markdown
Second, we register `NgFor` as a directive accessible to the template by updating the
`@Component` decorator with a `directives` array property whose only item is `NgFor`:
Let's look again at the few lines of HTML that perform this operation:
Now the heroes will appear in the view as an unordered list.
:markdown
Breaking this down:
- *ng-for : creates a DOM element for each item in an [iterable](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Iteration_protocols) like an array
- #hero : defines a local variable that refers to individual values of the iterable as 'hero'
- of heros : the iterable to use is called 'heroes' in the current component
Using this NgFor syntax, we can display data from any iterable object.
Before we get too much further, note that putting our data model directly in our component doesn’t follow best practices. We should separate the concerns by having another class serve the role of model and use it in the component.
We are defining our data directly inside our component.
That's fine for a demo but certainly isn't a best practice. It's not even a good practice.
We won't do anything about that in this chapter.
We’ll add this class to the app.ts file to minimize the number of files required for this demo. But you would want to define a separate hero.ts file for this.
```
class Hero {
id: number;
name: string;
}
```
Here we define a Hero class with two properties: id and name.
At the moment, we're binding to an array of strings. We do that occasionally in real applications but
most of the time we're displaying objects, potentially instances of classes.
We can then change the AppComponent to use this new class as a data type:
```
class AppComponent {
title: string;
myHero: Hero;
heroes: Hero[];
Let's turn our array of hero names into an array of `Hero` objects. For that we'll need a `Hero' class.
constructor() {
this.title = 'Tour of Heroes';
this.myHero = {
id: 1,
name: 'Windstorm'
};
this.heroes = [
{ "id": 1, "name": "Windstorm" },
{ "id": 15, "name": "Magneta" },
{ "id": 20, "name": "Tornado" }
];
}
}
```
We also need to change the template to access the appropriate class property:
Create a new file called `hero.ts` and add the following short snippet to it.
The application should work as before, but it now uses the Hero class to define the hero properties.
Don't forget the leading asterisk (\*) in front of `*ng-if`. It is an essential part of the syntax.
Learn more about this and `NgIf` in the [Template Syntax](./template-syntax.html#ng-if) chapter.
## Conditionally displaying data with NgIf
:markdown
The [template expression](./template-syntax.html#template-expressions) inside the double quotes
looks much like JavaScript and it is much like JavaScript.
When the component's list of heroes has more than 3 items, Angular adds the paragraph to the DOM and the message appears.
If there were 3 or fewer items, Angular omits the the paragraph and there is no message.
There may be times that the app needs to conditionally display data. For example, only display a message if a specific condition is true. We can conditionally display data using NgIf. The NgIf directive adds or removes elements from the DOM based on an expression.
.alert.is-helpful
:markdown
Angular isn't showing and hiding the message. It is adding and removing the paragraph element from the DOM.
That hardly matters here. It would matter a great deal from a performance perspective if
we were conditionally including or excluding a big chunk of HTML with many data bindings.
See it in action by adding a paragraph at the end of the template as shown below:
```
<p *ng-if="heroes.length > 3">There are many heroes!</p>
```
If the list of heroes has more than 3 items, the paragraph is added to the DOM and the message appears. If there are 3 or fewer items, the paragraph won’t be added to the DOM and no message appears.
:markdown
As with the `NgFor`, we must add the `NgIf` directive to the component's metadata.
As with the NgFor, we’ll need to add the NgIf directive so Angular knows to include it. Add it to the import:
```
import {Component, View, bootstrap, NgFor, NgIf} from 'angular2/angular2';
```
And add it to the directives array:
```
directives: [NgFor, NgIf]
```
Since there are four items in the array, the message should appear. Delete one of the elements from the array, refresh the browser and the message should no longer appear.
We should extend our `import` statement as before ...
In addition to NgFor and NgIf, there are other core Angular directives that are often used in Angular apps such as NgClass and NgSwitch. Instead of importing each Angular core directive separately as we did with NgFor and NgIf, Angular provides a constant called CORE_DIRECTIVES. This constant defines a collection of the Angular core directives. We can then use this constant instead of enumerating each built-in directive as part of the import statement and @View annotation.
:markdown
Try it out. We have four items in the array so the message should appear.
Delete one of the elements from the array, refresh the browser, and the message should no longer appear.
Using the CORE_DIRECTIVES constant we can change our import statement to:
```
import {Component, View, bootstrap, CORE_DIRECTIVES} from 'angular2/angular2';
```
And we can change our @View annotation to:
```
directives: [CORE_DIRECTIVES]
```
Use this constant instead of enumerating each Angular core directive any time you plan to use the built-in directives in your view.
.l-main-section
:markdown
## Use the CORE_DIRECTIVES Constant
There are other core Angular directives, such as `NgClass` and `NgSwitch`,
that we often use in our apps.
Extending the `import` statement and adding to the `directives` array for each one gets old.
Fortunately, Angular provides a constant array called `CORE_DIRECTIVES`
that includes many of the directives that we use all the time.
Let's simplify our lives, discard the `NgFor` and `NgIf`, use the constant for all of them.
We'll revise our `import` statement one last time.
Ward Bell
Naomi Black
