"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
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 } } .
## Showing component properties with interpolation
The easiest way to display a component property
is to bind the property name through interpolation.
With interpolation, we 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:
Let's build a small illustrative example together.
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:
We added two properties to the formerly empty component: `title` and `myHero`.
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 automatically updates the display whenever the property value changes.
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
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 is creating an instance for us. How?
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.
Notice the CSS `selector` in the `@Component` decorator that specifies an element named "my-app".
Remember back in QuickStart that we added the `<my-app>` element to the body of our `index.html`
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.
: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.
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.
## Template inline or template file?
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.
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.
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: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 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.
Now we use the Angular `NgFor` "repeater" Directive in the template to display
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 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.
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:
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]
})
Our presentation is the familiar HTML unordered list with `<ul>` and `<li>` tags. Let's focus on the `<li>` tag.
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.
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".
Let's look again at the few lines of HTML that perform this operation:
code-example(format="linenums" language="html").
<li *ng-for="#hero of heroes">
{{ hero }}
</li>
.alert.is-important
:markdown
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.
: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
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`.
Using this NgFor syntax, we can display data from any iterable object.
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.
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:markdown
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
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.
We have to register the `NgFor` directive with the component metadata by making two changes to the app.ts file.
First, we import the `NgFor` symbol from the Angular library by extending the existing `import` statement.
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:
code-example(format="linenums" language="html").
@View({
template: `
<h1>{{title}}</h1>
<h2>My favorite hero is: {{myHero.name}}</h2>
<p>Heroes:</p>
<ul>
<li *ng-for="#hero of heroes">
{{ hero.name }}
</li>
</ul>
`,
directives: [NgFor]
})
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.
We've defined a class with a constructor and two properties: `id` and `name`.
## Conditionally displaying data with NgIf
If we are new to TypeScript, it may not look like we have properties. But we do. We're taking
advantage of a TypeScript short-cut in our declaration of the constructor parameters.
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.
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
That brief syntax simultaneously
* declares a constructor parameter and its type
* declare a public property of the same name
* initializes that property with the corresponding argument when we "new" an instance of the class.
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.
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:markdown
## Use the Hero class
Let's redefine the heroes property in our component to return an array of these Heroes
and also set the `myHero` property with the first of these mock heroes.
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
Our display looks the same but we know how much better it is under the hood.
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.
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:markdown
## Conditional display with NgIf
Sometimes the app should display a view or a portion of a view only under prescribed circumstances.
In our example, we'd like to display a message if we have a large number of heroes ... say more than 3.
The Angular `NgIf` directive will insert or remove an element based on a truthy/falsey condition.
We can see it in action by adding the following paragraph at the bottom of the template:
Ward Bell
Naomi Black
