模板变量
Template variables
模板变量可以帮助你在模板的另一部分使用这个部分的数据。使用模板变量，你可以执行某些任务，比如响应用户输入或微调应用的表单。
Template variables help you use data from one part of a template in another part of the template. With template variables, you can perform tasks such as respond to user input or finely tune your application's forms.
模板变量可以引用这些东西：
A template variable can refer to the following:
模板中的 DOM 元素
a DOM element within a template
指令
a directive
元素
an element
语法
Syntax
在模板中，要使用井号 # 来声明一个模板变量。下列模板变量 #phone 语法在 <input> 元素上声明了一个名为 phone 的变量
In the template, you use the hash symbol, #, to declare a template variable. The following template variable, #phone, declares a phone variable on an <input> element.
<input #phone placeholder="phone number" />
你可以在组件模板中的任何地方引用某个模板变量。这里的 <button> 就引用了 phone 变量。
You can refer to a template variable anywhere in the component's template. Here, a <button> further down the template refers to the phone variable.
<input #phone placeholder="phone number" />
<!-- lots of other elements -->
<!-- phone refers to the input element; pass its `value` to an event handler -->
<button (click)="callPhone(phone.value)">Call</button>
Angular 是如何为模板变量赋值的
How Angular assigns values to template variables
Angular 根据你所声明的变量的位置给模板变量赋值：
Angular assigns a template variable a value based on where you declare the variable:
如果在组件上声明变量，该变量就会引用该组件实例。
If you declare the variable on a component, the variable refers to the component instance.
如果在标准的 HTML 标记上声明变量，该变量就会引用该元素。
If you declare the variable on a standard HTML tag, the variable refers to the element.
如果你在
<ng-template>元素上声明变量，该变量就会引用一个TemplateRef实例来代表此模板。关于<ng-template>的更多信息，请参阅结构型指令 中的 ng-template 部分。If you declare the variable on an
<ng-template>element, the variable refers to aTemplateRefinstance, which represents the template. For more information on<ng-template>, see the ng-template section of Structural directives.如果该变量在右侧指定了一个名字，比如
#var="ngModel"，那么该变量就会引用所在元素上具有这个exportAs名字的指令或组件。If the variable specifies a name on the right-hand side, such as
#var="ngModel", the variable refers to the directive or component on the element with a matchingexportAsname.
将 NgForm 与模板变量一起使用
Using NgForm with template variables
在大多数情况下，Angular 会把模板变量的值设置为它所在的元素。在前面的例子中， phone 引用的是电话号码 <input> 。该按钮的 click 处理程序会把这个 <input> 的值传给该组件的 callPhone() 方法。
In most cases, Angular sets the template variable's value to the element on which it occurs. In the previous example, phone refers to the phone number <input>. The button's click handler passes the <input> value to the component's callPhone() method.
这里的 NgForm 指令演示了如何通过引用指令的的 exportAs 名字来引用不同的值。在下面的例子中，模板变量 itemForm 在 HTML 中分别出现了三次。
The NgForm directive demonstrates getting a reference to a different value by reference a directive's exportAs name. In the following example, the template variable, itemForm, appears three times separated by HTML.
<form #itemForm="ngForm" (ngSubmit)="onSubmit(itemForm)">
<label for="name">Name <input class="form-control" name="name" ngModel required />
</label>
<button type="submit">Submit</button>
</form>
<div [hidden]="!itemForm.form.valid">
<p>{{ submitMessage }}</p>
</div>
如果没有 ngForm 这个属性值，itemForm 引用的值将是 HTMLFormElement 也就是 <form> 元素。而 Component 和 Directive 之间的差异在于 Angular 在没有指定属性值的情况下，Angular 会引用 Component，而 Directive 不会改变这种隐式引用（即它的宿主元素）。
Without the ngForm attribute value, the reference value of itemForm would be the HTMLFormElement, <form>. There is, however, a difference between a Component and a Directive in that Angular references a Component without specifying the attribute value, and a Directive does not change the implicit reference, or the element.
而使用了 NgForm 之后，itemForm 就是对 NgForm 指令的引用，可以用它来跟踪表单中每一个控件的值和有效性。
With NgForm, itemForm is a reference to the NgForm directive with the ability to track the value and validity of every control in the form.
与原生的 <form> 元素不同， NgForm 指令有一个 form 属性。如果 itemForm.form.valid 无效，那么 NgForm 的 form 属性就会让你禁用提交按钮。
Unlike the native <form> element, the NgForm directive has a form property. The NgForm form property allows you to disable the submit button if the itemForm.form.valid is invalid.
模板变量的作用域
Template variable scope
你可以在包含此模板变量的模板中的任何地方引用它。而 结构型指令（如 *ngIf 和 *ngFor 或 <ng-template> 同样充当了模板的边界。你不能在这些边界之外访问其中的模板变量。
You can refer to a template variable anywhere within its surrounding template. Structural directives, such as *ngIf and *ngFor, or <ng-template> act as a template boundary. You cannot access template variables outside of these boundaries.
同名变量在模板中只能定义一次，这样运行时它的值就是可预测的。
Define a variable only once in the template so the runtime value remains predictable.
在嵌套模板中访问
Accessing in a nested template
内部模板可以访问外模板定义的模板变量。
An inner template can access template variables that the outer template defines.
在下面的例子中，修改 <input> 中的文本值也会改变 <span> 中的值，因为 Angular 会立即通过模板变量 ref1 来更新这种变化。
In the following example, changing the text in the <input> changes the value in the <span> because Angular immediately updates changes through the template variable, ref1.
<input #ref1 type="text" [(ngModel)]="firstExample" />
<span *ngIf="true">Value: {{ ref1.value }}</span>
在这种情况下，有一个包含这个 <span> 的隐式 <ng-template>，而该变量的定义在该隐式模板之外。访问父模板中的模板变量是可行的，因为子模板会从父模板继承上下文。
In this case, there is an implied <ng-template> around the <span> and the definition of the variable is outside of it. Accessing a template variable from the parent template works because the child template inherits the context from the parent template.
我们用更啰嗦的形式重写上述的代码，可以明确地显示出 <ng-template>。
Rewriting the above code in a more verbose form explicitly shows the <ng-template>.
<input #ref1 type="text" [(ngModel)]="firstExample" />
<!-- New template -->
<ng-template [ngIf]="true">
<!-- Since the context is inherited, the value is available to the new template -->
<span>Value: {{ ref1.value }}</span>
</ng-template>
但是，从外部的父模板访问本模板中的变量是行不通的。
However, accessing a template variable from outside the parent template doesn't work.
<input *ngIf="true" #ref2 type="text" [(ngModel)]="secondExample" />
<span>Value: {{ ref2?.value }}</span> <!-- doesn't work -->
这个更啰嗦的形式表明 ref2 位于外部的父模板中。
The verbose form shows that ref2 is outside the parent template.
<ng-template [ngIf]="true">
<!-- The reference is defined within a template -->
<input #ref2 type="text" [(ngModel)]="secondExample" />
</ng-template>
<!-- ref2 accessed from outside that template doesn't work -->
<span>Value: {{ ref2?.value }}</span>
考虑下面这个带 *ngFor 的使用范例。
Consider the following example that uses *ngFor.
<ng-container *ngFor="let i of [1,2]">
<input #ref type="text" [value]="i" />
</ng-container>
{{ ref.value }}
这里，ref.value 不起作用。结构型指令 *ngFor 将模板实例化了两次，因为 *ngFor 在对数组中的两个条目进行迭代。因此不可能定义出 ref.value 指向的是谁。
Here, ref.value doesn't work. The structural directive, *ngFor instantiates the template twice because *ngFor iterates over the two items in the array. It is impossible to define what the ref.value reference signifies.
对于结构型指令，比如 *ngFor 或 *ngIf ，Angular 也无法知道模板是否曾被实例化过。
With structural directives, such as *ngFor or *ngIf, there is no way for Angular to know if a template is ever instantiated.
结果，Angular 无法访问该值并返回错误。
As a result, Angular isn't able to access the value and returns an error.
访问 <ng-template> 的模板变量
Accessing a template variable within <ng-template>
在 <ng-template> 上声明变量时，该变量会引用一个 TemplateRef 实例来表示该模板。
When you declare the variable on an <ng-template>, the variable refers to a TemplateRef instance, which represents the template.
<ng-template #ref3></ng-template>
<button (click)="log(ref3)">Log type of #ref</button>
在这个例子中，单击该按钮会调用 log() 函数，它把 #ref3 的值输出到控制台。因为 #ref 变量在 <ng-template> 上，所以它的值是一个 TemplateRef。
In this example, clicking the button calls the log() function, which outputs the value of #ref3 to the console. Because the #ref variable is on an <ng-template>, the value is TemplateRef.
下面是一个名为 TemplateRef 的 TemplateRef() 函数在浏览器控制台中展开时的输出。
The following is the expanded browser console output of the TemplateRef() function with the name of TemplateRef.
▼ ƒ TemplateRef()
name: "TemplateRef"
__proto__: Function
