範本變數
Template variables
範本變數可以幫助你在範本的另一部分使用這個部分的資料。使用範本變數，你可以執行某些任務，比如響應使用者輸入或微調應用的表單。
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.
範本變數可以參考這些東西：
A template variable can refer to the following:
範本中的 DOM 元素
a DOM element within a template
指令
a directive
元素
an element
語法
Syntax
在範本中，要使用井號 # 來宣告一個範本變數。下列範本變數 #phone 語法在 <input> 元素上聲明瞭一個名為 phone 的變數
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" />
你可以在元件範本中的任何地方參考某個範本變數。這裡的 <button> 就參考了 phone 變數。
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 是如何為範本變數賦值的
How Angular assigns values to template variables
Angular 根據你所宣告的變數的位置給範本變數賦值：
Angular assigns a template variable a value based on where you declare the variable:
如果在元件上宣告變數，該變數就會參考該元件實例。
If you declare the variable on a component, the variable refers to the component instance.
如果在標準的 HTML 標記上宣告變數，該變數就會參考該元素。
If you declare the variable on a standard HTML tag, the variable refers to the element.
如果你在
<ng-template>元素上宣告變數，該變數就會參考一個TemplateRef實例來代表此範本。關於<ng-template>的更多資訊，請參閱結構型指令 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.如果該變數在右側指定了一個名字，比如
#var="ngModel"，那麼該變數就會參考所在元素上具有這個exportAs名字的指令或元件。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.
將 NgForm 與範本變數一起使用
Using NgForm with template variables
在大多數情況下，Angular 會把範本變數的值設定為它所在的元素。在前面的例子中， phone 參考的是電話號碼 <input> 。該按鈕的 click 處理程式會把這個 <input> 的值傳給該元件的 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.
這裡的 NgForm 指令示範瞭如何透過參考指令的的 exportAs 名字來參考不同的值。在下面的例子中，範本變數 itemForm 在 HTML 中分別出現了三次。
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>
如果沒有 ngForm 這個屬性值，itemForm 參考的值將是 HTMLFormElement 也就是 <form> 元素。而 Component 和 Directive 之間的差異在於 Angular 在沒有指定屬性值的情況下，Angular 會參考 Component，而 Directive 不會改變這種隱式參考（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 之後，itemForm 就是對 NgForm 指令的參考，可以用它來追蹤表單中每一個控制元件的值和有效性。
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.
與原生的 <form> 元素不同， NgForm 指令有一個 form 屬性。如果 itemForm.form.valid 無效，那麼 NgForm 的 form 屬性就會讓你禁用提交按鈕。
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.
範本變數的作用域
Template variable scope
你可以在包含此範本變數的範本中的任何地方參考它。而 結構型指令（如 *ngIf 和 *ngFor 或 <ng-template> 同樣充當了範本的邊界。你不能在這些邊界之外訪問其中的範本變數。
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.
同名變數在範本中只能定義一次，這樣執行時它的值就是可預測的。
Define a variable only once in the template so the runtime value remains predictable.
在巢狀範本中訪問
Accessing in a nested template
內部範本可以訪問外範本定義的範本變數。
An inner template can access template variables that the outer template defines.
在下面的例子中，修改 <input> 中的文字值也會改變 <span> 中的值，因為 Angular 會立即透過範本變數 ref1 來更新這種變化。
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>
在這種情況下，有一個包含這個 <span> 的隱式 <ng-template>，而該變數的定義在該隱式範本之外。訪問父範本中的範本變數是可行的，因為子範本會從父範本繼承上下文。
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.
我們用更囉嗦的形式重寫上述的程式碼，可以明確地顯示出 <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，從外部的父範本訪問本範本中的變數是行不通的。
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 -->
這個更囉嗦的形式表明 ref2 位於外部的父範本中。
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>
考慮下面這個帶 *ngFor 的使用範例。
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 }}
這裡，ref.value 不起作用。結構型指令 *ngFor 將範本實例化了兩次，因為 *ngFor 在對陣列中的兩個條目進行迭代。因此不可能定義出 ref.value 指向的是誰。
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.
對於結構型指令，比如 *ngFor 或 *ngIf ，Angular 也無法知道範本是否曾被實例化過。
With structural directives, such as *ngFor or *ngIf, there is no way for Angular to know if a template is ever instantiated.
結果，Angular 無法訪問該值並返回錯誤。
As a result, Angular isn't able to access the value and returns an error.
訪問 <ng-template> 的範本變數
Accessing a template variable within <ng-template>
在 <ng-template> 上宣告變數時，該變數會參考一個 TemplateRef 實例來表示該範本。
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>
在這個例子中，單擊該按鈕會呼叫 log() 函式，它把 #ref3 的值輸出到控制檯。因為 #ref 變數在 <ng-template> 上，所以它的值是一個 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.
下面是一個名為 TemplateRef 的 TemplateRef() 函式在瀏覽器控制檯中展開時的輸出。
The following is the expanded browser console output of the TemplateRef() function with the name of TemplateRef.
▼ ƒ TemplateRef()
name: "TemplateRef"
__proto__: Function
