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 aTemplateRef
instance, 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 matchingexportAs
name.
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