SelectControlValueAccessor
该 ControlValueAccessor 用于写入 select 控件的值，并监听 select 控件的变化。该值访问器会被 FormControlDirective、FormControlName 和 NgModel 指令使用。
The ControlValueAccessor for writing select control values and listening to select control changes. The value accessor is used by the FormControlDirective, FormControlName, and NgModel directives.
Exported from
选择器
select:not([multiple])[formControlName]select:not([multiple])[formControl]select:not([multiple])[ngModel]
属性
| 属性 | 说明 |
|---|---|
@Input() | 只写 跟踪选项的比较算法，以在检查变更时跟踪其标识。 Tracks the option comparison algorithm for tracking identities when checking for changes. |
说明
在响应式表单中使用 select 控件
Using select controls in a reactive form
下面的例子演示了如何在响应式表单中使用 select 控件。
The following examples show how to use a select control in a reactive form.
import {Component} from '@angular/core';
import {FormControl, FormGroup} from '@angular/forms';
@Component({
selector: 'example-app',
template: `
<form [formGroup]="form">
<select formControlName="state">
<option *ngFor="let state of states" [ngValue]="state">
{{ state.abbrev }}
</option>
</select>
</form>
<p>Form value: {{ form.value | json }}</p>
<!-- {state: {name: 'New York', abbrev: 'NY'} } -->
`,
})
export class ReactiveSelectComp {
states = [
{name: 'Arizona', abbrev: 'AZ'},
{name: 'California', abbrev: 'CA'},
{name: 'Colorado', abbrev: 'CO'},
{name: 'New York', abbrev: 'NY'},
{name: 'Pennsylvania', abbrev: 'PA'},
];
form = new FormGroup({
state: new FormControl(this.states[3]),
});
}
在模板驱动表单中使用 select 控件
Using select controls in a template-driven form
要在模板驱动表单中使用 select，只要把 ngModel 和 name 属性加到 <select> 标签上即可。
To use a select in a template-driven form, simply add an ngModel and a name attribute to the main <select> tag.
import {Component} from '@angular/core';
@Component({
selector: 'example-app',
template: `
<form #f="ngForm">
<select name="state" ngModel>
<option value="" disabled>Choose a state</option>
<option *ngFor="let state of states" [ngValue]="state">
{{ state.abbrev }}
</option>
</select>
</form>
<p>Form value: {{ f.value | json }}</p>
<!-- example value: {state: {name: 'New York', abbrev: 'NY'} } -->
`,
})
export class SelectControlComp {
states = [
{name: 'Arizona', abbrev: 'AZ'},
{name: 'California', abbrev: 'CA'},
{name: 'Colorado', abbrev: 'CO'},
{name: 'New York', abbrev: 'NY'},
{name: 'Pennsylvania', abbrev: 'PA'},
];
}
自定义 option 的选择结果
Customizing option selection
Angular 使用对象标识作为选项。条目标识可能在其实质性数据没有变化的情况发生变化。比如，如果这些条目是通过 RPC 的方式从服务端取到的，当重新执行 RPC 时，就算数据没有变化，第二个响应也会生成一些具有不同对象标识的对象。
Angular uses object identity to select option. It's possible for the identities of items to change while the data does not. This can happen, for example, if the items are produced from an RPC to the server, and that RPC is re-run. Even if the data hasn't changed, the second response will produce objects with different identities.
要想自定义默认的选项比较算法，<select> 支持一个名叫 compareWith 的输入。 compareWith 接受一个函数，它具有两个参数：option1 和 option2。 如果指定了 compareWith，则 Angular 会根据该函数的返回值来选取一个选项。
To customize the default option comparison algorithm, <select> supports compareWith input. compareWith takes a function which has two arguments: option1 and option2. If compareWith is given, Angular selects option by the return value of the function.
const selectedCountriesControl = new FormControl();
<select [compareWith]="compareFn" [formControl]="selectedCountriesControl">
<option *ngFor="let country of countries" [ngValue]="country">
{{country.name}}
</option>
</select>
compareFn(c1: Country, c2: Country): boolean {
return c1 && c2 ? c1.id === c2.id : c1 === c2;
}
注意：我们要监听 change 事件，这是因为 input 事件不会在 Firefox 和 IE 的 select 元素上触发： https://bugzilla.mozilla.org/show_bug.cgi?id=1024350 https://developer.microsoft.com/en-us/microsoft-edge/platform/issues/4660045/
Note: We listen to the 'change' event because 'input' events aren't fired for selects in IE, see: https://developer.mozilla.org/en-US/docs/Web/API/HTMLElement/input_event#browser_compatibility
