[英]Proper way to propagate computed value on Angular2
我想通过@Input
属性使用计算值
但传播初始值并不奏效。
https://plnkr.co/edit/1MMpOYOKIouwnNc3uIuy
我创建App
(具有模板驱动形式的根组件)和NumComponent
(仅保存类型化值的子组件)组件。
当我将属性传递给NumComponent
如[useThree]="true"
那么我想将默认值'3'设置为NumComponent
但是我没有使用setTimeout
就找不到方法
有没有setTimeout传播初始值的方法?
编辑于5/5
应用组件
@Component({
selector: 'my-app',
template: `
<div>
<form novalidate #form="ngForm">
<app-num name="num" ngModel [useThree]="true"></app-num>
</form>
<pre>{{form.value | json}}</pre>
</div>
`
})
export class App {}
NumComponent
export const NumValueAccessor = {
provide: NG_VALUE_ACCESSOR,
useExisting: forwardRef(() => NumComponent),
multi: true
};
@Component({
selector: 'app-num',
template: `<input [(ngModel)]="num" type="text" (ngModelChange)="updateValue()" />`,
providers: [NumValueAccessor]
})
export class NumComponent implements ControlValueAccessor {
num = 0;
// I want set literal number 3 to `num` property
// when `useThree` is true.
@Input() useThree = false;
onChange = (_: any) => {};
updateValue(num = this.num) {
this.onChange(String(num));
}
writeValue(value: string): void {
if (this.useThree) {
/**********
* ISSUE
**********/
// this code is not work. after code ran, `NumComponent` has
// value 3 but AppComponent's internal FormComponent value
// is '' (empty string)
// this.num = 3;
// this.updateValue(3);
// ran code with `setTimeout` solve this problem. but
// I don't want using setTimeout for this feature.
// setTimeout(() => {
// this.num = 3;
// this.updateValue(3);
// }, 0);
// Is there any way to propagate computed initial value?
this.num = 3;
this.updateValue(3);
/**********
* ISSUE
**********/
this.useThree = false;
return;
}
this.num = Number(value);
}
registerOnChange(fn: any): void {
this.onChange = fn;
}
registerOnTouched(fn: any): void {}
setDisabledState(isDisabled: boolean): void {}
}
看起来父组件在初始化生命周期时没有实现传播值。
我不确定我是否完全理解了这个问题,但是你正在寻找的一个可能的解决方案可能是
private _useThree = false;
@Input() set useThree(value: boolean) {
this._useThree = value;
if (this._useThree) {
this.num = 3;
}
}
这样,只要您想从父组件设置输入useThree
属性的值,就可以实际执行上面定义的setter方法的代码。
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