使用*ngIf时,为什么angular2模板局部变量在模板中不可用？

Question

使用*ngIf时,第1部分"#test"未定义

当引用可以隐藏/"销毁"的输入时(因为使用了*ngIf并且某些元素被销毁),由angular2语法#(在下面的示例中为#test)创建的局部变量不起作用,即使元素存在于页面中.

代码是:

@Component({
    selector: 'my-app',
    template: `<h1>My First Angular 2 App</h1>
    <button (click)="focusOther(test)">test</button>
    <input #test *ngIf="boolValue" >
    `
})
export class AppComponent { 

  private isVisible = false;

  focusOther(testElement){
    this.isVisible = true;
    alert(testElement);
    testElement.focus();
  }

}

警报显示"未定义",因为没有任何内容传递给该函数.

是否有解决方案使其工作？我的目标是集中一个将要创建的元素.

Mark Rajcok给出的解决方案:使用一个使用elementRef并在元素上调用.focus()的afterViewInit创建一个指令.

有关第1部分的工作版本,请参阅此plunker:http://plnkr.co/edit/JmBBHMo1hXLe4jrbpVdv？p = preview

第2部分如何在初始创建后重新聚焦该元素

一旦修复了"创建后焦点"的问题,我需要一种重新聚焦()组件的方法,比如"test.focus()"(其中#test是输入的局部变量名,但不能像我之前演示的那样使用).

Mark Rajcok提供的多种解决方案

Answer 1

至于焦点问题的解决方案,您可以创建一个属性指令focusMe:

import {Component, Directive, ElementRef} from 'angular2/core';
@Directive({
  selector: '[focusMe]'
})
export class FocusDirective {
  constructor(private el: ElementRef) {}
  ngAfterViewInit() {
    this.el.nativeElement.focus();
  }
}
@Component({
    selector: 'my-app',
    directives: [FocusDirective],
    template: `<h1>My First Angular 2 App</h1>
      <button (click)="toggle()">toggle</button>
      <input focusMe *ngIf="isVisible">
    `
})
export class AppComponent { 
  constructor() { console.clear(); }
  private isVisible = false;
  toggle() {
    this.isVisible = !this.isVisible;
  }
}

Plunker

更新1:添加重新聚焦功能的解决方案:

import {Component, Directive, ElementRef, Input} from 'angular2/core';

@Directive({
  selector: '[focusMe]'
})
export class FocusMe {
    @Input('focusMe') hasFocus: boolean;
    constructor(private elementRef: ElementRef) {}
    ngAfterViewInit() {
      this.elementRef.nativeElement.focus();
    }
    ngOnChanges(changes) {
      //console.log(changes);
      if(changes.hasFocus && changes.hasFocus.currentValue === true) {
        this.elementRef.nativeElement.focus();
      }
    }
}
@Component({
    selector: 'my-app',
    template: `<h1>My First Angular 2 App</h1>
    <button (click)="showInput()">Make it visible</button>
    <input *ngIf="inputIsVisible" [focusMe]="inputHasFocus">
    <button (click)="focusInput()" *ngIf="inputIsVisible">Focus it</button>
    `,
    directives:[FocusMe]
})
export class AppComponent {
  private inputIsVisible = false;
  private inputHasFocus = false;
  constructor() { console.clear(); }
  showInput() {
    this.inputIsVisible = true;
  }
  focusInput() {
    this.inputHasFocus = true;
    setTimeout(() => this.inputHasFocus = false, 50);
  }
}

Plunker

使用setTimeout()重置焦点属性的替代方法false是在FocusDirective上创建事件/输出属性,并emit()在focus()调用时创建事件.然后,AppComponent将侦听该事件并重置焦点属性.

更新2:这是使用ViewChild添加重新聚焦功能的另一种/更好的方法.我们不需要以这种方式跟踪焦点状态,也不需要FocusMe指令的输入属性.

import {Component, Directive, ElementRef, Input, ViewChild} from 'angular2/core';

@Directive({
  selector: '[focusMe]'
})
export class FocusMe {
    constructor(private elementRef: ElementRef) {}
    ngAfterViewInit() {
      // set focus when element first appears
      this.setFocus();
    }
    setFocus() {
      this.elementRef.nativeElement.focus();
    }
}
@Component({
    selector: 'my-app',
    template: `<h1>My First Angular 2 App</h1>
    <button (click)="showInput()">Make it visible</button>
    <input *ngIf="inputIsVisible" focusMe>
    <button (click)="focusInput()" *ngIf="inputIsVisible">Focus it</button>
    `,
    directives:[FocusMe]
})
export class AppComponent {
  @ViewChild(FocusMe) child;
  private inputIsVisible = false;
  constructor() { console.clear(); }
  showInput() {
    this.inputIsVisible = true;
  }
  focusInput() {
    this.child.setFocus();
  }
}

Plunker

更新3:这是另一个不需要仍然使用ViewChild的指令的替代方案,但我们通过本地模板变量而不是属性指令访问子节点(感谢@alexpods 提示):

import {Component, ViewChild, NgZone} from 'angular2/core';

@Component({
    selector: 'my-app',
    template: `<h1>Focus test</h1>
    <button (click)="showInput()">Make it visible</button>
    <input #input1 *ngIf="input1IsVisible">
    <button (click)="focusInput1()" *ngIf="input1IsVisible">Focus it</button>
    `,
})
export class AppComponent {
  @ViewChild('input1') input1ElementRef;
  private input1IsVisible = false;
  constructor(private _ngZone: NgZone) { console.clear(); }
  showInput() {
    this.input1IsVisible = true;
    // Give ngIf a chance to render the <input>.
    // Then set the focus, but do this outside the Angualar zone to be efficient.
    // There is no need to run change detection after setTimeout() runs,
    // since we're only focusing an element.
    this._ngZone.runOutsideAngular(() => { 
      setTimeout(() => this.focusInput1(), 0);
   });
  }
  setFocus(elementRef) {
    elementRef.nativeElement.focus();
  }
  ngDoCheck() {
    // if you remove the ngZone stuff above, you'll see
    // this log 3 times instead of 1 when you click the
    // "Make it visible" button.
    console.log('doCheck');
  }
  focusInput1() {
    this.setFocus(this.input1ElementRef);
  }
}

Plunker

更新4:我更新了Update 3中的代码以使用NgZone,这样我们就不会在setTimeout()完成后运行Angular的更改检测算法.(有关更改检测的更多信息,请参阅此答案).

更新5:我更新了上面的plunker中的代码,使用Renderer使web worker安全.不鼓励focus()直接访问nativeElement.

focusInput1() {
  this._renderer.invokeElementMethod(
    this.input1ElementRef.nativeElement, 'focus', []);
}

我从这个问题中学到了很多东西.