为什么在使用* ngIf时,角色模板局部变量在模板中不可用

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,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()将focus属性重置为false的替代方法是在FocusDirective上创建一个事件/输出属性，并在调用focus()时发出一个事件。然后AppComponent将监听该事件并重置focus属性。

更新2：这是使用ViewChild添加重点焦点功能的替代/更好的方式。我们不需要以这种方式跟踪焦点状态，也不需要在FocusMe指令上输入属性。

import {Component,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：这是另一个不需要使用ViewChild的指令的替代方法，但是我们通过一个本地模板变量而不是一个属性指令来访问该子进程(感谢@alexpods为the tip)：

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：我更新了更新3中的代码以使用NgZone，以便在setTimeout()完成后，我们不会导致Angular的更改检测算法运行。 (有关更改检测的更多信息，请参阅this answer)。

更新5：我更新了上面的代码，使用Renderer来使web worker安全。不要直接在nativeElement上访问focus()。

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

我从这个问题中学到很多东西。