如何从类属性 TypeScript 中侦听值更改 - Angular

Question

在 AngularJS 中，我们可以使用 $watch、$digest... 监听变量变化，而新版本的 Angular (5, 6) 不再支持。

在 Angular 中，这种行为现在是组件生命周期的一部分。

我查看了官方文档、文章，尤其是Angular 5 change detection on mutable objects，以了解如何在TypeScript class / Angular 中监听变量（类属性）的变化

今天提议的是：

import { OnChanges, SimpleChanges, DoCheck } from '@angular/core';


@Component({
  selector: 'my-comp',
  templateUrl: 'my-comp.html',
  styleUrls: ['my-comp.css'],
  inputs:['input1', 'input2']
})
export class MyClass implements OnChanges, DoCheck, OnInit{

  //I can track changes for this properties
  @Input() input1:string;
  @Input() input2:string;
  
  //Properties what I want to track !
  myProperty_1: boolean
  myProperty_2: ['A', 'B', 'C'];
  myProperty_3: MysObject;

  constructor() { }

  ngOnInit() { }

  //Solution 1 - fired when Angular detects changes to the @Input properties
  ngOnChanges(changes: SimpleChanges) {
    //Action for change
  }

  //Solution 2 - Where Angular fails to detect the changes to the input property
  //the DoCheck allows us to implement our custom change detection
  ngDoCheck() {
    //Action for change
  }
}

这仅适用于@Input() 属性！

如果我想跟踪我的组件自身属性（myProperty_1、myProperty_2 或 myProperty_3）的更改，这将不起作用。

有人可以帮我解决这个问题吗？最好是与 Angular 5 兼容的解决方案

Answer 1

您仍然可以通过KeyValueDiffers 通过DoCheck lifehook 检查组件的字段成员值变化。

import { DoCheck, KeyValueDiffers, KeyValueDiffer } from '@angular/core';

differ: KeyValueDiffer<string, any>;
constructor(private differs: KeyValueDiffers) {
  this.differ = this.differs.find({}).create();
}

ngDoCheck() {
  const change = this.differ.diff(this);
  if (change) {
    change.forEachChangedItem(item => {
      console.log('item changed', item);
    });
  }
}

见demo。

Answer 2

我认为对您的问题最好的解决方案是使用一个装饰器，该装饰器自动将原始字段替换为一个属性，然后在设置器上，您可以创建一个类似于 angular 创建的 SimpleChanges 对象，以便使用与 Angular 相同的通知回调（或者，您可以为这些通知创建不同的接口，但适用相同的原则）

import { OnChanges, SimpleChanges, DoCheck, SimpleChange } from '@angular/core';

function Watch() : PropertyDecorator & MethodDecorator{
    function isOnChanges(val: OnChanges): val is OnChanges{
        return !!(val as OnChanges).ngOnChanges
    }
    return (target : any, key: string | symbol, propDesc?: PropertyDescriptor) => {
        let privateKey = "_" + key.toString();
        let isNotFirstChangePrivateKey = "_" + key.toString() + 'IsNotFirstChange';
        propDesc = propDesc || {
            configurable: true,
            enumerable: true,
        };
        propDesc.get = propDesc.get || (function (this: any) { return this[privateKey] });

        const originalSetter = propDesc.set || (function (this: any, val: any) { this[privateKey] = val });

        propDesc.set = function (this: any, val: any) {
            let oldValue = this[key];
            if(val != oldValue) {
                originalSetter.call(this, val);
                let isNotFirstChange = this[isNotFirstChangePrivateKey];
                this[isNotFirstChangePrivateKey] = true;
                if(isOnChanges(this)) {
                    var changes: SimpleChanges = {
                        [key]: new SimpleChange(oldValue, val, !isNotFirstChange)
                    }
                    this.ngOnChanges(changes);
                }
            }
        }
        return propDesc;
    }
}

// Usage
export class MyClass implements OnChanges {


    //Properties what I want to track !
    @Watch()
    myProperty_1: boolean  =  true
    @Watch()
    myProperty_2 =  ['A', 'B', 'C'];
    @Watch()
    myProperty_3 = {};

    constructor() { }
    ngOnChanges(changes: SimpleChanges) {
        console.log(changes);
    }
}

var myInatsnce = new MyClass(); // outputs original field setting with firstChange == true
myInatsnce.myProperty_2 = ["F"]; // will be notified on subsequent changes with firstChange == false

Answer 3

如你所说，你可以使用

public set myProperty_2(value: type): void {
 if(value) {
  //doMyCheck
 }

 this._myProperty_2 = value;
}

然后如果你需要检索它

public get myProperty_2(): type {
  return this._myProperty_2;
}

通过这种方式，您可以在设置/获取变量时进行所有您想要的检查，这样每次设置/获取 myProperty_2 属性时都会触发此方法。

小演示：https://stackblitz.com/edit/angular-n72qlu

Answer 4

我想我是来听 DOM 变化的，你可以得到对你的元素所做的任何变化，我真的希望这些提示和技巧能帮助你解决你的问题，遵循以下简单的步骤：

首先，您需要像这样引用您的元素：

在 HTML 中：

<section id="homepage-elements" #someElement>
....
</section>

并且在该组件的 TS 文件中：

@ViewChild('someElement')
public someElement: ElementRef;

第二，你需要创建一个观察者来监听那个元素的变化，你需要把你的组件ts文件变成implements AfterViewInit, OnDestroy，然后在那里实现ngAfterViewInit()（@987654331 @以后有工作）：

private changes: MutationObserver;
ngAfterViewInit(): void {
  console.debug(this.someElement.nativeElement);

  // This is just to demo 
  setInterval(() => {
    // Note: Renderer2 service you to inject with constructor, but this is just for demo so it is not really part of the answer
    this.renderer.setAttribute(this.someElement.nativeElement, 'my_custom', 'secondNow_' + (new Date().getSeconds()));
  }, 5000);

  // Here is the Mutation Observer for that element works
  this.changes = new MutationObserver((mutations: MutationRecord[]) => {
      mutations.forEach((mutation: MutationRecord) => {
        console.debug('Mutation record fired', mutation);
        console.debug(`Attribute '${mutation.attributeName}' changed to value `, mutation.target.attributes[mutation.attributeName].value);
      });
    }
  );

  // Here we start observer to work with that element
  this.changes.observe(this.someElement.nativeElement, {
    attributes: true,
    childList: true,
    characterData: true
  });
}

您将看到控制台将适用于对该元素的任何更改：

这是另一个示例，您将看到 2 个突变记录被触发并且类发生了变化：

// This is just to demo
setTimeout(() => {
   // Note: Renderer2 service you to inject with constructor, but this is just for demo so it is not really part of the answer
  this.renderer.addClass(this.someElement.nativeElement, 'newClass' + (new Date().getSeconds()));
  this.renderer.addClass(this.someElement.nativeElement, 'newClass' + (new Date().getSeconds() + 1));
}, 5000);

// Here is the Mutation Observer for that element works
this.changes = new MutationObserver((mutations: MutationRecord[]) => {
    mutations.forEach((mutation: MutationRecord) => {
      console.debug('Mutation record fired', mutation);
      if (mutation.attributeName == 'class') {
        console.debug(`Class changed, current class list`, mutation.target.classList);
      }
    });
  }
);

控制台日志：

还有家务，OnDestroy:

ngOnDestroy(): void {
  this.changes.disconnect();
}

最后，您可以查看此参考：Listening to DOM Changes Using MutationObserver in Angular

Answer 5

你可以导入 ChangeDetectorRef

 constructor(private cd: ChangeDetectorRef) {
          // detect changes on the current component
            // this.cd is an injected ChangeDetector instance
            this.cd.detectChanges();

            // or run change detection for the all app
            // this.appRef is an ApplicationRef instance
            this.appRef.tick();
}