如何在自定义组件中使用ngModel？

网址：https://embed.plnkr.co/nqKUSPWb6w5QXr8a0wEu/?show=preview

import { Component, forwardRef } from '@angular/core';
import { NG_VALUE_ACCESSOR, ControlValueAccessor } from '@angular/forms';

const noop = () => {
};

export const CUSTOM_INPUT_CONTROL_VALUE_ACCESSOR: any = {
    provide: NG_VALUE_ACCESSOR,
    useExisting: forwardRef(() => CustomInputComponent),
    multi: true
};

@Component({
    selector: 'custom-input',
    template: `<div class="form-group">
                    <label><ng-content></ng-content>
                        <input [(ngModel)]="value"  
                                class="form-control" 
                                (blur)="onBlur()" >
                    </label>
                </div>`,
    providers: [CUSTOM_INPUT_CONTROL_VALUE_ACCESSOR]
})
export class CustomInputComponent implements ControlValueAccessor {

    //The internal data model
    private innerValue: any = '';

    //Placeholders for the callbacks which are later provided
    //by the Control Value Accessor
    private onTouchedCallback: () => void = noop;
    private onChangeCallback: (_: any) => void = noop;

    //get accessor
    get value(): any {
        return this.innerValue;
    };

    //set accessor including call the onchange callback
    set value(v: any) {
        if (v !== this.innerValue) {
            this.innerValue = v;
            this.onChangeCallback(v);
        }
    }

    //Set touched on blur
    onBlur() {
        this.onTouchedCallback();
    }

    //From ControlValueAccessor interface
    writeValue(value: any) {
        if (value !== this.innerValue) {
            this.innerValue = value;
        }
    }

    //From ControlValueAccessor interface
    registerOnChange(fn: any) {
        this.onChangeCallback = fn;
    }

    //From ControlValueAccessor interface
    registerOnTouched(fn: any) {
        this.onTouchedCallback = fn;
    }

}

使用

//our root app component
import {Component} from '@angular/core'

@Component({
    selector: 'demo-app',
    template: `
        <p><span class="boldspan">Form data:</span>{{demoForm.value | json}}</p>
        <p><span class="boldspan">Model data:</span> {{dataModel}}</p>
        <form #demoForm="ngForm">
          <custom-input name="someValue" 
                          [(ngModel)]="dataModel">
              Write in this wrapper control:
          </custom-input>
        </form>`
})
export class AppComponent {
    dataModel: string = '';
}

原理：

• 需要继承ControlValueAccessor 。
• writeValue用来获取ngModel的值，onChangeCallback 用来将ngModel的值传出。每次改变value时都要触发这个函数。
• 注意当ngmodel传入的值没有改变时，不会触发writeValue。