Angular2 Tri-State Checkbox. Source: https://github.com/Sn3b/ng2-tri-state-checkbox

<!DOCTYPE html>
<html>

  <head>
    <base href="." />
    <title>angular2 playground</title>
    <link rel="stylesheet" href="style.css" />
    <script src="https://unpkg.com/core-js@2.4.1/client/shim.min.js"></script>
    <script src="https://unpkg.com/zone.js/dist/zone.js"></script>
    <script src="https://unpkg.com/zone.js/dist/long-stack-trace-zone.js"></script>
    <script src="https://unpkg.com/reflect-metadata@0.1.3/Reflect.js"></script>
    <script src="https://unpkg.com/systemjs@0.19.31/dist/system.js"></script>
    <script src="config.js"></script>
    <script>
    System.import('app')
      .catch(console.error.bind(console));
  </script>
  </head>

  <body>
    <my-app>
    loading...
  </my-app>
  </body>

</html>

/* Styles go here */

# ng2-tri-state-checkbox
A simple checkbox with indeterminate state to be used in Angular2 projects.

### Usage:
#### Model-driven form:

```html
<form [formGroup]="myForm">
  <ng2-tri-state-checkbox formControlName="myControl"></ng2-tri-state-checkbox>
</form>
```

```typescript
this.myForm = new FormGroup({
  myControl: new FormControl(null) // possible values: true / false / null
});
```

#### Template-driven form:

```html
<form #myForm="ngForm">
  <ng2-tri-state-checkbox [(ngModel)]="myModel" name="myControl" #myControl="ngModel"></ng2-tri-state-checkbox>
</form>
```

System.config({
  //use typescript for compilation
  transpiler: 'typescript',
  //typescript compiler options
  typescriptOptions: {
    emitDecoratorMetadata: true
  },
  paths: {
    'npm:': 'https://unpkg.com/'
  },
  //map tells the System loader where to look for things
  map: {
    
    'app': './src',
    
    '@angular/core': 'npm:@angular/core/bundles/core.umd.js',
    '@angular/common': 'npm:@angular/common/bundles/common.umd.js',
    '@angular/compiler': 'npm:@angular/compiler/bundles/compiler.umd.js',
    '@angular/platform-browser': 'npm:@angular/platform-browser/bundles/platform-browser.umd.js',
    '@angular/platform-browser-dynamic': 'npm:@angular/platform-browser-dynamic/bundles/platform-browser-dynamic.umd.js',
    '@angular/http': 'npm:@angular/http/bundles/http.umd.js',
    '@angular/router': 'npm:@angular/router/bundles/router.umd.js',
    '@angular/forms': 'npm:@angular/forms/bundles/forms.umd.js',
    
    '@angular/core/testing': 'npm:@angular/core/bundles/core-testing.umd.js',
    '@angular/common/testing': 'npm:@angular/common/bundles/common-testing.umd.js',
    '@angular/compiler/testing': 'npm:@angular/compiler/bundles/compiler-testing.umd.js',
    '@angular/platform-browser/testing': 'npm:@angular/platform-browser/bundles/platform-browser-testing.umd.js',
    '@angular/platform-browser-dynamic/testing': 'npm:@angular/platform-browser-dynamic/bundles/platform-browser-dynamic-testing.umd.js',
    '@angular/http/testing': 'npm:@angular/http/bundles/http-testing.umd.js',
    '@angular/router/testing': 'npm:@angular/router/bundles/router-testing.umd.js',
    
    'rxjs': 'npm:rxjs',
    'typescript': 'npm:typescript@2.0.2/lib/typescript.js'
  },
  //packages defines our app package
  packages: {
    app: {
      main: './main.ts',
      defaultExtension: 'ts'
    },
    rxjs: {
      defaultExtension: 'js'
    }
  }
});

//main entry point
import {platformBrowserDynamic} from '@angular/platform-browser-dynamic';
import {AppModule} from './app';

platformBrowserDynamic().bootstrapModule(AppModule)

//our root app component
import {Component, NgModule, OnInit} from '@angular/core'
import {BrowserModule} from '@angular/platform-browser'
import {FormsModule, ReactiveFormsModule, FormGroup, FormControl} from '@angular/forms';

import {Ng2TriStateCheckboxComponent} from './ng2-tri-state-checkbox.component'

@Component({
  selector: 'my-app',
  template: `
    <form [formGroup]="myModelDrivenForm">
      <ng2-tri-state-checkbox formControlName="myModelDrivenControl"></ng2-tri-state-checkbox>
    </form>
    <pre>{{myModelDrivenForm.value | json}}</pre>
    <form #myTemplateDrivenForm="ngForm">
      <ng2-tri-state-checkbox [(ngModel)]="isTemplateDrivenControlChecked"
        name="myTemplateDrivenControl" #myTemplateDrivenControl="ngModel"></ng2-tri-state-checkbox>
    </form>
    <pre>{{myTemplateDrivenForm.value | json}}</pre>`,
})
export class App implements OnInit {
  public myModelDrivenForm: FormGroup;
  public isTemplateDrivenControlChecked: boolean = null; // possible values: true / false / null
  
  constructor() { }

  public ngOnInit(): void {
    this.myModelDrivenForm = new FormGroup({
      myModelDrivenControl: new FormControl(null) // possible values: true / false / null
    });
 }
}

@NgModule({
  imports: [ BrowserModule, FormsModule, ReactiveFormsModule ],
  declarations: [ App, Ng2TriStateCheckboxComponent ],
  bootstrap: [ App ]
})
export class AppModule {}

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

@Component({
  selector: 'ng2-tri-state-checkbox',
  template: `<input #checkbox type="checkbox" (change)="onChange()" (click)="setState()" />`,
  providers: [
    {
      provide: NG_VALUE_ACCESSOR,
      useExisting: forwardRef(() => Ng2TriStateCheckboxComponent),
      multi: true
    }
  ]
})
export class Ng2TriStateCheckboxComponent implements ControlValueAccessor {

  get checked() {
    return this._checked;
  }
  set checked(checked: boolean) {
    this._checked = checked;
    this.render();
    this.onChangeCallback(this._checked);
  }

  @ViewChild('checkbox') private _checkbox: ElementRef;
  private _checked: boolean = null;

  public writeValue(value: any): void {
    if (value !== undefined) {
      this.checked = value;
    }
  }

  public registerOnChange(fn: (_: any) => void): void {
    this.onChangeCallback = fn;
  }

  public registerOnTouched(fn: () => void): void {
    this.onTouchedCallback = fn;
  }

  public onChange(): void {
    this.onTouchedCallback();
  }

  public setState(): void {
    if (this._checkbox.nativeElement.readOnly) {
      this._checkbox.nativeElement.checked = this._checkbox.nativeElement.readOnly = this.checked = false;
    } else if (!this._checkbox.nativeElement.checked) {
      this._checkbox.nativeElement.readOnly = this._checkbox.nativeElement.indeterminate = true;
      this.checked = null;
    } else {
      this.checked = true;
    }
  }

  private onChangeCallback = (_: any) => { }
  private onTouchedCallback = () => { }

  private render(): void {
    switch(this.checked) {
      case true:
        this._checkbox.nativeElement.readOnly = this._checkbox.nativeElement.indeterminate = false;
        this._checkbox.nativeElement.checked = true;
        break;
      case false:
        this._checkbox.nativeElement.checked = this._checkbox.nativeElement.readOnly = this._checkbox.nativeElement.indeterminate = false;
        break;
      case null:
      case undefined:
      default:
        this._checkbox.nativeElement.readOnly = this._checkbox.nativeElement.indeterminate = true;
        break;
    }
  }
}