Typescriptでタイプセーフなビルダーパターン

type-safe-builder.ts

type WidthDefined = {width: number};
type HeightDefined = {height:number};
type TitleDefined = {title:string};
type AllDefined = WidthDefined & HeightDefined & TitleDefined;

class WidgetBuilder<T>{
  constructor(private params: T){}
   width<D>(this:WidgetBuilder<D extends WidthDefined ? never : D>, width: number):WidgetBuilder<D & WidthDefined> {
     return new WidgetBuilder(Object.assign(this.params, {width}));
   }
   height<D>(this:WidgetBuilder<D extends HeightDefined ? never : D>, height: number):WidgetBuilder<D & HeightDefined> {
     return new WidgetBuilder(Object.assign(this.params, {height}));
   }
   title<D>(this:WidgetBuilder<D extends TitleDefined ? never : D>, title: string):WidgetBuilder<D & TitleDefined> {
     return new WidgetBuilder(Object.assign(this.params, {title}));
   }
   build(this:WidgetBuilder<AllDefined>): Widget {
     return new Widget(this.params.width, this.params.height, this.params.title);
   }
 }

class Widget {
  public constructor(
    private width: number,
    private height: number,
    private title: string){
  }

  public static builder():WidgetBuilder<object> {return new WidgetBuilder({});}
}

// OK
Widget.builder()
  .width(320)
  .height(240)
  .title("po")
  .build();

// エラー
Widget.builder()
  .width(320)
  .build();

// エラー
Widget.builder()
  .width(320)
  .height(240)
  .title("po")
  .height(480)
  .build();