Angular 2 Pipe under condition

问题

Is it possible in Angular 2 to apply a pipe under condition? I would like to do something like:

{{ variable.text | (variable.value ? SomePipe : OtherPipe) }}

If not, what is the preferred way to achieve this effect?

回答1:

You need to change the syntax a bit:

{{variable.value ? (variable.text | SomePipe) : (variable.text | pipe2)}}

Plunker example

回答2:

Since such syntax isn't supported, I think that the only way to do that is to implement another pipe to handle the condition:

@Pipe({
  name: 'condition'
})
export class ConditionPipe {
  transform(val,conditions) {
    let condition = conditions[0];
    let conditionValue = conditions[1];

    if (condition===conditionValue) {
      return new Pipe1().transform(val);
    } else {
      return new Pipe2().transform(val);
    }
  }
}

And use it this way:

@Component({
  selector: 'app'
  template: `
    <div>
      {{val | condition:cond:1}}<br/>
    </div>
  `,
  pipes: [ Pipe1, Pipe2, ConditionPipe ]
})
export class App {
  val:string = 'test';
  cond:number = 1;
}

See this plunkr: https://plnkr.co/edit/KPIA5ly515xZk4QZx4lp?p=preview.

回答3:

You could also use ngIf

<span *ngIf="variable.value; else elseBlock">{{ variable.text | SomePipe }}</span>
<ng-template #elseBlock>{{ variable.text | OtherPipe }}</ng-template>

I find it useful in case the line becomes too long.

回答4:

As others pointed out, you can use the syntax of {{condition ? (value | pipe1) : (value2 | pipe2 )}}.

But it is worth knowing that also the format parameter of a pipe can be dynamic. e.g. this is an example of a number which can be formatted with a high precision or a low precision. The condition is passed to a method, which will create a formatter text conditionally.

  // in template
  {{ value | number:getFormat(true) }}

  // in .ts
  public getFormat(highPrecision = false): string {
    const digits = highPrecision ? 3 : 2;
    return `1.${digits}-${digits}`;
  }

So, yes, you can use a condition to select between 2 pipes. But in some cases you may prefer (or only need) to use one pipe with a conditional format parameter..

来源：https://stackoverflow.com/questions/36928435/angular-2-pipe-under-condition