Dynamic pipe in Angular 2

问题:

I'm trying to create a component where you can pass which pipe that should be used for a list inside the component. From what I could find by testing and looking around for answers the only solution appears to create something like:

<my-component myFilter="sortByProperty"></my-component>

my-component template:

<li *ngFor="#item of list | getPipe:myFilter"></li>

Which then maps myFilter to the correct pipe logic and runs it, but this seems a bit dirty and not optimal.

I thought they would have come up with a better solution to this problem since Angular 1 where you would also do something along these lines.

Is there not a better way to do this in Angular 2?

回答1:

Unfortunately I don't think so. It's the same as in angular1 where you have a function return a string for the dynamic Pipe you want.

Looking at the docs that's exactly how they show it as well.

https://angular.io/docs/ts/latest/guide/pipes.html

template: `
   <p>The hero's birthday is {{ birthday | date:format }}</p>
   <button (click)="toggleFormat()">Toggle Format</button>
`

Then in the controller:

get format()   { return this.toggle ? 'shortDate' : 'fullDate'}

Alas, it could be worse! :)

回答2:

Building on borislemke's answer, here's a solution which does not need eval() and which I find rather clean:

dynamic.pipe.ts:

import {
    Injector,
    Pipe,
    PipeTransform
} from '@angular/core';


@Pipe({
  name: 'dynamicPipe'
})
export class DynamicPipe implements PipeTransform {

    public constructor(private injector: Injector) {
    }

    transform(value: any, pipeToken: any, pipeArgs: any[]): any {
        if (!pipeToken) {
            return value;
        }
        else {
            let pipe = this.injector.get(pipeToken);
            return pipe.transform(value, ...pipeArgs);
        }
    }
}

app.module.ts:

// …
import { DynamicPipe } from './dynamic.pipe';

@NgModule({
  declarations: [
    // …
    DynamicPipe,
  ],
  imports: [
    // …
  ],
  providers: [
    // list all pipes you would like to use
    PercentPipe,
    ],
  bootstrap: [AppComponent]
})
export class AppModule { }

app.component.ts:

import { Component, OnInit } from '@angular/core';
import { PercentPipe } from '@angular/common';

@Component({
  selector: 'app-root',
  template: `
    The following should be a percentage: 
    {{ myPercentage | dynamicPipe: myPipe:myPipeArgs }}
    `,
  providers: []
})

export class AppComponent implements OnInit {
  myPercentage = 0.5;
  myPipe = PercentPipe;
  myPipeArgs = [];
}

回答3:

I managed to get something working, it's a bit dirty and evil (with eval) but it does the trick for me. In my case, I have a table component with different data types in each row (e.g title, url, date, status). In my database, status is marked as either 1 as enabled or 0 for disabled. Of course, it is more preferable to be showing enabled/disabled to my user. Also, my title column is multilingual, which makes it an object with either en or id as it's key.

// Example row object:
title: {
    "en": "Some title in English",
    "id": "Some title in Indonesian"
},
status: 1 // either 1 or 0

Ideally, I need 2 different pipes to convert my data to show to my app's user. Something like translateTitle and getStatus will do fine. Let's call the parent's pipe dynamicPipe.

/// some-view.html
{{ title | dynamicPipe:'translateTitle' }}
{{ status | dynamicPipe:'getStatus' }}


/// dynamic.pipe.ts
//...import Pipe and PipeTransform

@Pipe({name:'dynamicPipe'})
export class DynamicPipe implements PipeTransform {

    transform(value:string, modifier:string) {
        if (!modifier) return value;
        return eval('this.' + modifier + '(' + value + ')')
    }

    getStatus(value:string|number):string {
        return value ? 'enabled' : 'disabled'
    }

    translateTitle(value:TitleObject):string {
        // defaultSystemLanguage is set to English by default
        return value[defaultSystemLanguage]
    }
}

I'll probably get a lot of hate on using eval. Hope it helps!

Update: when you might need it

posts = {
    content: [
        {
            title:
                {
                    en: "Some post title in English",
                    es: "Some post title in Spanish"
                },
            url: "a-beautiful-post",
            created_at: "2016-05-15 12:21:38",
            status: 1
        },
        {
            title:
                {
                    en: "Some post title in English 2",
                    es: "Some post title in Spanish 2"
                },
            url: "a-beautiful-post-2",
            created_at: "2016-05-13 17:53:08",
            status: 0
        }
    ],
    pipes: ['translateTitle', null, 'humanizeDate', 'getStatus']
}

<table>
    <tr *ngFor="let row in posts">
        <td *ngFor="let column in row; let i = index">{{ column | dynamicPipe:pipes[i] }}</td>
    </tr>
</table>

Will return:

| title          | url            | date           | status         |
| Some post t...   a-beautiful...   an hour ago      enabled
| Some post ...2   a-beautifu...2   2 days ago       disabled

回答4:

The easiest way to tackle this would be to not use the pipes in the HTML templates, but instead, inject the pipe into a component's constructor (using DI), then apply the transform functionally. This works quite well with an Observable map or similar rxjs streams.