问题描述:

I need to filter items inside an ngFor loop, by changing the category in a drop-down list. Therefore, when a particular category is selected from the list, it should only list the items containing that same category.

HTML Template:

<select>

<option *ngFor="let model of models">{{model.category}}</option>

</select>

<ul class="models">

<li *ngFor="let model of models" (click)="gotoDetail(model)">

<img [src]="model.image"/>

{{model.name}},{{model.category}}

</li>

</ul>

Items Array:

export var MODELS: Model[] = [

{ id: 1,

name: 'Model 1',

image: 'img1',

category: 'Cat1',

},

{ id: 2,

name: 'Model 2',

image: 'img2',

category: 'Cat3',

},

{ id: 3,

name: 'Model 3',

image: 'img3',

category: 'Cat1',

},

{ id: 4,

name: 'Model 4',

image: 'img4',

category: 'Cat4',

},

...

];

Also, the drop-down list contains repeated category names. It is necessary for it to list only unique categories (strings).

I know that creating a custom pipe would be the right way to do this, but I don't know how to write one.

Plunker: http://plnkr.co/edit/tpl:2GZg5pLaPWKrsD2JRted?p=preview

网友答案:

Here is a sample pipe:

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

@Pipe({
    name: 'matchesCategory'
})
export class MathcesCategoryPipe implements PipeTransform {
    transform(items: Array<any>, category: string): Array<any> {
        return items.filter(item => item.category === category);
    }
}

To use it:

<li *ngFor="let model; of models | matchesCategory:model.category" (click)="gotoDetail(model)">

===== for the plunkr example ====

You need your select changes to reflect in some variable

First define in your class a member:

selectedCategory: string;

then update your template:

<select (change)="selectedCategory = $event.target.value">
   <option *ngFor="let model of models ">{{model.category}}</option>
</select>

last, use the pipe:

  <li *ngFor="let model; of models | matchesCategory:selectedCategory" (click)="gotoDetail(model)">

==== comments after seeing the plunker ====

I noticed you used promise. Angular2 is more rxjs oriented. So the first thing I'd change is in your service, replace:

getModels(): Promise<Model[]> {
  return Promise.resolve(MODELS);
}

to:

getModels(): Observable<Array<Model>> {
  return Promise.resolve(MODELS);
}

and

getModels(id: number): Observable<Model> {
  return getModels().map(models => models.find(model.id === id);
}

then in your ModelsComponent

models$: Observable<Array<Model>> = svc.getModels();
uniqueCategories$: Observable<Array<Model>> = this.models$
  .map(models => models.map(model => model.category)
  .map(categories => Array.from(new Set(categories)));

Your options will become:

     <option *ngFor="let category; of uniqueCategories$ | async">{{model.category}}</option>

and your list:

      <li *ngFor="let model; of models$ | async | matchesCategory:selectedCategory" (click)="gotoDetail(model)">

This is a very drafty solution since you have many duplicates and you keep querying the service. Take this as a starting point and query the service only once, then derive specific values from the result you got.

If you'd like to keep you code, just implement a UniqueValuesPipe, its transform will get a single parameter and filter it to return unique categories using the Array.from(new Set(...)). You will need though to map it to strings (categories) first.

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