Angular 2+ - check if Pipe returns an empty subset of original list

问题

I have a list of strings that I want to iterate through, but I want to be able to filter them using a search term. Like this:

<div *ngFor="#item in list | search: searchTerm">{{ item }}</div>

My question is: how can I check if the pipe returns an empty subset of the list?

In other words if none of the strings matches the search term, I want to display a message saying: "No matches".

回答1:

<div *ngIf="(list | search: searchTerm).length === 0">
  "No matches"
</div>
<div *ngFor="#item in list | search: searchTerm">{{ item }}</div>

Alternatively you could modify your pipe to return a specific token that indicates that the list is empty

@Pipe({
  name: 'search'
})
export class SearchPipe {

  transform(value, searchTerm) {
    let result = ...
    if(result.length === 0) {
      return [-1];
    }
    return result;
  }
}

<ng-container *ngFor="let item of list | search: searchTerm">
  <div *ngIf="item === -1">"No matches"</div>
  <div *ngIf="item !== -1">{{ item }}</div>
</ng-container>

回答2:

One more way to achieve this is to check the html element for children or in my case the table for rows.

<table #myTable>
  <tr *ngFor="let item of list | somePipe : searchText">
      <td>{{ item.name }}</td>
  </tr>
</table>

<p *ngIf="!myTable.rows.length">No results</p>

回答3:

It's possible to leverage dependency injection into pipes. You could inject the component:

Then you can set a property on it to notify this:

@Pipe({
  name: 'search'
})
export class SearchPipe {
  constructor(@Inject(forwardRef(() => SomeComponent)) private comp:SomeComponent) {

  }

  transform(value) {
    var filtered = value.map((v) => v-1);
    this.comp.isEmpty = (filtered.length === 0);
    return filtered;
  }
}

The main drawback is that you link the pipe within the component. The advantage is that filtering is executed once.

回答4:

This is my code which modified a bit from @Günter Zöchbauer

<div  *ngFor="let filter_list of list | FilterItem" >
    <div *ngIf=" filter_list == -1 " class="alert alert-info">No item found</div>    
    <div *ngIf="filter_list !== -1" *ngFor="let item of filter_list  ; let i = index;" >
        {{ item }}
    </div>
</div>

Pipe code

@Pipe({
  name: 'FilterItem'
})
export class FilterItem {

  transform(list, args?) {
       let result = ...; 
       if ( result && result.length > 0 ){
          return [ result ];
       }else{
          return [ -1 ];
       }
  }
}

回答5:

If your purpose is just render an element than you could do with CSS query it self, i just fallowing Günter Zöchbauer code.

<ng-container *ngFor="let item of list | search: searchTerm">
  <div *ngIf="item !== -1">{{ item }}</div>
  <div class="empty">"No matches"</div>
</ng-container>

CSS

div.empty {
  display:none;
}
div.empty:first-child {
  display:block;
}

.list div.empty {
  display: none;
}

.list div.empty:first-child {
  display: block;
}

<h4>If you hava record to display than</h4>
<div class="list">
  <div>The first record.</div>
  <div>The second record.</div>
  <div>The third record.</div>
  <div class="empty">"No matches"</div>
</div>
<br>
<h4>If no record to show</h4>
<div class="list">
  <div class="empty">"No matches"</div>
</div>

回答6:

This is the cleanest solution I've been able to produce.

@Pipe({
  name: 'search'
})
export class SearchPipe {

  transform(value, searchTerm) {
    let result = ...
    if(result.length === 0) {
      return [undefined];
    }
    return result;
  }
}

By returning [undefined], checks in the DOM are much cleaner and easier to read.

<ng-container *ngFor="let item of list | search: searchTerm">
  <div *ngIf="!item">"No matches"</div>
  <div *ngIf="item">{{ item }}</div>
</ng-container>

回答7:

With angular now supporting ngIf with variables you could assign the pipe result to a new variable and then do the looping inside the if block

<ng-container *ngIf="search: searchTerm as results; else noItems">
 <!-- else is for cases where search:Search term is undefined or null  -->

    <div *ngFor="let item of results">{{item}}</div>

    <!-- the case where the pipe returns an empty array  -->
    <div *ngIf="!result.length">no items match the search</div>
</ng-container>
<ng-template #noItems>searching...</ng-template>

来源：https://stackoverflow.com/questions/37067138/angular-2-check-if-pipe-returns-an-empty-subset-of-original-list