Why do I need to call detectChanges() with the default change detection strategy?

问题

I am working on an Angular 4 app, but I am having a problem inasmuch as I am having to call this.changeDetectorRef.detectChanges(); to update the view when the model changes. For example, I have this code which is for pagination:

changePage(pageNumber) {
  this.currentPage = pageNumber;
  // why do I need to do this?
  this.changeDetectorRef.detectChanges();
}

I have expicitly set the change detection strategy on the component to ChangeDetectionStrategy.Default but this has no effect. It is happening here too when subscribing to an observable:

showResults(preference) {
  this.apiService.get('dining_autocomplete/', `?search=${preference}`)
    .subscribe((results) => {
      this.searchResults = results;
      // why do I need to do this?
      this.changeDetectorRef.detectChanges();
  });
}

If I console.log() this.searchResults in the TypeScript, I get the expected results, but if I use {{ searchResults }} in the HTML, it doesn't update until some other event happens, presumably when it goes through another digest cycle.

What could be going on?

== EDIT ===========================================================

The code for my component looks like this:

import {ChangeDetectorRef, Component, Input, OnChanges} from "@angular/core";

import * as _ from 'lodash';

@Component({
  selector: 'dining-search-results',
  templateUrl: './dining-search-results.template.html',
  styleUrls: ['./dining-search-results.style.scss'],

})
export class DiningSearchResultsComponent {
  @Input() searchResults: any[];
  @Input() hotTablesOnly: boolean = false;
  @Input() memberBenefitsOnly: boolean = false;

  numberOfTagsToDisplay: number = 3;
  resultsPerPage: number = 5;
  currentPage: number = 1;

  constructor(private changeDetectorRef: ChangeDetectorRef) {
  }

  get filteredResults() {
    return this.searchResults ?
      this.searchResults.filter((r) => !((this.hotTablesOnly && !r.has_hot_table)
        || (this.memberBenefitsOnly && !r.has_member_benefit))) : [];
  }

  get pagedResults() {
    return _.chain(this.filteredResults)
      .drop(this.resultsPerPage * (this.currentPage - 1))
      .take(this.resultsPerPage)
      .value();
  }

  get totalPages(): number {
    return Math.ceil(this.filteredResults.length / this.resultsPerPage);
  }

  getInitialTags(tagsArray: any[], count: number): any[] {
    return _.take(tagsArray, count);
  }

  changePage(pageNumber) {
    this.currentPage = pageNumber;
    // why do I need to do this?
    this.changeDetectorRef.detectChanges();
  }
}

When changePage() is called, and this.currentPage is updated, the changes are not reflected in the HTML unless I call detectChanges().

回答1:

I have solved the problem.

Another component sending notifications to this component was running Observable.fromEvent() outside the Angular zone, so change detection was not happening automatically in response to these events. This post on zones and this StackOverflow post on the issue held the solution!

回答2:

You can Input your searchResults into the child Components

@Input() searchResults;

after that you pass it trough the parent template

// parent template
<app-child [searchResults]="searchResults"></app-child>

you can use it in the child template

// child template 
<my-date-picker [ngModel]="searchResults"></my-date-picker>

after that you can 'listen' for changes in this property in the child Component

export class ChildComponent implements OnChanges {

  @Input() searchResults;

  constructor() { }

  ngOnChanges() {
    // some code
  }

Every time searchResults is changed the changes will be populated in the child component, the values in the child template will get the new value.

来源：https://stackoverflow.com/questions/45236671/why-do-i-need-to-call-detectchanges-with-the-default-change-detection-strategy

标签

angular

typescript

angular2-changedetection