Image preview with changedetection OnPush

Question

I want to image preview multiple images before upload with OnPush change detection strategy.

I try this https://stackblitz.com/edit/angular-mnltiv

When I add OnPush it stop working, I know that I should change the array in a inmutable way but not working

import { Component, ChangeDetectionStrategy } from '@angular/core';
import { FormsModule } from '@angular/forms';

@Component({
  selector: 'my-app',
  templateUrl: './app.component.html',
  styleUrls: ['./app.component.css'],
  changeDetection: ChangeDetectionStrategy.OnPush
})
export class AppComponent {
  urls = new Array<string>();
  detectFiles(event) {
    this.urls = [];
    let files = event.target.files;
    if (files) {
      for (let file of files) {
        let reader = new FileReader();
        reader.onload = (e: any) => {
          this.urls.push(e.target.result);
          this.urls = [...this.urls]
        }
        reader.readAsDataURL(file);
      }
    }
  }
}

I expect this with OnPush https://stackblitz.com/edit/angular-4jmjzh

Answer 1

you have to trigger change detection after updated your url array when using onPush

import { Component, ChangeDetectionStrategy, OnInit, ChangeDetectorRef } from '@angular/core';


constructor(private cdr: ChangeDetectorRef){}
...
detectFiles(event) {
this.urls = [];
let files = event.target.files;
if (files) {
  for (let file of files) {
    let reader = new FileReader();
    reader.onload = (e: any) => {
      this.urls = [...this.urls, e.target.result]
      this.cdr.detectChanges(); // add this and it should work 
    }
    reader.readAsDataURL(file);
  }
}