Angular 8 - 在前端更改检测问题上调整图像大小

Question

我正在尝试根据我在网上找到的几个教程构建一个图像压缩器服务。 service 本身按预期工作，它接收一个图像作为文件，然后压缩它并 returns 一个 可观察。一切都很好，只是我想在将压缩图像上传到服务器之前在我的组件中使用它。

该组件不会检测新的压缩图像何时通过 async 管道到达。如果我手动订阅 Observable，我会按预期获得图像数据，但如果我尝试用它更新组件属性，它不会立即更改视图，而是用旧的 'image data' 如果我尝试压缩新图像。

我发现如果部分代码在 ngZone 之外解析，则可能会出现此问题，因此我找到了一种解决方法（请参阅下面的代码）注入 ApplicationRef 并使用 .tick() 这实际上很好用，但使我的服务难以重用。

我的问题是： 服务代码的哪一部分在 ngZone 之外运行以及可能的修复或解决方法是什么，以便服务可以在其他组件中重用，而不必在每次服务发出数据时都注入 ApplicationRef 和 .tick()。

这是我的服务代码：

 import { Observable ,  Subscriber } from 'rxjs';
import { Injectable } from '@angular/core';
import { DomSanitizer, SafeUrl } from '@angular/platform-browser';

@Injectable({
  providedIn: 'root'
})

export class ImageCompressorService {

// globals
private _currentFile : File ;
private _currentImage : ICompressedImage = {} ;

// Constructor
constructor( private sanitizer : DomSanitizer) {}

// FileReader Onload callback
readerOnload(observer : Subscriber<ICompressedImage>)  {
 return (progressEvent : ProgressEvent) => {
  const img = new Image();
  img.src = (progressEvent.target as any).result;
  img.onload = this.imageOnload(img , observer);
}
}

// Image Onload callback
 imageOnload(image : HTMLImageElement , observer : Subscriber<ICompressedImage>) {
  return () => {
  const canvas = document.createElement('canvas');
  canvas.width = 100;
  canvas.height = 100;
  const context = <CanvasRenderingContext2D>canvas.getContext('2d');
  context.drawImage(image , 0 , 0 , 100 , 100);
  this.toICompressedImage(context , observer);
}}

// Emit CompressedImage
toICompressedImage(context : CanvasRenderingContext2D , observer : Subscriber<ICompressedImage> ) {
  context.canvas.toBlob(
    (blob) => {
      this._currentImage.blob = blob ;
      this._currentImage.image = new File([blob] , this._currentFile.name , {type : 'image/jpeg', lastModified : Date.now()} );
      this._currentImage.imgUrl = this.sanitizer.bypassSecurityTrustUrl(URL.createObjectURL(blob));
      this._currentImage.name = this._currentFile.name ;
      observer.next(this._currentImage);
      observer.complete();
    } ,
    'image/jpeg' ,
    1
  );
}

//  Compress function
 compress(file : File) : Observable<ICompressedImage> {
   this._currentFile = file ;
   return new Observable<ICompressedImage>(
     observer => {
       const currentFile = file;
       const reader = new FileReader();
       reader.readAsDataURL(currentFile);
       reader.onload = this.readerOnload(observer);
     }
   );
 }
}

// Image Data Interface
export interface ICompressedImage {
  name? : string;
  image? : File ;
  blob? : Blob ;
  imgUrl? : SafeUrl ;
}

这是我的 component.ts :

import { Component, OnInit, ApplicationRef } from '@angular/core';
import { ImageCompressorService, ICompressedImage } from 'src/app/shared/services/image-compressor.service';


@Component({
  selector: 'app-new-project',
  templateUrl: './new-project.component.html',
  styleUrls: ['./new-project.component.css']
})
export class NewProjectComponent implements OnInit  {
// globals
private selectedImage ;
compressedImage :  ICompressedImage = {name : 'No file selected'};


// Constructor
  constructor( private compressor : ImageCompressorService,
               private ar : ApplicationRef
             ) {}
// OnInit implementation
     ngOnInit(): void {}

// Compress method
  compress(fl : FileList) {
if (fl.length>0) {
    this.selectedImage = fl.item(0);
    this.compressor
    .compress(this.selectedImage)
    .subscribe(data => {
     this.compressedImage = data ;
     this.ar.tick();
    });
  } else {
    console.error('No file/s selected');

  }
  }


}

这是我的 HTML 模板 组件：

<div style='border : 1px solid green;'>
    <input type='file' #SelectedFile (change)="compress($event.target.files)" accept='image/*' >
</div>


<div
style = 'border : 1px solid blue ; height : 200px;'
*ngIf="compressedImage " >
 <strong>File Name : </strong>{{ compressedImage?.name }}

<img *ngIf="compressedImage?.imgUrl as src"
[src]= 'src' >
</div>

我展示代码的方式非常完美。尝试注释掉 this.ar.tick(); 在 Compress Method 中 component.ts 文件并查看更改 .

Answer 1

经过几个小时的挖掘，我找到了一个可行的解决方案。我在我的服务中注入了 NgZone 包装器。之后，在我的压缩方法中，我使用运行所有文件处理代码 zone.runOutsideAngular() ，从而故意防止 ChangeDetection ，一旦调整大小操作完成并且新的压缩图像可用，我运行观察者（订阅者）的下一个方法 zone.Run() ，它实际上在 Angular 的区域内运行代码，强制 ChangeDetection 。我已经测试了在我的组件中手动订阅生成的可观察对象，以及通过异步管道进行的订阅。两者都很有魅力。使用异步管道发布代码。

service.ts :

import { Observable ,  Subscriber } from 'rxjs';
import { Injectable, NgZone } from '@angular/core';
import { DomSanitizer, SafeUrl } from '@angular/platform-browser';

@Injectable({
  providedIn: 'root'
})

export class ImageCompressorService {

// globals
private _currentFile : File ;
private _currentImage : ICompressedImage = {} ;

// Constructor
constructor( private sanitizer : DomSanitizer , private _zone : NgZone) {

}

// FileReader Onload callback
readerOnload(observer : Subscriber<ICompressedImage>)  {
 return (progressEvent : ProgressEvent) => {
  const img = new Image();
  img.src = (progressEvent.target as any).result;
  img.onload = this.imageOnload(img , observer);
}
}

// Image Onload callback
 imageOnload(image : HTMLImageElement , observer : Subscriber<ICompressedImage>) {
  return () => {
  const canvas = document.createElement('canvas');
  canvas.width = 100;
  canvas.height = 100;
  const context = <CanvasRenderingContext2D>canvas.getContext('2d');
  context.drawImage(image , 0 , 0 , 100 , 100);
  this.toICompressedImage(context , observer);
}}

// Emit CompressedImage
toICompressedImage(context : CanvasRenderingContext2D , observer : Subscriber<ICompressedImage> ) {
  context.canvas.toBlob(
    (blob) => {
      this._currentImage.blob = blob ;
      this._currentImage.image = new File([blob] , this._currentFile.name , {type : 'image/jpeg', lastModified : Date.now()} );
      this._currentImage.imgUrl = this.sanitizer.bypassSecurityTrustUrl(URL.createObjectURL(blob));
      this._currentImage.name = this._currentFile.name ;
      this._zone.run(() => {
        observer.next(this._currentImage);
        observer.complete();
      })

    } ,
    'image/jpeg' ,
    1
  );
}

//  Compress function
 compress(file : File) : Observable<ICompressedImage> {
   this._currentFile = file ;
   return new Observable<ICompressedImage>(
     observer => {
       this._zone.runOutsideAngular(() => {
        const currentFile = file;

        const reader = new FileReader();
        reader.readAsDataURL(currentFile);
        reader.onload = this.readerOnload(observer);
       })

     }
   );
 }
}

// Image Data Interface
export interface ICompressedImage {
  name? : string;
  image? : File ;
  blob? : Blob ;
  imgUrl? : SafeUrl ;
}

component.ts :

import { Component, OnInit } from '@angular/core';
import { ImageCompressorService, ICompressedImage } from 'src/app/shared/services/image-compressor.service';
import { Observable } from 'rxjs';


@Component({
  selector: 'app-new-project',
  templateUrl: './new-project.component.html',
  styleUrls: ['./new-project.component.css']
})
export class NewProjectComponent implements OnInit  {
// globals
private selectedImage ;
compressedImage : Observable<ICompressedImage>;

// Constructor
  constructor( private compressor : ImageCompressorService) {}

// OnInit implementation
     ngOnInit(): void {}

// Compress method
  compress(fl : FileList) {
if (fl.length>0) {
    this.selectedImage = fl.item(0);
  this.compressedImage =  this.compressor.compress(this.selectedImage)
  } else {

    console.error('No file/s selected');

  }
  }
}

component.html :

<div style='border : 1px solid green;'>
    <input type='file' #SelectedFile (change)="compress($event.target.files)" accept='image/*' >
</div>


<div
style = 'border : 1px solid blue ; height : 200px;'
*ngIf="compressedImage | async as image" >
 <strong>File Name : </strong>{{ image.name }}

<img *ngIf="image.imgUrl as src"
[src]= 'src' >
</div>

Angular 8 - 在前端更改检测问题上调整图像大小

Angular 8 - Resizing image on the front-end change-detection issue

angular

asynchronous

auto-update

image-compression

ngzone