Android使用AsyncTask加载图片的操作流程

加载图片基本操作

一、创建asynctask子类

• 将imageview的弱引用设置为成员变量，创建构造函数传入imageview对象。
• 调用指定大小解析bitmap方法。
• 因为是弱引用，所以必须判断引用是否被回收。如果异步任务完成前，用户离开activity或者设置发生改变，imageview也可能不存在。

class bitmapworkertask extends asynctask<integer, void, bitmap> {
  private final weakreference<imageview> imageviewreference;
  private int data = 0;
  public bitmapworkertask(imageview imageview) {
    // use a weakreference to ensure the imageview can be garbage collected
    imageviewreference = new weakreference<imageview>(imageview);
  }
  // decode image in background.
  @override
  protected bitmap doinbackground(integer... params) {
    data = params[0];
    return decodesampledbitmapfromresource(getresources(), data, 100, 100));
  }
  // once complete, see if imageview is still around and set bitmap.
  @override
  protected void onpostexecute(bitmap bitmap) {
    if (imageviewreference != null && bitmap != null) {
      final imageview imageview = imageviewreference.get();
      if (imageview != null) {
        imageview.setimagebitmap(bitmap);
      }
    }
  }
}

二、创建异步任务实例对象，开始执行

public void loadbitmap(int resid, imageview imageview) {
  bitmapworkertask task = new bitmapworkertask(imageview);
  task.execute(resid);
}

处理并发

因为listview和gridview复用子布局，无法保证异步任务完成时，相关的布局没有被回收。也无法保证异步任务完成时间的先后顺序。所以必须处理并发事件。

一、创建bitmapdrawable子类

该类专门用来保存bitmap及其对应的异步任务

static class asyncdrawable extends bitmapdrawable {
  private final weakreference<bitmapworkertask> bitmapworkertaskreference;
  public asyncdrawable(resources res, bitmap bitmap,
      bitmapworkertask bitmapworkertask) {
    super(res, bitmap);
    bitmapworkertaskreference =
      new weakreference<bitmapworkertask>(bitmapworkertask);
  }
  public bitmapworkertask getbitmapworkertask() {
    return bitmapworkertaskreference.get();
  }
}

二、绑定asyncdrawable

创建asyncdrawable并传入bitmapworkertask对象，imageview设置为该asyncdrawable再开始异步任务

public void loadbitmap(int resid, imageview imageview) {
  if (cancelpotentialwork(resid, imageview)) {
    final bitmapworkertask task = new bitmapworkertask(imageview);
    final asyncdrawable asyncdrawable =
        new asyncdrawable(getresources(), mplaceholderbitmap, task);
    imageview.setimagedrawable(asyncdrawable);
    task.execute(resid);
  }
}

cancelpotentialwork这个方法用于调用方法获取控件对应异步任务，判断是否与当前任务一致

public static boolean cancelpotentialwork(int data, imageview imageview) {
  final bitmapworkertask bitmapworkertask = getbitmapworkertask(imageview);
  if (bitmapworkertask != null) {
    final int bitmapdata = bitmapworkertask.data;
    // if bitmapdata is not yet set or it differs from the new data
    if (bitmapdata == 0 || bitmapdata != data) {
      // cancel previous task
      bitmapworkertask.cancel(true);
    } else {
      // the same work is already in progress
      return false;
    }
  }
  // no task associated with the imageview, or an existing task was cancelled
  return true;
}

这个getbitmapworkertask()方法用于获取图片对应异步任务

private static bitmapworkertask getbitmapworkertask(imageview imageview) {
  if (imageview != null) {
    final drawable drawable = imageview.getdrawable();
    if (drawable instanceof asyncdrawable) {
      final asyncdrawable asyncdrawable = (asyncdrawable) drawable;
      return asyncdrawable.getbitmapworkertask();
    }
  }
  return null;
}

总结

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