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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