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。
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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
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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
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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
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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
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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
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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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原文链接:https://blog.csdn.net/timshinlee/article/details/53064305