加載圖片基本操作
一、創建AsyncTask子類
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、創建異步任務實例對象，開始執行
public void loadBitmap(int resId, ImageView imageView) { BitmapWorkerTask task = new BitmapWorkerTask(imageView); task.execute(resId);}處理并發
因為ListView和GridView復用子布局，無法保證異步任務完成時，相關的布局沒有被回收。也無法保證異步任務完成時間的先后順序。所以必須處理并發事件。
一、創建BitmapDrawable子類
該類專門用來保存Bitmap及其對應的異步任務
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、綁定AsyncDrawable
創建AsyncDrawable并傳入BitmapWorkerTask對象，imageView設置為該AsyncDrawable再開始異步任務
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這個方法用于調用方法獲取控件對應異步任務，判斷是否與當前任務一致
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;}這個getBitmapWorkerTask()方法用于獲取圖片對應異步任務
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;}總結
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