Android-异步加载图片

转载请标明出处：http://blog.csdn.net/goldenfish1919/article/details/38559205

原文：http://developer.android.com/training/displaying-bitmaps/process-bitmap.html

ListView中加载图片的时候，为了防止图片错位，通常会给图片设置一个tag，今天听同事说google官网上有一种很牛逼的实现，然后看了下，google给出的代码实现的非常巧妙，而且还不用设置额外的tag，它是把ImageView和AsyncTask绑定到一块，如果imageView重用就把task取消。

public void loadBitmap(int resId, ImageView imageView, Bitmap mPlaceHolderBitmap) {
		// 取消任务，重新绑定
		if (cancelPotentialWork(resId, imageView)) {
			// 任务绑定一个imageView
			final BitmapWorkerTask task = new BitmapWorkerTask(imageView);
			// imageView绑定一个任务，imageView可能会重用，但是task不会
			final AsyncDrawable asyncDrawable = new AsyncDrawable(getResources(), mPlaceHolderBitmap, task);
			imageView.setImageDrawable(asyncDrawable);
			// 执行任务
			task.execute(resId);
		}
	}

	public static boolean cancelPotentialWork(int latestData, 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 != latestData) {
				// 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;
	}
	
	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);
		}

		@Override
		protected void onPostExecute(Bitmap bitmap) {
			if (isCancelled()) {
				bitmap = null;
			}
			if (imageViewReference != null && bitmap != null) {
				final ImageView imageView = imageViewReference.get();
				final BitmapWorkerTask bitmapWorkerTask = getBitmapWorkerTask(imageView);
				if (this == bitmapWorkerTask && imageView != null) {
					imageView.setImageBitmap(bitmap);
				}
			}
		}
	}

	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();
		}
	}

	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;
	}

	public static Bitmap decodeSampledBitmapFromResource(Resources res,
			int resId, int reqWidth, int reqHeight) {

		// First decode with inJustDecodeBounds=true to check dimensions
		final BitmapFactory.Options options = new BitmapFactory.Options();
		options.inJustDecodeBounds = true;
		BitmapFactory.decodeResource(res, resId, options);

		try{
			Thread.sleep(new Random().nextInt(1000));
		}catch(Exception e){}
		
		// Calculate inSampleSize
		options.inSampleSize = calculateInSampleSize(options, reqWidth,
				reqHeight);

		// Decode bitmap with inSampleSize set
		options.inJustDecodeBounds = false;
		return BitmapFactory.decodeResource(res, resId, options);
	}

	public static int calculateInSampleSize(BitmapFactory.Options options,
			int reqWidth, int reqHeight) {
		// Raw height and width of image
		final int height = options.outHeight;
		final int width = options.outWidth;
		int inSampleSize = 1;

		if (height > reqHeight || width > reqWidth) {

			final int halfHeight = height / 2;
			final int halfWidth = width / 2;

			// Calculate the largest inSampleSize value that is a power of 2 and
			// keeps both
			// height and width larger than the requested height and width.
			while ((halfHeight / inSampleSize) > reqHeight
					&& (halfWidth / inSampleSize) > reqWidth) {
				inSampleSize *= 2;
			}
		}
		return inSampleSize;
	}

类似的实现还有：

http://blog.csdn.net/goldenfish1919/article/details/25374731 这个能防止多次下载

http://blog.csdn.net/lmj623565791/article/details/38476887 这个非常有意思，用子线程的Handler来实现，还可以实现先加载最后一页的图片。