正确的在GridView中使用Bitmap图像
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上面一篇文章《正确的在ViewPager中使用Bitmap图像》介绍了如何在ViewPager中加载bitmap图像。本篇将讲述如何将此技术用在GridView中。
使用列表来显示一组图像是十分有用的，你可以使用GridList控件来实现这个功能，你可以每次只显示一屏图像，其他的图像在用户做上下滚动操作的时候才会显示。实现这种控件的时候，你要确保UI元素流畅，内存不会超出最大限制，并且还要注意同步问题。
首先，我们以一个标准的使用Fragment包含ImageView的GridView实现。下面的实现看上去是十分理想的，你能看出怎样可以去优化它吗？
public class ImageGridFragment extends Fragment implements AdapterView.OnItemClickListener { private ImageAdapter mAdapter; // A static dataset to back the GridView adapter public final static Integer[] imageResIds = new Integer[] { R.drawable.sample_image_1, R.drawable.sample_image_2, R.drawable.sample_image_3, R.drawable.sample_image_4, R.drawable.sample_image_5, R.drawable.sample_image_6, R.drawable.sample_image_7, R.drawable.sample_image_8, R.drawable.sample_image_9}; // Empty constructor as per Fragment docs public ImageGridFragment() {} @Override public void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); mAdapter = new ImageAdapter(getActivity()); } @Override public View onCreateView( LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) { final View v = inflater.inflate(R.layout.image_grid_fragment, container, false); final GridView mGridView = (GridView) v.findViewById(R.id.gridView); mGridView.setAdapter(mAdapter); mGridView.setOnItemClickListener(this); return v; } @Override public void onItemClick(AdapterView<?> parent, View v, int position, long id) { final Intent i = new Intent(getActivity(), ImageDetailActivity.class); i.putExtra(ImageDetailActivity.EXTRA_IMAGE, position); startActivity(i); } private class ImageAdapter extends BaseAdapter { private final Context mContext; public ImageAdapter(Context context) { super(); mContext = context; } @Override public int getCount() { return imageResIds.length; } @Override public Object getItem(int position) { return imageResIds[position]; } @Override public long getItemId(int position) { return position; } @Override public View getView(int position, View convertView, ViewGroup container) { ImageView imageView; if (convertView == null) { // if it's not recycled, initialize some attributes imageView = new ImageView(mContext); imageView.setScaleType(ImageView.ScaleType.CENTER_CROP); imageView.setLayoutParams(new GridView.LayoutParams( LayoutParams.MATCH_PARENT, LayoutParams.MATCH_PARENT)); } else { imageView = (ImageView) convertView; } imageView.setImageResource(imageResIds[position]); // Load image into ImageView return imageView; } }}与上篇文章《正确的在ViewPager中使用Bitmap图像》中的问题一样，这里的图像都是在UI线程中加载的。如果你的图片都很小可能不会出现问题，但是如果加入更多处理的话你的UI线程还是会悲剧的挂掉。
在上个文章中使用的异步和缓存机制同样适用于此。但是，你要多考虑一下同步的问题，因为GridView会自动回收它包含的子View。要解决这个问题，我们可以使用在《Processing Bitmaps Off the UI Thread》这一课中学到的方法。下面是解决方法：
public class ImageGridFragment extends Fragment implements AdapterView.OnItemClickListener { ... private class ImageAdapter extends BaseAdapter { ... @Override public View getView(int position, View convertView, ViewGroup container) { ... loadBitmap(imageResIds[position], imageView) return imageView; } } 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); } } 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(); } } public static boolean cancelPotentialWork(int data, ImageView imageView) { final BitmapWorkerTask bitmapWorkerTask = getBitmapWorkerTask(imageView); if (bitmapWorkerTask != null) { final int bitmapData = bitmapWorkerTask.data; if (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; } 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; } ... // include updated BitmapWorkerTask class注意：上面的代码同样适用于ListView控件。
这个实现能够让你任意的处理和加载图片，并不会影响到UI的效果。在后台线程中你可以从网络上下载图片，或者裁剪从相机中导入的过大的照片，后台任务完成后才将图片显示出来。
翻译自：Displaying Bitmaps in Your UI