（DOC）Displaying Bitmaps Efficiently 3
Handle Configuration Changes运行时配置改变了，如屏幕的方向改变了，导致Android会销毁，重启。这就需要避免处理所有的图片了，南昌需要一个更缓和，更高效的办法。前面已经讨论过内存缓存了，这个缓存可以通过Fragment的setRetainInstance(true)得到，Activity重建以后，Fragment会重新加载，reattached附着到Activity中下面是一个使用Fragment与LruCache在配置改变时的例子。private LruCache mMemoryCache;@Overrideprotected void onCreate(Bundle savedInstanceState) { ... RetainFragment mRetainFragment = RetainFragment.findOrCreateRetainFragment(getFragmentManager()); mMemoryCache = RetainFragment.mRetainedCache; if (mMemoryCache == null) { mMemoryCache = new LruCache(cacheSize) { ... // Initialize cache here as usual } mRetainFragment.mRetainedCache = mMemoryCache; } ...}class RetainFragment extends Fragment { private static final String TAG = "RetainFragment"; public LruCache mRetainedCache; public RetainFragment() {} public static RetainFragment findOrCreateRetainFragment(FragmentManager fm) { RetainFragment fragment = (RetainFragment) fm.findFragmentByTag(TAG); if (fragment == null) { fragment = new RetainFragment(); } return fragment; } @Override public void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setRetainInstance(true); }}要测试的话可以旋转屏幕改变Fragment的获取方式Displaying Bitmaps in Your UI在你自己的ui中显示位图 ,这章节将前面的几章节综合讨论，在配置改变或并发时如何加载图片到gridview这样的组件中。在ViewPager中的实现这个非正式的view模式，是个不错的办法，可以使用ViewPager组件后端是PagerAdapter提供数据，一个更有效的是FragmentStatePagerAdapter，因为它会自动上和保存状态。如果只是一小部分的图片，且你相信他们不会超过内存限制，使用上述的adapter是个不错的选择。public class ImageDetailActivity extends FragmentActivity { public static final String EXTRA_IMAGE = "extra_image"; private ImagePagerAdapter mAdapter; private ViewPager mPager; // A static dataset to back the ViewPager 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}; @Override public void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.image_detail_pager); // Contains just a ViewPager mAdapter = new ImagePagerAdapter(getSupportFragmentManager(), imageResIds.length); mPager = (ViewPager) findViewById(R.id.pager); mPager.setAdapter(mAdapter); } public static class ImagePagerAdapter extends FragmentStatePagerAdapter { private final int mSize; public ImagePagerAdapter(FragmentManager fm, int size) { super(fm); mSize = size; } @Override public int getCount() { return mSize; } @Override public Fragment getItem(int position) { return ImageDetailFragment.newInstance(position); } }}具体的Fragmentpublic class ImageDetailFragment extends Fragment { private static final String IMAGE_DATA_EXTRA = "resId"; private int mImageNum; private ImageView mImageView; static ImageDetailFragment newInstance(int imageNum) { final ImageDetailFragment f = new ImageDetailFragment(); final Bundle args = new Bundle(); args.putInt(IMAGE_DATA_EXTRA, imageNum); f.setArguments(args); return f; } // Empty constructor, required as per Fragment docs public ImageDetailFragment() {} @Override public void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); mImageNum = getArguments() != null ? getArguments().getInt(IMAGE_DATA_EXTRA) : -1; } @Override public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) { // image_detail_fragment.xml contains just an ImageView final View v = inflater.inflate(R.layout.image_detail_fragment, container, false); mImageView = (ImageView) v.findViewById(R.id.imageView); return v; } @Override public void onActivityCreated(Bundle savedInstanceState) { super.onActivityCreated(savedInstanceState); final int resId = ImageDetailActivity.imageResIds[mImageNum]; mImageView.setImageResource(resId); // Load image into ImageView }}这里图片由ui线程来读取的，会导致程序的fc，这时可以你自己修改成从地方读取，用到前面的AsyncTask或缓存方法。public class ImageDetailActivity extends FragmentActivity { ... public void loadBitmap(int resId, ImageView imageView) { mImageView.setImageResource(R.drawable.image_placeholder); BitmapWorkerTask task = new BitmapWorkerTask(mImageView); task.execute(resId); } ... // include BitmapWorkerTask class}public class ImageDetailFragment extends Fragment { ... @Override public void onActivityCreated(Bundle savedInstanceState) { super.onActivityCreated(savedInstanceState); if (ImageDetailActivity.class.isInstance(getActivity())) { final int resId = ImageDetailActivity.imageResIds[mImageNum]; // Call out to ImageDetailActivity to load the bitmap in a background thread ((ImageDetailActivity) getActivity()).loadBitmap(resId, mImageView); } }}其它一些方法如确定大小，从网络获取可以在BitmapWorkerTask中处理了，可以加入缓存。public class ImageDetailActivity extends FragmentActivity { ... private LruCache mMemoryCache; @Override public void onCreate(Bundle savedInstanceState) { ... // initialize LruCache as per Use a Memory Cache section } public void loadBitmap(int resId, ImageView imageView) { final String imageKey = String.valueOf(resId); final Bitmap bitmap = mMemoryCache.get(imageKey); if (bitmap != null) { mImageView.setImageBitmap(bitmap); } else { mImageView.setImageResource(R.drawable.image_placeholder); BitmapWorkerTask task = new BitmapWorkerTask(mImageView); task.execute(resId); } } ... // include updated BitmapWorkerTask from Use a Memory Cache section}将上面合在一起就可以了。3.0以后就有Fragment，这是个不错的东西，我在微博程序中就大量使用，它可以有Activity类似的生命周期，可以保存一些状态便于重建等。Load Bitmaps into a GridView ，ListView是一样的处理。http://developer.android.com/design/building-blocks/grid-lists.html 这里有说到一些相关的知识，可以参考下。GridView会回收一些子元素，所以它能有足够的内存一直加载新的图片。现在来看看：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; } }}同样是使用了Fragment，上面也是在ui线程对图片解码，所以需要修改为缓存的方式：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 bitmapWorkerTaskReference; public AsyncDrawable(Resources res, Bitmap bitmap, BitmapWorkerTask bitmapWorkerTask) { super(res, bitmap); bitmapWorkerTaskReference = new WeakReference(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 classListView是一样的，就不说了。源码可以在示例程序中找到。至此，也写过几篇关于网络图片载入到ListView中的文章了，这里翻译了文档中的一些文章，结合了缓存使用，文档中的办法固然也算是一种解决方案，但也不是完美的，不同的情况还是要不同对待，分析问题后再选择相应的办法。文档只是提供了一些解决的办法，如何组合，如何应用不能一概而论的。因为它是一种通用型的。如产生很多的线程就是一个问题，这些文档中显然没有提到，还有可以把缓存设置为静态，全局变量，文档中没有提到可能是因为静态变量不提倡用吧（有一篇关于静态变量引用的文章），但是对于一个程序来说，多处地方用到相同的缓存图片也有可能，为了方便使用把缓存设置成全局变量也是可以的。