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;
@Override
protected 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Fragment
public 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 class
ListView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。
