在UI中显示图片
——Android官网原文翻译
这节课将前几节的内容汇聚到一起，向您展示怎样使用后台线程和图片缓存将多幅图片加载进ViewPager和GridView等组件中，同时处理并发性和系统设置变更。
实现加载图片到ViewPager
滑动视图设计是在画廊应用中导航细节View的一种非常棒的方式。您可以使用ViewPager组件和PagerAdapter实现这种模式。但是，ViewPager使用的Adapter是FragmentStatePagerAdapter的一个子类，它会在ViewPager的Fragment从屏幕中消失后自动的销毁它们并保存状态，以减少内存的使用。
注意：如果您有少量的图片并且确信它们在应用程序内存限制下都很合适，那么使用PagerAdapter或者FragmentPagerAdapter会更加的合适。
这里是ViewPager的实现，使用ImageView作为子视图。Main Activity保存ViewPager和Adapter。
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);
}
}
}
这里有一个详细Fragment的实现，它作为ImageView 的子视图。这可能看起来是一个非常合理的方式，但是您能否看到这种实现的缺点？该怎样改进呢？
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
}
}
希望您已经注意到了问题：图片在UI线程被读取，这会导致程序被挂起并且被强制停止。使用在UI线程以外处理图片这一节提到的AsyncTask，它可以很轻松地把图片加载和处理转移到后台线程去执行：
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);
}
}
}
任何附加的操作（像修改尺寸或者从网络读取图片）都会开启不影响主UI的响应能力的BitmapWorkerTask任务。如果后台线程不仅仅是直接从磁盘中加载图片的话，那么添加在缓存图片一节中提到的内存或者磁盘缓存是有好处的。这里是添加了内存缓存之后的修改：
public class ImageDetailActivity extends FragmentActivity {
...
private LruCache<String, Bitmap> 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
}
将这些片段汇总在一起回给您提供一个响应式的VIewPager实现，最小的加载延迟，和尽可能符合您在后台线程处理图片的需要的能力。
实现加载图片到GridView
网格列表构件用于显示图片数据集是非常有用的，使用GridView组件实现，可以同时在屏幕上显示多张图片，如果用户滚动还会有很多图片显示出来。当实现这种控件时，您要确保UI保持流动性，内存占用率在控制范围内，并且并发性被正确的处理（由于GridView回收子视图的机制）。
首先，这里有一个使用装载ImageView的Fragment作为GridView子视图的基础实现。然后，这将看起来像是一个非常合理的方式，但是我们改怎样让它更好呢？
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;
}
}
}
我们在这种实现方式上又看到了相同的问题，图片在UI线程中加载。当被用于加载小的简单的图片时（由于系统资源加载和缓存），如果任何附加的操作需要被完成，您的UI将会停止。
在前一节中使用的同样的异步操作和缓存方法同样可以在这里使用。但是，由于GridView回收子视图的机制，您也需要谨慎的处理并发问题。要解决这种情况，可以使用在UI线程以外处理图片这一节讨论的技术。这里是更新后的方案：
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：相同的代码也可以被适配ListView。
对于图片怎样加载和处理以使更加UI平滑，这种实现方式允许有一定的灵活性。在后台任务中您可以从网络加载图片或者把摄像头得到的照片重新设置大小，并且在任务处理完时显示图片。
