首页 > 代码库 > 高效地加载图片(五) 将图片展示在UI中
高效地加载图片(五) 将图片展示在UI中
这篇文章将前几篇使用的方法进行了整合,让我们能够在后台线程中加载以及缓存图片并在ViewPager和GridView中展示出来,并在这些过程中处理并发以及参数的设置.
将图片加载到ViewPager中
使用滑动视图来对图片详情进行导航是一种不错的方式.我们可以使用ViewPager和PagerAdapter来实现.但是,使用FragmentStatePagerAdapter可能会更好,它能够自动地保存ViewPager中Fragment的状态并控制它的创建和销毁,能够有效地利用内存.
注意:如果你仅仅需要加载少量的图片,并且能够确认这些图片不会造成内存溢出,则使用普通的PagerAdapter或者FragmenetPagerAdapter可能会更合适.
以下是使用ViewPager实现的图片展示:
- public class ImageDetailActivity extends FragmentActivity {
- public static final String EXTRA_IMAGE = "extra_image";
- private ImagePagerAdapter mAdapter;
- private ViewPager mPager;
- // 静态的图片资源数组
- 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); // 布局中仅仅包含一个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);
- }
- }
- }
以下是一个包含了ImageView的Fragment,用于展示图片.这部分代码看起来是一个十分理想的实现,但是你能够看出它的缺点吗?该怎么改进它?
- 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); // 将图片加载到ImageView中
- }
- }
从以上代码中,你可能发现了:图片是直接从资源文件中读取并展示到UI上的,这可能会造成应用挂起或者被强制关闭.此处使用AsyncTask在后台线程中加载图片会更好:
- 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];
- // 在后台线程中加载图片,而不是在Activity中直接读取图片
- ((ImageDetailActivity) getActivity()).loadBitmap(resId, mImageView);
- }
- }
- }
这样,我们可以在BitmapWorkerTask中实现任何附加的操作(如改变尺寸或者从网络读取图片资源)而不会映像UI线程的响应速度.就算使用后台线程加载图片的效率比直接加载图片低,我们也可以将使用的图片添加到缓存中以便复用.以下是添加缓存的方式:
- 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
- }
将以上组合起来使用,就可以实现高效率的ViewPager展示,这样可以耗费最少的时间和资源完成更多的任务.
将图片展示在GridView中的实现
网格结构的视图有利于展示多张图片,这种结果可以通过GridView来实现.而GridView可用通过用户的滑动操作及时地将图片显示出来.要实现这种效果,我们必须保证UI线程不被阻塞,内存的使用也要处于控制之中,而且图片的并发加载也要处理好(基于GridView的子View的循环使用).
开始之前,这里有一个使用GridView和Fragment(包含ImageView)的实现:
- 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;
- }
- }
- }
此处同样在UI线程中读取了图片.这种方式在图片较少并且尺寸规律时并无不妥(基于系统资源的加载和缓存方式),但是如果需要任何其他的附加处理,我们的UI线程就可能被阻塞.
同样,前述的异步加载图片并且缓存的方式也适用于这里.同时,我们也必须谨慎地处理并发操作,因为在GridView中,子View是可以被复用的.具体处理方式,可以参考Processing Bitmaps Off the UI Thread.如下是此处的处理:
- 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:此处的处理同样适用于ListView.
在此处的处理中,我们实现了对图片的灵活加载以及处理,并且保证了UI线程的流畅性.在后台线程中我们能够加载网络图片或者对大图片进行处理.