版权声明：本文为博主原创文章，未经博主允许不得转载。 https://blog.csdn.net/syusikoku/article/details/74359948
高效的加载图片
图片的缩放
public static int calculateInSampleSize( BitmapFactory.Options options, int reqWidth, int reqHeight) {
// Raw height and width of image
final int height = options.outHeight;
有效地加载大尺寸位图(Loading Large Bitmaps Efficiently)
读取位图的尺寸与类型(Read Bitmap Dimensions and Type)
加载一个按比例缩小的版本到内存中(Load a Scaled Down
Version into Memory)
final int width = options.outWidth;
int inSampleSize = 1;
if (height > reqHeight || width > reqWidth) {
final int halfHeight = height / 2;
final int halfWidth = width / 2;
// Calculate the largest inSampleSize value that is a power of 2 and keeps both
// height and width larger than the requested height and width.
while ((halfHeight / inSampleSize) > reqHeight && (halfWidth / inSampleSize) > reqWidth) {
inSampleSize *= 2;
}
}
return inSampleSize;
}
AsyncTask 类提供了一个简单的方法来在后台线程执行一些操作，并且可以把后台的结果呈现到UI线程。下面是一个加载大
图的示例：
class BitmapWorkerTask extends AsyncTask {
private final WeakReference imageViewReference;
private int data = 0;
public BitmapWorkerTask(ImageView imageView) {
// Use a WeakReference to ensure the ImageView can be garbage collected
imageViewReference = new WeakReference(imageView);
}
// Decode image in background.
@Override
protected Bitmap doInBackground(Integer... params) {
data = params[0];
return decodeSampledBitmapFromResource(getResources(), data, 100, 100));
}
// Once complete, see if ImageView is still around and set bitmap.
@Override
protected void onPostExecute(Bitmap bitmap) {
if (imageViewReference != null && bitmap != null) {
final ImageView imageView = imageViewReference.get();
if (imageView != null) {
imageView.setImageBitmap(bitmap);
}
}
}
}
为ImageView使用WeakReference 确保了 AsyncTask 所引用的资源可以被GC(garbage collected)。因为当任务结束时不能
确保 ImageView 仍然存在，因此你必须在 onPostExecute() 里面去检查引用。这个ImageView 也许已经不存在了，例如，
在任务结束时用户已经不在那个Activity或者是设备已经发生配置改变(旋转屏幕等)。
开始异步加载位图，只需要创建一个新的任务并执行它即可:
public void loadBitmap(int resId, ImageView imageView) {
BitmapWorkerTask task = new BitmapWorkerTask(imageView);
task.execute(resId);
}
处理并发问题(Handle Concurrency)
Multithreading for Performance 这篇博文更进一步的讨论了如何处理并发并且提供了一种解决方法，当任务结束时
ImageView 保存一个最近常使用的AsyncTask引用。使用类似的方法, AsyncTask 可以扩展出一个类似的模型。创建一个专
用的 Drawable 子类来保存一个可以回到当前工作任务的引用。在这种情况下，BitmapDrawable 被用来作为占位图片，它可
以在任务结束时显示到ImageView中。
创建一个专用的Drawable的子类来储存返回工作任务的引用。在这种情况下，当任务完成时BitmapDrawable会被使用，
placeholder image才会在ImageView中被显示:
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();
}
}
在执行BitmapWorkerTask 之前，你需要创建一个 AsyncDrawable 并且绑定它到目标组件 ImageView 中：
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);
}
}
在上面的代码示例中， cancelPotentialWork 方法检查确保了另外一个在ImageView中运行的任务得以取消。如果是这样，它
通过执行 cancel() 方法来取消之前的一个任务. 在小部分情况下, New出来的任务有可能已经存在，这样就不需要执行这个任
务了。下面演示了如何实现一个 cancelPotentialWork 。
public static boolean cancelPotentialWork(int data, ImageView imageView) {
final BitmapWorkerTask bitmapWorkerTask = getBitmapWorkerTask(imageView);
if (bitmapWorkerTask != null) {
final int bitmapData = bitmapWorkerTask.data;
if (bitmapData == 0 || 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;
}
在上面有一个帮助方法， getBitmapWorkerTask() , 被用作检索任务是否已经被分配到指定的 ImageView:
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;
}
最后一步是在BitmapWorkerTask 的 onPostExecute() 方法里面做更新操作:
class BitmapWorkerTask extends AsyncTask {
...
@Override
protected void onPostExecute(Bitmap bitmap) {
if (isCancelled()) {
bitmap = null;
}
if (imageViewReference != null && bitmap != null) {
final ImageView imageView = imageViewReference.get();
final BitmapWorkerTask bitmapWorkerTask = getBitmapWorkerTask(imageView);
if (this == bitmapWorkerTask && imageView != null) {
imageView.setImageBitmap(bitmap);
}
}
}
}
这个方法不仅仅适用于 ListView 与 GridView 组件，在那些需要循环利用子视图的组件中同样适用。只需要在设置图片到
ImageView的地方调用 loadBitmap 方法。例如，在GridView 中实现这个方法会是在 getView() 方法里面调用。
BitmapWorkerTask task = new BitmapWorkerTask(mImageView);
task.execute(resId);
完整的示例代码:
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;
}
}
声明：资源 摘自：Google Android官方培训教程中文版