【发布时间】：2014-01-15 15:59:32
【问题描述】：
我想从我的 sd 卡加载一些图像并将它们设置为我的应用程序布局的背景。我想有效地做到这一点，所以我不会得到 OutOfMemory Exception 并在后台进行。所以我在Processing Bitmaps 中阅读并使用了几乎关于使用位图的确切代码 在安卓开发者那里。这是我的代码：
public class ImageLoader {
private final Context context;
private int imageWidth;
private int imageHeight;
public ImageLoader(Context context, int imageWidth, int imageHeight) {
this.context = context;
setImageSize(imageWidth, imageHeight);
}
public ImageLoader(Context context, int imageSize) {
this.context = context;
setImageSize(imageSize);
}
public void setImageSize(int width, int height) {
imageWidth = width;
imageHeight = height;
}
public void setImageSize(int size) {
setImageSize(size, size);
}
public static int calculateInSampleSize(
BitmapFactory.Options options, int reqWidth, int reqHeight) {
// Raw height and width of image
final int height = options.outHeight;
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;
}
public static Bitmap decodeSampleBitmapFromResource(Resources res, int resId,
int reqWidth, int reqHeight) {
// First decode with inJustDecodeBounds=true to check dimensions
final BitmapFactory.Options options = new BitmapFactory.Options();
options.inJustDecodeBounds = true;
BitmapFactory.decodeResource(res, resId, options);
// Calculate inSampleSize
options.inSampleSize = calculateInSampleSize(options, reqWidth, reqHeight);
// Decode bitmap with inSampleSize set
options.inJustDecodeBounds = false;
return BitmapFactory.decodeResource(res, resId, options);
}
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;
}
public void loadBitmap(int resId, ImageView imageView) {
if (cancelPotentialWork(resId, imageView)) {
final BitmapWorkerTask task = new BitmapWorkerTask(imageView);
final AsyncDrawable asyncDrawable =
new AsyncDrawable(context.getResources(), ???mPlaceHoderBitmap???, task);
imageView.setImageDrawable(asyncDrawable);
task.execute(resId);
}
}
class BitmapWorkerTask extends AsyncTask<Integer, Void, Bitmap> {
private final WeakReference<ImageView> imageViewReference;
private int data = 0;
public BitmapWorkerTask(ImageView imageView) {
// Use a WeakReference to ensure the ImageView can be garbage collected
imageViewReference = new WeakReference<ImageView>(imageView);
}
// Decode image in background
@Override
protected Bitmap doInBackground(Integer... params) {
data = params[0];
return decodeSampleBitmapFromResource(context.getResources(), data, imageWidth, imageHeight);
}
@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);
}
}
}
}
static class AsyncDrawable extends BitmapDrawable {
private final WeakReference<BitmapWorkerTask> bitmapWorkerTaskReference;
public AsyncDrawable(Resources res, Bitmap bitmap,
BitmapWorkerTask bitmapWorkerTask) {
super(res, bitmap);
bitmapWorkerTaskReference =
new WeakReference<ImageLoader.BitmapWorkerTask>(bitmapWorkerTask);
}
public BitmapWorkerTask getBitmapWorkerTask() {
return bitmapWorkerTaskReference.get();
}
}
}
但是在 loadBitmap(int resId, Bitmap bitmap, ImageView imageView) 方法中我不知道如何传递位图。如果我使用 BitmapFactory.decode* 我可能会遇到异常。我应该如何为我的图像数据源传递位图参数？
【问题讨论】：
-
“如果我使用 BitmapFactory.decode* 我可能会得到异常”你是什么意思？你试过吗？
-
如果您直接解码图像文件以将其分配给位图而不考虑其大小和尺寸，它可能会变得太大并超出内存限制。我已经试过了。这就是我使用上面代码的原因。
-
这就是 doInBackground() 方法的作用，我也传递了 BitmapWorkerTask 对象。
-
如果您阅读了我在问题开头链接的文章，他们编写的 loadBitmap 没有参数位图，但传递了参数 mPlaceHolderBitmap，我不知道那是什么。这是我的问题。
-
我明白了，尝试在该参数中传递 null