Q:给定一个1000*20000的图片(宽1000dp,高20000dp),如何正常加载显示,并且不发生OOM异常.
解决这个问题有两种思路,第一种是使用BitmapFactory.Options的采样率inSampleSize,二是使用BitmapRegionDecoder每次加载一个矩形区域.
采样率加载
BitmapFactory.Options的几个字段
1.inJustDecodeBounds:为true的时候不会加载图片(返回一个null),但是会填充outXXX信息,常用来在不分配内存的情况下获取图片的宽高.
2.inSampleSize:采样率是用来对图片进行缩放的.如果采样率=4,加载到内存的图片宽高都会变成原来的1/4,内存则变成了原来图片的1/16.采样率只有在>1的时候才会对图片进行压缩. <=1的时候会把他看做是1.它使用的是2的幂次数,如果不是会向下取到2的幂次数.
这种方法是通过对图片按照一定的比例缩放,从而达到减少内存的效果.
private void setLargeViewWithInSampleSize() {
BitmapFactory.Options options = new BitmapFactory.Options();
//不获取图像,仅仅获取图片的一些信息,比如宽高
options.inJustDecodeBounds = true;
BitmapFactory.decodeStream(getInputStream("world.jpg"), null, options);
int imageWidth = options.outWidth;
int imageHeight = options.outHeight;
//获取屏幕宽高,然后根据屏幕大小设置采样率
DisplayMetrics displayMetrics = new DisplayMetrics();
getWindowManager().getDefaultDisplay().getMetrics(displayMetrics);
int screenWidth = displayMetrics.widthPixels;
int screenHeight = displayMetrics.heightPixels;
//计算采样率
int scaleX = imageWidth / screenWidth;
int scaleY = imageHeight / screenHeight;
int scale = 1;
if (scaleX > scaleY && scaleY >= 1) {
scale = scaleX;
}
if (scaleX < scaleY && scaleX >= 1) {
scale = scaleY;
}
options.inJustDecodeBounds = false;
options.inSampleSize = scale;
Bitmap bitmap = BitmapFactory.decodeStream(getInputStream("world.jpg"), null, options);
largeView.setImageBitmap(bitmap);
}
private InputStream getInputStream(String name) {
InputStream inputStream = null;
try {
inputStream = getAssets().open(name);
} catch (IOException e) {
e.printStackTrace();
}
return inputStream;
}
图片按区域加载
如果需求是完全展示,不对图片进行压缩,那么上面的方案就无法满足我们了,Android提供给了我们一个类BitmapRegionDecoder,它可以让我们每次加载一个矩形区域.
思路:主要思路就是在onTouchEvent中根据手势去用BitmapRegionDecoder加载imageView大小的图片区域.
具体的几个类如下,下面的做法是提升了手势事件的重用性,也可以将手势处理的过程直接写在onTouchEvent中.
public abstract class BaseGestureDetector {
protected boolean mGestureInProgress;
protected MotionEvent mPreMotionEvent;
protected MotionEvent mCurrentMotionEvent;
protected Context mContext;
public BaseGestureDetector(Context context) {
mContext = context;
}
public boolean onToucEvent(MotionEvent event) {
if (!mGestureInProgress) {
handleStartProgressEvent(event);
} else {
handleInProgressEvent(event);
}
return true;
}
protected abstract void handleInProgressEvent(MotionEvent event);
protected abstract void handleStartProgressEvent(MotionEvent event);
protected abstract void updateStateByEvent(MotionEvent event);
protected void resetState() {
if (mPreMotionEvent != null) {
mPreMotionEvent.recycle();
mPreMotionEvent = null;
}
if (mCurrentMotionEvent != null) {
mCurrentMotionEvent.recycle();
mCurrentMotionEvent = null;
}
mGestureInProgress = false;
}
}
public class MoveGestureDetector extends BaseGestureDetector {
private PointF mCurrentPointer;
private PointF mPrePointer;
//仅仅为了减少创建内存
private PointF mDeltaPointer = new PointF();
//用于记录最终结果，并返回
private PointF mExtenalPointer = new PointF();
private OnMoveGestureListener mListenter;
public MoveGestureDetector(Context context, OnMoveGestureListener listener) {
super(context);
mListenter = listener;
}
@Override
protected void handleInProgressEvent(MotionEvent event) {
int actionCode = event.getAction() & MotionEvent.ACTION_MASK;
switch (actionCode) {
case MotionEvent.ACTION_CANCEL:
case MotionEvent.ACTION_UP:
mListenter.onMoveEnd(this);
resetState();
break;
case MotionEvent.ACTION_MOVE:
updateStateByEvent(event);
boolean update = mListenter.onMove(this);
if (update) {
mPreMotionEvent.recycle();
mPreMotionEvent = MotionEvent.obtain(event);
}
break;
}
}
@Override
protected void handleStartProgressEvent(MotionEvent event) {
int actionCode = event.getAction() & MotionEvent.ACTION_MASK;
switch (actionCode) {
case MotionEvent.ACTION_DOWN:
resetState();//防止没有接收到CANCEL or UP ,保险起见
mPreMotionEvent = MotionEvent.obtain(event);
updateStateByEvent(event);
break;
case MotionEvent.ACTION_MOVE:
mGestureInProgress = mListenter.onMoveBegin(this);
break;
}
}
protected void updateStateByEvent(MotionEvent event) {
final MotionEvent prev = mPreMotionEvent;
mPrePointer = caculateFocalPointer(prev);
mCurrentPointer = caculateFocalPointer(event);
//Log.e("TAG", mPrePointer.toString() + " , " + mCurrentPointer);
boolean mSkipThisMoveEvent = prev.getPointerCount() != event.getPointerCount();
//Log.e("TAG", "mSkipThisMoveEvent = " + mSkipThisMoveEvent);
mExtenalPointer.x = mSkipThisMoveEvent ? 0 : mCurrentPointer.x - mPrePointer.x;
mExtenalPointer.y = mSkipThisMoveEvent ? 0 : mCurrentPointer.y - mPrePointer.y;
}
/**
* 根据event计算多指中心点
*
* @param event
* @return
*/
private PointF caculateFocalPointer(MotionEvent event) {
final int count = event.getPointerCount();
float x = 0, y = 0;
for (int i = 0; i < count; i++) {
x += event.getX(i);
y += event.getY(i);
}
x /= count;
y /= count;
return new PointF(x, y);
}
public float getMoveX() {
return mExtenalPointer.x;
}
public float getMoveY() {
return mExtenalPointer.y;
}
public interface OnMoveGestureListener {
public boolean onMoveBegin(MoveGestureDetector detector);
public boolean onMove(MoveGestureDetector detector);
public void onMoveEnd(MoveGestureDetector detector);
}
public static class SimpleMoveGestureDetector implements OnMoveGestureListener {
@Override
public boolean onMoveBegin(MoveGestureDetector detector) {
return true;
}
@Override
public boolean onMove(MoveGestureDetector detector) {
return false;
}
@Override
public void onMoveEnd(MoveGestureDetector detector) {
}
}
}
public class LargeImageView extends View {
private BitmapRegionDecoder mDecoder;
/**
* 图片的宽度和高度
*/
private int mImageWidth, mImageHeight;
/**
* 绘制的区域
*/
private volatile Rect mRect = new Rect();
private MoveGestureDetector mDetector;
private static final BitmapFactory.Options options = new BitmapFactory.Options();
static {
options.inPreferredConfig = Bitmap.Config.RGB_565;
}
public void setInputStream(InputStream is) {
try {
mDecoder = BitmapRegionDecoder.newInstance(is, false);
BitmapFactory.Options tmpOptions = new BitmapFactory.Options();
// Grab the bounds for the scene dimensions
tmpOptions.inJustDecodeBounds = true;
//解决获取图片宽高=-1.
is.reset();
BitmapFactory.decodeStream(is, null, tmpOptions);
mImageWidth = tmpOptions.outWidth;
mImageHeight = tmpOptions.outHeight;
requestLayout();
invalidate();
} catch (IOException e) {
e.printStackTrace();
} finally {
try {
if (is != null) is.close();
} catch (Exception e) {
}
}
}
public void init() {
mDetector = new MoveGestureDetector(getContext(), new MoveGestureDetector.SimpleMoveGestureDetector() {
@Override
public boolean onMove(MoveGestureDetector detector) {
int moveX = (int) detector.getMoveX();
int moveY = (int) detector.getMoveY();
if (mImageWidth > getWidth()) {
mRect.offset(-moveX, 0);
checkWidth();
invalidate();
}
if (mImageHeight > getHeight()) {
mRect.offset(0, -moveY);
checkHeight();
invalidate();
}
return true;
}
});
}
private void checkWidth() {
Rect rect = mRect;
int imageWidth = mImageWidth;
int imageHeight = mImageHeight;
if (rect.right > imageWidth) {
rect.right = imageWidth;
rect.left = imageWidth - getWidth();
}
if (rect.left < 0) {
rect.left = 0;
rect.right = getWidth();
}
}
private void checkHeight() {
Rect rect = mRect;
int imageWidth = mImageWidth;
int imageHeight = mImageHeight;
if (rect.bottom > imageHeight) {
rect.bottom = imageHeight;
rect.top = imageHeight - getHeight();
}
if (rect.top < 0) {
rect.top = 0;
rect.bottom = getHeight();
}
}
public LargeImageView(Context context, AttributeSet attrs) {
super(context, attrs);
init();
}
@Override
public boolean onTouchEvent(MotionEvent event) {
mDetector.onToucEvent(event);
return true;
}
@Override
protected void onDraw(Canvas canvas) {
if (mDecoder != null) {
Bitmap bm = mDecoder.decodeRegion(mRect, options);
canvas.drawBitmap(bm, 0, 0, null);
}
}
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
super.onMeasure(widthMeasureSpec, heightMeasureSpec);
int width = getMeasuredWidth();
int height = getMeasuredHeight();
int imageWidth = mImageWidth;
int imageHeight = mImageHeight;
//默认直接显示图片的中心区域，可以自己去调节
mRect.left = imageWidth / 2 - width / 2;
mRect.top = imageHeight / 2 - height / 2;
mRect.right = mRect.left + width;
mRect.bottom = mRect.top + height;
}
}