BitmapRegionDecoder主要用于显示图片的某一块矩形区域，如果你需要显示某个图片的指定区域，那么这个类非常合适。
对于该类的用法，非常简单，既然是显示图片的某一块区域，那么至少只需要一个方法去设置图片；一个方法传入显示的区域即可；详见：
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BitmapRegionDecoder提供了一系列的newInstance方法来构造对象，支持传入文件路径，文件描述符，文件的inputstrem等。
例如：
BitmapRegionDecoder bitmapRegionDecoder = BitmapRegionDecoder.newInstance(inputStream, false); -
上述解决了传入我们需要处理的图片，那么接下来就是显示指定的区域。
bitmapRegionDecoder.decodeRegion(rect, options);参数一很明显是一个rect，参数二是BitmapFactory.Options，你可以控制图片的
inSampleSize,inPreferredConfig等。
那么下面看一个超级简单的例子：
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.graphics.BitmapRegionDecoder;
import android.graphics.Rect;
import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.widget.ImageView;
import java.io.IOException;
import java.io.InputStream;
public class LargeImageViewActivity extends AppCompatActivity
{
private ImageView mImageView;
@Override
protected void onCreate(Bundle savedInstanceState)
{
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_large_image_view);
mImageView = (ImageView) findViewById(R.id.id_imageview);
try
{
InputStream inputStream = getAssets().open("tangyan.jpg");
//获得图片的宽、高
BitmapFactory.Options tmpOptions = new BitmapFactory.Options();
tmpOptions.inJustDecodeBounds = true;
BitmapFactory.decodeStream(inputStream, null, tmpOptions);
int width = tmpOptions.outWidth;
int height = tmpOptions.outHeight;
//设置显示图片的中心区域
BitmapRegionDecoder bitmapRegionDecoder = BitmapRegionDecoder.newInstance(inputStream, false);
BitmapFactory.Options options = new BitmapFactory.Options();
options.inPreferredConfig = Bitmap.Config.RGB_565;
Bitmap bitmap = bitmapRegionDecoder.decodeRegion(new Rect(width / 2 - 100, height / 2 - 100, width / 2 + 100, height / 2 + 100), options);
mImageView.setImageBitmap(bitmap);
} catch (IOException e)
{
e.printStackTrace();
}
}
}
上述代码，就是使用BitmapRegionDecoder去加载assets中的图片，调用bitmapRegionDecoder.decodeRegion解析图片的中间矩形区域，返回bitmap，最终显示在ImageView上。
根据上面的分析呢，我们这个自定义控件思路就非常清晰了：
- 提供一个设置图片的入口
- 重写onTouchEvent，在里面根据用户移动的手势，去更新显示区域的参数
- 每次更新区域参数后，调用invalidate，onDraw里面去regionDecoder.decodeRegion拿到bitmap，去draw
代码如下：
import android.content.Context;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.graphics.BitmapRegionDecoder;
import android.graphics.Canvas;
import android.graphics.Rect;
import android.util.AttributeSet;
import android.view.MotionEvent;
import android.view.View;
import java.io.IOException;
import java.io.InputStream;
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;
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)
{
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;
}
}
根据上述源码:
- setInputStream里面去获得图片的真实的宽度和高度，以及初始化我们的mDecoder
- onMeasure里面为我们的显示区域的rect赋值，大小为view的尺寸
- onTouchEvent里面我们监听move的手势，在监听的回调里面去改变rect的参数，以及做边界检查，最后invalidate
- 在onDraw里面就是根据rect拿到bitmap，然后draw了
ok，上面并不复杂，不过大家有没有注意到，这个监听用户move手势的代码写的有点奇怪，恩，这里模仿了系统的ScaleGestureDetector，编写了MoveGestureDetector，代码如下：
-
MoveGestureDetector
import android.content.Context; import android.graphics.PointF; import android.view.MotionEvent; 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) { } } } -
BaseGestureDetector
import android.content.Context; import android.view.MotionEvent; 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; } }你可能会说，一个move手势搞这么多代码，太麻烦了。的确是的，move手势的检测非常简单，那么之所以这么写呢，主要是为了可以复用，比如现在有一堆的
XXXGestureDetector，当我们需要监听什么手势，就直接拿个detector来检测多方便。我相信大家肯定也郁闷过Google，为什么只有ScaleGestureDetector而没有RotateGestureDetector呢。
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原文链接：Android使用BitmapRegionDecoder加载超大图片方案，转载请注明来源！