Android使用BitmapRegionDecoder加载超大图片方案

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：

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。

可以下载测试代码查看