BitmapRegionDecoder，从API10就引入了。如下图：

 

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看官方文档的说明，这个类就是用来显示指定区域的图像，当原始图像大，你只需要部分图像时，BitmapRegionDecoder特别有用。

二、使用

 

J1_7H5)MI{M2PV{){{3V97V.png

最主要的就是BitmapRegionDecode.newInstance(...)获取一个对象，然后通过这个对象去调用decodeRegion（...）得到bitmap，最后就可以显示在屏幕上了。考虑到用户可以触摸移动图像，我们用手势控制器GestureDetector来控制图片显示的区域。

public class LargeImageView extends View implements GestureDetector.OnGestureListener {

    private static final String TAG = "LargeImageView";

    private BitmapRegionDecoder mDecoder;

    //绘制的区域

    private volatile Rect mRect = new Rect();

    private int mScaledTouchSlop;

    // 分别记录上次滑动的坐标

    private int mLastX = 0;

    private int mLastY = 0;

    //图片的宽度和高度

    private int mImageWidth, mImageHeight;

   //手势控制器

    private GestureDetector mGestureDetector;

    private BitmapFactory.Options options;

    public LargeImageView(Context context) {

        super(context);

        init(context, null);

    }

    public LargeImageView(Context context, @Nullable AttributeSet attrs) {

        super(context, attrs);

        init(context, attrs);

    }

    public LargeImageView(Context context, @Nullable AttributeSet attrs, int defStyleAttr) {

        super(context, attrs, defStyleAttr);

        init(context, attrs);

    }

    private void init(Context context, AttributeSet attrs) {

        //设置显示图片的参数，如果对图片质量有要求，就选择ARGB_8888模式

        options = new BitmapFactory.Options();

        options.inPreferredConfig = Bitmap.Config.RGB_565;

        mScaledTouchSlop = ViewConfiguration.get(getContext())

                .getScaledTouchSlop();

        Log.d(TAG, "sts:" + mScaledTouchSlop);

        //初始化手势控制器

        mGestureDetector = new GestureDetector(context, this);

        //获取图片的宽高

        InputStream is = null;

        try {

            is = context.getResources().getAssets().open("timg.jpg");

            //初始化BitmapRegionDecode，并用它来显示图片

            mDecoder = BitmapRegionDecoder

                    .newInstance(is, false);

            BitmapFactory.Options tmpOptions = new BitmapFactory.Options();

            // 设置为true则只获取图片的宽高等信息，不加载进内存

            tmpOptions.inJustDecodeBounds = true;

            BitmapFactory.decodeStream(is, null, tmpOptions);

            mImageWidth = tmpOptions.outWidth;

            mImageHeight = tmpOptions.outHeight;

        } catch (IOException e) {

            e.printStackTrace();

        } finally {

            try {

                if (is != null) {

                    is.close();

                }

            } catch (Exception e) {

                e.printStackTrace();

            }

        }

    }

    @Override

    public boolean onTouchEvent(MotionEvent event) {

        //把触摸事件交给手势控制器处理

        return mGestureDetector.onTouchEvent(event);

    }

    @Override

    public boolean onDown(MotionEvent e) {

        mLastX = (int) e.getRawX();

        mLastY = (int) e.getRawY();

        return true;

    }

    @Override

    public void onShowPress(MotionEvent e) {

    }

    @Override

    public boolean onSingleTapUp(MotionEvent e) {

        return false;

    }

    @Override

    public boolean onScroll(MotionEvent e1, MotionEvent e2, float distanceX, float distanceY) {

        int x = (int) e2.getRawX();

        int y = (int) e2.getRawY();

        move(x, y);

        return true;

    }

    /**

     * 移动的时候更新图片显示的区域

     *

     * @param x

     * @param y

     */

    private void move(int x, int y) {

        int deltaX = x - mLastX;

        int deltaY = y - mLastY;

        Log.d(TAG, "move, deltaX:" + deltaX + " deltaY:" + deltaY);

        //如果图片宽度大于屏幕宽度

        if (mImageWidth > getWidth()) {

            //移动rect区域

            mRect.offset(-deltaX, 0);

            //检查是否到达图片最右端

            if (mRect.right > mImageWidth) {

                mRect.right = mImageWidth;

                mRect.left = mImageWidth - getWidth();

            }

            //检查左端

            if (mRect.left < 0) {

                mRect.left = 0;

                mRect.right = getWidth();

            }

            invalidate();

        }

        //如果图片高度大于屏幕高度

        if (mImageHeight > getHeight()) {

            mRect.offset(0, -deltaY);

            //是否到达最底部

            if (mRect.bottom > mImageHeight) {

                mRect.bottom = mImageHeight;

                mRect.top = mImageHeight - getHeight();

            }

            if (mRect.top < 0) {

                mRect.top = 0;

                mRect.bottom = getHeight();

            }

            //重绘

            invalidate();

        }

        mLastX = x;

        mLastY = y;

    }

    @Override

    public void onLongPress(MotionEvent e) {

        mLastX = (int) e.getRawX();

        mLastY = (int) e.getRawY();

    }

    @Override

    public boolean onFling(MotionEvent e1, MotionEvent e2, float velocityX, float velocityY) {

        int x = (int) e2.getRawX();

        int y = (int) e2.getRawY();

        move(x, y);

        return true;

    }

    @Override

    protected void onDraw(Canvas canvas) {

        //显示图片

        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;

        //默认显示图片的中心区域，开发者可自行选择

        mRect.left = imageWidth / 2 - width / 2;

        mRect.top = imageHeight / 2 - height / 2;

        mRect.right = mRect.left + width;

        mRect.bottom = mRect.top + height;

    }

}