手势识别兼容Android 1.x和2.x的代码

 public abstract class VersionedGestureDetector {
    private static final String TAG = "VersionedGestureDetector";

    OnGestureListener mListener;

    public static VersionedGestureDetector newInstance(Context context,
            OnGestureListener listener) {  //设计实例化构造方法，这里Android123提示大家目前有3种API的实现方法，我们需要逐一考虑最优的解决方法，以满足高平台更多的功能实现。

        final int sdkVersion = Integer.parseInt(Build.VERSION.SDK); //使用android.os.Build判断API Level，但需要将字符串转换为整形
        VersionedGestureDetector detector = null;
        if (sdkVersion < Build.VERSION_CODES.ECLAIR) { //如果版本小于2.0则使用1.5版本的API，可以兼容1.5和1.6
            detector = new CupcakeDetector();
        } else if (sdkVersion < Build.VERSION_CODES.FROYO) { //如果版本小于2.1则使用2.0版本的API，可以兼容2.0,2.0.1和2.1这三个版本
            detector = new EclairDetector();
        } else {  //否则使用2.2开始的新的触控API

            detector = new FroyoDetector(context);
        }

        Log.d(TAG, "Created new " + detector.getClass()); //判断最终选择的到底是哪个版本的类
        detector.mListener = listener;

        return detector;
    }

    public abstract boolean onTouchEvent(MotionEvent ev); //我们需要根据版本决定onTouchEvent的实现

    public interface OnGestureListener { //手势判断接口主要是实现两个方法
        public void onDrag(float dx, float dy);  //拖拽
        public void onScale(float scaleFactor); //缩放
    }

    private static class CupcakeDetector extends VersionedGestureDetector { //针对Android 1.5和1.6设计的兼容方式
        float mLastTouchX;
        float mLastTouchY;

        float getActiveX(MotionEvent ev) { //获得当前X坐标
            return ev.getX();
        }

        float getActiveY(MotionEvent ev) { //获得当前Y坐标
            return ev.getY();
        }

        boolean shouldDrag() { //是否是拖拽中或者说移动中
            return true;
        }

        @Override
        public boolean onTouchEvent(MotionEvent ev) { //重写onTouchEvent方法
            switch (ev.getAction()) {
            case MotionEvent.ACTION_DOWN: {  //向下
                mLastTouchX = getActiveX(ev);
                mLastTouchY = getActiveY(ev);
                break;
            }
            case MotionEvent.ACTION_MOVE: { //Android开发网提醒大家，由于1.x时代的API比较简单，很多手势没有封装，我们只能从ACTION_MOVE中根据坐标变化判断手势样式
                final float x = getActiveX(ev);
                final float y = getActiveY(ev);

                if (shouldDrag()) {
                    mListener.onDrag(x - mLastTouchX, y - mLastTouchY); //处理拖拽移动
                }

                mLastTouchX = x;
                mLastTouchY = y;
                break;
            }
            }
            return true;
        }
    }

    private static class EclairDetector extends CupcakeDetector { //这个是针对Android 2.0,2.0.1和2.1提供的解决方法，可以看到有很多多点触控相关API出现
        private static final int INVALID_POINTER_ID = -1;
        private int mActivePointerId = INVALID_POINTER_ID;
        private int mActivePointerIndex = 0;

        @Override
        float getActiveX(MotionEvent ev) {
            return ev.getX(mActivePointerIndex);
        }

        @Override
        float getActiveY(MotionEvent ev) {
            return ev.getY(mActivePointerIndex);
        }

        @Override
        public boolean onTouchEvent(MotionEvent ev) {
            final int action = ev.getAction();
            switch (action & MotionEvent.ACTION_MASK) {
            case MotionEvent.ACTION_DOWN:
                mActivePointerId = ev.getPointerId(0);
                break;
            case MotionEvent.ACTION_CANCEL:
            case MotionEvent.ACTION_UP:
                mActivePointerId = INVALID_POINTER_ID;
                break;
            case MotionEvent.ACTION_POINTER_UP: //有个点松开
                final int pointerIndex = (ev.getAction() & MotionEvent.ACTION_POINTER_INDEX_MASK)
                        >> MotionEvent.ACTION_POINTER_INDEX_SHIFT;
                final int pointerId = ev.getPointerId(pointerIndex); //获取第几个点
                if (pointerId == mActivePointerId) {
                    final int newPointerIndex = pointerIndex == 0 ? 1 : 0;
                    mActivePointerId = ev.getPointerId(newPointerIndex);
                    mLastTouchX = ev.getX(newPointerIndex); //处理第newPointerIndex个点的x位置
                    mLastTouchY = ev.getY(newPointerIndex);
                }
                break;
            }

            mActivePointerIndex = ev.findPointerIndex(mActivePointerId);
            return super.onTouchEvent(ev);
        }
    }

    private static class FroyoDetector extends EclairDetector { //从Android 2.2开始可以很好的处理多点触控的缩放问题
        private ScaleGestureDetector mDetector;

        public FroyoDetector(Context context) {
            mDetector = new ScaleGestureDetector(context,
                    new ScaleGestureDetector.SimpleOnScaleGestureListener() {
                @Override public boolean onScale(ScaleGestureDetector detector) {
                    mListener.onScale(detector.getScaleFactor()); //根据 ScaleGestureDetector.SimpleOnScaleGestureListener这个系统类处理缩放情况通过onScale方法
                    return true;
                }
            });
        }

        @Override
        boolean shouldDrag() {
            return !mDetector.isInProgress();
        }

        @Override
        public boolean onTouchEvent(MotionEvent ev) {
            mDetector.onTouchEvent(ev);
            return super.onTouchEvent(ev);
        }
    }
}