在Android中获取onTouch ACTION_MOVE事件的速度

Question

这应该是一件非常简单的事情.用户将手指放在屏幕上并将其拖到屏幕上.onTouch上有两个事件触发:

• MotionEvent.ACTION_DOWN
• MotionEvent.ACTION_MOVE

现在,我该如何计算ACTION_MOVE手势的速度？用户在手势期间拖动手指的速度越来越慢,所以我想我需要计算两个中间触摸点之间的速度:lastTouchedPointX,lastTouchedPointY和event.getX(),event.getY().

有没有人这样做过？

Answer 1

您需要的是使用标准VelocityTracker类.有关Google 用户输入最佳做法的详细信息,同时在此处跟踪移动情况.下面的大部分代码(VelocityTracker通过在每个fling/move的X和Y轴上显示速度来演示使用)取自上一个资源链接:

import android.os.Bundle;
import android.app.Activity;
import android.support.v4.view.VelocityTrackerCompat;
import android.view.MotionEvent;
import android.view.VelocityTracker;
import android.widget.TextView;

public class MainActivity extends Activity {

    private VelocityTracker mVelocityTracker = null;
    private TextView mTextView;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        mTextView = new TextView(this);
        mTextView.setText("Move finger on screen to get velocity.");
        setContentView(mTextView);
    }

    @Override
    public boolean onTouchEvent(MotionEvent event) {
        int index = event.getActionIndex();
        int action = event.getActionMasked();
        int pointerId = event.getPointerId(index);

        switch (action) {
        case MotionEvent.ACTION_DOWN:
            if (mVelocityTracker == null) {

                // Retrieve a new VelocityTracker object to watch the velocity
                // of a motion.
                mVelocityTracker = VelocityTracker.obtain();
            } else {

                // Reset the velocity tracker back to its initial state.
                mVelocityTracker.clear();
            }

            // Add a user's movement to the tracker.
            mVelocityTracker.addMovement(event);
            break;
        case MotionEvent.ACTION_MOVE:
            mVelocityTracker.addMovement(event);
            // When you want to determine the velocity, call
            // computeCurrentVelocity(). Then call getXVelocity()
            // and getYVelocity() to retrieve the velocity for each pointer ID.
            mVelocityTracker.computeCurrentVelocity(1000);

            // Log velocity of pixels per second
            // Best practice to use VelocityTrackerCompat where possible.
            mTextView.setText("X velocity: "
                    + VelocityTrackerCompat.getXVelocity(mVelocityTracker,
                            pointerId)
                    + "\nY velocity: "
                    + VelocityTrackerCompat.getYVelocity(mVelocityTracker,
                            pointerId));
            break;
        case MotionEvent.ACTION_UP:
            break;
        case MotionEvent.ACTION_CANCEL:
            // Return a VelocityTracker object back to be re-used by others.
            mVelocityTracker.recycle();
            break;
        }
        return true;
    }

}

Answer 2

@Override
        public boolean onTouchEvent(MotionEvent event, MapView mapView) {


            if(event.getAction() == MotionEvent.ACTION_DOWN) {

                oldX = event.getX();
                oldY = event.getY();
                    //start timer

            } else if (event.getAction() == MotionEvent.ACTION_UP) {   

                //long timerTime = getTime between two event down to Up
                newX = event.getX();
                newY = event.getY();

                float distance = Math.sqrt((newX-oldX) * (newX-oldX) + (newY-oldY) * (newY-oldY));
                float speed = distance / timerTime;

            }
}