Android 如何提前知道fling velocity的距离、时间 根据滑动距离得到velocity

为什么

    现在Android提供了fling(velocity)函数，但如果需要fling(distance)，fling滑动一段距离呢？

    如果需要知道fling(velocity)花了多少毫秒呢？

just do it

    根据Android源代码，我们可以通过高中数学来实现这些功能。直接上代码

mTouchSlop = ViewConfiguration.get(context).getScaledTouchSlop();
        mMaximumVelocity = ViewConfiguration.get(context)
                .getScaledMaximumFlingVelocity();
        mMinimumVelocity = ViewConfiguration.get(context)
                .getScaledMinimumFlingVelocity();

        float ppi = context.getResources().getDisplayMetrics().density * 160.0f;
        mPhysicalCoeff = SensorManager.GRAVITY_EARTH // g (m/s^2)
                * 39.37f // inch/meter
                * ppi
                * 0.84f;

//        double distanceEs = getSplineFlingDistance(1000);
//        Log.e("getSplineFlingDistance", "getSplineFlingDistance " + distanceEs);
//        int velocityRs = getVelocityByDistance(116.64459);
//        Log.e("getVelocityByDistance", "getVelocityByDistance " + velocityRs);
    }

    private static final float INFLEXION = 0.35f; // Tension lines cross at (INFLEXION, 1)
    // Fling friction
    private static float mFlingFriction = ViewConfiguration.getScrollFriction();
    private static float mPhysicalCoeff;
    private static float DECELERATION_RATE = (float) (Math.log(0.78) / Math.log(0.9));

    private double getSplineDeceleration(int velocity) {
        return Math.log(INFLEXION * Math.abs(velocity) / (mFlingFriction * mPhysicalCoeff));
    }

    private static double getSplineDecelerationByDistance(double distance) {
        final double decelMinusOne = DECELERATION_RATE - 1.0;
        return decelMinusOne * (Math.log(distance / (mFlingFriction * mPhysicalCoeff))) / DECELERATION_RATE;
    }

//通过初始速度获取最终滑动距离
    private double getSplineFlingDistance(int velocity) {
        final double l = getSplineDeceleration(velocity);
        final double decelMinusOne = DECELERATION_RATE - 1.0;
        return mFlingFriction * mPhysicalCoeff * Math.exp(DECELERATION_RATE / decelMinusOne * l);
    }

//通过需要滑动的距离获取初始速度
    public static int getVelocityByDistance(double distance) {
        final double l = getSplineDecelerationByDistance(distance);
        int velocity = (int) (Math.exp(l) * mFlingFriction * mPhysicalCoeff / INFLEXION);
        return Math.abs(velocity);
    }

//获取滑动的时间
    /* Returns the duration, expressed in milliseconds */
    private int getSplineFlingDuration(int velocity) {
        final double l = getSplineDeceleration(velocity);
        final double decelMinusOne = DECELERATION_RATE - 1.0;
        return (int) (1000.0 * Math.exp(l / decelMinusOne));
    }

使用实例在该项目中哦 https://github.com/smallnew/PullFling