AnimateUtils.java

package com.tencent.util;

/**
 * 粘性流体算法. 从scroller里扣出来的. 使用这个算法处理滚动
 * 视觉效果会比AccelerateDecelerateInterpolator视觉效果要好
 * 
 *
 */
public final class AnimateUtils
{
    private static float START_TENSION = 0.4f; // Tension at start: (0.4 * total T, 1.0 * Distance)
    private static float END_TENSION = 1.0f - START_TENSION;
    private static final int NB_SAMPLES = 100;
    private static final float[] SPLINE = new float[NB_SAMPLES + 1];

    private static float sViscousFluidScale;
    private static float sViscousFluidNormalize;

    static
    {
        float x_min = 0.0f;
        for (int i = 0; i <= NB_SAMPLES; i++)
        {
            final float t = (float) i / NB_SAMPLES;
            float x_max = 1.0f;
            float x, tx, coef;
            while (true)
            {
                x = x_min + (x_max - x_min) / 2.0f;
                coef = 3.0f * x * (1.0f - x);
                tx = coef * ((1.0f - x) * START_TENSION + x * END_TENSION) + x * x * x;
                if (Math.abs(tx - t) < 1E-5)
                    break;
                if (tx > t)
                    x_max = x;
                else
                    x_min = x;
            }
            final float d = coef + x * x * x;
            SPLINE[i] = d;
        }
        SPLINE[NB_SAMPLES] = 1.0f;

        // This controls the viscous fluid effect (how much of it)
        sViscousFluidScale = 8.0f;
        // must be set to 1.0 (used in viscousFluid())
        sViscousFluidNormalize = 1.0f;
        sViscousFluidNormalize = 1.0f / viscousFluid(1.0f);
    }

    private AnimateUtils()
    {

    }

    /**
     * for滚动动画, 这个算法会比较平滑
     * 
     * @param x
     * @return
     */
    public static float viscousFluid(float x)
    {
        x *= sViscousFluidScale;
        if (x < 1.0f)
        {
            x -= (1.0f - (float) Math.exp(-x));
        }
        else
        {
            float start = 0.36787944117f; // 1/e == exp(-1)
            x = 1.0f - (float) Math.exp(1.0f - x);
            x = start + x * (1.0f - start);
        }
        x *= sViscousFluidNormalize;
        return x;
    }
}