Scroller类源码解析及其应用(一)

滑动是我们在自定义控件时候经常遇见的难题,让新手们倍感困惑,这篇文章主要介绍Scroller类的源码,告诉打击这个到底有什么用,怎么使用它来控制滑动。另外,我还会结合一个简单的例子,来看一下这个类的应用。

要说明Scroller类,我们往往要从另外两个方法说起,一个是ScrollTo(),一个是ScrollBy()

这两个方法我们可以在View的源码看到,我们知道其实每个控件都有滚动条,只是有的我们将它隐藏,所以我们看不见

下面是ScrollTo方法

  /**
     * Set the scrolled position of your view. This will cause a call to
     * {@link #onScrollChanged(int, int, int, int)} and the view will be
     * invalidated.
     * @param x the x position to scroll to
     * @param y the y position to scroll to
     */
    public void scrollTo(int x, int y) {//滑动到的目标坐标
        if (mScrollX != x || mScrollY != y) {
            int oldX = mScrollX;//已经滑动到的X
            int oldY = mScrollY;//已经滑动到的Y
            mScrollX = x;
            mScrollY = y;
            onScrollChanged(mScrollX, mScrollY, oldX, oldY);//调用说明状态改变
            if (!awakenScrollBars()) {
                invalidate();//通知视图进行重绘
            }
        }
    }
ScrollTo是一个public方法,说明我们可以在外部调用它,而正是我们调用了它,才能实现滑动,它的滑动效果是瞬间的,也就是说一步到位。

传进去的x,y是目的坐标,mScrollX,mScrollY是之前已经滑动到位置,调用这个函数以后,我们可以看到mScrollX,mScrollY被我们重新赋值,接下来会调用invalidate()进行重绘

那么滑动的本质是什么?根据动画移动的基本原理,对于一个控件来说,它的大小是有限的(例如我们可以自己设定,或者说被父控件所束缚)。所以我们绘图时会在这个有限的大小内进行绘制,但是控件的Canvas本质上是无限的,也就是一个控件的画布大小是无限,控件的大小就像一个窗口,我们只是通过这个窗口去看这块画布。

所以滑动的本质是,在窗口所见的画布部分,我们画些什么。

mScrollX,mScrollY起始是0,0,也就是说,窗口看见的,从画布的左上角开始的,然后我们改变两个值,例如100,100,窗口看见的,就是从100,100这个坐标开始绘制的东西(我们也可能没有在这块区域绘制东西,所以显示空白,因为画布是无限的)

说了这么多不知道大家有没有听懂,总之ScrollTo就是重新定义绘制起点坐标,从而实现滑动,由于绘制的一瞬间就开始的,所以ScrollTo造成的效果也是瞬间的

再来看看ScrollBy()方法

 /**
     * Move the scrolled position of your view. This will cause a call to
     * {@link #onScrollChanged(int, int, int, int)} and the view will be
     * invalidated.
     * @param x the amount of pixels to scroll by horizontally
     * @param y the amount of pixels to scroll by vertically
     */
    public void scrollBy(int x, int y) {
        scrollTo(mScrollX + x, mScrollY + y);
    }
它里面调用了ScrollTo(),那样我们就好理解了。传入的参数是mScrollX+x,也就是说这次x是一个增量,所以scrollBy实现的效果就是,在原来的起始位置,偏移x距离,再重新绘制,这是ScrollTo()和ScrollBy()的重要区别。


OK,上面讲了这么多,我们知道了用ScrollTo()就可以实现滑动了,但是我们需要的不是这样的滑动,我们希望滑动是有过程的,要缓慢的滑,要有feeling。

做javascript的朋友可能会想到,我使用一个for循环,然后每次调用ScrollBy()移动一定距离,每次移动后,让进程沉睡一段时间,不久可以实现动画的效果了吗?

是的,这是一般的思维,开始这样我们会面临两个问题,一个是让主线程沉睡(这是万万不能),一个是假设我们要实现多样的滑动算法(先快后慢,先慢后快等),怎么办?

其实cpu每个一定时间就会从新绘制画布(如果控件需要重新绘制的话),所以我们不必要让主线程沉睡,至于后面一个问题确实值得考虑。


直接地说,解决办法就是Scroller类,很多人看到这个类名,会不知道它跟滑动有什么关系。下面说一下,下面这段是Scroller的本质,只要听懂了,这篇文章就算没有白看了。

Scroller不能让控件滑动,之所以不能,因为本质上使控件滑动的是scrollTo()和scrollBy()方法

根据上面实现动画的思想,我们有必要每次调用ScrollTo方法前,计算出的新的x,y值,而Scroller就是替我们计算这两个值的

我们可以设定Scroller的内部算法(用于实现滑动物理的效果,我们不需要了解具体算法内容),然后调用Scroller的startScroll方法,这方法传入起始坐标,目的坐标,和滑动完成所需的时间,一旦调用这个方法以后,我们就可以通过它的getCurrX(),getCurrY()方法获得新的x,y坐标(这个两个坐标,是通过内部算法得来的)。

所以我们每次获得新坐标,然后scrollTo就行了,就可以实现滑动了。那么我们每次什么时候滑动呢(按照什么频率?难道真的写for循环?),当然是cpu绘制的频率最合理。

View里面有一个computeScroll()方法,在每次draw之前会调用,我们可以在调用这个方法时滑动(这个方法其实就是为我们滑动准备的啊!)

@Override
	public void computeScroll() {
        }

上面说的有点多,下面直接看scroller类源码

首先是一些属性,重要的我都有注释,没有注释的,是默认算法需要的,我们不必理睬

/**
	 * 模式,有SCROLL_MODE和FLING_MODE
	 */
    private int mMode;
    /**
     * 起始x方向偏移
     */
    private int mStartX;
    /**
     * 起始y方向偏移
     */
    private int mStartY;
    /**
     * 终点x方向偏移
     */
    private int mFinalX;
    /**
     * 终点y方向偏移
     */
    private int mFinalY;

    private int mMinX;
    private int mMaxX;
    private int mMinY;
    private int mMaxY;

    /**
     * 当前x方向偏移
     */
    private int mCurrX;
    /**
     * 当前y方向偏移
     */
    private int mCurrY;
    /**
     * 起始时间
     */
    private long mStartTime;
    /**
     * 滚动持续时间
     */
    private int mDuration;
    /**
     * 持续时间的倒数
     */
    private float mDurationReciprocal;
    /**
     * x方向应该滚动的距离,mDeltaX=mFinalX-mStartX
     */
    private float mDeltaX;
    /**
     * y方向应该滚动的距离,mDeltaY=mFinalY-mStartY
     */
    private float mDeltaY;
    /**
     * 是否结束
     */
    private boolean mFinished;
    /**
     * 插值器
     */
    private Interpolator mInterpolator;
    /**
     * 调速轮
     */
    private boolean mFlywheel;

    private float mVelocity;
    /**
     * 默认滑动时间
     */
    private static final int DEFAULT_DURATION = 250;
    /**
     * 滑动模式
     */
    private static final int SCROLL_MODE = 0;
    /**
     * 猛冲模式
     */
    private static final int FLING_MODE = 1;

    private static float DECELERATION_RATE = (float) (Math.log(0.75) / Math.log(0.9));
    private static float ALPHA = 800; // pixels / seconds
    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];
    /**
     * 减速率
     */
    private float mDeceleration;
    /**
     * pixels per inch
     */
    private final float mPpi;

然后来看构造函数

/**
     * Create a Scroller with the specified interpolator. If the interpolator is
     * null, the default (viscous) interpolator will be used. Specify whether or
     * not to support progressive "flywheel" behavior in flinging.
     * 通过Ppi和滑动摩擦因素,计算减速率
     */
    public Scroller(Context context, Interpolator interpolator, boolean flywheel) {
        mFinished = true;
        mInterpolator = interpolator;
        mPpi = context.getResources().getDisplayMetrics().density * 160.0f;
        mDeceleration = computeDeceleration(ViewConfiguration.getScrollFriction());
        mFlywheel = flywheel;
    }
为控件创建scroller时,需要传入context,另外interpolator是插值器,不同的插值器实现不同的动画算法,如果我们不传,则使用scroller内部算法,接下面我就会看到

另外flywheel是flywheel,有版本的限制,一般也不会用到,也是用于滑动动画算法的实现

可以看到构造函数里面,只是做了一些设定,mFinished表示滑动是否结束

再来看很重要的startScroll()方法

/**
     * Start scrolling by providing a starting point and the distance to travel.
     * The scroll will use the default value of 250 milliseconds for the
     * duration.
     * 
     * @param startX Starting horizontal scroll offset in pixels. Positive
     *        numbers will scroll the content to the left.
     * @param startY Starting vertical scroll offset in pixels. Positive numbers
     *        will scroll the content up.
     * @param dx Horizontal distance to travel. Positive numbers will scroll the
     *        content to the left.
     * @param dy Vertical distance to travel. Positive numbers will scroll the
     *        content up.
     * 使用默认滑动时间完成滑动
     */
    public void startScroll(int startX, int startY, int dx, int dy) {
        startScroll(startX, startY, dx, dy, DEFAULT_DURATION);
    }

    /**
     * Start scrolling by providing a starting point and the distance to travel.
     * 
     * @param startX Starting horizontal scroll offset in pixels. Positive
     *        numbers will scroll the content to the left.
     *        滑动起始X坐标
     * @param startY Starting vertical scroll offset in pixels. Positive numbers
     *        will scroll the content up.
     *        滑动起始Y坐标
     * @param dx Horizontal distance to travel. Positive numbers will scroll the
     *        content to the left.
     *        X方向滑动距离
     * @param dy Vertical distance to travel. Positive numbers will scroll the
     *        content up.
     *        Y方向滑动距离
     * @param duration Duration of the scroll in milliseconds.
     * 		      完成滑动所需的时间     
     * 从起始点开始滑动一定距离
     */
    public void startScroll(int startX, int startY, int dx, int dy, int duration) {
        mMode = SCROLL_MODE;
        mFinished = false;
        mDuration = duration;
        mStartTime = AnimationUtils.currentAnimationTimeMillis();//获取当前时间作为滑动的起始时间
        mStartX = startX;
        mStartY = startY;
        mFinalX = startX + dx;
        mFinalY = startY + dy;
        mDeltaX = dx;
        mDeltaY = dy;
        mDurationReciprocal = 1.0f / (float) mDuration;
    }

说它非常重要,但是内部却很简单,就是定义滑动的起始坐标,目的坐标和滑动时间,所以说根本没有实现滑动的效果

那么算法是在哪个方法里面调用的呢?

是computeScrollOffset()方法,这个方法返回false,说明滑动没有结束,但是它更重要的作用是,利用内部算法,起始坐标,目的坐标和滑动时间,从滑动开始到当前经过的时间,计算出新的坐标

所以说,我们在获取新的坐标前,一定要调用这个函数,这个函数的功能非常重要,而且并不简单!

/**
     * Call this when you want to know the new location.  If it returns true,
     * the animation is not yet finished.  loc will be altered to provide the
     * new location.
     * 调用这个函数获得新的位置坐标(滑动过程中)。如果它返回true,说明滑动没有结束。
     * getCurX(),getCurY()方法就可以获得计算后的值。
     */ 
    public boolean computeScrollOffset() {
        if (mFinished) {//是否结束
            return false;
        }

        int timePassed = (int)(AnimationUtils.currentAnimationTimeMillis() - mStartTime);//滑动开始,经过了多长时间
    
        if (timePassed < mDuration) {//如果经过的时间小于动画完成所需时间
            switch (mMode) {
            case SCROLL_MODE:
                float x = timePassed * mDurationReciprocal;
    
                if (mInterpolator == null)//如果没有设置插值器,利用默认算法
                    x = viscousFluid(x); 
                else//否则利用插值器定义的算法
                    x = mInterpolator.getInterpolation(x);
    
                mCurrX = mStartX + Math.round(x * mDeltaX);//计算当前X坐标
                mCurrY = mStartY + Math.round(x * mDeltaY);//计算当前Y坐标
                break;
            case FLING_MODE:
                final float t = (float) timePassed / mDuration;
                final int index = (int) (NB_SAMPLES * t);
                final float t_inf = (float) index / NB_SAMPLES;
                final float t_sup = (float) (index + 1) / NB_SAMPLES;
                final float d_inf = SPLINE[index];
                final float d_sup = SPLINE[index + 1];
                final float distanceCoef = d_inf + (t - t_inf) / (t_sup - t_inf) * (d_sup - d_inf);
                
                mCurrX = mStartX + Math.round(distanceCoef * (mFinalX - mStartX));
                // Pin to mMinX <= mCurrX <= mMaxX
                mCurrX = Math.min(mCurrX, mMaxX);
                mCurrX = Math.max(mCurrX, mMinX);
                
                mCurrY = mStartY + Math.round(distanceCoef * (mFinalY - mStartY));
                // Pin to mMinY <= mCurrY <= mMaxY
                mCurrY = Math.min(mCurrY, mMaxY);
                mCurrY = Math.max(mCurrY, mMinY);

                if (mCurrX == mFinalX && mCurrY == mFinalY) {
                    mFinished = true;
                }

                break;
            }
        }
        else {
            mCurrX = mFinalX;
            mCurrY = mFinalY;
            mFinished = true;
        }
        return true;
    }

从代码可以看到,如果我们没有设置插值器,就会调用内部默认算法。

/**
     * 函数翻译是粘性流体
     * 估计是一种算法
     */
    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;
    }
接着是两个重要的get方法

/**
     * Returns the current X offset in the scroll. 
     * 
     * @return The new X offset as an absolute distance from the origin.
     * 获得当前X方向偏移
     */
    public final int getCurrX() {
        return mCurrX;
    }
    
    /**
     * Returns the current Y offset in the scroll. 
     * 
     * @return The new Y offset as an absolute distance from the origin.
     * 获得当前Y方向偏移
     */
    public final int getCurrY() {
        return mCurrY;
    }

从scroller的整个源码,我们都可以看到,根本没有对控件进行重绘的操作。scroller只是一个利用滑动算法为控件提供新的绘制位置的工具,真正引起重绘的,是scrollTo方法,而这个方法需要我们主动调用(在computeScroll()里面)


另外scroller内部还有许多的get方法,还有强制停止动画效果的方法等,下面贴出scroller的完整代码和注释(我翻译得很烂,大家海涵),大家可以看看一下

/*
 * Copyright (C) 2006 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package android.widget;

import android.content.Context;
import android.hardware.SensorManager;
import android.os.Build;
import android.util.FloatMath;
import android.view.ViewConfiguration;
import android.view.animation.AnimationUtils;
import android.view.animation.Interpolator;


/**
 * This class encapsulates scrolling.  The duration of the scroll
 * can be passed in the constructor and specifies the maximum time that
 * the scrolling animation should take.  Past this time, the scrolling is 
 * automatically moved to its final stage and computeScrollOffset()
 * will always return false to indicate that scrolling is over.
 * 这个类封装了滑动。滑动时间可以通过构造函数传入,并且制定滑动动画完成所需的最大时间。
 * 这个时间以后,将会自动滑动到它的最终状态,并且computeScrollOffset()将返回false,表示滑动结束
 */
public class Scroller  {
	/**
	 * 模式,有SCROLL_MODE和FLING_MODE
	 */
    private int mMode;
    /**
     * 起始x方向偏移
     */
    private int mStartX;
    /**
     * 起始y方向偏移
     */
    private int mStartY;
    /**
     * 终点x方向偏移
     */
    private int mFinalX;
    /**
     * 终点y方向偏移
     */
    private int mFinalY;

    private int mMinX;
    private int mMaxX;
    private int mMinY;
    private int mMaxY;

    /**
     * 当前x方向偏移
     */
    private int mCurrX;
    /**
     * 当前y方向偏移
     */
    private int mCurrY;
    /**
     * 起始时间
     */
    private long mStartTime;
    /**
     * 滚动持续时间
     */
    private int mDuration;
    /**
     * 持续时间的倒数
     */
    private float mDurationReciprocal;
    /**
     * x方向应该滚动的距离,mDeltaX=mFinalX-mStartX
     */
    private float mDeltaX;
    /**
     * y方向应该滚动的距离,mDeltaY=mFinalY-mStartY
     */
    private float mDeltaY;
    /**
     * 是否结束
     */
    private boolean mFinished;
    /**
     * 插值器
     */
    private Interpolator mInterpolator;
    /**
     * 调速轮
     */
    private boolean mFlywheel;

    private float mVelocity;
    /**
     * 默认滑动时间
     */
    private static final int DEFAULT_DURATION = 250;
    /**
     * 滑动模式
     */
    private static final int SCROLL_MODE = 0;
    /**
     * 猛冲模式
     */
    private static final int FLING_MODE = 1;

    private static float DECELERATION_RATE = (float) (Math.log(0.75) / Math.log(0.9));
    private static float ALPHA = 800; // pixels / seconds
    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];
    /**
     * 减速率
     */
    private float mDeceleration;
    /**
     * pixels per inch
     */
    private final float mPpi;

    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 static float sViscousFluidScale;
    private static float sViscousFluidNormalize;

    /**
     * Create a Scroller with the default duration and interpolator.
     * 默认滑动时间和插值器
     */
    public Scroller(Context context) {
        this(context, null);
    }

    /**
     * Create a Scroller with the specified interpolator. If the interpolator is
     * null, the default (viscous) interpolator will be used. "Flywheel" behavior will
     * be in effect for apps targeting Honeycomb or newer.
     * 调速轮只能在Honeycomb以上的版本有效
     */
    public Scroller(Context context, Interpolator interpolator) {
        this(context, interpolator,
                context.getApplicationInfo().targetSdkVersion >= Build.VERSION_CODES.HONEYCOMB);
    }

    /**
     * Create a Scroller with the specified interpolator. If the interpolator is
     * null, the default (viscous) interpolator will be used. Specify whether or
     * not to support progressive "flywheel" behavior in flinging.
     * 通过Ppi和滑动摩擦因素,计算减速率
     */
    public Scroller(Context context, Interpolator interpolator, boolean flywheel) {
        mFinished = true;
        mInterpolator = interpolator;
        mPpi = context.getResources().getDisplayMetrics().density * 160.0f;
        mDeceleration = computeDeceleration(ViewConfiguration.getScrollFriction());
        mFlywheel = flywheel;
    }

    /**
     * The amount of friction applied to flings. The default value
     * is {@link ViewConfiguration#getScrollFriction}.
     * 
     * @param friction A scalar dimension-less value representing the coefficient of
     *         friction.
     * 设置fling方法的摩擦因素大小        
     */
    public final void setFriction(float friction) {
        mDeceleration = computeDeceleration(friction);
    }
    
    /**
     * 计算减速率
     */
    private float computeDeceleration(float friction) {
        return SensorManager.GRAVITY_EARTH   // g (m/s^2)
                      * 39.37f               // inch/meter
                      * mPpi                 // pixels per inch
                      * friction;
    }

    /**
     * 
     * Returns whether the scroller has finished scrolling.
     * 
     * @return True if the scroller has finished scrolling, false otherwise.
     * 判断滑动是否停止
     */
    public final boolean isFinished() {
        return mFinished;
    }
    
    /**
     * Force the finished field to a particular value.
     *  
     * @param finished The new finished value.
     * 强制滑动停止
     */
    public final void forceFinished(boolean finished) {
        mFinished = finished;
    }
    
    /**
     * Returns how long the scroll event will take, in milliseconds.
     * 
     * @return The duration of the scroll in milliseconds.
     * 获得滑动时间
     */
    public final int getDuration() {
        return mDuration;
    }
    
    /**
     * Returns the current X offset in the scroll. 
     * 
     * @return The new X offset as an absolute distance from the origin.
     * 获得当前X方向偏移
     */
    public final int getCurrX() {
        return mCurrX;
    }
    
    /**
     * Returns the current Y offset in the scroll. 
     * 
     * @return The new Y offset as an absolute distance from the origin.
     * 获得当前Y方向偏移
     */
    public final int getCurrY() {
        return mCurrY;
    }
    
    /**
     * Returns the current velocity.
     *
     * @return The original velocity less the deceleration. Result may be
     * negative.
     */
    public float getCurrVelocity() {
        return mVelocity - mDeceleration * timePassed() / 2000.0f;
    }

    /**
     * Returns the start X offset in the scroll. 
     * 
     * @return The start X offset as an absolute distance from the origin.
     */
    public final int getStartX() {
        return mStartX;
    }
    
    /**
     * Returns the start Y offset in the scroll. 
     * 
     * @return The start Y offset as an absolute distance from the origin.
     */
    public final int getStartY() {
        return mStartY;
    }
    
    /**
     * Returns where the scroll will end. Valid only for "fling" scrolls.
     * 
     * @return The final X offset as an absolute distance from the origin.
     */
    public final int getFinalX() {
        return mFinalX;
    }
    
    /**
     * Returns where the scroll will end. Valid only for "fling" scrolls.
     * 
     * @return The final Y offset as an absolute distance from the origin.
     */
    public final int getFinalY() {
        return mFinalY;
    }

    /**
     * Call this when you want to know the new location.  If it returns true,
     * the animation is not yet finished.  loc will be altered to provide the
     * new location.
     * 调用这个函数获得新的位置坐标(滑动过程中)。如果它返回true,说明滑动没有结束。
     * getCurX(),getCurY()方法就可以获得计算后的值。
     */ 
    public boolean computeScrollOffset() {
        if (mFinished) {//是否结束
            return false;
        }

        int timePassed = (int)(AnimationUtils.currentAnimationTimeMillis() - mStartTime);//滑动开始,经过了多长时间
    
        if (timePassed < mDuration) {//如果经过的时间小于动画完成所需时间
            switch (mMode) {
            case SCROLL_MODE:
                float x = timePassed * mDurationReciprocal;
    
                if (mInterpolator == null)//如果没有设置插值器,利用默认算法
                    x = viscousFluid(x); 
                else//否则利用插值器定义的算法
                    x = mInterpolator.getInterpolation(x);
    
                mCurrX = mStartX + Math.round(x * mDeltaX);//计算当前X坐标
                mCurrY = mStartY + Math.round(x * mDeltaY);//计算当前Y坐标
                break;
            case FLING_MODE:
                final float t = (float) timePassed / mDuration;
                final int index = (int) (NB_SAMPLES * t);
                final float t_inf = (float) index / NB_SAMPLES;
                final float t_sup = (float) (index + 1) / NB_SAMPLES;
                final float d_inf = SPLINE[index];
                final float d_sup = SPLINE[index + 1];
                final float distanceCoef = d_inf + (t - t_inf) / (t_sup - t_inf) * (d_sup - d_inf);
                
                mCurrX = mStartX + Math.round(distanceCoef * (mFinalX - mStartX));
                // Pin to mMinX <= mCurrX <= mMaxX
                mCurrX = Math.min(mCurrX, mMaxX);
                mCurrX = Math.max(mCurrX, mMinX);
                
                mCurrY = mStartY + Math.round(distanceCoef * (mFinalY - mStartY));
                // Pin to mMinY <= mCurrY <= mMaxY
                mCurrY = Math.min(mCurrY, mMaxY);
                mCurrY = Math.max(mCurrY, mMinY);

                if (mCurrX == mFinalX && mCurrY == mFinalY) {
                    mFinished = true;
                }

                break;
            }
        }
        else {
            mCurrX = mFinalX;
            mCurrY = mFinalY;
            mFinished = true;
        }
        return true;
    }
    
    /**
     * Start scrolling by providing a starting point and the distance to travel.
     * The scroll will use the default value of 250 milliseconds for the
     * duration.
     * 
     * @param startX Starting horizontal scroll offset in pixels. Positive
     *        numbers will scroll the content to the left.
     * @param startY Starting vertical scroll offset in pixels. Positive numbers
     *        will scroll the content up.
     * @param dx Horizontal distance to travel. Positive numbers will scroll the
     *        content to the left.
     * @param dy Vertical distance to travel. Positive numbers will scroll the
     *        content up.
     * 使用默认滑动时间完成滑动
     */
    public void startScroll(int startX, int startY, int dx, int dy) {
        startScroll(startX, startY, dx, dy, DEFAULT_DURATION);
    }

    /**
     * Start scrolling by providing a starting point and the distance to travel.
     * 
     * @param startX Starting horizontal scroll offset in pixels. Positive
     *        numbers will scroll the content to the left.
     *        滑动起始X坐标
     * @param startY Starting vertical scroll offset in pixels. Positive numbers
     *        will scroll the content up.
     *        滑动起始Y坐标
     * @param dx Horizontal distance to travel. Positive numbers will scroll the
     *        content to the left.
     *        X方向滑动距离
     * @param dy Vertical distance to travel. Positive numbers will scroll the
     *        content up.
     *        Y方向滑动距离
     * @param duration Duration of the scroll in milliseconds.
     * 		      完成滑动所需的时间     
     * 从起始点开始滑动一定距离
     */
    public void startScroll(int startX, int startY, int dx, int dy, int duration) {
        mMode = SCROLL_MODE;
        mFinished = false;
        mDuration = duration;
        mStartTime = AnimationUtils.currentAnimationTimeMillis();//获取当前时间作为滑动的起始时间
        mStartX = startX;
        mStartY = startY;
        mFinalX = startX + dx;
        mFinalY = startY + dy;
        mDeltaX = dx;
        mDeltaY = dy;
        mDurationReciprocal = 1.0f / (float) mDuration;
    }

    /**
     * Start scrolling based on a fling gesture. The distance travelled will
     * depend on the initial velocity of the fling.
     * 
     * @param startX Starting point of the scroll (X)
     * @param startY Starting point of the scroll (Y)
     * @param velocityX Initial velocity of the fling (X) measured in pixels per
     *        second.
     * @param velocityY Initial velocity of the fling (Y) measured in pixels per
     *        second
     * @param minX Minimum X value. The scroller will not scroll past this
     *        point.
     * @param maxX Maximum X value. The scroller will not scroll past this
     *        point.
     * @param minY Minimum Y value. The scroller will not scroll past this
     *        point.
     * @param maxY Maximum Y value. The scroller will not scroll past this
     *        point.
     * 开始基于滑动手势的滑动。根据初始的滑动手势速度,决定滑动的距离(滑动的距离,不能大于设定的最大值,不能小于设定的最小值)       
     */
    public void fling(int startX, int startY, int velocityX, int velocityY,
            int minX, int maxX, int minY, int maxY) {
        // Continue a scroll or fling in progress
        if (mFlywheel && !mFinished) {
            float oldVel = getCurrVelocity();

            float dx = (float) (mFinalX - mStartX);
            float dy = (float) (mFinalY - mStartY);
            float hyp = FloatMath.sqrt(dx * dx + dy * dy);

            float ndx = dx / hyp;
            float ndy = dy / hyp;

            float oldVelocityX = ndx * oldVel;
            float oldVelocityY = ndy * oldVel;
            if (Math.signum(velocityX) == Math.signum(oldVelocityX) &&
                    Math.signum(velocityY) == Math.signum(oldVelocityY)) {
                velocityX += oldVelocityX;
                velocityY += oldVelocityY;
            }
        }

        mMode = FLING_MODE;
        mFinished = false;

        float velocity = FloatMath.sqrt(velocityX * velocityX + velocityY * velocityY);
     
        mVelocity = velocity;
        final double l = Math.log(START_TENSION * velocity / ALPHA);
        mDuration = (int) (1000.0 * Math.exp(l / (DECELERATION_RATE - 1.0)));
        mStartTime = AnimationUtils.currentAnimationTimeMillis();
        mStartX = startX;
        mStartY = startY;

        float coeffX = velocity == 0 ? 1.0f : velocityX / velocity;
        float coeffY = velocity == 0 ? 1.0f : velocityY / velocity;

        int totalDistance =
                (int) (ALPHA * Math.exp(DECELERATION_RATE / (DECELERATION_RATE - 1.0) * l));
        
        mMinX = minX;
        mMaxX = maxX;
        mMinY = minY;
        mMaxY = maxY;

        mFinalX = startX + Math.round(totalDistance * coeffX);
        // Pin to mMinX <= mFinalX <= mMaxX
        mFinalX = Math.min(mFinalX, mMaxX);
        mFinalX = Math.max(mFinalX, mMinX);
        
        mFinalY = startY + Math.round(totalDistance * coeffY);
        // Pin to mMinY <= mFinalY <= mMaxY
        mFinalY = Math.min(mFinalY, mMaxY);
        mFinalY = Math.max(mFinalY, mMinY);
    }
    
    /**
     * 函数翻译是粘性流体
     * 估计是一种算法
     */
    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;
    }
    
    /**
     * Stops the animation. Contrary to {@link #forceFinished(boolean)},
     * aborting the animating cause the scroller to move to the final x and y
     * position
     *
     * @see #forceFinished(boolean)     
     * 强制停止滑动动画,并且将目的坐标设置为当前坐标
     */
    public void abortAnimation() {
        mCurrX = mFinalX;
        mCurrY = mFinalY;
        mFinished = true;
    }
    
    /**
     * Extend the scroll animation. This allows a running animation to scroll
     * further and longer, when used with {@link #setFinalX(int)} or {@link #setFinalY(int)}.
     *
     * @param extend Additional time to scroll in milliseconds.
     * @see #setFinalX(int)
     * @see #setFinalY(int)
     * 延长滚动时间
     */
    public void extendDuration(int extend) {
        int passed = timePassed();
        mDuration = passed + extend;
        mDurationReciprocal = 1.0f / mDuration;
        mFinished = false;
    }

    /**
     * Returns the time elapsed since the beginning of the scrolling.
     *
     * @return The elapsed time in milliseconds.
     * 获得已经滚动的时间
     */
    public int timePassed() {
        return (int)(AnimationUtils.currentAnimationTimeMillis() - mStartTime);
    }

    /**
     * Sets the final position (X) for this scroller.
     *
     * @param newX The new X offset as an absolute distance from the origin.
     * @see #extendDuration(int)
     * @see #setFinalY(int)
     * 设置mScroller最终停留的水平位置,没有动画效果,直接跳到目标位置
     */
    public void setFinalX(int newX) {
        mFinalX = newX;
        mDeltaX = mFinalX - mStartX;
        mFinished = false;
    }

    /**
     * Sets the final position (Y) for this scroller.
     *
     * @param newY The new Y offset as an absolute distance from the origin.
     * @see #extendDuration(int)
     * @see #setFinalX(int)
     * 设置mScroller最终停留的竖直位置,没有动画效果,直接跳到目标位置
     */
    public void setFinalY(int newY) {
        mFinalY = newY;
        mDeltaY = mFinalY - mStartY;
        mFinished = false;
    }

    /**
     * @hide
     */
    public boolean isScrollingInDirection(float xvel, float yvel) {
        return !mFinished && Math.signum(xvel) == Math.signum(mFinalX - mStartX) &&
                Math.signum(yvel) == Math.signum(mFinalY - mStartY);
    }
}


更重要的是创建scroller来解决问题的这种思想,有点像记账簿模式(记不清了,但肯定是设计模式的一种体现)

转载请注明出处http://blog.csdn.net/crazy__chen/article/details/45896961

对应scroller的具体应用,还有手势滑动等内容,大家可以参看下一篇文章


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