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Xint oldY = mScrollY;//已经滑动到的YmScrollX = 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！)

@Overridepublic 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 / secondsprivate 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 <= mMaxXmCurrX = Math.min(mCurrX, mMaxX);mCurrX = Math.max(mCurrX, mMinX);mCurrY = mStartY + Math.round(distanceCoef * (mFinalY - mStartY));// Pin to mMinY <= mCurrY <= mMaxYmCurrY = 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 / secondsprivate 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 <= mMaxXmCurrX = Math.min(mCurrX, mMaxX);mCurrX = Math.max(mCurrX, mMinX);mCurrY = mStartY + Math.round(distanceCoef * (mFinalY - mStartY));// Pin to mMinY <= mCurrY <= mMaxYmCurrY = 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 progressif (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 <= mMaxXmFinalX = Math.min(mFinalX, mMaxX);mFinalX = Math.max(mFinalX, mMinX);mFinalY = startY + Math.round(totalDistance * coeffY);// Pin to mMinY <= mFinalY <= mMaxYmFinalY = 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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