Android中scrollTo()和scrollBy()的区别以及Scroller源码解析

文章转自http://blog.csdn.net/crazy__chen/article/details/45896961。

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

[java] view
plain copy

/**

* 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

[java] view
plain copy

/**

* 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！)

[java] view
plain copy

@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

[java] view
plain copy

/**

* 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

[java] view
plain copy

/**

* 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

[java] view
plain copy

/**

* 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！

[java] view
plain copy

/**

* 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。

[java] view
plain copy

/**

* 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

[java] view
plain copy

/**

* 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

[java] view
plain copy

/*

* 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)