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滑动是我们在自定义控件时候经常遇见的难听，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，我们知道其实每个空间都有滚动条，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() {
}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;
}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;
}/**
* 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;
}
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)
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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原文地址：http://blog.csdn.net/crazy__chen/article/details/45896961
