【Android】View的滚动——Scroller

当一个View超出我们屏幕大小的时候，肯定只能显示View的一部分，这个时候必然涉及到移动一个View。在View的源码中，有两个这样子的方法：

 /**
     * 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) {
        if (mScrollX != x || mScrollY != y) {
            int oldX = mScrollX;
            int oldY = mScrollY;
            mScrollX = x;
            mScrollY = y;
            onScrollChanged(mScrollX, mScrollY, oldX, oldY);
            if (!awakenScrollBars()) {
                invalidate();
            }
        }
    }

    /**
     * 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);
    }

归根结底，这两个方法调用的都是下面这个方法：

    /**
     * This is called in response to an internal scroll in this view (i.e., the
     * view scrolled its own contents). This is typically as a result of
     * {@link #scrollBy(int, int)} or {@link #scrollTo(int, int)} having been
     * called.
     *
     * @param l Current horizontal scroll origin.
     * @param t Current vertical scroll origin.
     * @param oldl Previous horizontal scroll origin.
     * @param oldt Previous vertical scroll origin.
     */
    protected void onScrollChanged(int l, int t, int oldl, int oldt) {
        mBackgroundSizeChanged = true;

        final AttachInfo ai = mAttachInfo;
        if (ai != null) {
            ai.mViewScrollChanged = true;
        }
    }

这里具体发生了什么事情，不得而知，但是我们可以基本认为：View是提供给外部滚动自己内容的方式的（好吧，这里弱爆了。。。）

当我们去滚动一个View的时候，我们有两种方式去滚动一个View，第一种是瞬间移动到我们想要移动到的地方，第二种则是，以动画方式移动，并且设定一定的时长。当然第二种方式更加用户友好。我们需要手动控制这个过程。时刻了解一些信息，比如：

1）滚动过了多长时间；

2）现在滚动了多少距离了；

3）滚动是否结束了；

在Android中有一个类，其源码如下：

/*
 * 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.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.
 */
public class Scroller  {
    private int mMode;

    private int mStartX;
    private int mStartY;
    private int mFinalX;
    private int mFinalY;

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

    private int mCurrX;
    private int mCurrY;
    private long mStartTime;
    private int mDuration;
    private float mDurationReciprocal;
    private float mDeltaX;
    private float mDeltaY;
    private float mViscousFluidScale;
    private float mViscousFluidNormalize;
    private boolean mFinished;
    private Interpolator mInterpolator;

    private float mCoeffX = 0.0f;
    private float mCoeffY = 1.0f;
    private float mVelocity;

    private static final int DEFAULT_DURATION = 250;
    private static final int SCROLL_MODE = 0;
    private static final int FLING_MODE = 1;

    private final float mDeceleration;

    /**
     * Create a Scroller with the default duration and interpolator.
     */
    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.
     */
    public Scroller(Context context, Interpolator interpolator) {
        mFinished = true;
        mInterpolator = interpolator;
        float ppi = context.getResources().getDisplayMetrics().density * 160.0f;
        mDeceleration = SensorManager.GRAVITY_EARTH   // g (m/s^2)
                      * 39.37f                        // inch/meter
                      * ppi                           // pixels per inch
                      * ViewConfiguration.getScrollFriction();
    }
    
    /**
     * 
     * Returns whether the scroller has finished scrolling.
     * 
     * @return True if the scroller has finished scrolling, false otherwise.
     */
    public final boolean isFinished() {
        return mFinished;
    }
    
    /**
     * Force the finished field to a particular value.
     *  
     * @param finished The new finished value.
     */
    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.
     */
    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.
     */
    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.
     */
    public final int getCurrY() {
        return mCurrY;
    }
    
    /**
     * @hide
     * 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.
     */ 
    public boolean computeScrollOffset() {
        if (mFinished) {
            return false;
        }

        int timePassed = (int)(AnimationUtils.currentAnimationTimeMillis() - mStartTime);
    
        if (timePassed < mDuration) {
            switch (mMode) {
            case SCROLL_MODE:
                float x = (float)timePassed * mDurationReciprocal;
    
                if (mInterpolator == null)
                    x = viscousFluid(x); 
                else
                    x = mInterpolator.getInterpolation(x);
    
                mCurrX = mStartX + Math.round(x * mDeltaX);
                mCurrY = mStartY + Math.round(x * mDeltaY);
                break;
            case FLING_MODE:
                float timePassedSeconds = timePassed / 1000.0f;
                float distance = (mVelocity * timePassedSeconds)
                        - (mDeceleration * timePassedSeconds * timePassedSeconds / 2.0f);
                
                mCurrX = mStartX + Math.round(distance * mCoeffX);
                // Pin to mMinX <= mCurrX <= mMaxX
                mCurrX = Math.min(mCurrX, mMaxX);
                mCurrX = Math.max(mCurrX, mMinX);
                
                mCurrY = mStartY + Math.round(distance * mCoeffY);
                // Pin to mMinY <= mCurrY <= mMaxY
                mCurrY = Math.min(mCurrY, mMaxY);
                mCurrY = Math.max(mCurrY, mMinY);
                
                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.
     */
    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.
     * @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.
     * @param duration Duration of the scroll in milliseconds.
     */
    public void startScroll(int startX, int startY, int dx, int dy, int duration) {
        mMode = SCROLL_MODE;
        mFinished = false;
        mDuration = duration;
        mStartTime = AnimationUtils.currentAnimationTimeMillis();
        mStartX = startX;
        mStartY = startY;
        mFinalX = startX + dx;
        mFinalY = startY + dy;
        mDeltaX = dx;
        mDeltaY = dy;
        mDurationReciprocal = 1.0f / (float) mDuration;
        // This controls the viscous fluid effect (how much of it)
        mViscousFluidScale = 8.0f;
        // must be set to 1.0 (used in viscousFluid())
        mViscousFluidNormalize = 1.0f;
        mViscousFluidNormalize = 1.0f / viscousFluid(1.0f);
    }

    /**
     * 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.
     */
    public void fling(int startX, int startY, int velocityX, int velocityY,
            int minX, int maxX, int minY, int maxY) {
        mMode = FLING_MODE;
        mFinished = false;

        float velocity = (float)Math.hypot(velocityX, velocityY);
     
        mVelocity = velocity;
        mDuration = (int) (1000 * velocity / mDeceleration); // Duration is in
                                                            // milliseconds
        mStartTime = AnimationUtils.currentAnimationTimeMillis();
        mStartX = startX;
        mStartY = startY;

        mCoeffX = velocity == 0 ? 1.0f : velocityX / velocity; 
        mCoeffY = velocity == 0 ? 1.0f : velocityY / velocity;

        int totalDistance = (int) ((velocity * velocity) / (2 * mDeceleration));
        
        mMinX = minX;
        mMaxX = maxX;
        mMinY = minY;
        mMaxY = maxY;
        
        
        mFinalX = startX + Math.round(totalDistance * mCoeffX);
        // Pin to mMinX <= mFinalX <= mMaxX
        mFinalX = Math.min(mFinalX, mMaxX);
        mFinalX = Math.max(mFinalX, mMinX);
        
        mFinalY = startY + Math.round(totalDistance * mCoeffY);
        // Pin to mMinY <= mFinalY <= mMaxY
        mFinalY = Math.min(mFinalY, mMaxY);
        mFinalY = Math.max(mFinalY, mMinY);
    }
    
    
    
    private float viscousFluid(float x)
    {
        x *= mViscousFluidScale;
        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 *= mViscousFluidNormalize;
        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)
     */
    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)
     */
    public void extendDuration(int extend) {
        int passed = timePassed();
        mDuration = passed + extend;
        mDurationReciprocal = 1.0f / (float)mDuration;
        mFinished = false;
    }

    /**
     * Returns the time elapsed since the beginning of the scrolling.
     *
     * @return The elapsed time in milliseconds.
     */
    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)
     */
    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)
     */
    public void setFinalY(int newY) {
        mFinalY = newY;
        mDeltaY = mFinalY - mStartY;
        mFinished = false;
    }
}

这个类可以精确记录我们滚动过程中的所有信息，并且通过它，我们可以实现相应的动画。下面解释这个类的几个重要方面：

1）新建这个类的时候，可以传入加速器，这个加速器可以被用来计算滚动过程中的已经滚动的距离；

/**
     * Create a Scroller with the specified interpolator. If the interpolator is
     * null, the default (viscous) interpolator will be used.
     */
    public Scroller(Context context, Interpolator interpolator) {
        mFinished = true;
        mInterpolator = interpolator;
        float ppi = context.getResources().getDisplayMetrics().density * 160.0f;
        mDeceleration = SensorManager.GRAVITY_EARTH   // g (m/s^2)
                      * 39.37f                        // inch/meter
                      * ppi                           // pixels per inch
                      * ViewConfiguration.getScrollFriction();
    }

上面是构造方法，其中有一个比较复杂的计算，源码中毫无注释，先不管；

2）最重要的方法就是下面这个：

/**
     * 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.
     */ 
    public boolean computeScrollOffset() {
        if (mFinished) {
            return false;
        }

        int timePassed = (int)(AnimationUtils.currentAnimationTimeMillis() - mStartTime);
    
        if (timePassed < mDuration) {
            switch (mMode) {
            case SCROLL_MODE:
                float x = (float)timePassed * mDurationReciprocal;
    
                if (mInterpolator == null)
                    x = viscousFluid(x); 
                else
                    x = mInterpolator.getInterpolation(x);
    
                mCurrX = mStartX + Math.round(x * mDeltaX);
                mCurrY = mStartY + Math.round(x * mDeltaY);
                break;
            case FLING_MODE:
                float timePassedSeconds = timePassed / 1000.0f;
                float distance = (mVelocity * timePassedSeconds)
                        - (mDeceleration * timePassedSeconds * timePassedSeconds / 2.0f);
                
                mCurrX = mStartX + Math.round(distance * mCoeffX);
                // Pin to mMinX <= mCurrX <= mMaxX
                mCurrX = Math.min(mCurrX, mMaxX);
                mCurrX = Math.max(mCurrX, mMinX);
                
                mCurrY = mStartY + Math.round(distance * mCoeffY);
                // Pin to mMinY <= mCurrY <= mMaxY
                mCurrY = Math.min(mCurrY, mMaxY);
                mCurrY = Math.max(mCurrY, mMinY);
                
                break;
            }
        }
        else {
            mCurrX = mFinalX;
            mCurrY = mFinalY;
            mFinished = true;
        }
        return true;
    }

只要在滚动，也就是mFinished=false的时候，这个方法就会不停的计算当前应该处于的位置！

3）第三个是启动滚动的类：

/**
     * 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.
     */
    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.
     * @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.
     * @param duration Duration of the scroll in milliseconds.
     */
    public void startScroll(int startX, int startY, int dx, int dy, int duration) {
        mMode = SCROLL_MODE;
        mFinished = false;
        mDuration = duration;
        mStartTime = AnimationUtils.currentAnimationTimeMillis();
        mStartX = startX;
        mStartY = startY;
        mFinalX = startX + dx;
        mFinalY = startY + dy;
        mDeltaX = dx;
        mDeltaY = dy;
        mDurationReciprocal = 1.0f / (float) mDuration;
        // This controls the viscous fluid effect (how much of it)
        mViscousFluidScale = 8.0f;
        // must be set to 1.0 (used in viscousFluid())
        mViscousFluidNormalize = 1.0f;
        mViscousFluidNormalize = 1.0f / viscousFluid(1.0f);
    }

其实这个方法只是设置一些初始的参数，以便于用于计算。

我想，之所以Scroller难以理解，其实是因为很多人以为Scroller类控制View的滚动，其实不是这个样子的！Scroller类只是当你告诉它你要在一定的时间（默认250ms）内将View滚动一定距离，任何时刻View存在的位置！具体的滚动，其实是View调用自己的ScrollTo和ScrollBy方法实现的！

讲到这里，我想读者一定有一些想法了，假设我现在调用Scroller的startScroll方法，我就可实时获得该动画在任何时刻的位置，那么，假设我在View上要施加一个动画移动，那么我只要产生一个循环，不停去问Scroller  View应该存在的位置，然后将View移动到需要移动到的地方，自然就成为动画移动了！

在View的源码中，有方法如下：

/**
     * Called by a parent to request that a child update its values for mScrollX
     * and mScrollY if necessary. This will typically be done if the child is
     * animating a scroll using a {@link android.widget.Scroller Scroller}
     * object.
     */
    public void computeScroll() {
    }

是个空方法，应该是被用来覆盖的。并且明确提出经典用法是通过Scroller方法来滚动一个view。OK，我们来看看到底如何实现上面的循环。现在我们先离开上面，来看看ViewGroup里面的一个方法：

/**
     * Draw one child of this View Group. This method is responsible for getting
     * the canvas in the right state. This includes clipping, translating so
     * that the child's scrolled origin is at 0, 0, and applying any animation
     * transformations.
     *
     * @param canvas The canvas on which to draw the child
     * @param child Who to draw
     * @param drawingTime The time at which draw is occuring
     * @return True if an invalidate() was issued
     */
    protected boolean drawChild(Canvas canvas, View child, long drawingTime) {
        boolean more = false;

        final int cl = child.mLeft;
        final int ct = child.mTop;
        final int cr = child.mRight;
        final int cb = child.mBottom;

        final int flags = mGroupFlags;

        if ((flags & FLAG_CLEAR_TRANSFORMATION) == FLAG_CLEAR_TRANSFORMATION) {
            if (mChildTransformation != null) {
                mChildTransformation.clear();
            }
            mGroupFlags &= ~FLAG_CLEAR_TRANSFORMATION;
        }

        Transformation transformToApply = null;
        final Animation a = child.getAnimation();
        boolean concatMatrix = false;

        if (a != null) {
            if (mInvalidateRegion == null) {
                mInvalidateRegion = new RectF();
            }
            final RectF region = mInvalidateRegion;

            final boolean initialized = a.isInitialized();
            if (!initialized) {
                a.initialize(cr - cl, cb - ct, getWidth(), getHeight());
                a.initializeInvalidateRegion(0, 0, cr - cl, cb - ct);
                child.onAnimationStart();
            }

            if (mChildTransformation == null) {
                mChildTransformation = new Transformation();
            }
            more = a.getTransformation(drawingTime, mChildTransformation);
            transformToApply = mChildTransformation;

            concatMatrix = a.willChangeTransformationMatrix();

            if (more) {
                if (!a.willChangeBounds()) {
                    if ((flags & (FLAG_OPTIMIZE_INVALIDATE | FLAG_ANIMATION_DONE)) ==
                            FLAG_OPTIMIZE_INVALIDATE) {
                        mGroupFlags |= FLAG_INVALIDATE_REQUIRED;
                    } else if ((flags & FLAG_INVALIDATE_REQUIRED) == 0) {
                        // The child need to draw an animation, potentially offscreen, so
                        // make sure we do not cancel invalidate requests
                        mPrivateFlags |= DRAW_ANIMATION;
                        invalidate(cl, ct, cr, cb);
                    }
                } else {
                    a.getInvalidateRegion(0, 0, cr - cl, cb - ct, region, transformToApply);

                    // The child need to draw an animation, potentially offscreen, so
                    // make sure we do not cancel invalidate requests
                    mPrivateFlags |= DRAW_ANIMATION;

                    final int left = cl + (int) region.left;
                    final int top = ct + (int) region.top;
                    invalidate(left, top, left + (int) region.width(), top + (int) region.height());
                }
            }
        } else if ((flags & FLAG_SUPPORT_STATIC_TRANSFORMATIONS) ==
                FLAG_SUPPORT_STATIC_TRANSFORMATIONS) {
            if (mChildTransformation == null) {
                mChildTransformation = new Transformation();
            }
            final boolean hasTransform = getChildStaticTransformation(child, mChildTransformation);
            if (hasTransform) {
                final int transformType = mChildTransformation.getTransformationType();
                transformToApply = transformType != Transformation.TYPE_IDENTITY ?
                        mChildTransformation : null;
                concatMatrix = (transformType & Transformation.TYPE_MATRIX) != 0;
            }
        }

        // Sets the flag as early as possible to allow draw() implementations
        // to call invalidate() successfully when doing animations
        child.mPrivateFlags |= DRAWN;

        if (!concatMatrix && canvas.quickReject(cl, ct, cr, cb, Canvas.EdgeType.BW) &&
                (child.mPrivateFlags & DRAW_ANIMATION) == 0) {
            return more;
        }

        child.computeScroll();

        final int sx = child.mScrollX;
        final int sy = child.mScrollY;

        boolean scalingRequired = false;
        Bitmap cache = null;
        if ((flags & FLAG_CHILDREN_DRAWN_WITH_CACHE) == FLAG_CHILDREN_DRAWN_WITH_CACHE ||
                (flags & FLAG_ALWAYS_DRAWN_WITH_CACHE) == FLAG_ALWAYS_DRAWN_WITH_CACHE) {
            cache = child.getDrawingCache(true);
            if (mAttachInfo != null) scalingRequired = mAttachInfo.mScalingRequired;
        }

        final boolean hasNoCache = cache == null;

        final int restoreTo = canvas.save();
        if (hasNoCache) {
            canvas.translate(cl - sx, ct - sy);
        } else {
            canvas.translate(cl, ct);
            if (scalingRequired) {
                // mAttachInfo cannot be null, otherwise scalingRequired == false
                final float scale = 1.0f / mAttachInfo.mApplicationScale;
                canvas.scale(scale, scale);
            }
        }

        float alpha = 1.0f;

        if (transformToApply != null) {
            if (concatMatrix) {
                int transX = 0;
                int transY = 0;
                if (hasNoCache) {
                    transX = -sx;
                    transY = -sy;
                }
                // Undo the scroll translation, apply the transformation matrix,
                // then redo the scroll translate to get the correct result.
                canvas.translate(-transX, -transY);
                canvas.concat(transformToApply.getMatrix());
                canvas.translate(transX, transY);
                mGroupFlags |= FLAG_CLEAR_TRANSFORMATION;
            }

            alpha = transformToApply.getAlpha();
            if (alpha < 1.0f) {
                mGroupFlags |= FLAG_CLEAR_TRANSFORMATION;
            }

            if (alpha < 1.0f && hasNoCache) {
                final int multipliedAlpha = (int) (255 * alpha);
                if (!child.onSetAlpha(multipliedAlpha)) {
                    canvas.saveLayerAlpha(sx, sy, sx + cr - cl, sy + cb - ct, multipliedAlpha,
                            Canvas.HAS_ALPHA_LAYER_SAVE_FLAG | Canvas.CLIP_TO_LAYER_SAVE_FLAG);
                } else {
                    child.mPrivateFlags |= ALPHA_SET;
                }
            }
        } else if ((child.mPrivateFlags & ALPHA_SET) == ALPHA_SET) {
            child.onSetAlpha(255);
        }

        if ((flags & FLAG_CLIP_CHILDREN) == FLAG_CLIP_CHILDREN) {
            if (hasNoCache) {
                canvas.clipRect(sx, sy, sx + (cr - cl), sy + (cb - ct));
            } else {
                if (!scalingRequired) {
                    canvas.clipRect(0, 0, cr - cl, cb - ct);
                } else {
                    canvas.clipRect(0, 0, cache.getWidth(), cache.getHeight());
                }
            }
        }

        if (hasNoCache) {
            // Fast path for layouts with no backgrounds
            if ((child.mPrivateFlags & SKIP_DRAW) == SKIP_DRAW) {
                if (ViewDebug.TRACE_HIERARCHY) {
                    ViewDebug.trace(this, ViewDebug.HierarchyTraceType.DRAW);
                }
                child.mPrivateFlags &= ~DIRTY_MASK;
                child.dispatchDraw(canvas);
            } else {
                child.draw(canvas);
            }
        } else {
            final Paint cachePaint = mCachePaint;
            if (alpha < 1.0f) {
                cachePaint.setAlpha((int) (alpha * 255));
                mGroupFlags |= FLAG_ALPHA_LOWER_THAN_ONE;
            } else if  ((flags & FLAG_ALPHA_LOWER_THAN_ONE) == FLAG_ALPHA_LOWER_THAN_ONE) {
                cachePaint.setAlpha(255);
                mGroupFlags &= ~FLAG_ALPHA_LOWER_THAN_ONE;
            }
            if (Config.DEBUG && ViewDebug.profileDrawing) {
                EventLog.writeEvent(60003, hashCode());
            }
            canvas.drawBitmap(cache, 0.0f, 0.0f, cachePaint);
        }

        canvas.restoreToCount(restoreTo);

        if (a != null && !more) {
            child.onSetAlpha(255);
            finishAnimatingView(child, a);
        }

        return more;
    }

这个方法是ViewGroup用来绘制子View的，第100行！调用了子View的computeScroll()方法，从Android的View树来看，我们会知道，所有的View的computeScroll方法一定会被调用，而在View被刷新（也就是Draw()方法被调用的时候）也会调用这个方法（这里有待探究，我在源码里面追踪了很久没有找到很直接的方法调用路径）（当我们执行ontouch或invalidate(）或postInvalidate()都会导致这个方法的执行所以我们像下面这样调用，postInvalidate执行后，会去调computeScroll 方法，而这个方法里再去调postInvalidate，这样就可以不断地去调用scrollTo方法了，直到mScroller动画结束，当然第一次时，我们需要手动去调用一次postInvalidate才会去调用 ），所以，假设我们存在一个方法可以调用Scroller的startScroll方法，然后立刻调用invalidate方法，这个时候就回去调用computeScroll方法，而在覆盖这个方法的时候，我们如下操作：

@Override  
public void computeScroll() {  
    if (mScroller.computeScrollOffset()) { // 如果返回true，表示动画还没有结束
        scrollTo(mScroller.getCurrX(), mScroller.getCurrY()); 
        postInvalidate();  
    } else { //如果返回false，表示startScroll完成  
        Log.i(tag, " scoller has finished -----");  
}  

这个方法一方面调用scrollTo方法，一方面，我们调用postInvalidate方法形成循环，直到MScroller.computeScrollOffset方法返回false，表示滚动结束的时候，我们停止刷新！另一个方法大致如下：

public void scroll(){ 
   mScroller.startScroll(0, 0, menuItemWidth, 0,3000);  
   invalidate();  
}  

这样就可以通过View本身的滚动方法和Scroller配合，实现View的动画滚动。完整的例子： 

import android.content.Context;  
import android.util.AttributeSet;  
import android.view.View;  
import android.view.ViewGroup;  
import android.widget.LinearLayout;  
import android.widget.Scroller;  
  
public class MyViewGroup extends LinearLayout {  
    private boolean s1=true;  
    Scroller mScroller=null;  
    public MyViewGroup(Context context, AttributeSet attrs) {  
        super(context, attrs);  
        mScroller=new Scroller(context);  
        // TODO Auto-generated constructor stub  
    }  
    @Override  
    public void computeScroll() {  
        if (mScroller.computeScrollOffset()) {  
            scrollTo(mScroller.getCurrX(), 0);  
            postInvalidate();  
        }  
    }  
    public void beginScroll(){  
        if (!s1) {  
            mScroller.startScroll(0, 0, 0, 0, 1000);  
            s1 = true;  
        } else {  
            mScroller.startScroll(0, 0, -500, 0, 1000);  
            s1 = false;  
        }  
        invalidate();  
    }  
}  

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