[UI]抽屉菜单DrawerLayout分析（三）

在[UI]抽屉菜单DrawerLayout分析（一）和[UI]抽屉菜单DrawerLayout分析（二）中分别介绍了DrawerLayout得基本框架结构和ViewDragerHelper的作用以及手势分发，本文一起来分析其中的Scroller的使用情况。

在ViewDragerHelper中可以发现private ScrollerCompat mScroller;说明抽屉菜单的具体滑动也是依赖于Scroller的使用，检索一下mScroller的引用，定位到forceSettleCapturedViewAt，这个方法回调用Scroller的startScroll来计算位移，它本身适用于计算和保存位移在特定时间的变化情况，最终的在绘制view时我可以获取其保存的x，y坐标值。

/**

 * Settle the captured view at the given (left, top) position.

 *

 * @param finalLeft Target left position for the captured view

 * @param finalTop Target top position for the captured view

 * @param xvel Horizontal velocity

 * @param yvel Vertical velocity

 * @return true if animation should continue through {@link #continueSettling(boolean)} calls

 */

private boolean forceSettleCapturedViewAt(int finalLeft, int finalTop, int xvel, int yvel) {

final int startLeft = mCapturedView.getLeft();

final int startTop = mCapturedView.getTop();

final int dx = finalLeft - startLeft;

final int dy = finalTop - startTop;

if (dx == 0 && dy == 0) {

// Nothing to do. Send callbacks, be done.

mScroller.abortAnimation();

setDragState(STATE_IDLE);

returnfalse;

}

final int duration = computeSettleDuration(mCapturedView, dx, dy, xvel, yvel);

mScroller.startScroll(startLeft, startTop, dx, dy, duration);

setDragState(STATE_SETTLING);

returntrue;

}

这里用的是v4扩展包里的ScrollerCompat用于低版本兼容，它继承自ScrollerCompatImpl，可以看到里面主要的方法声明：

interface ScrollerCompatImpl{

Object createScroller(Context context, Interpolator interpolator);

boolean isFinished(Object scroller);

int getCurrX(Object scroller);

int getCurrY(Object scroller);

float getCurrVelocity(Object scroller);

boolean computeScrollOffset(Object scroller);

void startScroll(Object scroller, int startX, int startY, int dx, int dy);

void startScroll(Object scroller, int startX, int startY, int dx, int dy, int duration);

void fling(Object scroller, int startX, int startY, int velX, int velY,

int minX, int maxX, int minY, int maxY);

void fling(Object scroller, int startX, int startY, int velX, int velY,

int minX, int maxX, int minY, int maxY, int overX, int overY);

void abortAnimation(Object scroller);

void notifyHorizontalEdgeReached(Object scroller, int startX, int finalX, int overX);

void notifyVerticalEdgeReached(Object scroller, int startY, int finalY, int overY);

boolean isOverScrolled(Object scroller);

int getFinalX(Object scroller);

int getFinalY(Object scroller);

}

从Scroller一直往上追溯，可以得到如图的调用流程。

当滑动屏幕时，DrawerLayout中的手势分发被触发，先执行onInterceptTouchEvent根据返回结果确定是否执行onTouchEvent,之后就是一些和ViewDragHelper之间的回调接口处理。

接下来追踪一下什么时候从Scroller中取出x，y来使用：

在View里面有一个实现为空的computeScroll，DrawerLayout对它进行重写，这个方法应该是在view自动重绘是会被调用，回到continueSettling：

/**

 * Move the captured settling view by the appropriate amount for the current time.

 * If <code>continueSettling</code> returns true, the caller should call it again

 * on the next frame to continue.

 *

 * @param deferCallbacks true if state callbacks should be deferred via posted message.

 *                       Set this to true if you are calling this method from

 *                       {@link android.view.View#computeScroll()} or similar methods

 *                       invoked as part of layout or drawing.

 * @return true if settle is still in progress

 */

public boolean continueSettling(boolean deferCallbacks) {

if (mDragState == STATE_SETTLING) {

boolean keepGoing = mScroller.computeScrollOffset();

final int x = mScroller.getCurrX();

final int y = mScroller.getCurrY();

final int dx = x - mCapturedView.getLeft();

final int dy = y - mCapturedView.getTop();

if (dx != 0) {

mCapturedView.offsetLeftAndRight(dx);

}

if (dy != 0) {

mCapturedView.offsetTopAndBottom(dy);

}

if (dx != 0 || dy != 0) {

mCallback.onViewPositionChanged(mCapturedView, x, y, dx, dy);

}

if (keepGoing && x == mScroller.getFinalX() && y == mScroller.getFinalY()) {

// Close enough. The interpolator/scroller might think we're still moving

// but the user sure doesn't.

mScroller.abortAnimation();

keepGoing = mScroller.isFinished();

}

if (!keepGoing) {

if (deferCallbacks) {

mParentView.post(mSetIdleRunnable);

} else {

setDragState(STATE_IDLE);

}

}

}

return mDragState == STATE_SETTLING;

}

当状态处于STATE_SETTLING时开始获取Scroller中的x，y值，结合当前运动view的left，top位置，计算出偏移量，通过offsetLeftAndRight设置，里面是一些具体的位置改变，挺复杂的。

/**

 * Offset this view's horizontal location by the specified amount of pixels.

 *

 * @param offset the number of pixels to offset the view by

 */

public void offsetLeftAndRight(int offset) {

if (offset != 0) {

updateMatrix();

final boolean matrixIsIdentity = mTransformationInfo == null

|| mTransformationInfo.mMatrixIsIdentity;

if (matrixIsIdentity) {

if (mDisplayList != null) {

invalidateViewProperty(false, false);

} else {

final ViewParent p = mParent;

if (p != null && mAttachInfo != null) {

final Rect r = mAttachInfo.mTmpInvalRect;

int minLeft;

int maxRight;

if (offset < 0) {

minLeft = mLeft + offset;

maxRight = mRight;

} else {

minLeft = mLeft;

maxRight = mRight + offset;

}

r.set(0, 0, maxRight - minLeft, mBottom - mTop);

p.invalidateChild(this, r);

}

}

} else {

invalidateViewProperty(false, false);

}

mLeft += offset;

mRight += offset;

if (mDisplayList != null) {

mDisplayList.offsetLeftAndRight(offset);

invalidateViewProperty(false, false);

} else {

if (!matrixIsIdentity) {

invalidateViewProperty(false, true);

}

invalidateParentIfNeeded();

}

}

}

小结

至此DrawerLayout的基本工作流程分析完毕，简单做一个总结，v4包提供了ViewDragHelper类，里面封装了对 scroller合view的位移操作，和Callback接口，通过DrawerLayout内的onInterceptTouchEvent和 onTouchEvent的重载，触发ViewDragHelper内的相关方法，同时在DrawerLayout内实现 ViewDragHelp.Callback.

作者：小文字

出处：http://www.cnblogs.com/avenwu/

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