[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);
}

DragActionMethodFlow
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：

Scroller

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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