ViewPager部分源码分析三：scroll

手指在屏幕上滑动，触发到onTouchEvent()，执行case MotionEvent.ACTION_MOVE: 。。。

public boolean onTouchEvent(MotionEvent ev) {
        if (mFakeDragging) {
            // A fake drag is in progress already, ignore this real one
            // but still eat the touch events.
            // (It is likely that the user is multi-touching the screen.)
            return true;
        }

        if (ev.getAction() == MotionEvent.ACTION_DOWN && ev.getEdgeFlags() != 0) {
            // Don't handle edge touches immediately -- they may actually belong to one of our
            // descendants.
            return false;
        }

        if (mAdapter == null || mAdapter.getCount() == 0) {
            // Nothing to present or scroll; nothing to touch.
            return false;
        }

        if (mVelocityTracker == null) {
            mVelocityTracker = VelocityTracker.obtain();
        }
        mVelocityTracker.addMovement(ev);

        final int action = ev.getAction();
        boolean needsInvalidate = false;

        switch (action & MotionEventCompat.ACTION_MASK) {
            case MotionEvent.ACTION_DOWN: {
                mScroller.abortAnimation();
                mPopulatePending = false;
                populate();

                // Remember where the motion event started
                mLastMotionX = mInitialMotionX = ev.getX();
                mLastMotionY = mInitialMotionY = ev.getY();
                mActivePointerId = MotionEventCompat.getPointerId(ev, 0);
                break;
            }
            case MotionEvent.ACTION_MOVE:
                if (!mIsBeingDragged) {
                    final int pointerIndex = MotionEventCompat.findPointerIndex(ev, mActivePointerId);
                    if (pointerIndex == -1) {
                        // A child has consumed some touch events and put us into an inconsistent state.
                        needsInvalidate = resetTouch();
                        break;
                    }
                    final float x = MotionEventCompat.getX(ev, pointerIndex);
                    final float xDiff = Math.abs(x - mLastMotionX);
                    final float y = MotionEventCompat.getY(ev, pointerIndex);
                    final float yDiff = Math.abs(y - mLastMotionY);
                    if (DEBUG) Log.v(TAG, "Moved x to " + x + "," + y + " diff=" + xDiff + "," + yDiff);
                    if (xDiff > mTouchSlop && xDiff > yDiff) {
                        if (DEBUG) Log.v(TAG, "Starting drag!");
                        mIsBeingDragged = true;
                        requestParentDisallowInterceptTouchEvent(true);
                        mLastMotionX = x - mInitialMotionX > 0 ? mInitialMotionX + mTouchSlop :
                                mInitialMotionX - mTouchSlop;
                        mLastMotionY = y;
                        setScrollState(SCROLL_STATE_DRAGGING);
                        setScrollingCacheEnabled(true);

                        // Disallow Parent Intercept, just in case
                        ViewParent parent = getParent();
                        if (parent != null) {
                            parent.requestDisallowInterceptTouchEvent(true);
                        }
                    }
                }
                // Not else! Note that mIsBeingDragged can be set above.
                if (mIsBeingDragged) {
                    // Scroll to follow the motion event
                    final int activePointerIndex = MotionEventCompat.findPointerIndex(
                            ev, mActivePointerId);
                    final float x = MotionEventCompat.getX(ev, activePointerIndex);
                    needsInvalidate |= performDrag(x);
                }
                break;
            case MotionEvent.ACTION_UP:
                if (mIsBeingDragged) {
                    final VelocityTracker velocityTracker = mVelocityTracker;
                    velocityTracker.computeCurrentVelocity(1000, mMaximumVelocity);
                    int initialVelocity = (int) VelocityTrackerCompat.getXVelocity(
                            velocityTracker, mActivePointerId);
                    mPopulatePending = true;
                    final int width = getClientWidth();
                    final int scrollX = getScrollX();
                    final ItemInfo ii = infoForCurrentScrollPosition();
                    final float marginOffset = (float) mPageMargin / width;
                    final int currentPage = ii.position;
                    final float pageOffset = (((float) scrollX / width) - ii.offset)
                            / (ii.widthFactor + marginOffset);
                    final int activePointerIndex =
                            MotionEventCompat.findPointerIndex(ev, mActivePointerId);
                    final float x = MotionEventCompat.getX(ev, activePointerIndex);
                    final int totalDelta = (int) (x - mInitialMotionX);
                    int nextPage = determineTargetPage(currentPage, pageOffset, initialVelocity,
                            totalDelta);
                    setCurrentItemInternal(nextPage, true, true, initialVelocity);

                    needsInvalidate = resetTouch();
                }
                break;
            case MotionEvent.ACTION_CANCEL:
                if (mIsBeingDragged) {
                    scrollToItem(mCurItem, true, 0, false);
                    needsInvalidate = resetTouch();
                }
                break;
            case MotionEventCompat.ACTION_POINTER_DOWN: {
                final int index = MotionEventCompat.getActionIndex(ev);
                final float x = MotionEventCompat.getX(ev, index);
                mLastMotionX = x;
                mActivePointerId = MotionEventCompat.getPointerId(ev, index);
                break;
            }
            case MotionEventCompat.ACTION_POINTER_UP:
                onSecondaryPointerUp(ev);
                mLastMotionX = MotionEventCompat.getX(ev,
                        MotionEventCompat.findPointerIndex(ev, mActivePointerId));
                break;
        }
        if (needsInvalidate) {
            ViewCompat.postInvalidateOnAnimation(this);
        }
        return true;
    }

onTouchEvent(MotionEvent ev)

--> performDrag()

private boolean performDrag(float x) {
        boolean needsInvalidate = false;

        final float deltaX = mLastMotionX - x;
        mLastMotionX = x;

        float oldScrollX = getScrollX();
        float scrollX = oldScrollX + deltaX;
        final int width = getClientWidth();

        float leftBound = width * mFirstOffset;
        float rightBound = width * mLastOffset;
        boolean leftAbsolute = true;
        boolean rightAbsolute = true;

        final ItemInfo firstItem = mItems.get(0);
        final ItemInfo lastItem = mItems.get(mItems.size() - 1);
        if (firstItem.position != 0) {
            leftAbsolute = false;
            leftBound = firstItem.offset * width;
        }
        if (lastItem.position != mAdapter.getCount() - 1) {
            rightAbsolute = false;
            rightBound = lastItem.offset * width;
        }

        if (scrollX < leftBound) {
            if (leftAbsolute) {
                float over = leftBound - scrollX;
                needsInvalidate = mLeftEdge.onPull(Math.abs(over) / width);
            }
            scrollX = leftBound;
        } else if (scrollX > rightBound) {
            if (rightAbsolute) {
                float over = scrollX - rightBound;
                needsInvalidate = mRightEdge.onPull(Math.abs(over) / width);
            }
            scrollX = rightBound;
        }
        // Don't lose the rounded component
        mLastMotionX += scrollX - (int) scrollX;
        scrollTo((int) scrollX, getScrollY());
        pageScrolled((int) scrollX);

        return needsInvalidate;
    }

performDrag(float x)

--> pageScrolled()

private boolean pageScrolled(int xpos) {
        if (mItems.size() == 0) {
            mCalledSuper = false;
            onPageScrolled(0, 0, 0);
            if (!mCalledSuper) {
                throw new IllegalStateException(
                        "onPageScrolled did not call superclass implementation");
            }
            return false;
        }
        final ItemInfo ii = infoForCurrentScrollPosition();
        final int width = getClientWidth();
        final int widthWithMargin = width + mPageMargin;
        final float marginOffset = (float) mPageMargin / width;
        final int currentPage = ii.position;
        final float pageOffset = (((float) xpos / width) - ii.offset) /
                (ii.widthFactor + marginOffset);
        final int offsetPixels = (int) (pageOffset * widthWithMargin);

        mCalledSuper = false;
        onPageScrolled(currentPage, pageOffset, offsetPixels);
        if (!mCalledSuper) {
            throw new IllegalStateException(
                    "onPageScrolled did not call superclass implementation");
        }
        return true;
    }

pageScrolled(int xpos)

--> onPageScrolled()

protected void onPageScrolled(int position, float offset, int offsetPixels) {
        // Offset any decor views if needed - keep them on-screen at all times.
        if (mDecorChildCount > 0) {
            final int scrollX = getScrollX();
            int paddingLeft = getPaddingLeft();
            int paddingRight = getPaddingRight();
            final int width = getWidth();
            final int childCount = getChildCount();
            for (int i = 0; i < childCount; i++) {
                final View child = getChildAt(i);
                final LayoutParams lp = (LayoutParams) child.getLayoutParams();
                if (!lp.isDecor) continue;

                final int hgrav = lp.gravity & Gravity.HORIZONTAL_GRAVITY_MASK;
                int childLeft = 0;
                switch (hgrav) {
                    default:
                        childLeft = paddingLeft;
                        break;
                    case Gravity.LEFT:
                        childLeft = paddingLeft;
                        paddingLeft += child.getWidth();
                        break;
                    case Gravity.CENTER_HORIZONTAL:
                        childLeft = Math.max((width - child.getMeasuredWidth()) / 2,
                                paddingLeft);
                        break;
                    case Gravity.RIGHT:
                        childLeft = width - paddingRight - child.getMeasuredWidth();
                        paddingRight += child.getMeasuredWidth();
                        break;
                }
                childLeft += scrollX;

                final int childOffset = childLeft - child.getLeft();
                if (childOffset != 0) {
                    child.offsetLeftAndRight(childOffset);
                }
            }
        }

        dispatchOnPageScrolled(position, offset, offsetPixels);

        if (mPageTransformer != null) {
            final int scrollX = getScrollX();
            final int childCount = getChildCount();
            for (int i = 0; i < childCount; i++) {
                final View child = getChildAt(i);
                final LayoutParams lp = (LayoutParams) child.getLayoutParams();

                if (lp.isDecor) continue;
                final float transformPos = (float) (child.getLeft() - scrollX) / getClientWidth();
                mPageTransformer.transformPage(child, transformPos);
            }
        }

        mCalledSuper = true;
    }

onPageScrolled(int position, float offset, int offsetPixels)

--> dispatchOnPageScrolled() 处理OnPageChangeListener

private void dispatchOnPageScrolled(int position, float offset, int offsetPixels) {
        if (mOnPageChangeListener != null) {
            mOnPageChangeListener.onPageScrolled(position, offset, offsetPixels);
        }
        if (mOnPageChangeListeners != null) {
            for (int i = 0, z = mOnPageChangeListeners.size(); i < z; i++) {
                OnPageChangeListener listener = mOnPageChangeListeners.get(i);
                if (listener != null) {
                    listener.onPageScrolled(position, offset, offsetPixels);
                }
            }
        }
        if (mInternalPageChangeListener != null) {
            mInternalPageChangeListener.onPageScrolled(position, offset, offsetPixels);
        }
    }

dispatchOnPageScrolled(int position, float offset, int offsetPixels)

--> onPageScrolled()中，执行完dispatchOnPageScrolled()之后，对mPageTransformer处理的代码是进行动画处理，因此自定义的动画类（继承PageTransformer，并实现transformPage()方法）在这里得到执行。可以看到，通过for循环对每一个child都进行动画处理。

在分析这个过程是，对scrollX不太明白，就对原来的一个例子（Android Tab -- 使用ViewPager、Fragment、FragmentPagerAdapter来实现）作了下调整，进行跟踪分析。

-->

根据跟踪到的数据对onPageScrolled(int position, float positionOffset, int positionOffsetPixels)进行分析：

先看看scrollX：

scrollX = 已经滑过去的标签宽度之和（old scrollX，其实就是对应图中标签1的宽度） + 本次滑动的宽度

本次滑动的宽度 = 当前左边显示的标签的宽度（child的width） - 当前左边显示的标签剩下的宽度（fragment2滑动后剩下的宽度）

再来说三个参数：

position:  当前左边显示的标签位置（从0开始），图片对应的是1。

positionOffset: 本次滑动的宽度 / 当前左边显示的标签的宽度（child的width）

positionOffsetSetPixels: 本次滑动的宽度

如果没有重写PagerAdapter.getPageWidth()方法，就可以简单按以上对三个参数参数的理解。

然而，但重写了PagerAdapter.getPageWidth()方法后发现不太一样了！！

public float getPageWidth(int position) {
    if(position == 2)
        return 1.f;
    return 0.4f;
}

除了Fragment3是1倍宽度，其他的都是0.4倍宽度。

-->

在这种情况下得到的:

scrollX：同上一种情况，依旧是每次滑动距离之和。

position：同上一种情况，依旧是当前最左边标签的序号。

positionOffset：依旧可以简单的理解为：本次滑动的宽度 / 当前左边显示的标签的宽度（child的width），对应图片中：Fragment1滑动的距离 / Fragment1的宽度

但是，

positionOffsetSetPixels: 并不是 本次滑动的宽度， 而是positionOffset反映到ViewPager的宽度上，按源码的意思是应该就是 positionOffset * getClientWidth() 。

当然，源码还有对mPageMargin即page之间的间距的处理，本示例为0。

因此positionOffsetSetPixels并不是327(494 - 167)，而是817（跟踪到的数据，嘿嘿。。。）