ViewPager实现源码分析

1、松手自动计算当前位置，并自动滑动到合适的position的页面

分析：松手属于View的事件分发机制，事件分发的入口在dispatchTouchEvent和onTouchEvent，大概的分发机制如下：

    public boolean dispatchTouchEvent(MotionEvent ev) {        boolean consume = false;        if(onInterceptTouchEvent(ev)) {            consume = onTouchEvent(ev);        } else {            consume = child.dispatchTouchEvent(ev);        }        return consume;    }

    因此，可以在onTouchEvent中处理松手事件思路1：查看RecyclerView的源码，看系统源码是否提供ACTION_UP的事件的回调接口，重写该接口，实现松手的相应业务逻辑思路2：覆盖掉onTouchEvent，ACTION_UP事件自己处理，其他事件交由系统默认处理（本项目采用的方法）    类比ViewPager的实现ACTION_UP时候的实现方式思路3：查看下RecyclerView的事件处理是否交由其他专门的事件处理类去处理（既在onTouchEvent中是否将事件处理转发到其他专门类处理），查找相应类和回调接口并覆盖掉回调接口方法    分析RecyclerView的onTouchEvent函数，并没发现使用其他事件处理类去接收事件，因此该方法排除。

2.松手自动滑动到指定page

方法一：根据松手的位置距离中心点的位置是否超过一半，确定最终位置，并用smoothScrollToPosition实现滑动到指定位置.

缺点：虽然能实现松手滑动到指定位置，但是滑动效果和ViewPager的松手滑动不大一样

实现代码：

     int widthWithMargin = getWidth() + mPageMargin;        // calculate scrollX        int scrollX = mMyScrollX;        int currentPage = scrollX / widthWithMargin;        int offset = scrollX - currentPage*widthWithMargin;        final int dstPage = currentPage + ((offset>widthWithMargin/2) ? 1 : 0);        getHandler().post(new Runnable() {            @Override            public void run() {                LogUtils.i("run to position");                smoothScrollToPosition(dstPage);            }        });

方法二：参考ViewPager的松手实现方法：VelocityTracker+Scroller+Interpolator+回调函数computeScroll,根据用户滑动速度动态改变松手滑动的快慢（本工程采用该方法）

参考源码：ViewPager源码

问题：滑动的自然性和使用ViewPager的滑动自然性不同？

分析：能够准确滑动，但是自然性不足，可能是因为滑动过程的算法参考的代码是4.0的ViewPager的滑动效果

解决：滑动过程的实现算法采用最新版本的ViewPager代码，滑动自然性体验和现有的ViewPager一致。

3.实现接口的回调（类OnPageChangeListener,如：当前滑动到第几页，当前滑动的状态）

源码中查看下该接口的三个函数，分别在哪个点被回调(使用AS的快捷键查看调用层级树)。然后在自定义的RecyclerView中相应的位置回调这三个方法（还未实现。。）

 /**     * Callback interface for responding to changing state of the selected page.     */    public interface OnPageChangeListener {        /**         * This method will be invoked when the current page is scrolled, either as part         * of a programmatically initiated smooth scroll or a user initiated touch scroll.         *         * @param position Position index of the first page currently being displayed.         *                 Page position+1 will be visible if positionOffset is nonzero.         * @param positionOffset Value from [0, 1) indicating the offset from the page at position.         * @param positionOffsetPixels Value in pixels indicating the offset from position.         */        void onPageScrolled(int position, float positionOffset, int positionOffsetPixels);        /**         * This method will be invoked when a new page becomes selected. Animation is not         * necessarily complete.         *         * @param position Position index of the new selected page.         */        void onPageSelected(int position);        /**         * Called when the scroll state changes. Useful for discovering when the user         * begins dragging, when the pager is automatically settling to the current page,         * or when it is fully stopped/idle.         *         * @param state The new scroll state.         * @see ViewPager#SCROLL_STATE_IDLE         * @see ViewPager#SCROLL_STATE_DRAGGING         * @see ViewPager#SCROLL_STATE_SETTLING         */        void onPageScrollStateChanged(int state);    }

3.1 ViewPager中mOnPageChangeListener.onPageScrolled的调用过程

    public void computeScroll()     // implement in ViewPagerRecyclerView    private void completeScroll(boolean postEvents)    private boolean performDrag(float x)    // implement in ViewPagerRecyclerView    public void fakeDragBy(float xOffset)        private boolean pageScrolled(int xpos)             -->protected void onPageScrolled(int position, float offset, int offsetPixels)                -->private void dispatchOnPageScrolled(int position, float offset, int offsetPixels)                    -->mOnPageChangeListener.onPageScrolled(position, offset, offsetPixels);    /*        dispatchOnPageScrolled是mOnPageChangeListener.onPageScrolled的唯一调用入口    */     private void dispatchOnPageScrolled(int position, float offset, int offsetPixels) {            if (mOnPageChangeListener != null) {                mOnPageChangeListener.onPageScrolled(position, offset, offsetPixels); // callback onPageScrolled            }            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);            }        }

3.2 ViewPager中mOnPageChangeListener.onPageSelected的调用过程

    public void onRestoreInstanceState(Parcelable state)    public boolean onTouchEvent(MotionEvent ev) case MotionEvent.ACTION_UP:// implement in ViewPagerRecyclerView    public void endFakeDrag()    public void setAdapter(PagerAdapter adapter)    public void setCurrentItem(int item)    // implement in ViewPagerRecyclerView    void dataSetChanged()        -->void setCurrentItemInternal(int item, boolean smoothScroll, boolean always, int velocity)            -->private void dispatchOnPageSelected(int position)             /*                dispatchOnPageSelected是mOnPageChangeListener.onPageSelected的唯一调用入口            */            private void dispatchOnPageSelected(int position) {            if (mOnPageChangeListener != null) {                mOnPageChangeListener.onPageSelected(position);            }            if (mOnPageChangeListeners != null) {                for (int i = 0, z = mOnPageChangeListeners.size(); i < z; i++) {                    OnPageChangeListener listener = mOnPageChangeListeners.get(i);                    if (listener != null) {                        listener.onPageSelected(position);                    }                }            }            if (mInternalPageChangeListener != null) {                mInternalPageChangeListener.onPageSelected(position);  // callback onPageSelected            }        }

3.3 ViewPager中mOnPageChangeListener.onPageScrollStateChanged的调用过程

    private final Runnable mEndScrollRunnable    void smoothScrollTo(int x, int y, int velocity) // implement in ViewPagerRecyclerView    public boolean onInterceptTouchEvent(MotionEvent ev)    public boolean onTouchEvent(MotionEvent ev) // implement in ViewPagerRecyclerView    public boolean beginFakeDrag()         private void setScrollState(int newState)             -->private void dispatchOnScrollStateChanged(int state)     /*        dispatchOnPageSelected是mOnPageChangeListener.onPageSelected的唯一调用入口    */   private void dispatchOnScrollStateChanged(int state) {        if (mOnPageChangeListener != null) {            mOnPageChangeListener.onPageScrollStateChanged(state);        }        if (mOnPageChangeListeners != null) {            for (int i = 0, z = mOnPageChangeListeners.size(); i < z; i++) {                OnPageChangeListener listener = mOnPageChangeListeners.get(i);                if (listener != null) {                    listener.onPageScrollStateChanged(state);                }            }        }        if (mInternalPageChangeListener != null) {            mInternalPageChangeListener.onPageScrollStateChanged(state);        }    }

问题：测试的时候，状态为IDLE的时候会有问题
问题：如何测试onPageScrolled

4.设置当前选中Item

ViewPager：如果当前是第一次设置的话，则启动requestLayout，并且设置当前选中的item，在布局中重新布局
requestLayout(); ===》使用标志位isFirstLayout

5.setPagerMargin的实现

5.1.ViewPager的setPagerMargin实现

/*    Set the margin between pages.    Parameters:    marginPixels Distance between adjacent pages in pixels*/429     public void More ...setPageMargin(int marginPixels) {430         final int oldMargin = mPageMargin;            // record PageMargin for Layout431         mPageMargin = marginPixels;432 433         final int width = getWidth();434         recomputeScrollPosition(width, width, marginPixels, oldMargin);435 436         requestLayout();437     }899     @Override900     protected void More ...onLayout(boolean changed, int l, int t, int r, int b) {901         mInLayout = true;902         populate();903         mInLayout = false;904 905         final int count = getChildCount();906         final int width = r-l;907 908         for (int i = 0; i < count; i++) {909             View child = getChildAt(i);910             ItemInfo ii;                // Use the mPageMargin parameter in the layout911             if (child.getVisibility() != GONE && (ii=infoForChild(child)) != null) {912                 int loff = (width + mPageMargin) * ii.position;913                 int childLeft = getPaddingLeft() + loff;914                 int childTop = getPaddingTop();                    ...918                 child.layout(childLeft, childTop,919                         childLeft + child.getMeasuredWidth(),920                         childTop + child.getMeasuredHeight());921             }922         }923         mFirstLayout = false;924     }

5.2 RecyclerView的setPagerMargin的实现

思路：查看RecyclerView中如何布局子View。问题：查看onLayout后发现布局过程有点复杂，代码中调用dispatchLayoutStep1();，dispatchLayoutStep2();，dispatchLayoutStep3();分析：既然从入口函数到目标（View的布局过程）有点复杂（代码量有点多），那么就从后往前推。首先，先查看RecyclerView中哪个地方调用了子View的布局，    既：RecyclerView中应该有调用child.layout或者view.layout,然后顺藤摸瓜，往上找出调用过程栈。经过查找，子view的布局过程如下：    onFocusSearchFailed    onLayoutChildren（LayoutManager中必须要覆盖的方法）    LinearLayoutManager.scrollBy        LinearLayoutManager.fill            LinearLayoutManager.layoutChunk                LayoutManager.layoutDecoratedWithMargins 该方法为public方法，可以覆盖掉                    child.layout想重新布局，但是RecyclerView的布局方式和ViewPager的布局方式不同，滚动的时候布局会乱。解决： 当用户设置pageMargin的时候，自定义PageMarginItemDecoration，让pageMargin的大小和ItemDecoration的mPageMarginWidth大小相等 然后整个RecyclerView控件的高度 + 上下分割线的总和  让控件高度超出屏幕的高度 在当前屏幕就看不到分割线了 滑动的时候，又可以看到分割线

每个子View的测量和布局过程void layoutChunk(RecyclerView.Recycler recycler, RecyclerView.State state,            LayoutState layoutState, LayoutChunkResult result) {        View view = layoutState.next(recycler);        if (view == null) {            if (DEBUG && layoutState.mScrapList == null) {                throw new RuntimeException("received null view when unexpected");            }            // if we are laying out views in scrap, this may return null which means there is            // no more items to layout.            result.mFinished = true;            return;        }        LayoutParams params = (LayoutParams) view.getLayoutParams();        if (layoutState.mScrapList == null) {            if (mShouldReverseLayout == (layoutState.mLayoutDirection                    == LayoutState.LAYOUT_START)) {                addView(view);            } else {                addView(view, 0);            }        } else {            if (mShouldReverseLayout == (layoutState.mLayoutDirection                    == LayoutState.LAYOUT_START)) {                addDisappearingView(view);            } else {                addDisappearingView(view, 0);            }        }        // 1.measure child view        measureChildWithMargins(view, 0, 0);        result.mConsumed = mOrientationHelper.getDecoratedMeasurement(view);        int left, top, right, bottom;        if (mOrientation == VERTICAL) {            if (isLayoutRTL()) {                right = getWidth() - getPaddingRight();                left = right - mOrientationHelper.getDecoratedMeasurementInOther(view);            } else {                left = getPaddingLeft();                right = left + mOrientationHelper.getDecoratedMeasurementInOther(view);            }            if (layoutState.mLayoutDirection == LayoutState.LAYOUT_START) {                bottom = layoutState.mOffset;                top = layoutState.mOffset - result.mConsumed;            } else {                top = layoutState.mOffset;                bottom = layoutState.mOffset + result.mConsumed;            }        } else {            top = getPaddingTop();            bottom = top + mOrientationHelper.getDecoratedMeasurementInOther(view);            if (layoutState.mLayoutDirection == LayoutState.LAYOUT_START) {                right = layoutState.mOffset;                left = layoutState.mOffset - result.mConsumed;            } else {                left = layoutState.mOffset;                right = layoutState.mOffset + result.mConsumed;            }        }        // 2.layout child view        // We calculate everything with View's bounding box (which includes decor and margins)        // To calculate correct layout position, we subtract margins.        layoutDecoratedWithMargins(view, left, top, right, bottom);        // Consume the available space if the view is not removed OR changed        if (params.isItemRemoved() || params.isItemChanged()) {            result.mIgnoreConsumed = true;        }        result.mFocusable = view.isFocusable();    } public void measureChildWithMargins(View child, int widthUsed, int heightUsed) {      final LayoutParams lp = (LayoutParams) child.getLayoutParams();      final Rect insets = mRecyclerView.getItemDecorInsetsForChild(child);//use ItemDecoration      widthUsed += insets.left + insets.right;      heightUsed += insets.top + insets.bottom;      final int widthSpec = getChildMeasureSpec(getWidth(), getWidthMode(),              getPaddingLeft() + getPaddingRight() +                      lp.leftMargin + lp.rightMargin + widthUsed, lp.width,              canScrollHorizontally());      final int heightSpec = getChildMeasureSpec(getHeight(), getHeightMode(),              getPaddingTop() + getPaddingBottom() +                      lp.topMargin + lp.bottomMargin + heightUsed, lp.height,              canScrollVertically());      if (shouldMeasureChild(child, widthSpec, heightSpec, lp)) {          child.measure(widthSpec, heightSpec);      }  }

setMargin/setPadding均失败，查询代码，设置padding

转换思路：让RecyclerView控件的宽度大于屏幕宽度，而不是去考虑更改ItemDecoration的位置（源码不可动）

参考文献：

如何获取Android RecyclerView滑动的距离

如何获取 RecyclerView 的滑动距离？

RecyclerView

ViewPager

ViewPager源码

演示视频