HorizontalListView源码解读

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HorizontalListView源码解读

[TOC]

前言

​ 随着RecycleView进入人们的视野，其强大的功能让人们逐渐忘记了曾经的王者”AdapterView”,但这绝不影响它对我们的吸引力，要知道当年流行的Gallery，Spinner，ListView，GridView等等都是它的孩子。

​ 那么，假如今天我们要在它的孩子中增加一个横向滚动控件，我们应该怎么实现呢？HorizontalListView给出了我们想要的答案。

把ListView横过来

需要什么类辅助

  • Scroller

    一个平滑滚动的辅助工具,可以随时获取当前横向滚动距离,这里使用为了跟踪fling事件。

    • 调用关系

      onLayout -> computerScrollOffset //获得新的滚动位置x,y。

      onFling -> fling //根据手势和速度,计算滚动距离。

  • GestureListener

    一个用来跟踪触摸，点击，长按事件的监听器。

    [onDown,onFling,onScrol,onSingleTapConfirm,onLongPress]

  • ListAdapter

    一个非常常见的可用于填充数据和视图的适配器。

  • EdgeEffectCompat

    一个处理两边边界动画效果的工具,在合适的时机传入距离,速度等参数进行渲染。

    [onAbsorb,onPull,onRelease]

    • 调用关系

      onScroll -> updateOverscrollAnimation -> onPull

      dispatchDraw -> drawEdgeGlow ->[setSize,draw]

      onLayout -> onAbsorb

      onTouchEvent -> releaseEdgeGlow -> onRelease

布局处理

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/**
 * 1.判断是否需要重置数据
 * 2.恢复之前的滚动状态
 * 3.滚动状态的合法性校验
 * 4.获得deltx之后处理滚动后布局问题
 *   4.1 remove 滚动后的最左边子视图右边距小于0，最右边的子视图左边距大于父宽度
 *   4.2 add 向右边，向左边添加滚动后可见的新的子视图
 *   4.3 根据dx重新layout这些子视图
 *
 * @param changed
 * @param left
 * @param top
 * @param right
 * @param bottom
 */
@SuppressLint("WrongCall")
@Override
protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
    super.onLayout(changed, left, top, right, bottom);

    if (mAdapter == null) {
        return;
    }

    // Force the OS to redraw this view
    invalidate();

    // If the data changed then reset everything and render from scratch at the same offset as last time
    if (mDataChanged) {
        int oldCurrentX = mCurrentX;
        initView();
        removeAllViewsInLayout();
        mNextX = oldCurrentX;
        mDataChanged = false;
    }

    // If restoring from a rotation
    if (mRestoreX != null) {
        mNextX = mRestoreX;
        mRestoreX = null;
    }

    // If in a fling
    if (mFlingTracker.computeScrollOffset()) {
        // Compute the next position
        mNextX = mFlingTracker.getCurrX();
    }

    // Prevent scrolling past 0 so you can't scroll past the end of the list to the left
    if (mNextX < 0) {
        mNextX = 0;

        // Show an edge effect absorbing the current velocity
        if (mEdgeGlowLeft.isFinished()) {
            mEdgeGlowLeft.onAbsorb((int) determineFlingAbsorbVelocity());
        }

        mFlingTracker.forceFinished(true);
        setCurrentScrollState(OnScrollStateChangedListener.ScrollState.SCROLL_STATE_IDLE);
    } else if (mNextX > mMaxX) {
        // Clip the maximum scroll position at mMaxX so you can't scroll past the end of the list to the right
        mNextX = mMaxX;

        // Show an edge effect absorbing the current velocity
        if (mEdgeGlowRight.isFinished()) {
            mEdgeGlowRight.onAbsorb((int) determineFlingAbsorbVelocity());
        }

        mFlingTracker.forceFinished(true);
        setCurrentScrollState(OnScrollStateChangedListener.ScrollState.SCROLL_STATE_IDLE);
    }

    // Calculate our delta from the last time the view was drawn
    int dx = mCurrentX - mNextX;
    removeNonVisibleChildren(dx);
    fillList(dx);
    positionChildren(dx);

    // Since the view has now been drawn, update our current position
    mCurrentX = mNextX;

    // If we have scrolled enough to lay out all views, then determine the maximum scroll position now
    if (determineMaxX()) {
        // Redo the layout pass since we now know the maximum scroll position
        onLayout(changed, left, top, right, bottom);
        return;
    }

    // If the fling has finished
    if (mFlingTracker.isFinished()) {
        // If the fling just ended
        if (mCurrentScrollState == OnScrollStateChangedListener.ScrollState.SCROLL_STATE_FLING) {
            setCurrentScrollState(OnScrollStateChangedListener.ScrollState.SCROLL_STATE_IDLE);
        }
    } else {
        // Still in a fling so schedule the next frame
        ViewCompat.postOnAnimation(this, mDelayedLayout);
    }
}

手势处理

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 private class GestureListener extends GestureDetector.SimpleOnGestureListener {
    @Override
    public boolean onDown(MotionEvent e) {
        return HorizontalListView.this.onDown(e);
    }

    /**
     * 跟踪fling事件入口
     * @param e1
     * @param e2
     * @param velocityX
     * @param velocityY
     * @return
     */
    @Override
    public boolean onFling(MotionEvent e1, MotionEvent e2, float velocityX, float velocityY) {
        return HorizontalListView.this.onFling(e1, e2, velocityX, velocityY);
    }

    /**
     * 更新滚动状态计算获得新的nextX 
     * @param e1
     * @param e2
     * @param distanceX
     * @param distanceY
     * @return
     */
    @Override
    public boolean onScroll(MotionEvent e1, MotionEvent e2, float distanceX, float distanceY) {
        // Lock the user into interacting just with this view
        requestParentListViewToNotInterceptTouchEvents(true);

        setCurrentScrollState(OnScrollStateChangedListener.ScrollState.SCROLL_STATE_TOUCH_SCROLL);
        unpressTouchedChild();
        mNextX += (int) distanceX;
        updateOverscrollAnimation(Math.round(distanceX));
        requestLayout();

        return true;
    }

    /**
     * 根据 MotionEvent找到真正的childIndex，获得child后传递click事件
     * @param e
     * @return
     */
    @Override
    public boolean onSingleTapConfirmed(MotionEvent e) {
        unpressTouchedChild();
        OnItemClickListener onItemClickListener = getOnItemClickListener();

        final int index = getChildIndex((int) e.getX(), (int) e.getY());

        // If the tap is inside one of the child views, and we are not blocking touches
        if (index >= 0 && !mBlockTouchAction) {
            View child = getChildAt(index);
            int adapterIndex = mLeftViewAdapterIndex + index;

            if (onItemClickListener != null) {
                onItemClickListener.onItemClick(HorizontalListView.this, child, adapterIndex, mAdapter.getItemId(adapterIndex));
                return true;
            }
        }

        if (mOnClickListener != null && !mBlockTouchAction) {
            mOnClickListener.onClick(HorizontalListView.this);
        }

        return false;
    }

    /**
     * 根据 MotionEvent找到真正的childIndex，获得child后传递LongClick事件
     * @param e
     * @return
     */
    @Override
    public void onLongPress(MotionEvent e) {
        unpressTouchedChild();

        final int index = getChildIndex((int) e.getX(), (int) e.getY());
        if (index >= 0 && !mBlockTouchAction) {
            View child = getChildAt(index);
            OnItemLongClickListener onItemLongClickListener = getOnItemLongClickListener();
            if (onItemLongClickListener != null) {
                int adapterIndex = mLeftViewAdapterIndex + index;
                boolean handled = onItemLongClickListener.onItemLongClick(HorizontalListView.this, child, adapterIndex, mAdapter
                        .getItemId(adapterIndex));

                if (handled) {
                    // BZZZTT!!1!
                    performHapticFeedback(HapticFeedbackConstants.LONG_PRESS);
                }
            }
        }
    }
};

其他

复用机制

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//每一个Adapter的type对应一个queue，把同一个type的view插入到同一个queue中,然后通过queue的offer/poll接口进行复用
private List<Queue<View>> mRemovedViewsCache = new ArrayList<Queue<View>>();

  • 调用关系

    setAdapter -> initializeRecycledViewCache(typeCount) -> [clear,add]
    [fillListLeft,fillRight] -> getRecycledView -> [poll]
    removeNonVisibleChildren -> recycleView -> [offer]