Android自定义组件系列【7】——进阶实践（4）

上一篇《》中补充了关于Android中事件分发的过程知识，这一篇我们接着来分析任老师的《可下拉的PinnedHeaderExpandableListView的实现》。

一、StickyLayout中的OnGiveUpTouchEventListener接口的作用是什么？

    public interface OnGiveUpTouchEventListener {
        public boolean giveUpTouchEvent(MotionEvent event);
    }

    public void setOnGiveUpTouchEventListener(OnGiveUpTouchEventListener l) {
        mGiveUpTouchEventListener = l;
    }

    @Override
    public boolean giveUpTouchEvent(MotionEvent event) {
        if (expandableListView.getFirstVisiblePosition() == 0) {
            View view = expandableListView.getChildAt(0);
            if (view != null && view.getTop() >= 0) {
                return true;
            }
        }
        return false;
    }

这里返回的true和false有什么不同呢？向下看

    @Override
    public boolean onInterceptTouchEvent(MotionEvent event) {
        int intercepted = 0;
        int x = (int) event.getX();
        int y = (int) event.getY();

        switch (event.getAction()) {
        case MotionEvent.ACTION_DOWN: {
            mLastXIntercept = x;
            mLastYIntercept = y;
            mLastX = x;
            mLastY = y;
            intercepted = 0;
            break;
        }
        case MotionEvent.ACTION_MOVE: {
            int deltaX = x - mLastXIntercept;
            int deltaY = y - mLastYIntercept;
            if (mStatus == STATUS_EXPANDED && deltaY <= -mTouchSlop) {
                intercepted = 1;
            } else if (mGiveUpTouchEventListener != null) {
                if (mGiveUpTouchEventListener.giveUpTouchEvent(event) && deltaY >= mTouchSlop) {
                    intercepted = 1;
                }
            }
            break;
        }
        case MotionEvent.ACTION_UP: {
            intercepted = 0;
            mLastXIntercept = mLastYIntercept = 0;
            break;
        }
        default:
            break;
        }

        Log.d(TAG, "intercepted=" + intercepted);
        return intercepted != 0;
    }

            if (mStatus == STATUS_EXPANDED && deltaY <= -mTouchSlop) {
                intercepted = 1;
            } else if (mGiveUpTouchEventListener != null) {
                if (mGiveUpTouchEventListener.giveUpTouchEvent(event) && deltaY >= mTouchSlop) {
                    intercepted = 1;
                }
            }

二、PinnedHeaderExpandableListView对OnScrollLister的实现

    @Override
    public void onScrollStateChanged(AbsListView view, int scrollState) {
        if (mHeaderView != null && scrollState == SCROLL_STATE_IDLE) {
            int firstVisiblePos = getFirstVisiblePosition();
            if (firstVisiblePos == 0) {
                mHeaderView.layout(0, 0, mHeaderWidth, mHeaderHeight);
            }
        }
        if (mScrollListener != null) {
            mScrollListener.onScrollStateChanged(view, scrollState);
        }
    }

    @Override
    public void onScroll(AbsListView view, int firstVisibleItem,
            int visibleItemCount, int totalItemCount) {
        if (totalItemCount > 0) {
            refreshHeader();
        }
        if (mScrollListener != null) {
            mScrollListener.onScroll(view, firstVisibleItem, visibleItemCount, totalItemCount);
        }
    }

在onScrollStateChanged(AbsListView view, int scrollState)中，scrollState有三种状态，分别是：

1、SCROLL_STATE_FLING:开始滚动

2、SCROLL_STATE_TOUCH_SCROLL：正在滚动

3、SCROLL_STATE_IDLE：已经停止

onScroll()方法在列表滚动时一直回调，知道滚动停止才停止回调，另外单击时也回调一次。而OnScrollStateChanged的意思是上面三种状态改变时回调，回调顺序如下：

1. 第1次：scrollState = SCROLL_STATE_TOUCH_SCROLL(1) 正在滚动  
2. 第2次：scrollState = SCROLL_STATE_FLING(2) 手指做了抛的动作（手指离开屏幕前，用力滑了一下）  
3. 第3次：scrollState = SCROLL_STATE_IDLE(0) 停止滚动  
4. 上面的OnScrollStateChanged方法中在滚动停止后再次布局（绘制）了列表的头。在onScroll方法中主要是为了调用前面提到的refreshHeader()方法去回调并刷新列表头。
5. 四、如何处理下面列表头将上面列表头顶上去？

6.     protected void refreshHeader() {
           if (mHeaderView == null) {
               return;
           }
           int firstVisiblePos = getFirstVisiblePosition();
           int pos = firstVisiblePos + 1;
           int firstVisibleGroupPos = getPackedPositionGroup(getExpandableListPosition(firstVisiblePos));
           int group = getPackedPositionGroup(getExpandableListPosition(pos));
   
           if (group == firstVisibleGroupPos + 1) {
               View view = getChildAt(1);
               if (view.getTop() <= mHeaderHeight) {
                   int delta = mHeaderHeight - view.getTop();
                   mHeaderView.layout(0, -delta, mHeaderWidth, mHeaderHeight - delta);
               }
           } else {
               mHeaderView.layout(0, 0, mHeaderWidth, mHeaderHeight);
           }
   
           if (mHeaderUpdateListener != null) {
               mHeaderUpdateListener.updatePinnedHeader(firstVisibleGroupPos);
           }
       }

   refreshHeader()方法中可以看到，先判断上面的是不是多个列表头如果是则重新设置下面列表头的位置到“标准位置”，这样就感觉有一种顶上去的感觉了。
   
7. 三、如何展开收缩列表

8.     @Override
       public boolean dispatchTouchEvent(MotionEvent ev) {
           int x = (int) ev.getX();
           int y = (int) ev.getY();
           Log.d(TAG, "dispatchTouchEvent");
           int pos = pointToPosition(x, y);
           if (y >= mHeaderView.getTop() && y <= mHeaderView.getBottom()) {
               if (ev.getAction() == MotionEvent.ACTION_DOWN) {
                   mActionDownHappened = true;
               } else if (ev.getAction() == MotionEvent.ACTION_UP) {
                   int groupPosition = getPackedPositionGroup(getExpandableListPosition(pos));
                   if (groupPosition != INVALID_POSITION && mActionDownHappened) {
                       if (isGroupExpanded(groupPosition)) {
                           collapseGroup(groupPosition);
                       } else {
                           expandGroup(groupPosition);
                       }
                       mActionDownHappened = false;
                   }
                   
               }
               return true;
           }
   
           return super.dispatchTouchEvent(ev);
       }

   可以看到PinnedHeaderExpandableListView中的事件分发函数中有如下代码：

9. if (y >= mHeaderView.getTop() && y <= mHeaderView.getBottom()) 

   限制高度（在列表头位置）

10. if (isGroupExpanded(groupPosition)) {
                            collapseGroup(groupPosition);
                        } else {
                            expandGroup(groupPosition);
                        }

    如果点击则展开或闭合。

Android自定义组件系列【7】——进阶实践（4）,,5-wow.com