Android ListView onTouchEvent源码分析

Android ListView  onTouchEvent源码简单分析，在看代码之前先来看下代码结构图


一、onTouchEvent源码

    @Override
    public boolean onTouchEvent(MotionEvent ev) {
        if (!isEnabled()) {
            // A disabled view that is clickable still consumes the touch
            // events, it just doesn't respond to them.
            return isClickable() || isLongClickable();
        }
       
        // AbsListView 绘制与控制手指快速滚动的辅助类
        if (mFastScroller != null) {
            boolean intercepted = mFastScroller.onTouchEvent(ev);
            if (intercepted) {
                return true;
            }
        }

        final int action = ev.getAction();

        View v;
        int deltaY;

         // 获取触摸滚动时的速率
        if (mVelocityTracker == null) {
            mVelocityTracker = VelocityTracker.obtain();
        }
        mVelocityTracker.addMovement(ev);

        // ListView触屏事件主要从ACTION操作划分
        switch (action & MotionEvent.ACTION_MASK) {
        case MotionEvent.ACTION_DOWN: {
             ......
             break;
        }
        case MotionEvent.ACTION_MOVE: {
            ......
            break;
        }

        case MotionEvent.ACTION_UP: {
            switch (mTouchMode) {
            case TOUCH_MODE_DOWN:
            case TOUCH_MODE_TAP:
            case TOUCH_MODE_DONE_WAITING:
                ......
                mTouchMode = TOUCH_MODE_REST;
                break;
            case TOUCH_MODE_SCROLL:
             ......
                break;
            }

            setPressed(false);

            // Need to redraw since we probably aren't drawing the selector anymore
            invalidate();

            final Handler handler = getHandler();
            if (handler != null) {
                handler.removeCallbacks(mPendingCheckForLongPress);
            }

            if (mVelocityTracker != null) {
                mVelocityTracker.recycle();
                mVelocityTracker = null;
            }
           
            mActivePointerId = INVALID_POINTER;

            if (PROFILE_SCROLLING) {
                if (mScrollProfilingStarted) {
                    Debug.stopMethodTracing();
                    mScrollProfilingStarted = false;
                }
            }
            break;
        }

        case MotionEvent.ACTION_CANCEL: {
            mTouchMode = TOUCH_MODE_REST;
            ......
            break;
        }
       
        case MotionEvent.ACTION_POINTER_UP: {
            ......
            break;
        }
       
        }

        return true;
    }

二、ACTION_DOWN具体操作源码分析，主要是CheckForTap

        case MotionEvent.ACTION_DOWN: {
            mActivePointerId = ev.getPointerId(0);
            final int x = (int) ev.getX();
            final int y = (int) ev.getY();
           
             // 手指按下时x,y坐标，获取当前选中的item
            int motionPosition = pointToPosition(x, y);
            // 如果ListView 数据未发生变化
            if (!mDataChanged) {
                if ((mTouchMode != TOUCH_MODE_FLING) && (motionPosition >= 0)
                        && (getAdapter().isEnabled(motionPosition))) {
                    // User clicked on an actual view (and was not stopping a fling). It might be a
                    // click or a scroll. Assume it is a click until proven otherwise
                    mTouchMode = TOUCH_MODE_DOWN;
                   
                    // TAP机制，主要是用于去除手指点击抖动
                    // 使Item处于按下状态
                    if (mPendingCheckForTap == null) {
                        mPendingCheckForTap = new CheckForTap();
                    }
                    // 添加到消息队列并延时ViewConfiguration.getTapTimeout()执行此runnable
                    postDelayed(mPendingCheckForTap, ViewConfiguration.getTapTimeout());
                } else {
                    if (ev.getEdgeFlags() != 0 && motionPosition < 0) {
                        // If we couldn't find a view to click on, but the down event was touching
                        // the edge, we will bail out and try again. This allows the edge correcting
                        // code in ViewRoot to try to find a nearby view to select
                        return false;
                    }

                      // 之前处于Fling模式
                    if (mTouchMode == TOUCH_MODE_FLING) {
                        // Stopped a fling. It is a scroll.
                        createScrollingCache();
                        // 更改为scroll
                        mTouchMode = TOUCH_MODE_SCROLL;
                        mMotionCorrection = 0;
                        motionPosition = findMotionRow(y);
                        reportScrollStateChange(OnScrollListener.SCROLL_STATE_TOUCH_SCROLL);
                    }
                }
            }

             // 对于ACTION_MOVE,ACTION_UP会使用的触屏位置信息进行记录
            if (motionPosition >= 0) {
                // Remember where the motion event started
                v = getChildAt(motionPosition - mFirstPosition);
                mMotionViewOriginalTop = v.getTop();
            }
            mMotionX = x;
            mMotionY = y;
            mMotionPosition = motionPosition;
            mLastY = Integer.MIN_VALUE;
            break;
        }

三、ACTION_MOVE具体操作源码分析，主要是startScrollIfNeeded和trackMotionScroll

        case MotionEvent.ACTION_MOVE: {
            final int pointerIndex = ev.findPointerIndex(mActivePointerId);
            final int y = (int) ev.getY(pointerIndex);
            // 获取y轴当前与前一次的偏移值
            deltaY = y - mMotionY;
            switch (mTouchMode) {
            case TOUCH_MODE_DOWN:
            case TOUCH_MODE_TAP:
            case TOUCH_MODE_DONE_WAITING:
                // 必须移动一段距离后才会执行滚动
                startScrollIfNeeded(deltaY);
                break;
            case TOUCH_MODE_SCROLL:
                if (PROFILE_SCROLLING) {
                    if (!mScrollProfilingStarted) {
                        Debug.startMethodTracing("AbsListViewScroll");
                        mScrollProfilingStarted = true;
                    }
                }

                 // 手指移动
                if (y != mLastY) {
                    deltaY -= mMotionCorrection;
                    int incrementalDeltaY = mLastY != Integer.MIN_VALUE ? y - mLastY : deltaY;
                   
                    // No need to do all this work if we're not going to move anyway
                    boolean atEdge = false;
                    if (incrementalDeltaY != 0) {
                      // 滚动的重要方法，滚动的具体处理就是这里
                        atEdge = trackMotionScroll(deltaY, incrementalDeltaY);
                    }

                    // ListView滚动到边界后不不能再进行移动
                    if (atEdge && getChildCount() > 0) {
                        // Treat this like we're starting a new scroll from the current
                        // position. This will let the user start scrolling back into
                        // content immediately rather than needing to scroll back to the
                        // point where they hit the limit first.
                        int motionPosition = findMotionRow(y);
                        if (motionPosition >= 0) {
                            final View motionView = getChildAt(motionPosition - mFirstPosition);
                            mMotionViewOriginalTop = motionView.getTop();
                        }
                        mMotionY = y;
                        mMotionPosition = motionPosition;
                        invalidate();
                    }
                    // 记录当前Y值，用于下次计算偏移量
                    mLastY = y;
                }
                break;
            }

            break;
        }

四、ACTION_UP具体操作源码分析，主要是PerformClick, mPendingCheckForLongPress, mFlingRunnable

        case MotionEvent.ACTION_UP: {
            switch (mTouchMode) {
            case TOUCH_MODE_DOWN:
            case TOUCH_MODE_TAP:
            case TOUCH_MODE_DONE_WAITING:
                final int motionPosition = mMotionPosition;
                final View child = getChildAt(motionPosition - mFirstPosition);
                if (child != null && !child.hasFocusable()) {
                    // 清理Item按下状态
                    if (mTouchMode != TOUCH_MODE_DOWN) {
                        child.setPressed(false);
                    }

                      // 执行Item Click
                    if (mPerformClick == null) {
                        mPerformClick = new PerformClick();
                    }

                    final AbsListView.PerformClick performClick = mPerformClick;
                    performClick.mChild = child;
                    performClick.mClickMotionPosition = motionPosition;
                    performClick.rememberWindowAttachCount();

                    mResurrectToPosition = motionPosition;

                    if (mTouchMode == TOUCH_MODE_DOWN || mTouchMode == TOUCH_MODE_TAP) {
                        final Handler handler = getHandler();
                        if (handler != null) {
                          // 清理tap或者long press长按
                            handler.removeCallbacks(mTouchMode == TOUCH_MODE_DOWN ?
                                    mPendingCheckForTap : mPendingCheckForLongPress);
                        }
                        mLayoutMode = LAYOUT_NORMAL;
                        if (!mDataChanged && mAdapter.isEnabled(motionPosition)) {
                            mTouchMode = TOUCH_MODE_TAP;
                            setSelectedPositionInt(mMotionPosition);
                            layoutChildren();
                            child.setPressed(true);
                            positionSelector(child);
                            setPressed(true);
                            if (mSelector != null) {
                                Drawable d = mSelector.getCurrent();
                                if (d != null && d instanceof TransitionDrawable) {
                                    ((TransitionDrawable) d).resetTransition();
                                }
                            }
                            postDelayed(new Runnable() {
                                public void run() {
                                    child.setPressed(false);
                                    setPressed(false);
                                    if (!mDataChanged) {
                                        post(performClick);
                                    }
                                    mTouchMode = TOUCH_MODE_REST;
                                }
                            }, ViewConfiguration.getPressedStateDuration());
                        } else {
                            mTouchMode = TOUCH_MODE_REST;
                        }
                        return true;
                    } else if (!mDataChanged && mAdapter.isEnabled(motionPosition)) {
                        post(performClick);
                    }
                }
                mTouchMode = TOUCH_MODE_REST;
                break;
            case TOUCH_MODE_SCROLL:
                final int childCount = getChildCount();
                if (childCount > 0) {
                    if (mFirstPosition == 0 && getChildAt(0).getTop() >= mListPadding.top &&
                            mFirstPosition + childCount < mItemCount &&
                            getChildAt(childCount - 1).getBottom() <=
                                    getHeight() - mListPadding.bottom) {
                        mTouchMode = TOUCH_MODE_REST;
                        reportScrollStateChange(OnScrollListener.SCROLL_STATE_IDLE);
                    } else {
                        // 是否执行ListView Scroll Fling
                        final VelocityTracker velocityTracker = mVelocityTracker;
                        velocityTracker.computeCurrentVelocity(1000, mMaximumVelocity);
                        // 获取当前触屏滚动速率
                        final int initialVelocity = (int) velocityTracker.getYVelocity(mActivePointerId);
   
                        if (Math.abs(initialVelocity) > mMinimumVelocity) {
                            if (mFlingRunnable == null) {
                                mFlingRunnable = new FlingRunnable();
                            }
                            reportScrollStateChange(OnScrollListener.SCROLL_STATE_FLING);
                           
                            // 执行ListView 快速滚动（Scroll Fling）
                            mFlingRunnable.start(-initialVelocity);
                        } else {
                            mTouchMode = TOUCH_MODE_REST;
                            reportScrollStateChange(OnScrollListener.SCROLL_STATE_IDLE);
                        }
                    }
                } else {
                    mTouchMode = TOUCH_MODE_REST;
                    reportScrollStateChange(OnScrollListener.SCROLL_STATE_IDLE);
                }
                break;
            }

            setPressed(false);

            // Need to redraw since we probably aren't drawing the selector anymore
            invalidate();

            final Handler handler = getHandler();
            if (handler != null) {
                handler.removeCallbacks(mPendingCheckForLongPress);
            }

            if (mVelocityTracker != null) {
                mVelocityTracker.recycle();
                mVelocityTracker = null;
            }
           
            mActivePointerId = INVALID_POINTER;

            if (PROFILE_SCROLLING) {
                if (mScrollProfilingStarted) {
                    Debug.stopMethodTracing();
                    mScrollProfilingStarted = false;
                }
            }
            break;
        }

说明：本文为转载并整理，某些地方讲的并不完善，比如没有提到对onInterceptTouchEvent(MotionEvent ev)方法的处理，在滚动过程中child view是如何布局的，ListView中滚动条加载机制，ListView是如何实现高效缓存的，及如何自定义ListView可显示多列并每个child View高度自适应，等等。先做个记录，后续有空继续研究。