ViewGroup的手势和绘制机制（个人看法）

    学习最好的办法就是从实际出发，所以我选择了ScrollLayout（左右滑动切换屏幕控件）来作为例子，讲述一下我对ViewGroup的一些机制的个人理解。

首先先贴一下ScrollLayout的代码：

1package cn.edu.scau.hci.widget;
2
3import android.content.Context;
4import android.util.AttributeSet;
5import android.view.MotionEvent;
6import android.view.VelocityTracker;
7import android.view.View;
8import android.view.ViewConfiguration;
9import android.view.ViewGroup;
10import android.widget.Scroller;
11
12/**
13 * 左右滑动切换屏幕控件
14 * 
15 * @author Yao.GUET date: 2011-05-04
16 * @modify liux (http://my.oschina.net/liux)
17 */
18public class ScrollLayout extends ViewGroup {
19  private Scroller mScroller;
20  private VelocityTracker mVelocityTracker;
21  private int mCurScreen;
22  private int mDefaultScreen = 0;
23  private static final int TOUCH_STATE_REST = 0;
24  private static final int TOUCH_STATE_SCROLLING = 1;
25  private static final int SNAP_VELOCITY = 600;
26  private int mTouchState = TOUCH_STATE_REST;
27  private int mTouchSlop;
28  private float mLastMotionX;
29  private float mLastMotionY;
30  private OnViewChangeListener mOnViewChangeListener;
31
32  /**
33   * 设置是否可左右滑动
34   * 
35   * @author liux
36   */
37  private boolean isScroll = true;
38
39  public void setIsScroll(boolean b) {
40    this.isScroll = b;
41  }
42
43  public ScrollLayout(Context context, AttributeSet attrs) {
44    this(context, attrs, 0);
45  }
46
47  public ScrollLayout(Context context, AttributeSet attrs, int defStyle) {
48    super(context, attrs, defStyle);
49    mScroller = new Scroller(context);
50    mCurScreen = mDefaultScreen;
51    mTouchSlop = ViewConfiguration.get(getContext()).getScaledTouchSlop();
52  }
53
54  @Override
55  protected void onLayout(boolean changed, int l, int t, int r, int b) {
56    int childLeft = 0;
57    final int childCount = getChildCount();
58    for (int i = 0; i < childCount; i++) {
59      final View childView = getChildAt(i);
60      if (childView.getVisibility() != View.GONE) {
61        final int childWidth = childView.getMeasuredWidth();
62        childView.layout(childLeft, 0, childLeft + childWidth, childView.getMeasuredHeight());
63        childLeft += childWidth;
64      }
65    }
66  }
67
68  @Override
69  protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
70    // Log.e(TAG, "onMeasure");
71    super.onMeasure(widthMeasureSpec, heightMeasureSpec);
72    final int width = MeasureSpec.getSize(widthMeasureSpec);
73    final int widthMode = MeasureSpec.getMode(widthMeasureSpec);
74    if (widthMode != MeasureSpec.EXACTLY) {
75      throw new IllegalStateException("ScrollLayout only canmCurScreen run at EXACTLY mode!");
76    }
77    final int heightMode = MeasureSpec.getMode(heightMeasureSpec);
78    if (heightMode != MeasureSpec.EXACTLY) {
79      throw new IllegalStateException("ScrollLayout only can run at EXACTLY mode!");
80    }
81
82    // The children are given the same width and height as the scrollLayout
83    final int count = getChildCount();
84    for (int i = 0; i < count; i++) {
85      getChildAt(i).measure(widthMeasureSpec, heightMeasureSpec);
86    }
87    // Log.e(TAG, "moving to screen "+mCurScreen);
88    scrollTo(mCurScreen * width, 0);
89  }
90
91  /**
92   * According to the position of current layout scroll to the destination page.
93   */
94  public void snapToDestination() {
95    final int screenWidth = getWidth();
96    final int destScreen = (getScrollX() + screenWidth / 2) / screenWidth;
97    snapToScreen(destScreen);
98  }
99
100  public void snapToScreen(int whichScreen) {
101    // 是否可滑动
102    if (!isScroll) {
103      this.setToScreen(whichScreen);
104      return;
105    }
106
107    scrollToScreen(whichScreen);
108  }
109
110  public void scrollToScreen(int whichScreen) {
111    // get the valid layout page
112    whichScreen = Math.max(0, Math.min(whichScreen, getChildCount() - 1));
113    if (getScrollX() != (whichScreen * getWidth())) {
114      final int delta = whichScreen * getWidth() - getScrollX();
115      mScroller.startScroll(getScrollX(), 0, delta, 0, Math.abs(delta) * 1);// 持续滚动时间 以毫秒为单位
116      mCurScreen = whichScreen;
117
118      invalidate(); // Redraw the layout
119
120      if (mOnViewChangeListener != null) {
121        mOnViewChangeListener.OnViewChange(mCurScreen);
122      }
123    }
124
125  }
126
127  public void setToScreen(int whichScreen) {
128    whichScreen = Math.max(0, Math.min(whichScreen, getChildCount() - 1));
129    mCurScreen = whichScreen;
130    scrollTo(whichScreen * getWidth(), 0);
131
132    if (mOnViewChangeListener != null) {
133      mOnViewChangeListener.OnViewChange(mCurScreen);
134    }
135  }
136
137  public int getCurScreen() {
138    return mCurScreen;
139  }
140
141  @Override
142  public void computeScroll() {
143    if (mScroller.computeScrollOffset()) {
144      scrollTo(mScroller.getCurrX(), mScroller.getCurrY());
145      postInvalidate();
146    }
147  }
148
149  @Override
150  public boolean onTouchEvent(MotionEvent event) {
151    // 是否可滑动
152    if (!isScroll) {
153      return false;
154    }
155
156    if (mVelocityTracker == null) {
157      mVelocityTracker = VelocityTracker.obtain();
158    }
159    mVelocityTracker.addMovement(event);
160    final int action = event.getAction();
161    final float x = event.getX();
162    final float y = event.getY();
163    switch (action) {
164      case MotionEvent.ACTION_DOWN:
165        // Log.e(TAG, "event down!");
166        if (!mScroller.isFinished()) {
167          mScroller.abortAnimation();
168        }
169        mLastMotionX = x;
170
171        // ---------------New Code----------------------
172        mLastMotionY = y;
173        // ---------------------------------------------
174
175        break;
176      case MotionEvent.ACTION_MOVE:
177        int deltaX = (int) (mLastMotionX - x);
178
179        // ---------------New Code----------------------
180        int deltaY = (int) (mLastMotionY - y);
181        if (Math.abs(deltaX) < 200 && Math.abs(deltaY) > 10) break;
182        mLastMotionY = y;
183        // -------------------------------------
184
185        mLastMotionX = x;
186        scrollBy(deltaX, 0);
187        break;
188      case MotionEvent.ACTION_UP:
189        // Log.e(TAG, "event : up");
190        // if (mTouchState == TOUCH_STATE_SCROLLING) {
191        final VelocityTracker velocityTracker = mVelocityTracker;
192        velocityTracker.computeCurrentVelocity(1000);
193        int velocityX = (int) velocityTracker.getXVelocity();
194        // Log.e(TAG, "velocityX:" + velocityX);
195        if (velocityX > SNAP_VELOCITY && mCurScreen > 0) {
196          // Fling enough to move left
197          // Log.e(TAG, "snap left");
198          snapToScreen(mCurScreen - 1);
199        } else if (velocityX < -SNAP_VELOCITY && mCurScreen < getChildCount() - 1) {
200          // Fling enough to move right
201          // Log.e(TAG, "snap right");
202          snapToScreen(mCurScreen + 1);
203        } else {
204          snapToDestination();
205        }
206        if (mVelocityTracker != null) {
207          mVelocityTracker.recycle();
208          mVelocityTracker = null;
209        }
210        // }
211        mTouchState = TOUCH_STATE_REST;
212        break;
213      case MotionEvent.ACTION_CANCEL:
214        mTouchState = TOUCH_STATE_REST;
215        break;
216    }
217    return true;
218  }
219
220  @Override
221  public boolean onInterceptTouchEvent(MotionEvent ev) {
222    // Log.e(TAG, "onInterceptTouchEvent-slop:" + mTouchSlop);
223    final int action = ev.getAction();
224    if ((action == MotionEvent.ACTION_MOVE) && (mTouchState != TOUCH_STATE_REST)) {
225      return true;
226    }
227    final float x = ev.getX();
228    final float y = ev.getY();
229    switch (action) {
230      case MotionEvent.ACTION_MOVE:
231        final int xDiff = (int) Math.abs(mLastMotionX - x);
232        if (xDiff > mTouchSlop) {
233          mTouchState = TOUCH_STATE_SCROLLING;
234        }
235        break;
236      case MotionEvent.ACTION_DOWN:
237        mLastMotionX = x;
238        mLastMotionY = y;
239        mTouchState = mScroller.isFinished() ? TOUCH_STATE_REST : TOUCH_STATE_SCROLLING;
240        break;
241      case MotionEvent.ACTION_CANCEL:
242      case MotionEvent.ACTION_UP:
243        mTouchState = TOUCH_STATE_REST;
244        break;
245    }
246    return mTouchState != TOUCH_STATE_REST;
247  }
248
249  /**
250   * 设置屏幕切换监听器
251   * 
252   * @param listener
253   */
254  public void SetOnViewChangeListener(OnViewChangeListener listener) {
255    mOnViewChangeListener = listener;
256  }
257
258  /**
259   * 屏幕切换监听器
260   * 
261   * @author liux
262   */
263  public interface OnViewChangeListener {
264    public void OnViewChange(int view);
265  }
266}

下面我就根据我的理解讲述一下整个滑动的过程

首先是整个组建的初始化，调用构造函数（这个就不详细说了）

调用onMeasure(int widthMeasureSpec, int heightMeasureSpec)函数，循环计算没一个子View的宽高。

调用onLayout(boolean changed, int l, int t, int r, int b)设置每一个子View的布局

调用onDraw方法开始画图。在调用onDraw方法时候会调用computeScroll()方法，假如正在滑动的时候就调用使ViewscrollTo方法滑动，然后调用postInvalidate更新界面。

用户滑动的时候，会先到用onInterceptTouchEvent(MotionEvent ev)，第一次ACTION_DOWN的时候，onInterceptTouchEvent方法会返回false，表示ViewGroup不拦截这个之后的手势（即这一系列的手势会传到其子View），然后会触发ScrollLayout的onTouchEvent(MotionEvent event)方法（子View的onTouchEvent均返回false的情况下）。

onTouchEvent(MotionEvent event)方法中

ACTION_DOWN事件：

记下事件的坐标。

ACTION_MOVE事件：

计算滑动的距离，当水平华东大于200,垂直滑动大于10时候组建滑动。

ACTION_UP事件：

使用VelocityTracker计算滑动的速度，当速度大于600 千像素/微秒 时候，根据速度方向滑动。然后注销VelocityTracker。当速度达不到的时候，调用snapToDestination()方法判断应该返回到那一个页面。

参考链接

onMeasure和onLayout：http://blog.csdn.net/czh0766/article/details/5846460

Scroller：http://www.cnblogs.com/over140/archive/2010/12/16/1907528.html，http://ipjmc.iteye.com/blog/1615828

onInterceptTouchEvent和onTouchEvent：http://blog.csdn.net/ddna/article/details/5473293