用手机淘宝浏览商品详情时，商品图片是放在后面的，在第一个ScrollView滚动到最底下时会有提示，继续拖动才能浏览图片。仿照这个效果写一个出来并不难，只要定义一个Layout管理两个ScrollView就行了，当第一个ScrollView滑到底部时，再次向上滑动进入第二个ScrollView。效果如下：
需要注意的地方是：
1、如果是手动滑到底部需要再次按下才能继续往下滑，自动滚动到底部则不需要
2、在由上一个ScrollView滑动到下一个ScrollView的过程中多只手指相继拖动也不会导致布局的剧变，也就是多个pointer的滑动不会导致move距离的剧变。
这个Layout的实现思路是：
在布局中放置两个ScrollView，并为其设置OnTouchListener，时刻判断ScrollView的滚动距离，一旦第一个ScrollView滚动到底部，则标识改为可向上拖动，此时开始记录滑动距离mMoveLen，根据mMoveLen重新layout两个ScrollView；同理，监听第二个ScrollView是否滚动到顶部，以往下拖动。
OK，明白了原理之后可以看代码了：
package com.jingchen.tbviewer;
import java.util.Timer;
import java.util.TimerTask;
import android.content.Context;
import android.os.Handler;
import android.os.Message;
import android.util.AttributeSet;
import android.view.MotionEvent;
import android.view.VelocityTracker;
import android.view.View;
import android.widget.RelativeLayout;
import android.widget.ScrollView;
public class ScrollViewContainer extends RelativeLayout {
public static final int AUTO_UP = 0;
public static final int AUTO_DOWN = 1;
public static final int DONE = 2;
public static final float SPEED = 6.5f;
private boolean isMeasured = false;
private VelocityTracker vt;
private int mViewHeight;
private int mViewWidth;
private View topView;
private View bottomView;
private boolean canPullDown;
private boolean canPullUp;
private int state = DONE;
private int mCurrentViewIndex = 0;
private float mMoveLen;
private MyTimer mTimer;
private float mLastY;
private int mEvents;
private Handler handler = new Handler() {
@Override
public void handleMessage(Message msg) {
if (mMoveLen != 0) {
if (state == AUTO_UP) {
mMoveLen -= SPEED;
if (mMoveLen <= -mViewHeight) {
mMoveLen = -mViewHeight;
state = DONE;
mCurrentViewIndex = 1;
}
} else if (state == AUTO_DOWN) {
mMoveLen += SPEED;
if (mMoveLen >= 0) {
mMoveLen = 0;
state = DONE;
mCurrentViewIndex = 0;
}
} else {
mTimer.cancel();
}
}
requestLayout();
}
};
public ScrollViewContainer(Context context) {
super(context);
init();
}
public ScrollViewContainer(Context context, AttributeSet attrs) {
super(context, attrs);
init();
}
public ScrollViewContainer(Context context, AttributeSet attrs, int defStyle) {
super(context, attrs, defStyle);
init();
}
private void init() {
mTimer = new MyTimer(handler);
}
@Override
public boolean dispatchTouchEvent(MotionEvent ev) {
switch (ev.getActionMasked()) {
case MotionEvent.ACTION_DOWN:
if (vt == null)
vt = VelocityTracker.obtain();
else
vt.clear();
mLastY = ev.getY();
vt.addMovement(ev);
mEvents = 0;
break;
case MotionEvent.ACTION_POINTER_DOWN:
case MotionEvent.ACTION_POINTER_UP:
// 多一只手指按下或抬起时舍弃将要到来的第一个事件move，防止多点拖拽的bug
mEvents = -1;
break;
case MotionEvent.ACTION_MOVE:
vt.addMovement(ev);
if (canPullUp && mCurrentViewIndex == 0 && mEvents == 0) {
mMoveLen += (ev.getY() - mLastY);
// 防止上下越界
if (mMoveLen > 0) {
mMoveLen = 0;
mCurrentViewIndex = 0;
} else if (mMoveLen < -mViewHeight) {
mMoveLen = -mViewHeight;
mCurrentViewIndex = 1;
}
if (mMoveLen < -8) {
// 防止事件冲突
ev.setAction(MotionEvent.ACTION_CANCEL);
}
} else if (canPullDown && mCurrentViewIndex == 1 && mEvents == 0) {
mMoveLen += (ev.getY() - mLastY);
// 防止上下越界
if (mMoveLen < -mViewHeight) {
mMoveLen = -mViewHeight;
mCurrentViewIndex = 1;
} else if (mMoveLen > 0) {
mMoveLen = 0;
mCurrentViewIndex = 0;
}
if (mMoveLen > 8 - mViewHeight) {
// 防止事件冲突
ev.setAction(MotionEvent.ACTION_CANCEL);
}
} else
mEvents++;
mLastY = ev.getY();
requestLayout();
break;
case MotionEvent.ACTION_UP:
mLastY = ev.getY();
vt.addMovement(ev);
vt.computeCurrentVelocity(700);
// 获取Y方向的速度
float mYV = vt.getYVelocity();
if (mMoveLen == 0 || mMoveLen == -mViewHeight)
break;
if (Math.abs(mYV) < 500) {
// 速度小于一定值的时候当作静止释放，这时候两个View往哪移动取决于滑动的距离
if (mMoveLen <= -mViewHeight / 2) {
state = AUTO_UP;
} else if (mMoveLen > -mViewHeight / 2) {
state = AUTO_DOWN;
}
} else {
// 抬起手指时速度方向决定两个View往哪移动
if (mYV < 0)
state = AUTO_UP;
else
state = AUTO_DOWN;
}
mTimer.schedule(2);
try {
vt.recycle();
} catch (Exception e) {
e.printStackTrace();
}
break;
}
super.dispatchTouchEvent(ev);
return true;
}
@Override
protected void onLayout(boolean changed, int l, int t, int r, int b) {
topView.layout(0, (int) mMoveLen, mViewWidth,
topView.getMeasuredHeight() + (int) mMoveLen);
bottomView.layout(0, topView.getMeasuredHeight() + (int) mMoveLen,
mViewWidth, topView.getMeasuredHeight() + (int) mMoveLen
+ bottomView.getMeasuredHeight());
}
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
super.onMeasure(widthMeasureSpec, heightMeasureSpec);
if (!isMeasured) {
isMeasured = true;
mViewHeight = getMeasuredHeight();
mViewWidth = getMeasuredWidth();
topView = getChildAt(0);
bottomView = getChildAt(1);
bottomView.setOnTouchListener(bottomViewTouchListener);
topView.setOnTouchListener(topViewTouchListener);
}
}
private OnTouchListener topViewTouchListener = new OnTouchListener() {
@Override
public boolean onTouch(View v, MotionEvent event) {
ScrollView sv = (ScrollView) v;
if (sv.getScrollY() == (sv.getChildAt(0).getMeasuredHeight() - sv
.getMeasuredHeight()) && mCurrentViewIndex == 0)
canPullUp = true;
else
canPullUp = false;
return false;
}
};
private OnTouchListener bottomViewTouchListener = new OnTouchListener() {
@Override
public boolean onTouch(View v, MotionEvent event) {
ScrollView sv = (ScrollView) v;
if (sv.getScrollY() == 0 && mCurrentViewIndex == 1)
canPullDown = true;
else
canPullDown = false;
return false;
}
};
class MyTimer {
private Handler handler;
private Timer timer;
private MyTask mTask;
public MyTimer(Handler handler) {
this.handler = handler;
timer = new Timer();
}
public void schedule(long period) {
if (mTask != null) {
mTask.cancel();
mTask = null;
}
mTask = new MyTask(handler);
timer.schedule(mTask, 0, period);
}
public void cancel() {
if (mTask != null) {
mTask.cancel();
mTask = null;
}
}
class MyTask extends TimerTask {
private Handler handler;
public MyTask(Handler handler) {
this.handler = handler;
}
@Override
public void run() {
handler.obtainMessage().sendToTarget();
}
}
}
}
注释写的很清楚了，有几个关键点需要讲一下：
1、由于这里为两个ScrollView设置了OnTouchListener，所以在其他地方不能再设置了，否则就白搭了。
2、两个ScrollView的layout参数统一由mMoveLen决定。
3、变量mEvents有两个作用：一是防止手动滑到底部或顶部时继续滑动而改变布局，必须再次按下才能继续滑动；二是在新的pointer down或up时把mEvents设置成-1可以舍弃将要到来的第一个move事件，防止mMoveLen出现剧变。为什么会出现剧变呢？因为假设一开始只有一只手指在滑动，记录的坐标值是这个pointer的事件坐标点，这时候另一只手指按下了导致事件又多了一个pointer，这时候到来的move事件的坐标可能就变成了新的pointer的坐标，这时计算与上一次坐标的差值就会出现剧变，变化的距离就是两个pointer间的距离。所以要把这个move事件舍弃掉，让mLastY值记录这个pointer的坐标再开始计算mMoveLen。pointer up的时候也一样。
理解了这几点，看起来就没什么难度了，代码量也很小。
MainActivity的布局：
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent" >
<com.jingchen.tbviewer.ScrollViewContainer
android:layout_width="match_parent"
android:layout_height="match_parent" >
<ScrollView
android:layout_width="match_parent"
android:layout_height="match_parent" >
<RelativeLayout
android:layout_width="wrap_content"
android:layout_height="wrap_content" >
<LinearLayout
android:id="@+id/imagesLayout"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:gravity="center_horizontal"
android:orientation="vertical" >
<ImageView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:background="@drawable/h" />
<ImageView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:background="@drawable/i" />
<ImageView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:background="@drawable/j" />
<ImageView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:background="@drawable/k" />
<ImageView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:background="@drawable/l" />
<ImageView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:background="@drawable/m" />
</LinearLayout>
<TextView
android:layout_width="match_parent"
android:layout_height="60dp"
android:layout_below="@id/imagesLayout"
android:background="#eeeeee"
android:gravity="center"
android:text="继续拖动，查看更多美女"
android:textSize="20sp" />
</RelativeLayout>
</ScrollView>
<ScrollView
android:layout_width="match_parent"
android:layout_height="match_parent"
android:background="#000000" >
<LinearLayout
android:layout_width="match_parent"
android:layout_height="match_parent"
android:gravity="center_horizontal"
android:orientation="vertical" >
<ImageView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:background="@drawable/a" />
<ImageView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:background="@drawable/b" />
<ImageView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:background="@drawable/c" />
<ImageView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:background="@drawable/d" />
<ImageView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:background="@drawable/e" />
<ImageView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:background="@drawable/f" />
<ImageView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:background="@drawable/g" />
</LinearLayout>
</ScrollView>
</com.jingchen.tbviewer.ScrollViewContainer>
</RelativeLayout>
在ScrollView中放了几张图片而已。
MainActivity的代码：
package com.jingchen.tbviewer;
import android.app.Activity;
import android.os.Bundle;
import android.view.Menu;
public class MainActivity extends Activity
{
@Override
protected void onCreate(Bundle savedInstanceState)
{
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
}
@Override
public boolean onCreateOptionsMenu(Menu menu)
{
getMenuInflater().inflate(R.menu.main, menu);
return true;
}
}
