★理论知识
●View在屏幕上显示出来要先经过measure（计算）和layout（布局）。
◆当控件的父元素正要放置该控件时调用，父元素会问子控件“你想要用多大的地方？”，然后传入两个参数widthMeasureSpec和heightMeasureSpec，这两个参数指明控件可获得的空间以及关于这个空间描述的元数据。
◆更好的方法是你传递View的高度和宽度到setMeasureDimension方法里，这样可以直接告诉父控件需要多大地方放置子控件。
◆示例：
@Override
protected void onMeasure(int widthMeasureSpec,int heightMeasureSpec){
int measuredHeight=measureHeight(heightMeasureSpec);
int measuredWidth=measureWidth(widthMeasureSpec);
setMeasuredDimension(measureHeight,measuredWidth);
}
private int measureHeight(int measureSpec){
//Return measured widget height. int specMode=MeasureSpec.getMode(measureSpec);
int specSize=MeasureSpec.getSize(measureSpec);
int result=0;
if(specMode == MeasureSpec.AT_MOST){
result=specSize;
}else if(specMode==MeasureSpec.EXACTLY){
result=specSize;
}
return result;
}
private int measureWidth(int measureSpec){
//Return measured widget width. int specMode=MeasureSpec.getMode(measureSpec);
int specSize=MeasureSpec.getSize(measureSpec);
int result=0;
if(specMode == MeasureSpec.AT_MOST){
result=specSize;
}else if(specMode==MeasureSpec.EXACTLY){
result=specSize;
}
return result;
}
边界参数widthMeasureSpec和heightMeasureSpec效率原因以整数的方式传入。在使用之前首先要做的是使用MeasureSpec类的静态方法getMode和getSize来译解。
int specMode=MeasureSpec.getMode(measureSpec);
int specSize=MeasureSpec.getSize(measureSpec);
根据specMode的值（MeasureSpec有3种模式分别是UNSPECIFIED、EXACTLY、AT_MOST）
如果是AT_MOST，specSize代表的是最大可获得的空间
如果是EXACTLY，specSize代表的是精确的尺寸
如果是UNSPECIFIED，对于控件尺寸来说，没有任何参考意义
◆这些模式和设置的layout参数的fill_parent、wrap_content是什么关系呢？
当设置width或height为fill_parent时，容器在布局时调用子view的measure方法，传入的模式是EXACTLY，因为子view会占据剩余容器的空间，所以它大小是确定的。当设置为wrap_content时，容器传进去的是AT_MOST，表示子view的大小最多是多少，这样子view会根据上限来设置自己的尺寸。当子view的大小设置为精确值时，容器传入的是EXACTLY。
●VelocityTracker用来追踪触摸事件（flinging事件和其他手势事件）的速率。
◆用obtain()函数来获得类的实例，用addMovement(MotionEvent)函数将motion event加入到VelocityTracker类实例中，当你使用到速率时，使用computeCurrentVelocity(int)初始化速率的单位，并获得当前的事件的速率，然后使用getXVelocity()或getYVelocity()获得横向和竖向的速率。
◆VelocityTracker.computeCurrentVelocity(int units,float maxVelocity)用来计算已经发生触摸事件点的当前速率。这个函数只有在你需要得到速率消息的情况下才调用，因为使用它需要消耗很大的性能。参数units为速率单位，值为1的意思是以一毫秒运动了多少个像素的速率，1000表示一秒时间内运动了多少个像素。maxVelocity代表这个方法能计算出事件的最大速率，值和units的值具有一样的单位。
◆示例：
private VelocityTracker velocityTracker;
//在onTouchEvent(MotionEvent ev)中
if(velocityTracker==null){
velocityTracker=VelocityTracker.obtain();//获得VelocityTracker类实例
}
velocityTracker.addMovement(ev);//将事件加入到VelocityTracker类实例中
★实现代码：
import java.util.ArrayList;import java.util.List;import android.content.Context;import android.graphics.Canvas;import android.graphics.Color;import android.os.Handler;import android.os.Message;import android.view.MotionEvent;import android.view.VelocityTracker;import android.view.View;import android.view.ViewConfiguration;import android.view.ViewGroup;import android.widget.LinearLayout;import android.widget.TextView;public class TouchPageTransfer extends ViewGroup { /**
* 手势临界速度，当速度超过这个时切换到下一屏
*/ private final static int SNAP_VELOCITY = 100; /**
* 停止状态
*/ private final static int TOUCH_STATE_REST = 0; /**
* 滚动状态
*/ private final static int TOUCH_STATE_MOVING = 1; /**
* 减速停止状态
*/ private final static int TOUCH_STATE_SLOWING = 2; /**
* 移动的阈值
*/ private static final int TOUCH_SLOP = 10; private Context context; /**
* 速度跟踪
*/ private VelocityTracker velocityTracker; /**
* 速度最大值
*/ private int maximumVelocity; /**
* 当前触摸状态
*/ private int touchState = TOUCH_STATE_REST; /**
* 触摸起始点坐标
*/ private float lastMotionX; private float lastMotionY; /**
* 是否移动了
*/ private boolean isMoved; private List<LinearLayout> views; private LinearLayout view1; private LinearLayout view2; private LinearLayout view3; public TouchPageTransfer(Context context) { super(context); // TODO Auto-generated constructor stub
this.context = context; init(); } private void init() { views = new ArrayList<LinearLayout>(); view1 = new LinearLayout(context); view1.setBackgroundColor(Color.YELLOW); this.addView(view1); TextView tv = new TextView(context); tv.setText("测试"); view1.addView(tv); views.add(view1); view2 = new LinearLayout(context); view2.setBackgroundColor(Color.RED); this.addView(view2); views.add(view2); view3 = new LinearLayout(context); view3.setBackgroundColor(Color.GREEN); this.addView(view3); views.add(view3); final ViewConfiguration configuration = ViewConfiguration .get(getContext()); maximumVelocity = configuration.getScaledMaximumFlingVelocity(); } @Override protected void onLayout(boolean changed, int l, int t, int r, int b) { // TODO Auto-generated method stub
int viewLeft = -1; int count = views.size(); //水平从左到右放置View
for (int i = 0; i < count; i++) { View view = views.get(i); if (view.getVisibility() != View.GONE) {//View处于显示状态
int viewWidth = view.getMeasuredWidth(); if (viewLeft == -1) { viewLeft = -viewWidth; } view.layout(viewLeft, 0, viewLeft + viewWidth, view.getMeasuredHeight()); viewLeft += viewWidth; } } } @Override protected void onDraw(Canvas canvas) { //水平从左到右放置
int count = views.size(); for (int i = 0; i < count; i++) { View view = views.get(i); drawChild(canvas, view, getDrawingTime()); } } @Override protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) { // TODO Auto-generated method stub
super.onMeasure(widthMeasureSpec, heightMeasureSpec); int count = views.size(); for (int i = 0; i < count; i++) { View view = views.get(i); view.measure(widthMeasureSpec, heightMeasureSpec); } } @Override public boolean onTouchEvent(MotionEvent event) { if (velocityTracker == null) { velocityTracker = VelocityTracker.obtain(); } velocityTracker.addMovement(event); int action = event.getAction(); float x = event.getX(); float y = event.getY(); switch (action) { case MotionEvent.ACTION_DOWN: if (touchState == TOUCH_STATE_REST) { //记录按下去的坐标
lastMotionX = x; lastMotionY = y; //改变触摸状态
touchState = TOUCH_STATE_MOVING; isMoved = false; } break; case MotionEvent.ACTION_MOVE: if (touchState == TOUCH_STATE_MOVING) { //计算位移量，向左为负，向右为正
float offsetX = x - lastMotionX; float offsetY = y - lastMotionY; if (isMoved) { lastMotionX = x; lastMotionY = y; int count = views.size(); for (int i = 0; i < count; i++) { View view = views.get(i); if (view.getVisibility() != View.GONE) {//对于显示的View才进行布局
int viewWidth = view.getMeasuredWidth(); int viewLeft = view.getLeft() + (int) offsetX; view.layout(viewLeft, 0, viewLeft + viewWidth, view.getMeasuredHeight()); viewLeft += viewWidth; } } } else if (Math.abs(offsetX) > TOUCH_SLOP || Math.abs(offsetY) > TOUCH_SLOP) {//移动超过阈值，则表示移动了
isMoved = true; } } break; case MotionEvent.ACTION_UP: if (isMoved) { if (touchState == TOUCH_STATE_MOVING) { touchState = TOUCH_STATE_SLOWING; int sign = 0;//标志有足够能力移动的移动方向，向左为1，向右为-1，否则为0
//计算当前速度
velocityTracker.computeCurrentVelocity(1000, maximumVelocity); //x方向速度
int velocityX = (int) velocityTracker.getXVelocity(); if (velocityX > SNAP_VELOCITY) {//足够的能力向左
sign = 1; } else if (velocityX < -SNAP_VELOCITY) {//足够的能力向右
sign = -1; } else { sign = 0; } moveToFitView(sign); if (velocityTracker != null) { velocityTracker.recycle();//回收VelocityTracker对象
velocityTracker = null; } } } break; default: break; } return true; } /**
* 总的位移量，向右为正，向左为负
*/ int tOffsetX = 0; private void moveToFitView(int sign) { swapView(sign); int left = views.get(1).getLeft(); tOffsetX = 0; if (left != 0) { tOffsetX = -left; } moveView(); } FlipAnimationHandler flipAnimationHandler; int ovv = 40;//每次预期moveView位移量
/**
* 移动View
*/ private void moveView() { int count = views.size(); if (tOffsetX != 0) { int ov = 0;//本次moveView位移量，向右为正，向左为负
if (tOffsetX > 0) {//向右
ov = ovv; } else {//向左
ov = -1 * ovv; } ovv -= 3; if (ovv < 1) { ovv = 3; } if (Math.abs(tOffsetX) < Math.abs(ov)) {//总位移量的绝对值小于本次位移量的绝对值
ov = tOffsetX; tOffsetX = 0; } else { tOffsetX -= ov; } //水平从左到右放置
for (int i = 0; i < count; i++) { View view = views.get(i); int viewWidth = view.getMeasuredWidth(); int viewLeft = view.getLeft() + ov; view.layout(viewLeft, 0, viewLeft + viewWidth, view.getMeasuredHeight()); viewLeft += viewWidth; } if (flipAnimationHandler == null) { flipAnimationHandler = new FlipAnimationHandler(); } flipAnimationHandler.sleep(1); } else { ovv = 40; touchState = TOUCH_STATE_REST; } } class FlipAnimationHandler extends Handler { @Override public void handleMessage(Message msg) { TouchPageTransfer.this.moveView();//
} public void sleep(long millis) { this.removeMessages(0); sendMessageDelayed(obtainMessage(0), millis); } } /**
* 向左移动时将原来最左边的View移到最右边，向右移动时将原来最右边的View移到最左边，并根据当前其他的View布局移动的这个View
* @param sign 1代表向左移动，-1代表向右移动，0代表不移动
* @return
*/ private boolean swapView(int sign) { boolean flag = false; if (sign == -1) {//向左
View view0 = views.get(0); if (view0.getLeft() <= -1 * view0.getMeasuredWidth()) { swapViewIndex(sign); View view1 = views.get(1); View view2 = views.get(2); int viewLeft = view1.getLeft() + view1.getMeasuredWidth(); view2.layout(viewLeft, 0, viewLeft + view2.getMeasuredWidth(), view2.getMeasuredHeight()); flag = true; } } else if (sign == 1) {//向右
View view2 = views.get(2); if (view2.getLeft() >= view2.getMeasuredWidth()) { swapViewIndex(sign); View view0 = views.get(0); View view1 = views.get(1); int viewLeft = view1.getLeft() - view0.getMeasuredWidth(); view0.layout(viewLeft, 0, viewLeft + view0.getMeasuredWidth(), view0.getMeasuredHeight()); flag = true; } } return flag; } /**
* 向左移动时将原来最左边的View移到最右边，向右移动时将原来最右边的View移到最左边
* @param sign 1代表向左移动，-1代表向右移动，0代表不移动
*/ private void swapViewIndex(int sign) { if (sign == -1) {//向左
LinearLayout v = views.remove(0); views.add(v); } else if (sign == 1) {//向右
LinearLayout v = views.remove(views.size() - 1); views.add(0, v); } }}
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