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ViewPager的setOnPageChangeListener方法详解

2017-07-28 14:38 281 查看

参考：

ViewPager的setOnPageChangeListener的三个方法的执行顺序

先看ViewPager 源码：

public class ViewPager extends ViewGroup {

/**
* 页面停滞，无动画效果
*/
public static final int SCROLL_STATE_IDLE = 0;

/**
* 页面正被用户拖拽
*/
public static final int SCROLL_STATE_DRAGGING = 1;

/**
* 页面已设置成当前页面
*/
public static final int SCROLL_STATE_SETTLING = 2;

public interface OnPageChangeListener {

/**
*
* 该方法在当前页面滚动，或部分启动的平滑滚动或用户启动的触摸滚动时将被调用。
*
* 参数position ：当前显示的第一个页面的位置position，如果第二个参数positionOffset不为零，则position+1页面将可见
*
* 参数positionOffset ：从当前页面偏移的值，值得范围 [0, 1)，
*                     左滑成功从0到1，下一页设置成功就会立即由无限接近1变成0，
*                     右滑成功从1到0。
*
* 参数positionOffsetPixels ：从当前位置偏移的像素值
*                          左滑成功从0开始，下一页设置成功时又会立即重置变为0
*                          右滑从位置（像素单位）开始减少一直到0。
*/
void onPageScrolled(int position, float positionOffset, int positionOffsetPixels);

/**
* 当新页面设置成功时被调用，动画可能还未完成。即：代表哪个页面已经确定被选中。
*
* 滚动下一页失败时，不会调用
*
* 参数： 新页面的position
*/
void onPageSelected(int position);

/**
* 滚动状态改变时调用，可发现用户开始拖拽、设置成当前页面、停滞的时机。
*
* 参数： 滚动的状态state
* @see ViewPager#SCROLL_STATE_IDLE==0，页面滚动后停滞
* @see ViewPager#SCROLL_STATE_DRAGGING==1，拖拽
* @see ViewPager#SCROLL_STATE_SETTLING==2，已确定设置哪个页面
*/
void onPageScrollStateChanged(int state);
}
}


三个方法调用顺序测试：

mViewPager.addOnPageChangeListener(new ViewPager.OnPageChangeListener() {

@Override
public void onPageSelected(int position) {
Log.e("111", "position===" + position);
}

@Override
public void onPageScrolled(int position, float positionOffset, int positionOffsetPixels) {
Log.e("222", "position===" + position +
"    positionOffset==" + positionOffset +
"     positionOffsetPixels==" + positionOffsetPixels);
}

@Override
public void onPageScrollStateChanged(int state) {
Log.e("333", "state===" + state);
}
});


setCurrentItem时：调用如下

//onPageSelected
position==0

//onPageScrolled
position==0   positionOffset==0.0    positionOffsetPixels==0


成功左滑一页

用手指拖动翻页时，最先执行一遍onPageScrollStateChanged（1），

然后不断执行onPageScrolled，

放手指的时候，直接立即执行一次onPageScrollStateChanged（2），

然后立即执行一次onPageSelected，

然后再不断执行onPageScrolled，

最后执行一次onPageScrollStateChanged（0）。

333: state===1  //手按住开始拖拽

222: position===5000    positionOffset==0.017105263     positionOffsetPixels==13
222: position===5000    positionOffset==0.12105264     positionOffsetPixels==92
222: position===5000    positionOffset==0.16973683     positionOffsetPixels==129
222: position===5000    positionOffset==0.21052632     positionOffsetPixels==160

333: state===2  //已确定页面，滑动下一页成功

111: position===5001    //已确定下一页，页面滑动动画还在继续

222: position===5000    positionOffset==0.35526314     positionOffsetPixels==270
222: position===5000    positionOffset==0.4881579     positionOffsetPixels==371
222: position===5000    positionOffset==0.5973684     positionOffsetPixels==454
222: position===5000    positionOffset==0.7065789     positionOffsetPixels==537
222: position===5000    positionOffset==0.7947368     positionOffsetPixels==604
222: position===5000    positionOffset==0.8592105     positionOffsetPixels==653
222: position===5000    positionOffset==0.9078947     positionOffsetPixels==689
222: position===5000    positionOffset==0.9394737     positionOffsetPixels==714
222: position===5000    positionOffset==0.96315783     positionOffsetPixels==731
222: position===5000    positionOffset==0.97894734     positionOffsetPixels==744
222: position===5000    positionOffset==0.98815787     positionOffsetPixels==751
222: position===5000    positionOffset==0.9947368     positionOffsetPixels==755
222: position===5000    positionOffset==0.9973684     positionOffsetPixels==758
222: position===5000    positionOffset==0.99868417     positionOffsetPixels==758
222: position===5001    positionOffset==0.0     positionOffsetPixels==0
// 设置下一页 position=5001 成功，位置偏移重置。

333: state===0      //设置下一页成功，页面停滞。


左滑一页失败：

333: state===1  //手按住开始拖拽

222: position===5001    positionOffset==0.031578917     positionOffsetPixels==23
222: position===5001    positionOffset==0.06842106     positionOffsetPixels==52
222: position===5001    positionOffset==0.19999996     positionOffsetPixels==151
222: position===5001    positionOffset==0.2105263     positionOffsetPixels==159
222: position===5001    positionOffset==0.21710522     positionOffsetPixels==164
222: position===5001    positionOffset==0.25263155     positionOffsetPixels==191
222: position===5001    positionOffset==0.25921047     positionOffsetPixels==196
222: position===5001    positionOffset==0.2657894     positionOffsetPixels==201
222: position===5001    positionOffset==0.27105263     positionOffsetPixels==206
222: position===5001    positionOffset==0.34605262     positionOffsetPixels==263
222: position===5001    positionOffset==0.36052626     positionOffsetPixels==273
222: position===5001    positionOffset==0.36184207     positionOffsetPixels==274

333: state===2  //已确定页面，滑动下一页失败，开始返回。

222: position===5001    positionOffset==0.35526314     positionOffsetPixels==270
222: position===5001    positionOffset==0.2921052     positionOffsetPixels==221
222: position===5001    positionOffset==0.25657886     positionOffsetPixels==194
222: position===5001    positionOffset==0.111842126     positionOffsetPixels==85
222: position===5001    positionOffset==0.09342106     positionOffsetPixels==71
222: position===5001    positionOffset==0.0526316     positionOffsetPixels==40
222: position===5001    positionOffset==0.04210526     positionOffsetPixels==31
222: position===5001    positionOffset==0.03421053     positionOffsetPixels==26
222: position===5001    positionOffset==0.027631609     positionOffsetPixels==21
222: position===5001    positionOffset==0.021052573     positionOffsetPixels==15
222: position===5001    positionOffset==0.0039473083     positionOffsetPixels==2
222: position===5001    positionOffset==0.002631501     positionOffsetPixels==1
222: position===5001    positionOffset==0.001315807     positionOffsetPixels==1
222: position===5001    positionOffset==0.0     positionOffsetPixels==0

333: state===0      //返回成功，页面停滞。


右滑一页成功：

333: state===1  //按住拖动

222: position===4999    positionOffset==0.82236844     positionOffsetPixels==625
222: position===4999    positionOffset==0.6657895     positionOffsetPixels==506
222: position===4999    positionOffset==0.33289477     positionOffsetPixels==253
222: position===4999    positionOffset==0.29473686     positionOffsetPixels==224

333: state===2  //确定页面

111: position===4999    //

222: position===4999    positionOffset==0.26184216     positionOffsetPixels==199
222: position===4999    positionOffset==0.16578948     positionOffsetPixels==126
222: position===4999    positionOffset==0.06973687     positionOffsetPixels==53
222: position===4999    positionOffset==0.0263158     positionOffsetPixels==20
222: position===4999    positionOffset==0.007894729     positionOffsetPixels==5
222: position===4999    positionOffset==0.001315807     positionOffsetPixels==1
222: position===4999    positionOffset==0.0     positionOffsetPixels==0

333: state===0  //停滞


右滑一页失败：

333: state===1  //按住拖动

222: position===4998    positionOffset==0.9828947     positionOffsetPixels==747
222: position===4998    positionOffset==0.94605273     positionOffsetPixels==719
222: position===4998    positionOffset==0.83157897     positionOffsetPixels==632
222: position===4998    positionOffset==0.8092106     positionOffsetPixels==615
222: position===4998    positionOffset==0.77236843     positionOffsetPixels==587
222: position===4998    positionOffset==0.7631579     positionOffsetPixels==580
222: position===4998    positionOffset==0.7539474     positionOffsetPixels==573
222: position===4998    positionOffset==0.74473685     positionOffsetPixels==566
222: position===4998    positionOffset==0.731579     positionOffsetPixels==556
222: position===4998    positionOffset==0.71842116     positionOffsetPixels==546
222: position===4998    positionOffset==0.7131579     positionOffsetPixels==542
222: position===4998    positionOffset==0.7118421     positionOffsetPixels==541
222: position===4998    positionOffset==0.7118421     positionOffsetPixels==541
222: position===4998    positionOffset==0.7105263     positionOffsetPixels==540
222: position===4998    positionOffset==0.7105263     positionOffsetPixels==540
222: position===4998    positionOffset==0.7078948     positionOffsetPixels==538
222: position===4998    positionOffset==0.70657897     positionOffsetPixels==537
222: position===4998    positionOffset==0.70657897     positionOffsetPixels==537

283 333: state===2      //确定页面

222: position===4998    positionOffset==0.7144737     positionOffsetPixels==543
222: position===4998    positionOffset==0.83289474     positionOffsetPixels==633
222: position===4998    positionOffset==0.8578948     positionOffsetPixels==652
222: position===4998    positionOffset==0.88026315     positionOffsetPixels==669
222: position===4998    positionOffset==0.92894745     positionOffsetPixels==706
222: position===4998    positionOffset==0.9421053     positionOffsetPixels==716
222: position===4998    positionOffset==0.96052635     positionOffsetPixels==730
222: position===4998    positionOffset==0.9684211     positionOffsetPixels==736
222: position===4998    positionOffset==0.975     positionOffsetPixels==741
222: position===4998    positionOffset==0.98026323     positionOffsetPixels==745
222: position===4998    positionOffset==0.9934211     positionOffsetPixels==755
222: position===4998    positionOffset==0.9973685     positionOffsetPixels==758
222: position===4998    positionOffset==0.99868417     positionOffsetPixels==758
222: position===4999    positionOffset==0.0     positionOffsetPixels==0

333: state===0  //停滞
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