java-刷新ViewPager片段中的数据
作者：互联网
这是我的情况：
我的Activity中有ViewPager,该ViewPager承载6个Fragment.我通过用手指轻扫来禁用分页,所以每当我想滑动时,我都会使用相关的按钮和：
viewPager.setCurrentItem(viewPager.getCurrentItem() + 1, true);
在要刷卡的每个片段中(刷卡完成后),我想向服务器发送一个GET请求,并获取一些数据并在该片段中显示它.为此：
第一种方法：我在片段中使用了以下代码,该代码在片段可见后立即运行：
@Override
public void setUserVisibleHint(boolean isVisibleToUser) {
super.setUserVisibleHint(isVisibleToUser);
if(isVisibleToUser)
{
sendGetRequest();
}
}
但是,这是一个问题：setUserVisibleHint准确地在可见片段时执行,并且由于滑动动画带有一定的滞后性(不够流畅).
所以我用了第二种方法：
我在托管活动中向ViewPager添加了一个OnPageChangeListener(),如下所示：
viewPager.addOnPageChangeListener(new ViewPager.OnPageChangeListener() {
int CurrentPossition = 0;
@Override
public void onPageScrolled(int position, float positionOffset, int positionOffsetPixels) { }
@Override
public void onPageSelected(int position) {
CurrentPossition = position;
}
@Override
public void onPageScrollStateChanged(int state) {
if(state == ViewPager.SCROLL_STATE_IDLE && CurrentPossition != 0){
Toast.makeText(getBaseContext(),"finished" , Toast.LENGTH_SHORT).show();
try{
new fragment_two().sendGetRequest();;
}catch(Exception ex){
ex.printStackTrace();
}
}
}
});
它运行良好,并且吐司一扫就显示,但是与完全可见的片段不同,当sendGetRequest()运行时,我得到NullPointerException.
这是StackTrace：
04-08 20:15:37.840 12848-12848/com.example.mohamad.travelagency W/System.err: java.lang.NullPointerException
04-08 20:15:37.850 12848-12848/com.example.mohamad.travelagency W/System.err: at android.content.ContextWrapper.getApplicationInfo(ContextWrapper.java:152)
04-08 20:15:37.850 12848-12848/com.example.mohamad.travelagency W/System.err: at android.view.ContextThemeWrapper.getTheme(ContextThemeWrapper.java:103)
04-08 20:15:37.850 12848-12848/com.example.mohamad.travelagency W/System.err: at android.app.AlertDialog.resolveDialogTheme(AlertDialog.java:143)
04-08 20:15:37.850 12848-12848/com.example.mohamad.travelagency W/System.err: at android.app.AlertDialog.<init>(AlertDialog.java:98)
04-08 20:15:37.850 12848-12848/com.example.mohamad.travelagency W/System.err: at android.app.ProgressDialog.<init>(ProgressDialog.java:77)
04-08 20:15:37.850 12848-12848/com.example.mohamad.travelagency W/System.err: at com.example.mohamad.travelagency.fragment_two.GetServetData_L1(fragment_two.java:458)
04-08 20:15:37.850 12848-12848/com.example.mohamad.travelagency W/System.err: at com.example.mohamad.travelagency.MainActivity$1.onPageScrollStateChanged(MainActivity.java:124)
04-08 20:15:37.860 12848-12848/com.example.mohamad.travelagency W/System.err: at android.support.v4.view.ViewPager.dispatchOnScrollStateChanged(ViewPager.java:1811)
04-08 20:15:37.860 12848-12848/com.example.mohamad.travelagency W/System.err: at android.support.v4.view.ViewPager.setScrollState(ViewPager.java:404)
04-08 20:15:37.860 12848-12848/com.example.mohamad.travelagency W/System.err: at android.support.v4.view.ViewPager.access$000(ViewPager.java:91)
04-08 20:15:37.860 12848-12848/com.example.mohamad.travelagency W/System.err: at android.support.v4.view.ViewPager$3.run(ViewPager.java:250)
04-08 20:15:37.860 12848-12848/com.example.mohamad.travelagency W/System.err: at android.view.Choreographer$CallbackRecord.run(Choreographer.java:761)
04-08 20:15:37.860 12848-12848/com.example.mohamad.travelagency W/System.err: at android.view.Choreographer.doCallbacks(Choreographer.java:574)
04-08 20:15:37.860 12848-12848/com.example.mohamad.travelagency W/System.err: at android.view.Choreographer.doFrame(Choreographer.java:543)
04-08 20:15:37.860 12848-12848/com.example.mohamad.travelagency W/System.err: at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:747)
04-08 20:15:37.860 12848-12848/com.example.mohamad.travelagency W/System.err: at android.os.Handler.handleCallback(Handler.java:733)
04-08 20:15:37.860 12848-12848/com.example.mohamad.travelagency W/System.err: at android.os.Handler.dispatchMessage(Handler.java:95)
04-08 20:15:37.860 12848-12848/com.example.mohamad.travelagency W/System.err: at android.os.Looper.loop(Looper.java:136)
04-08 20:15:37.870 12848-12848/com.example.mohamad.travelagency W/System.err: at android.app.ActivityThread.main(ActivityThread.java:5271)
04-08 20:15:37.870 12848-12848/com.example.mohamad.travelagency W/System.err: at java.lang.reflect.Method.invokeNative(Native Method)
04-08 20:15:37.870 12848-12848/com.example.mohamad.travelagency W/System.err: at java.lang.reflect.Method.invoke(Method.java:515)
04-08 20:15:37.870 12848-12848/com.example.mohamad.travelagency W/System.err: at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:851)
04-08 20:15:37.870 12848-12848/com.example.mohamad.travelagency W/System.err: at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:667)
04-08 20:15:37.870 12848-12848/com.example.mohamad.travelagency W/System.err: at dalvik.system.NativeStart.main(Native Method)
任何想法都很棒.
最好的祝福
答：
Daniel Nugent的代码很好用,除了在我使用的发送GET请求期间显示ProgressDialog之外：
final ProgressDialog dialog = new ProgressDialog(new MainActivity());
这段代码也返回了NullPointerException,我将其删除,并且现在运行良好.
解决方法:
使用ViewPager.OnPageChangeListener是正确的方法,但是您将需要稍微重构适配器以保留对FragmentPagerAdapter中包含的每个Fragment的引用.
您可以使用适配器中的InstantiateItem()方法重写来做到这一点,这是一个简化的示例：
class PagerAdapter extends FragmentPagerAdapter {
String tabTitles[] = new String[] { "One", "Two", "Three", "Four"};
Context context;
//This will contain your Fragment references:
public Fragment[] fragments = new Fragment[tabTitles.length];
public PagerAdapter(FragmentManager fm, Context context) {
super(fm);
this.context = context;
}
@Override
public int getCount() {
return tabTitles.length;
}
@Override
public Fragment getItem(int position) {
switch (position) {
case 0:
return new FragmentOne();
case 1:
return new FragmentTwo();
case 2:
return new FragmentThree();
case 3:
return new FragmentFour();
}
return null;
}
//This populates your Fragment reference array:
@Override
public Object instantiateItem(ViewGroup container, int position) {
Fragment createdFragment = (Fragment) super.instantiateItem(container, position);
fragments[position] = createdFragment;
return createdFragment;
}
@Override
public CharSequence getPageTitle(int position) {
// Generate title based on item position
return tabTitles[position];
}
}
然后,不要使用适配器中包含的片段来创建新的片段：
mViewPager.addOnPageChangeListener(new ViewPager.OnPageChangeListener() {
@Override
public void onPageScrolled(int position, float positionOffset, int positionOffsetPixels) { }
@Override
public void onPageSelected(int position) {
// do this instead, assuming your adapter reference
// is named mAdapter:
Fragment frag = mAdapter.fragments[position];
if (frag != null && frag instanceof FragmentTwo) {
((FragmentTwo)frag).sendGetRequest();
}
}
@Override
public void onPageScrollStateChanged(int state) { }
});
请注意,如果您在适配器中使用不同的Fragment类,则可以实现定义sendGetRequest()的接口,并且在每个Fragment类中都实现sendGetRequest()方法.
如果不采用接口方法,则需要将Fragment转换为自己的Fragment类型,如上面的示例所示,即：
if (frag instanceof FragmentTwo) {
((FragmentTwo)frag).sendGetRequest();
}
标签：android-fragments,android-viewpager,java,android 来源： https://codeday.me/bug/20191118/2031119.html