从 Activity 访问 ViewPager Fragment 方法 [英] Access ViewPager Fragment method from Activity
问题描述
这是我的场景:
我的 Activity 中有 ViewPager，它承载了 6 个 Fragment.我通过用手指滑动来禁用分页，所以每当我想滑动时，我都会使用相关按钮和 :
I have ViewPager in my Activity which hosts 6 Fragment. I disabled paging by swiping with finger, so whenever I want to swipe I use related button and :
viewPager.setCurrentItem(viewPager.getCurrentItem() + 1, true);
在滑动的每个片段中(滑动完成后)，我想向我的服务器发送一个 GET 请求并获取一些数据并将其显示在该片段中.这样做:
In each fragment that is swiped (after swipe is finished) I want to send a GET request to my server and fetch some data and show it in that fragment. for doing that:
第一种方法:我在我的片段中使用了这段代码，它会在片段变得可见时立即运行:
First Approach : I used this code in my fragments which runs as soon as fragment become visible:
@Override
public void setUserVisibleHint(boolean isVisibleToUser) {
super.setUserVisibleHint(isVisibleToUser);
if(isVisibleToUser)
{
sendGetRequest();
}
}
但是，这里有一个问题:setUserVisibleHint 恰好在片段可见时执行，因此滑动动画会出现一些滞后(不够流畅).
But , here was a problem : that setUserVisibleHint executes exactly whene the fragment visible , and because of that the animation of swiping came with some lag(it wasn't smooth enough).
所以我使用了第二种方法:我在托管活动中向 ViewPager 添加了一个 OnPageChangeListener()，如下所示:
So I used Second Approach :
I added an OnPageChangeListener() to ViewPager in hosted activity like this :
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();
}
}
}
});
效果很好，只要滑动完成，toast 就会显示，但与完全可见的片段不同，当 sendGetRequest() 运行时，我得到 NullPointerException.
It works great, and toast shows as soon as swipe finished, but unlike fragment which visible completely , when sendGetRequest() runs i get NullPointerException.
这里是堆栈跟踪:
here is 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)
任何想法都会很棒.最好的问候
any idea would be great. best regards
答案:Daniel Nugent 的代码运行良好，除了在我使用的发送 GET 请求期间显示 ProgressDialog 之外:
Answer :
Daniel Nugent's code worked well , beside for showing ProgressDialog during sending GET request i used :
final ProgressDialog dialog = new ProgressDialog(new MainActivity());
此代码也返回 NullPointerException，我将其删除，现在运行良好.
this code returned NullPointerException too , i removed it and now working well.
推荐答案
使用 ViewPager.OnPageChangeListener 是正确的方法，但您需要稍微重构一下您的适配器以保持对包含在 FragmentPagerAdapter 中的每个 Fragment 的引用.
Using the ViewPager.OnPageChangeListener is the correct way to go, but you will need to refactor your adapter a bit in order to keep a reference to each Fragment contained in the FragmentPagerAdapter.
您可以使用适配器中的 instantiateItem() 方法覆盖来执行此操作，这里是一个简化示例:
You do that using the instantiateItem() method override in the adapter, here is a simplified example:
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，不要创建一个新的 Fragment，而是使用包含在适配器中的那个:
Then, instead of creating a new Fragment, use the one contained in the adapter:
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，您可以实现一个定义 sendGetRequest() 的接口，并在您的每个 Fragment 类中实现 sendGetRequest() 方法.
Note that if you're using different Fragment classes in your adapter, you can implement an interface that defines sendGetRequest(), and in each of your Fragment classes implement the sendGetRequest() method.
如果您不采用接口方法，则需要将 Fragment 转换为您自己的 Fragment 类型，如上例所示，即:
If you don't go with the interface approach, you will need to cast the Fragment to your own Fragment type as shown in the example above, i.e.:
if (frag instanceof FragmentTwo) {
((FragmentTwo)frag).sendGetRequest();
}
更新
对于使用 ViewPager2 和 Kotlin，如下所示:
For using ViewPager2 and Kotlin, here is how it would look:
viewPager.registerOnPageChangeCallback(
object: ViewPager2.OnPageChangeCallback() {
override fun onPageSelected(position: Int) {
super.onPageSelected(position)
val frag: Fragment = mAdapter.fragments[position]
if (frag != null && frag is FragmentTwo) {
(frag as FragmentTwo).sendGetRequest()
}
}
}
)
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