刷新ViewPager Fragment中的数据 [英] Refresh data in ViewPager Fragment

问题描述

这是我的情景：

我的活动中有 ViewPager ，其中包含6个片段。我通过用手指轻扫来禁用分页，所以每当我想要滑动时我都使用相关按钮并且：

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);

在刷卡的每个片段中（刷卡完成后）我想发送 GET 请求我的服务器并获取一些数据并将其显示在该片段中。为此：

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，片段一旦可见就会运行：

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，这是一个问题： setUserVisibleHint 正好在片段可见的情况下执行，因此刷卡的动画带来了一些延迟（它不够平滑）。

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).

所以我使用第二种方法：
我在托管活动中向ViewPager添加了 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();
                    }
                }
            }
        });

它很好用，一旦刷卡完成就会显示吐司，但与完全可见的片段不同， sendGetRequest（）运行我得到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StackTrace：

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，除了在发送 GET期间显示ProgressDialog 请求我使用：

Answer : Daniel Nugent's code worked well , beside for showing ProgressDialog during sending GET request i used :

final ProgressDialog dialog = new ProgressDialog(new MainActivity());

此代码也返回了NullPointerException，我删除了它，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，但你需要重构你的适配器一点，以保持对FragmentPagerAdapter中包含的每个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.

使用 instantiateItem（）适配器中的方法覆盖，这是一个简化的例子：

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，不要创建新的片段，而是使用适配器中包含的一个：

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，如果在适配器中使用不同的Fragment类，则可以实现定义的接口 sendGetRequest（），并在每个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.

如果你不喜欢使用接口方法，您需要将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();
}

这篇关于刷新ViewPager Fragment中的数据的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！