从活动中访问ViewPager片段方法

Question

这是我的场景：

我在我的活动中有ViewPager，其中包含6个片段。我用手指滑动来禁用分页，所以每当我想滑动时，我使用相关的按钮和：

viewPager.setCurrentItem(viewPager.getCurrentItem() + 1, true);

在每一个swipe片段中（swipe完成后），我希望向服务器发送一个GET请求，并获取一些数据并在该片段中显示它。因为这样做：

@Override
    public void setUserVisibleHint(boolean isVisibleToUser) {
        super.setUserVisibleHint(isVisibleToUser);
        if(isVisibleToUser)
        {
          sendGetRequest();
        }
}

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();
                    }
                }
            }
        });

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)

答：Daniel Nugent的代码运行良好，除了在发送GET请求时显示ProgressDialog之外，我使用了:

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

这段代码也返回NullPointerException，我删除了它，现在运行良好。

Answer 1

使用viewpager.onpageChangelistener是正确的方法，但是您需要对适配器进行一些重构，以便保留对FragmentPagerAdapter中包含的每个片段的引用。

您可以使用适配器中的instantiateItem()方法重写来完成此操作，下面是一个简化的示例：

 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，不创建新片段，而是使用适配器中包含的片段：

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) {  }
});

if (frag instanceof FragmentTwo) {
    ((FragmentTwo)frag).sendGetRequest();
}

    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()
                    }
                }
            }
    )