防止重新创建导航抽屉片段

Question

最近我正在开发我的应用程序，以使其加载更快并更好地工作，我在我的 MainActivity 中使用导航抽屉：

    @Override
public boolean onNavigationItemSelected(MenuItem item) {

    // Handle navigation view item clicks here.
    int id = item.getItemId();

    if (id == R.id.nav_camara) {
        LoadJson asyncTask = (LoadJson) new LoadJson(new LoadJson.AsyncResponse() {
            @Override
            public void processFinish(JSONArray output) {
                //Here you will receive the result fired from async class
                //of onPostExecute(result) method.
                //Set the fragment initially
                MainFragment fragment = new MainFragment(output);
                FragmentTransaction fragmentTransaction =
                        getSupportFragmentManager().beginTransaction();
                fragmentTransaction.add(R.id.fragment_container, fragment);
                fragmentTransaction.commit();
                // Handle the camera action
            }
        }).execute();


    } else if (id == R.id.nav_gallery) {
        //Set the fragment initially
        GalleryFragment fragment = new GalleryFragment();
        FragmentTransaction fragmentTransaction =
                getSupportFragmentManager().beginTransaction();
        fragmentTransaction.replace(R.id.fragment_container, fragment);
        fragmentTransaction.commit();


    } else if (id == R.id.nav_search) {
        //Set the fragment initially
        FetchResualt fragment = new FetchResualt();
        FragmentTransaction fragmentTransaction =
                getSupportFragmentManager().beginTransaction();
        fragmentTransaction.replace(R.id.fragment_container, fragment);
        fragmentTransaction.commit();
        // Handle the camera action

    } else if (id == R.id.nav_manage) {//
         Bundle bundle = new Bundle();//
        bundle.putInt("someStr",ID_OF_BEACH);//
        //Set the fragment initially//
        FragmentBeach fragment = new FragmentBeach();//
        fragment.setArguments(bundle);//
        FragmentTransaction fragmentTransaction =//
                getSupportFragmentManager().beginTransaction();//
        fragmentTransaction.replace(R.id.fragment_container, fragment);//
        fragmentTransaction.commit();//
        // Handle the camera action

    } else if (id == R.id.nav_share) {

    } else if (id == R.id.nav_send) {

    }

    DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
    drawer.closeDrawer(GravityCompat.START);
    return true;
}

如您所见，如果我们按下第一个菜单 if (id == R.id.nav_camara)，它会将 JSONArray 传递给 Fragment 类，并且加载大约需要 4-5 秒。因此，如果用户每次转到 nav_camara 时都在菜单之间导航，它会使应用程序冻结 4 秒才能加载，并且正如您所看到的，每次我们选择一个菜单项时，它都会重新创建一个片段的新副本，所以即使我让：setRetainInstance(true); 在我的片段类中它不起作用。

您建议什么解决方案来防止应用在每次选择菜单项时重新创建新片段？

Answer 1

您需要保留 SparseArray<Fragment> 以将实例保存在内存中。

按照以下步骤操作：

1. 在您的活动中创建一个字段：

   SparseArray<Fragment> myFragments;
   

2. 在onCreate() 中初始化它：

   myFragments = new SparseArray<Fragment>();
   

3. 更新您的onNavigationItemSelected()：

   @Override
   public boolean onNavigationItemSelected(MenuItem item) {
       // Handle navigation view item clicks here.
       int id = item.getItemId();
   
       if (id == R.id.nav_camara) {
           // get cached instance of the fragment
           fragment = myFragments.get(INT_CONSTANT_FOR_CAM_FRAGMENT);
   
           // if fragment doesn't exist in myFragments, create one and add to it
           if (fragment == null) {
               fragment = new MainFragment();
               myFragments.put(INT_CONSTANT_FOR_CAM_FRAGMENT, fragment);
           }
   
           // now load the fragment
           FragmentTransaction fragmentTransaction =
                       getSupportFragmentManager().beginTransaction();
           fragmentTransaction.replace(R.id.fragment_container, fragment);
           fragmentTransaction.commit();
   
       }
       // do the rest with others as well.
   }
   

4. 将AsyncTask 移动到您的MainFragment 的onActivityCreated() 或类似的生命周期方法中。

以上方式可以让您重用片段并将逻辑移动到正确的位置（您的 Activity 不应该知道 MainFragment 如何加载数据以启动自身，但当然您仍然可以将其保留在那里，尽管不推荐） .

Answer 2

您可以在将新片段推送到容器时添加TAG。然后使用FragmentManager#findFragmentByTag 查找之前添加了相同标签的任何片段。

        FragmentManager manager = getSupportFragmentManager();
        FragmentTransaction transaction = manager.beginTransaction();
        Fragment oldFragment = manager.findFragmentByTag(tag);
        if (oldFragment != null)
            transaction.replace(R.id.fragment_container, oldFragment, tag);
        else
            transaction.replace(R.id.fragment_container, newInstanceOfFragment, tag);
        transaction.commit();

Answer 3

全局声明它们

GalleryFragment galleryFrag = new GalleryFragment();
FragmentTransaction ft;
FragmentManager fm;

@Override
protected void onCreate(Bundle savedInstanceState) {
    ...

    fm = getSupportFragmentManager();

    ...
}

然后在您的导航选择上

@Override
public boolean onNavigationItemSelected(MenuItem item) {

// Handle navigation view item clicks here.
int id = item.getItemId();

if (id == R.id.nav_search) {
    ft = fm.beginTransaction();
    ft.replace(R.id.fragment_container, galleryFrag).commit();
} 

DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
drawer.closeDrawer(GravityCompat.START);
return true;
}