Question

我遇到麻烦了，我不知道如何解决。一切运行正常，并且方式正确，但是如果该应用程序位于堆栈中，并且操作系统需要资源，则它将终止该应用程序，而我遇到了问题。

我有一个带有FAB，选项卡和ViewPager的活动，其中包含2个片段。 FAB取决于当前的Tab调用Fragments的方法，我使用它的接口，而我的方法使用Fragments中的上下文。

活动方案是

public class MyActivity extends AppCompatActivity {

private MyInterface mMyInterface;
private FloatingActionButton mFab;
private Toolbar mToolbar;
private DrawerLayout mDrawerLayout;
private NavigationView mNavigationView;
private ViewPager mViewPager;
private TabLayout mTabLayout;
private PagerAdapter mAdapter;
private FirstFragment mFirstFrg;
private SecondFragment mSecondFrg;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    initToolbar();
    initComponent();
}

private void initToolbar() {
    mToolbar = (Toolbar) findViewById(R.id.toolbar);
    mToolbar.setNavigationIcon(R.drawable.ic_menu);
    setSupportActionBar(mToolbar);
    getSupportActionBar().setTitle("My App");
    getSupportActionBar().setDisplayHomeAsUpEnabled(true);
}

private void initComponent() {

    mDrawerLayout = (DrawerLayout)findViewById(R.id.drawer);
    mNavigationView = (NavigationView) findViewById(R.id.navigation);
    mViewPager = (ViewPager) findViewById(R.id.view_pager);
    setupViewPager(mViewPager);

    mTabLayout = (TabLayout) findViewById(R.id.tab_layout);
    mTabLayout.setupWithViewPager(mViewPager);

    mFirstFrg = (FirstFragment) mAdapter.getItem(0);
    mSecondFrg = (SecondFragment) mAdapter.getItem(1);

    mFab = (FloatingActionButton)findViewById(R.id.fab);

    mNavigationView.setNavigationItemSelectedListener(new NavigationView.OnNavigationItemSelectedListener() {
        @Override
        public boolean onNavigationItemSelected(MenuItem item) {

            if(item.getItemId() == R.id.drawer_one) {
                showToast("Menu one");
            }else{
                showToast("Menu two");
            }
            item.setChecked(true);
            mDrawerLayout.closeDrawers();
            return true;
        }
    });

    mViewPager.addOnPageChangeListener(new ViewPager.SimpleOnPageChangeListener() {
        @Override
        public void onPageSelected(int position) {
            changeFabIcon(position);
            super.onPageSelected(position);
        }
    });

    mFab.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            switch (mViewPager.getCurrentItem()) {
                case 0: {       
                        showToast("Test!!!");
                    }
                    break;
                }
                case 1: {
                        if(mSecondFrg.getContext() == null){                
                            showToast("context is Null!");
                        }
                        mMyInterface.doSomething();
                    break;
                }
            }
        }
    });
}

private void setupViewPager(ViewPager viewPager) {
    FirstFragment firstFragment = new FirstFragment();
    SecondFragment secondFragment = new SecondFragment();
    this.mMyInterface = secondFragment;
    mAdapter = new PagerAdapter(getSupportFragmentManager());
    mAdapter.addFragment(firstFragment, "One");
    mAdapter.addFragment(secondFragment, "Two");
    viewPager.setAdapter(mAdapter);
}

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    if(item.getItemId() == android.R.id.home)
    {
        mDrawerLayout.openDrawer(GravityCompat.START);
        return true;
    }
    return super.onOptionsItemSelected(item);
}

private void changeFabIcon(final int index) {
    mFab.hide();
    new Handler().postDelayed(new Runnable() {
        @Override
        public void run() {
            switch (index) {
                case 0: {
                    mFab.setImageResource(R.drawable.ic_plus);
                    break;
                }
                case 1: {
                    mFab.setImageResource(R.drawable.ic_text);
                    break;
                }
            }
            mFab.show();
        }
    }, 300);
}

private void showToast(String message){
    Toast.makeText(this, message, Toast.LENGTH_LONG).show();
}}

第二个方案是

public class SecondFragment extends Fragment implements MyInterface {

private RecyclerView mRecycler;
private Context mContext;

@Nullable
@Override
public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
    View fragmentView = inflater.inflate(R.layout.fragment_second, container, false);
    mRecycler = (RecyclerView) placeView.findViewById(R.id.recycler);
    LinearLayoutManager linearLayoutManager = new LinearLayoutManager(mContext);
    linearLayoutManager.setOrientation(LinearLayoutManager.VERTICAL);
    mRecycler.setLayoutManager(linearLayoutManager);

    return fragmentView;
}

@Override
public void onAttach(Context context) {
    super.onAttach(context);
    mContext = context;
}

@Override
public void onCreate(@Nullable Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setRetainInstance(true);
}

private void getData(){// Some action and Update the UI
}
@Override
public void doSomething(){ 
    getData();
}}

问题是每次系统杀死Stack中的应用程序时，我得到空的Context和Fragment方法都不起作用:( 我在那里检查

if(mSecondFrg.getContext() == null){                
                            showToast("context is Null!");
                        }
                        mMyInterface.doSomething();

也是我的适配器代码

public class PagerAdapter extends FragmentPagerAdapter {

private final List<Fragment> mFragmentList = new ArrayList<>();
private final List<String> mFragmentTitleList = new ArrayList<>();

public PagerAdapter(FragmentManager manager) {
    super(manager);
}

@Override
public Fragment getItem(int position) {
    return mFragmentList.get(position);
}

@Override
public int getCount() {
    return mFragmentList.size();
}

public void addFragment(Fragment fragment, String title) {
    mFragmentList.add(fragment);
    mFragmentTitleList.add(title);
}

@Override
public CharSequence getPageTitle(int position) {
    return mFragmentTitleList.get(position);
}}

感谢任何提示或帮助

更新并修复 我解决了，问题出在适配器上请从文档中阅读

对PagerAdapter方法startUpdate（ViewGroup）的调用指示ViewPager的内容即将更改。随后将对instantiateItem（ViewGroup，int）和/或destroyItem（ViewGroup，int，Object）进行一次或多次调用，并通过对finishUpdate（ViewGroup）的调用来发出更新结束的信号。

我删除了有关在Activity中创建Fragment的所有代码，并添加了

mAdapter.startUpdate(mViewPager);
mFirstFrg = (FirstFragment) mAdapter.instantiateItem(mViewPager,0);
mSecondFrg = (SecondFragment) mAdapter.instantiateItem(mViewPager,1);
mAdapter.finishUpdate(mViewPager);

更新适配器类

public class PagerAdapter extends FragmentPagerAdapter {
public PagerAdapter(FragmentManager manager) {
    super(manager);
}
@Override
public Fragment getItem(int position) {
    if (position == 0) {
        return new FirstFragment();
    }else {
        return new SecondFragment();
    }
}

@Override
public int getCount() {
    return 2;
}

@Override
public CharSequence getPageTitle(int position) {
    if (position == 0){
        return "One";
    }else {
        return "Two";
    }
}

您还可以使用FragmentStatePagerAdapter，您可以在此处Difference between FragmentPagerAdapter and FragmentStatePagerAdapter

进行了解。

如果系统杀死了堆栈中的应用，则来自ViewPager的片段将返回Null上下文

0 个答案: