从活动访问片段时，Android罕见的NPE

Question

我在Android中有一个包含框架布局的活动。其中一个框架布局用一个片段进行了膨胀。在片段的onResume()中，对在"活动"中实现的侦听器进行调用。侦听器然后调用片段的方法。在这一点上，NPE发生在对片段的引用上。

这种情况很少发生，但它已经被复制了至少2次。我怀疑这个问题与活动和片段的生命周期有关。当活动仍处于生命周期的onCreate()步骤时，对片段进行引用，这可能是在片段初始化之前。

我的分析正确吗？如何预防NPE？

这是代码(请记住，我已经重命名了很多代码，并删除了看似无关的部分(：

活动：

FoodsActivity extends AppCompatActivity implements FruitStateFragment.OnAppleSelectedListener {
private Context mContext;
private FruitsManager mFruitsManagr;
private FruitStateFragment mFruitStateFragment;

@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.foods_activity);
if (savedInstanceState != null) {
return;
}
if (mContext == null) {
mContext = getApplicationContext();
}
mFruitsManagr = FruitsManager.get(mContext);
if (findViewById(R.id.fl_fruits_status) != null) {
mFruitStateFragment = new FruitStateFragment();
mFruitStateFragment.setArguments(getIntent().getExtras());
getSupportFragmentManager().beginTransaction()
.add(R.id.fl_fruits_status, mFruitStateFragment, "FruitStateFragment").commit();
}
mFruitsManagr.setApple(0);
}
@Override
public void onAttachFragment(Fragment fragment) {
if (fragment instanceof FruitStateFragment) {
FruitStateFragment fruitStateFragment = (FruitStateFragment) fragment;
fruitStateFragment.setOnAppleSelectedListener(this);
}
}
public void onAppleSelected(Integer appleNum) {
FruitsManager fManager = FruitsManager.get(mContext);
fManager.setApple(appleNum);
// NPE on mFruitStateFragment
mFruitStateFragment.updateBasketUi(fManager.getActiveBasketName());
}   
}

片段：

FruitStateFragment extends Fragment {
private Context mContext;
FruitsManager mFruitsManagr;
OnAppleSelectedListener mAppleCallback;
public void setOnAppleSelectedListener(OnAppleSelectedListener callback) {
this.mAppleCallback = callback;
}
public interface OnAppleSelectedListener {
void onAppleSelected(Integer appleNum);
}
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
if (mContext == null) {
mContext = getActivity().getApplicationContext();
}
mFruitsManagr = FruitsManager.get(mContext);
}
@Override
public void onResume() {
super.onResume();
mAppleCallback.onAppleSelected(mFruitsManagr.getActiveApple());
}
void updateBasketUi(String basketName) {
}
}

经理：

public class FruitsManager {
private static FruitsManager sMe;
private Context mContext;
private static int mActiveApple = 0;
private static String mActiveBasketName = "";
private FruitsManager(Context context) {
mContext = context;
initInterfaces();
}
public int getActiveApple() {
return mActiveApple;
}
public int getActiveBasketName() {
return mActiveBasketName;
}
}

日志：

D FRUITS: 0001 onCreate (FoodsActivity%onCreate:)
D FRUITS: 0002 onCreate (FruitStateFragment%onCreate:)
D FRUITS: 0003 onCreateView (FruitStateFragment%onCreateView:)
D FRUITS: 0004 onActivityCreated (FruitStateFragment%onActivityCreated:)
D FRUITS: 0005 initViews (FruitStateFragment%initViews:)
D FRUITS: 0006 onResume (FruitStateFragment%onResume:)
E AndroidRuntime: java.lang.RuntimeException: Unable to resume activity {com.hi/FoodsActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void com.hi.FruitStateFragment.updateBasketUi(java.lang.String)' on a null object reference

Answer 1

这些行：

if (savedInstanceState != null) {
return;
}

意思是，在重新创建活动时，您永远不会设置mFruitStateFragment变量——您只在第一次添加片段时设置它。这也意味着您的mFruitsManagr和mContext也没有设置，因为它也在该行之后。如果您希望这些在每次创建活动时都可用，那么您应该删除该复选框，只包装应该只发生一次的事情。

这意味着你的活动应该看起来像

@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.foods_activity);
if (mContext == null) {
mContext = getApplicationContext();
}
mFruitsManagr = FruitsManager.get(mContext);
if (savedInstanceState == null) {
mFruitStateFragment = new FruitStateFragment();
mFruitStateFragment.setArguments(getIntent().getExtras());
getSupportFragmentManager().beginTransaction()
.add(R.id.fl_fruits_status, mFruitStateFragment, "FruitStateFragment").commit();
// I assume this shouldn't be set every time the activity
// is recreated
mFruitsManagr.setApple(0);
} else {
// Get the Fragment that is already created from the FragmentManager
mFruitStateFragment = getSupportFragmentManager()
.findFragmentById(R.id.fl_fruits_status);
}
}

当然，由于您使用的是onAttachFragment()，您还可以从中获得一个引用，因为它将在每次重新创建活动时调用：

@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.foods_activity);
if (mContext == null) {
mContext = getApplicationContext();
}
mFruitsManagr = FruitsManager.get(mContext);
if (savedInstanceState == null) {
// We'll store a reference to this in onAttachFragment()
FruitStateFragment fruitStateFragment = new FruitStateFragment();
fruitStateFragment.setArguments(getIntent().getExtras());
getSupportFragmentManager().beginTransaction()
.add(R.id.fl_fruits_status, fruitStateFragment, "FruitStateFragment").commit();
// I assume this shouldn't be set every time the activity
// is recreated
mFruitsManagr.setApple(0);
}
}
@Override
public void onAttachFragment(Fragment fragment) {
if (fragment instanceof FruitStateFragment) {
// This get called every time the activity is created,
// ensuring that mFruitStateFragment is always set
mFruitStateFragment = (FruitStateFragment) fragment;
mFruitStateFragment.setOnSimNumSelectedListener(this);
}
}