我开发了一个简单的android应用程序。当我回到应用程序时,为什么getActivity返回null?
活动:
public class MainActivity extends AppCompatActivity {
ArrayList<Fragment> fgs = new ArrayList<>();
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
fgs.add(new MyFragment1());
fgs.add(new MyFragment2());
ViewPager viewPager = (ViewPager) findViewById(R.id.container);
viewPager.setAdapter(new FragmentPagerAdapter(getSupportFragmentManager()) {
@Override
public Fragment getItem(int position) {
switch (position) {
case 0:
return fgs.get(position);
case 1:
return fgs.get(position);
default:
return null;
}
}
@Override
public int getCount() {
return fgs.size();
}
});
viewPager.addOnPageChangeListener(new ViewPager.OnPageChangeListener() {
@Override
public void onPageScrolled(int position, float positionOffset, int positionOffsetPixels) {
}
@Override
public void onPageSelected(int position) {
switch (position) {
case 0:
((MyFragment1) fgs.get(0)).display();
return;
}
}
@Override
public void onPageScrollStateChanged(int state) {}
});
}
}
片段1
public class MyFragment1 extends Fragment {
String mString;
@Nullable
@Override
public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
View v = inflater.inflate(R.layout.my_fragment, container, false);
mString = "MyFragment1";
((TextView)v.findViewById(R.id.text)).setText(mString);
return v;
}
public void display()
{
Toast.makeText(getActivity(), mString, Toast.LENGTH_SHORT).show();
}
}
Fragment2
public class MyFragment2 extends Fragment {
String mString;
@Nullable
@Override
public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
View v = inflater.inflate(R.layout.my_fragment, container, false);
mString = "MyFragment2";
((TextView)v.findViewById(R.id.text)).setText(mString);
return v;
}
}
我测试了一下它的内存是由操作系统发布的情况下,这个程序behavir(我做了“不要保持活动“检查。)
在此sitiaion下,当我关闭此应用程序并回到应用程序并滚动到Fragment1时,发生了NullPointerException。
我以为,当我回到应用程序,一切工作正常。因为当我回到应用程序,应用程序重新开始。
这是为什么发生?我如何解决这个问题?
错误日志:
Exception in MessageQueue callback: handleReceiveCallback java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.res.Resources android.content.Context.getResources()' on a null object reference
at android.widget.Toast.<init>(Toast.java:102)
at android.widget.Toast.makeText(Toast.java:260)
at knkj28.com.experiment.MyFragment1.display(MyFragment1.java:33)
at knkj28.com.experiment.MainActivity$2.onPageSelected(MainActivity.java:52)
at android.support.v4.view.ViewPager.dispatchOnPageSelected(ViewPager.java:1967)
at android.support.v4.view.ViewPager.scrollToItem(ViewPager.java:685)
at android.support.v4.view.ViewPager.setCurrentItemInternal(ViewPager.java:669)
at android.support.v4.view.ViewPager.onTouchEvent(ViewPager.java:2284)
at android.view.View.dispatchTouchEvent(View.java:9991)
at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2663)
at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2344)
at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2669)
at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2358)
at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2669)
at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2358)
at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2669)
at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2358)
at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2669)
at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2358)
at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2669)
at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2358)
at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2669)
at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2358)
at com.android.internal.policy.DecorView.superDispatchTouchEvent(DecorView.java:413)
at com.android.internal.policy.PhoneWindow.superDispatchTouchEvent(PhoneWindow.java:1810)
at android.app.Activity.dispatchTouchEvent(Activity.java:3102)
at android.support.v7.view.WindowCallbackWrapper.dispatchTouchEvent(WindowCallbackWrapper.java:71)
at com.android.internal.policy.DecorView.dispatchTouchEvent(DecorView.java:375)
at android.view.View.dispatchPointerEvent(View.java:10211)
at android.view.ViewRootImpl$ViewPostImeInputStage.processPointerEvent(ViewRootImpl.java:4444)
at android.view.ViewRootImpl$ViewPostImeInputStage.onProcess(ViewRootImpl.java:4312)
at android.view.ViewRootImpl$InputStage.deliver(ViewRootImpl.java:3859)
at android.view.ViewRootImpl$InputStage.onDeliverToNext(ViewRootImpl.java:3912)
at android.view.ViewRootImpl$InputStage.forward(ViewRootImpl.java:3878)
at android.view.ViewRootImpl$AsyncInputStage.forward(ViewRootImpl.java:4005)
at android.view.ViewRootImpl$InputStage.apply(ViewRootImpl.java:3886)
at android.view.ViewRootImpl$AsyncInputStage.apply(ViewRootImpl.java:4062)
at android.view.ViewRootImpl$InputStage.deliver(ViewRootImpl.java:3859)
at android.view.ViewRootImpl$InputStage.onDeliverToNext(ViewRootImpl.java:3912)
at android.view.ViewRootImpl$InputStage.forward(ViewRootImpl.java:3878)
at android.view.ViewRootImpl$InputStage.apply(ViewRootImpl.java:3886)
at android.view.ViewRootImpl$InputStage.deliver(ViewRootImpl.java:3859)
at android.view.ViewRootImpl.deliverInputEvent(ViewRootImpl.java:6220)
at android.view.ViewRootImpl.doProcessInputEvents(ViewRootImpl.java:6194)
at android.view.ViewRootImpl.enqueueInputEvent(ViewRootImpl.java:6155)
at android.view.ViewRootImpl$WindowInputEventReceiver.onInputEvent(ViewRootImpl.java:6323)
at android.view.InputEventReceiver.dispatchInputEvent(InputEventReceiver.java:185)
at android.os.MessageQueue.nativePollOnce(Native Method)
at android.os.MessageQueue.next(MessageQueue.java:331)
at android.os.Looper.loop(Looper.java:219)
at android.app.ActivityThread.main(ActivityThread.java:6217)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:865)
at com.android.internal.os.
Shutting down VM
--------- beginning of crash
FATAL EXCEPTION: main
Process: knkj28.com.experiment, PID: 6025
java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.res.Resources android.content.Context.getResources()' on a null object reference
at android.widget.Toast.<init>(Toast.java:102)
at android.widget.Toast.makeText(Toast.java:260)
at knkj28.com.experiment.MyFragment1.display(MyFragment1.java:33)
at knkj28.com.experiment.MainActivity$2.onPageSelected(MainActivity.java:52)
at android.support.v4.view.ViewPager.dispatchOnPageSelected(ViewPager.java:1967)
at android.support.v4.view.ViewPager.scrollToItem(ViewPager.java:685)
at android.support.v4.view.ViewPager.setCurrentItemInternal(ViewPager.java:669)
at android.support.v4.view.ViewPager.onTouchEvent(ViewPager.java:2284)
at android.view.View.dispatchTouchEvent(View.java:9991)
at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2663)
at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2344)
at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2669)
at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2358)
at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2669)
at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2358)
at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2669)
at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2358)
at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2669)
at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2358)
at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2669)
at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2358)
at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2669)
at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2358)
at com.android.internal.policy.DecorView.superDispatchTouchEvent(DecorView.java:413)
at com.android.internal.policy.PhoneWindow.superDispatchTouchEvent(PhoneWindow.java:1810)
at android.app.Activity.dispatchTouchEvent(Activity.java:3102)
at android.support.v7.view.WindowCallbackWrapper.dispatchTouchEvent(WindowCallbackWrapper.java:71)
at com.android.internal.policy.DecorView.dispatchTouchEvent(DecorView.java:375)
at android.view.View.dispatchPointerEvent(View.java:10211)
at android.view.ViewRootImpl$ViewPostImeInputStage.processPointerEvent(ViewRootImpl.java:4444)
at android.view.ViewRootImpl$ViewPostImeInputStage.onProcess(ViewRootImpl.java:4312)
at android.view.ViewRootImpl$InputStage.deliver(ViewRootImpl.java:3859)
at android.view.ViewRootImpl$InputStage.onDeliverToNext(ViewRootImpl.java:3912)
at android.view.ViewRootImpl$InputStage.forward(ViewRootImpl.java:3878)
at android.view.ViewRootImpl$AsyncInputStage.forward(ViewRootImpl.java:4005)
at android.view.ViewRootImpl$InputStage.apply(ViewRootImpl.java:3886)
at android.view.ViewRootImpl$AsyncInputStage.apply(ViewRootImpl.java:4062)
at android.view.ViewRootImpl$InputStage.deliver(ViewRootImpl.java:3859)
at android.view.ViewRootImpl$InputStage.onDeliverToNext(ViewRootImpl.java:3912)
at android.view.ViewRootImpl$InputStage.forward(ViewRootImpl.java:3878)
at android.view.ViewRootImpl$InputStage.apply(ViewRootImpl.java:3886)
at android.view.ViewRootImpl$InputStage.deliver(ViewRootImpl.java:3859)
at android.view.ViewRootImpl.deliverInputEvent(ViewRootImpl.java:6220)
at android.view.ViewRootImpl.doProcessInputEvents(ViewRootImpl.java:6194)
at android.view.ViewRootImpl.enqueueInputEvent(ViewRootImpl.java:6155)
at android.view.ViewRootImpl$WindowInputEventReceiver.onInputEvent(ViewRootImpl.java:6323)
at android.view.InputEventReceiver.dispatchInputEvent(InputEventReceiver.java:185)
at android.os.MessageQueue.nativePollOnce(Native Method)
at android.os.MessageQueue.next(MessageQueue.java:331)
at android.os.Looper.loop(Looper.java:219)
at android.app.ActivityThread.main(ActivityThread.java:6217)
at java.lang.reflect.Method.invoke(Native Method)
08-16 13:32:16.701 6025-6025/knkj28.com.experiment E/AndroidRuntime: at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:865)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:755)
我明白了。但为什么我的代码无法将片段与活动相关联?它 第一次正常工作。即使操作系统强制执行该应用程序,它也应该正常工作,因为活动和片段都会再次创建,并且生命周期像第一次一样再次启动。我误解了一些问题吗? – user1692121
这取决于您如何将片段绑定到活动。看看你的代码发生了什么(可能显示给我们)。另外,调试getActivity()并查看返回的null。使代码健壮并为其做好准备,因为即使这次你做错了什么,一般而言,在片段内部,你不能保证你的宿主活动不会在任何时候被系统所杀死,而且仍然某些片段的代码(如回调)可能会延迟执行。 – Alessio