活动和片段的生命周期及方向更改

Question

我最近遇到了一些有关Fragments和屏幕旋转的奇怪问题，导致应用崩溃并且不符合逻辑。

于是我写了一个简单的Activity和Fragment生命周期调试应用程序，它通过记录每个步骤到logcat来实现了Activity生命周期和Fragment生命周期的所有过程。

这里是TestActivity和TestFragment类：

TestActivity

public class TestActivity extends Activity {
    Context ct = null;


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

        Log.e("ACT", "onCreate called");

        ct = getApplicationContext();

        FrameLayout fl = new FrameLayout(ct);
        fl.setId(1000);

        TestFragment tf = new TestFragment();
        getFragmentManager().beginTransaction().add(fl.getId(), tf, "").commit();

        setContentView(fl);
    }

    @Override
    protected void onStart() {
        Log.e("ACT", "onStart called");
        super.onStart();
    }

    @Override
    protected void onResume() {
        Log.e("ACT", "onResume called");
        super.onResume();
    }

    @Override
    protected void onPause() {
        Log.e("ACT", "onPause called");
        super.onPause();
    }

    @Override
    protected void onStop() {
        Log.e("ACT", "onStop called");
        super.onStop();
    }

    @Override
    protected void onDestroy() {
        Log.e("ACT", "onDestroy called");
        super.onDestroy();
    }

    @Override
    protected void onRestart() {
        Log.e("ACT", "onRestart called");
        super.onRestart();
    }
}

测试片段

public class TestFragment extends Fragment {
    Context ctFrag = null;

    @Override
    public void onAttach(Activity activity) {
        Log.e("FRAG", "onAttach called");
        super.onAttach(activity);
    }

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        Log.e("FRAG", "onCreate called");

    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
            Bundle savedInstanceState) {
        Log.e("FRAG", "onCreateView called");

        ctFrag = ((TestActivity) getActivity()).ct;

        TextView tv = new TextView(ctFrag);
        tv.setText("My test TextView");

        return tv;
    }

    @Override
    public void onActivityCreated(Bundle savedInstanceState) {
        Log.e("FRAG", "onActivityCreated called");
        super.onActivityCreated(savedInstanceState);
    }

    @Override
    public void onStart() {
        Log.e("FRAG", "onStart called");
        super.onStart();
    }

    @Override
    public void onResume() {
        Log.e("FRAG", "onResume called");
        super.onResume();
    }

    @Override
    public void onPause() {
        Log.e("FRAG", "onPause called");
        super.onPause();
    }

    @Override
    public void onStop() {
        Log.e("FRAG", "onStop called");
        super.onStop();
    }

    @Override
    public void onDestroyView() {
        Log.e("FRAG", "onDestroyView called");
        super.onDestroyView();
    }

    @Override
    public void onDestroy() {
        Log.e("FRAG", "onDestroy called");
        super.onDestroy();
    }

    @Override
    public void onDetach() {
        Log.e("FRAG", "onDetach called");
        super.onDetach();
    }
}

初始化时，Logcat 的输出按照预期的顺序进行（启动Activity，当 Fragment 被附加时，其生命周期的调用发生等）：

01-29 10:12:50.270: E/ACT(3321): onCreate called
01-29 10:12:50.760: E/FRAG(3321): onAttach called
01-29 10:12:50.760: E/FRAG(3321): onCreate called
01-29 10:12:50.760: E/FRAG(3321): onCreateView called
01-29 10:12:50.770: E/FRAG(3321): onActivityCreated called
01-29 10:12:50.770: E/ACT(3321): onStart called
01-29 10:12:50.770: E/FRAG(3321): onStart called
01-29 10:12:50.770: E/ACT(3321): onResume called
01-29 10:12:50.770: E/FRAG(3321): onResume called

然而问题是，当发生方向变化时，Android文档说:

当这种变化发生时，Android会重新启动运行中的Activity(调用onDestroy()，然后调用onCreate())。

这表明它应该关闭Activity和其内容，就像生命周期建议的那样（它确实这样做）。但是随后它并没有按照同样的顺序过程重新创建新方向的Activity。相反，它似乎尝试重新创建Fragment，然后在活动重新创建时再创建一个新的Fragment。

01-29 10:17:52.249: E/FRAG(3321): onPause called
01-29 10:17:52.259: E/ACT(3321): onPause called
01-29 10:17:52.269: E/FRAG(3321): onStop called
01-29 10:17:52.269: E/ACT(3321): onStop called
01-29 10:17:52.279: E/FRAG(3321): onDestroyView called
01-29 10:17:52.299: E/FRAG(3321): onDestroy called
01-29 10:17:52.299: E/FRAG(3321): onDetach called
01-29 10:17:52.299: E/ACT(3321): onDestroy called
01-29 10:17:52.650: E/FRAG(3321): onAttach called
01-29 10:17:52.650: E/FRAG(3321): onCreate called
01-29 10:17:52.650: E/ACT(3321): onCreate called
01-29 10:17:53.020: E/FRAG(3321): onCreateView called
01-29 10:17:53.020: E/FRAG(3321): onActivityCreated called
01-29 10:17:53.030: E/FRAG(3321): onAttach called
01-29 10:17:53.030: E/FRAG(3321): onCreate called
01-29 10:17:53.030: E/FRAG(3321): onCreateView called
01-29 10:17:53.030: E/FRAG(3321): onActivityCreated called
01-29 10:17:53.060: E/ACT(3321): onStart called
01-29 10:17:53.060: E/FRAG(3321): onStart called
01-29 10:17:53.060: E/FRAG(3321): onStart called
01-29 10:17:53.060: E/ACT(3321): onResume called
01-29 10:17:53.060: E/FRAG(3321): onResume called
01-29 10:17:53.060: E/FRAG(3321): onResume called

显然有很多解决方案可以解决这个问题，但我的问题是为什么会发生这种情况？为什么一个“Fragment”引用在应该被销毁并重新创建的“Activity”中被保留和重新创建？我可以通过“Fragment”有意地与活动分开来证明这一点。但是导致问题的原因是，“Activity”之前为什么会先重新连接和重新创建原始的“Fragment”？这似乎不遵循Android进程的逻辑生命周期的其他部分。

- btalb

Answer 1

这是因为活动在被销毁之前调用了 onSaveInstanceState(Bundle)。默认情况下，该活动在此方法中保存其片段的状态。

稍后，当活动被重新创建时，旧的碎片将在活动的 onCreate(Bundle savedInstanceState) 方法中被重新创建。

您可能需要检查源代码这里和这里以更好地理解此行为。

- Kéza

---

没错，谢谢。我的问题似乎只取决于我在onCreate调用中何时调用super.onCreate()。现在我确切地知道了为什么会发生这种情况，我可以解决它了，谢谢。 - btalb

Answer 2

这是因为在活动重新创建时，您不断地添加片段。您可以在活动的onCreate方法中使用以下代码来避免重新创建片段：

if(savedInstanceState == null) 
{
    mFragmentManager = getSupportFragmentManager();
    FragmentTransaction fragmentTransaction = mFragmentManager.beginTransaction();

    FragmentOne fragment = new FragmentOne();

    fragmentTransaction.add(R.id.fragment_container, fragment);
    fragmentTransaction.commit();
}

当配置更改发生时，旧的 Fragment 不会被销毁——它会在重新创建时将自己添加回 Activity。

- Smeet