FragmentManager保留了什么？

Question

我有一个用于实例化一个片段的活动的抽象基类：

<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
         android:id="@+id/fragmentContainer" android:layout_width="match_parent" android:layout_height="match_parent" >
</FrameLayout>


public abstract class SingleFragmentActivity extends Activity {
    protected abstract Fragment createFragment();

    private Fragment m_fragment;
    public Fragment getSingleFragment() { return m_fragment; }

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_fragment);

        FragmentManager fm = getFragmentManager();
        m_fragment = fm.findFragmentById( R.id.fragmentContainer );
        Log.d("SingleFragment", "fetching a fragment");
        if ( m_fragment == null ) {
            Log.d("SingleFragment", "SingleFragment requires creating a new one");
            m_fragment = createFragment();
            fm.beginTransaction().add( R.id.fragmentContainer, m_fragment ).commit();
        } else {
            Log.d("SingleFragment", "SingleFragment reusing one from fragment manager");
        }
    }
}

...plus是一个简单的活动和片段：

public class LoginActivity extends SingleFragmentActivity {
    private static final String TAG = "LOGIN ACTIVITY";

    private LoginFragment m_loginFragment;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        Log.d(TAG,"onCreate");
        super.onCreate(savedInstanceState);

        m_loginFragment = (LoginFragment) getSingleFragment();
        Log.d(TAG,"...login fragment foo = "+m_loginFragment.getFoo());
        m_loginFragment.setFoo( "Here's a new foo");
    }

    @Override
    public void onPause() { super.onPause(); Log.d(TAG,"onPause"); }

    @Override
    public void onDestroy() { super.onDestroy(); Log.d(TAG,"onDestroy"); }

    @Override
    public void onResume() { super.onResume(); Log.d(TAG,"onResume"); }


    @Override
    protected Fragment createFragment() {
        Log.d(TAG,"createFragment()");
        return new LoginFragment();
    }
}

public class LoginFragment extends Fragment {
    private static final String TAG = "LOGIN FRAGMENT";

    private String foo = "not set yet";
    public String getFoo() { return foo; }
    public void setFoo(String s) { foo = s; }

    @Override
    public void onCreate( Bundle savedInstanceState ) {
        super.onCreate(savedInstanceState);
        Log.d(TAG,"onCreate");
    }

    @Override
    public void onPause() { super.onPause(); Log.d(TAG,"onPause"); }

    @Override
    public void onDestroy() { super.onDestroy(); Log.d(TAG,"onDestroy"); }

    @Override
    public void onResume() { super.onResume(); Log.d(TAG,"onResume"); }
}

如果我运行这段代码，并在手机启动后旋转它，我会看到以下日志：

08-07 15:02:51.310: LOGIN ACTIVITY: onCreate
08-07 15:02:51.340: SingleFragment: fetching a fragment
08-07 15:02:51.340: SingleFragment: SingleFragment requires creating a new one
08-07 15:02:51.340: LOGIN ACTIVITY: createFragment()
08-07 15:02:51.340: LOGIN ACTIVITY: ...login fragment foo = not set yet
08-07 15:02:51.340: LOGIN FRAGMENT: onCreate
08-07 15:02:51.340: LOGIN ACTIVITY: onResume
08-07 15:02:51.340: LOGIN FRAGMENT: onResume
08-07 15:02:59.799: LOGIN FRAGMENT: onPause
08-07 15:02:59.799: LOGIN ACTIVITY: onPause
08-07 15:02:59.809: LOGIN FRAGMENT: onDestroy
08-07 15:02:59.809: LOGIN ACTIVITY: onDestroy
08-07 15:02:59.829: LOGIN ACTIVITY: onCreate
08-07 15:02:59.829: LOGIN FRAGMENT: onCreate
08-07 15:02:59.869: SingleFragment: fetching a fragment
08-07 15:02:59.869: SingleFragment: SingleFragment reusing one from fragment manager
08-07 15:02:59.869: LOGIN ACTIVITY: ...login fragment foo = not set yet
08-07 15:02:59.869: LOGIN ACTIVITY: onResume
08-07 15:02:59.869: LOGIN FRAGMENT: onResume

一切正常工作:旋转电话会导致活动和碎片被破坏和重新创建；我看到碎片管理器恢复了一些东西，但我的问题是“什么？”没有保留成员变量foo，那么正在还原的是什么？

Answer 1

我也有同样的问题，有时还会在After screen rotation, findFragmentById() returns a fragment, even if there's no such ID inside the layout上发帖。

当时，http://developer.android.com/training/basics/fragments/creating.html的文档提到了以下内容：

当配置更改导致承载这些片段的活动重新启动时，其新实例可能会使用与以前的布局不包含相同片段的不同布局。在本例中，前面的所有片段仍将在新实例中实例化和运行。但是，与视图层次结构中的标记不再关联的任何内容视图都不会被创建，并且将从isInLayout()返回false。(这里的代码还展示了如何确定放置在容器中的片段是否已不再运行在带有该容器的布局中，并避免在这种情况下创建其视图层次结构。)

这个部分在那个文档中不存在。

您可以使用isInLayout()检查这个片段是否已经添加到布局中。