Android支持的DialogFragment在屏幕旋转时崩溃

Question

我正在尝试将我的应用程序切换到使用对话框片段，但是当旋转屏幕时，出现应用程序崩溃而且对话框仍可见。我可以在以下非常简单的应用程序中重现此问题。在Android Studio中创建一个新项目并添加一个DialogFragment如下：

public class MainActivity extends ActionBarActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        if (savedInstanceState == null) {
            getSupportFragmentManager().beginTransaction()
                    .add(R.id.container, new PlaceholderFragment())
                    .commit();
        }
        if (savedInstanceState == null) {
            (new Handler()).postDelayed(new Runnable() {
                @Override
                public void run() {
                    AlertDialog dialog = new AlertDialog.Builder(MainActivity.this)
                            .setMessage("Alert")
                            .setTitle("My Alert")
                            .create();
                    MyDialogFragment dialogFragment = new MyDialogFragment();
                    dialogFragment.setDialog(dialog);
                    dialogFragment.show(getSupportFragmentManager(), "dialog");

                }
            }, 1000);
        }
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.main, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();
        if (id == R.id.action_settings) {
            return true;
        }
        return super.onOptionsItemSelected(item);
    }

    /**
     * A placeholder fragment containing a simple view.
     */
    public static class PlaceholderFragment extends Fragment {

        public PlaceholderFragment() {
        }

        @Override
        public View onCreateView(LayoutInflater inflater, ViewGroup container,
                                 Bundle savedInstanceState) {
            View rootView = inflater.inflate(R.layout.fragment_main, container, false);
            return rootView;
        }

        @Override
        public void onActivityCreated(Bundle savedState) {
            super.onActivityCreated(savedState);
        }
    }

    public static class MyDialogFragment extends DialogFragment {
        private Dialog mDialog;

        public MyDialogFragment() {
            super();
            mDialog = null;
        }

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

        // Set the dialog to display
        public void setDialog(Dialog dialog) {
            mDialog = dialog;
        }

        // Return a Dialog to the DialogFragment.
        @Override
        public Dialog onCreateDialog(Bundle savedInstanceState) {
            return mDialog;
        }
    }
}

现在运行应用程序，在对话框显示出来后（加载1秒钟后），旋转屏幕。请注意，我只在上面的initial onCreate中创建对话框。

以下是我收到的异常：

01-30 11:19:40.199  31986-31986/? E/AndroidRuntime﹕ FATAL EXCEPTION: main
    Process: com.example.testdialogs, PID: 31986
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.testdialogs/com.example.testdialogs.MainActivity}: java.lang.NullPointerException
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2215)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2265)
            at android.app.ActivityThread.handleRelaunchActivity(ActivityThread.java:3758)
            at android.app.ActivityThread.access$900(ActivityThread.java:145)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1212)
            at android.os.Handler.dispatchMessage(Handler.java:102)
            at android.os.Looper.loop(Looper.java:136)
            at android.app.ActivityThread.main(ActivityThread.java:5081)
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:515)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:781)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:597)
            at dalvik.system.NativeStart.main(Native Method)
     Caused by: java.lang.NullPointerException
            at android.support.v4.app.DialogFragment.onActivityCreated(DialogFragment.java:368)
            at android.support.v4.app.Fragment.performActivityCreated(Fragment.java:1508)
            at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:947)
            at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1104)
            at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1086)
            at android.support.v4.app.FragmentManagerImpl.dispatchActivityCreated(FragmentManager.java:1884)
            at android.support.v4.app.FragmentActivity.onStart(FragmentActivity.java:566)
            at android.app.Instrumentation.callActivityOnStart(Instrumentation.java:1171)
            at android.app.Activity.performStart(Activity.java:5241)
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2178)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2265)
            at android.app.ActivityThread.handleRelaunchActivity(ActivityThread.java:3758)
            at android.app.ActivityThread.access$900(ActivityThread.java:145)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1212)
            at android.os.Handler.dispatchMessage(Handler.java:102)
            at android.os.Looper.loop(Looper.java:136)
            at android.app.ActivityThread.main(ActivityThread.java:5081)
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:515)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:781)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:597)
            at dalvik.system.NativeStart.main(Native Method)
            
            
            
            

我的Gradle脚本如下所示：

apply plugin: 'android'

android {
    compileSdkVersion 19
    buildToolsVersion "19.0.1"

    defaultConfig {
        minSdkVersion 9
        targetSdkVersion 19
        versionCode 1
        versionName "1.0"
    }

    compileOptions {
        sourceCompatibility JavaVersion.VERSION_1_7
        targetCompatibility JavaVersion.VERSION_1_7
    }
    buildTypes {
        release {
            runProguard false
            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.txt'
        }
    }
}

dependencies {
    compile 'com.android.support:appcompat-v7:+'
}

当您创建新项目并使用最新的Android支持jar时，这只是标准构建脚本。

我可以在DialogFragment的onCreate方法中设置setRetainInstance，然后它就不会崩溃了，但是对话框会在旋转时被关闭。虽然这比崩溃好，但不是我想要的结果。

我不太确定使用DialogFragments的首选方式是什么，但我从一些直接来自Google（用于Google服务SDK）的示例代码中得到了这个想法。我认为他们知道他们在做什么，所以我会使用相同的概念。

- Matt Wolfe

Answer 1

我通过子类化DialogFragment类并重写下面两个生命周期回调方法来解决这样的问题：

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

@Override
public void onDestroyView() {
    if (getDialog() != null && getRetainInstance()) {
        getDialog().setDismissMessage(null);
    }
    super.onDestroyView();
}

- Andrew

Answer 2

我不太愿意回答自己的问题，但我找到了崩溃的原因。我需要在DialogFragment的onCreateDialog中实际创建对话框，而不是从我的Activity中设置它。不太确定为什么会这样。也许在旋转时，Android系统会擦除与旧Activity实例相关联的引用。这并不完全理想，但我可以通过将用于创建对话框的数据传递进去来解决这个问题。

以下是更新后的代码，在旋转时不会崩溃：

public class MainActivity extends ActionBarActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        if (savedInstanceState == null) {
            getSupportFragmentManager().beginTransaction()
                    .add(R.id.container, new PlaceholderFragment())
                    .commit();
        }
        if (savedInstanceState == null) {
            (new Handler()).postDelayed(new Runnable() {
                @Override
                public void run() {
                    MyDialogFragment dialogFragment = new MyDialogFragment();
                    dialogFragment.show(getSupportFragmentManager(), "dialog");

                }
            }, 1000);
        }
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.main, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();
        if (id == R.id.action_settings) {
            return true;
        }
        return super.onOptionsItemSelected(item);
    }

    /**
     * A placeholder fragment containing a simple view.
     */
    public static class PlaceholderFragment extends Fragment {

        public PlaceholderFragment() {
        }

        @Override
        public View onCreateView(LayoutInflater inflater, ViewGroup container,
                                 Bundle savedInstanceState) {
            View rootView = inflater.inflate(R.layout.fragment_main, container, false);
            return rootView;
        }

        @Override
        public void onActivityCreated(Bundle savedState) {
            super.onActivityCreated(savedState);
        }
    }

    public static class MyDialogFragment extends DialogFragment {
        public MyDialogFragment() {
            super();
        }

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


        // Return a Dialog to the DialogFragment.
        @Override
        public Dialog onCreateDialog(Bundle savedInstanceState) {
            return AlertDialog dialog = new AlertDialog.Builder(MainActivity.this)
                        .setMessage("Alert")
                        .setTitle("My Alert")
                        .create();
        }
    }  
}

- Matt Wolfe

Answer 3

当活动首次创建时，savedInstanceState为空，因为它没有先前的状态需要保存。当屏幕旋转时，活动被销毁并再次调用onCreate(savedInstanceState)，其中一些状态被保存（例如您的可见DialogFragment状态）。

因此，首先您的savedInstaceState为空，当屏幕旋转时，实例被保存，然后在onCreate(savedInstanceState)上恢复，因此它不再为空，任何if(savedInstanceState == null)内的内容都将被调用，然后您会得到NullPointerException。

要解决问题，请删除此验证：

if(savedInstanceState == null){

    ///blablalba

}

- Murillo Ferreira

---

不，完全不是这样的。我不想显示一个新的对话框，我想恢复之前的那个。实际上我已经弄清楚了，只是还没有提交答案。我必须在DialogFragment的onCreateDialog中创建对话框。我传递给对话框的实例似乎引起了问题。不过具体原因我也不太确定。 - Matt Wolfe

@MattWolfe，你应该在MyDialogFragment类中使用先前的对话状态，而不是在Activity类中，你试过了吗？ - Murillo Ferreira