Question

我忠实地遵循了这里的指导方针：http://developer.android.com/guide/topics/resources/runtime-changes.html。我很惊讶地发现使用RetainedFragment处理配置更改是多么容易。一切都在正常的事件流中正常运作。但是有一个小故障。如果我最小化应用程序并在很长一段时间后启动它，它会崩溃：

E/AndroidRuntime( 5734): FATAL EXCEPTION: main^M                                                                               E/AndroidRuntime( 5734): Process: net.citibuzz.app.scool, PID: 5734^M                                                
E/AndroidRuntime( 5734): java.lang.RuntimeException: Unable to start activity ComponentInfo{net.citibuzz.app.scool/net.citibuzz.app.scool.MapActivity}: android.support.v4.app.Fragment$InstantiationException: Unable to instantiate fragment net.citibuzz.app.scool.RetainedFragment: make sure class name exists, is public, and has an empty constructor that is public^M
E/AndroidRuntime( 5734):        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2209)^M      
E/AndroidRuntime( 5734):        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2269)^M       
E/AndroidRuntime( 5734):        at android.app.ActivityThread.access$800(ActivityThread.java:139)^M                  
E/AndroidRuntime( 5734):        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1210)^M            
E/AndroidRuntime( 5734):        at android.os.Handler.dispatchMessage(Handler.java:102)^M                            
E/AndroidRuntime( 5734):        at android.os.Looper.loop(Looper.java:136)^M                                         
E/AndroidRuntime( 5734):        at android.app.ActivityThread.main(ActivityThread.java:5102)^M                       
E/AndroidRuntime( 5734):        at java.lang.reflect.Method.invokeNative(Native Method)^M                            
E/AndroidRuntime( 5734):        at java.lang.reflect.Method.invoke(Method.java:515)^M                                
E/AndroidRuntime( 5734):        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:785)^M         
E/AndroidRuntime( 5734):        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:601)^M                    
E/AndroidRuntime( 5734):        at dalvik.system.NativeStart.main(Native Method)^M                                       
E/AndroidRuntime( 5734): Caused by: android.support.v4.app.Fragment$InstantiationException: Unable to instantiate fragment net.citibuzz.app.scool.RetainedFragment: make sure class name exists, is public, and has an empty constructor that is public^M
E/AndroidRuntime( 5734):        at android.support.v4.app.Fragment.instantiate(Fragment.java:413)^M                  
E/AndroidRuntime( 5734):        at android.support.v4.app.FragmentState.instantiate(Fragment.java:97)^M               
E/AndroidRuntime( 5734):        at android.support.v4.app.FragmentManagerImpl.restoreAllState(FragmentManager.java:1790)^M
E/AndroidRuntime( 5734):        at android.support.v4.app.FragmentActivity.onCreate(FragmentActivity.java:213)^M     
E/AndroidRuntime( 5734):        at android.support.v7.app.ActionBarActivity.onCreate(ActionBarActivity.java:97)^M    
E/AndroidRuntime( 5734):        at net.citibuzz.app.scool.MapActivity.onCreate(MapActivity.java:134)^M               
E/AndroidRuntime( 5734):        at android.app.Activity.performCreate(Activity.java:5248)^M                          
E/AndroidRuntime( 5734):        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1110)^M     
E/AndroidRuntime( 5734):        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2173)^M      
E/AndroidRuntime( 5734):        ... 11 more^M                                                                            
E/AndroidRuntime( 5734): Caused by: java.lang.InstantiationException: can't instantiate class net.citibuzz.app.scool.RetainedFragment; no empty constructor^M 
E/AndroidRuntime( 5734):        at java.lang.Class.newInstanceImpl(Native Method)^M                                  
E/AndroidRuntime( 5734):        at java.lang.Class.newInstance(Class.java:1208)^M                                    
E/AndroidRuntime( 5734):        at android.support.v4.app.Fragment.instantiate(Fragment.java:402)^M                  
E/AndroidRuntime( 5734):        ... 19 more^M

显然系统资源耗尽，杀死了活动实例，然后尝试在重启时重新创建它。这对我来说有点混乱。看起来系统正在以不同的方式处理两种情况：一种情况是由于配置改变而破坏活动，另一种情况是由于资源紧张而破坏它。我认为这一定是个常见问题。我该如何处理？

如果需要，我很乐意提供代码段，但正如我在开头所说的那样，我已将Google的示例代码添加到T中。

编辑：好的，我意识到我确实做了一些改变。我添加了一个构造函数： - （

public class RetainedFragment extends Fragment {

    // data object we want to retain
    private MapState data;
    private Context mContext;

    public RetainedFragment(Context context) {
        mContext = context;
    }

    // this method is only called once for this fragment
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        // retain this fragment
        setRetainInstance(true);
    }

    public void setData(MapState data) {
        this.data = data;
    }

    public MapState getData() {
        return data;
    }

    @Override
    public void onDestroy() {
        super.onDestroy();

    }

    ...
}

Answer 1

Caused by: java.lang.InstantiationException: can't instantiate class net.citibuzz.app.scool.RetainedFragment;

你不能在片段中有一个自定义构造函数，它需要调用它的空构造函数，如果它没有定义则会出现该异常

这是关于它的documentation：

Fragment的所有子类都必须包含一个公共空构造函数框架通常会在需要时重新实例化一个片段类，特别是在状态恢复期间，需要能够找到这个构造函数来实例化它。如果没有空构造函数可用时，在某些情况下会发生运行时异常状态恢复。

顺便说一句，您可以通过调用getActivity();调用活动的上下文来调用片段中对上下文的引用

如果要实例化片段，则需要创建一个静态方法，该方法将返回新创建片段的引用

public static RetainedFragment newInstance(int index) {
    RetainedFragment f = new RetainedFragment();

    // Supply index input as an argument.
    Bundle args = new Bundle();
    args.putInt("index", index);
    f.setArguments(args);

    return f;
}

Android应用在重启时崩溃

1 个答案: