片段MyFragment未附加到Activity

Question

我创建了一个代表我的问题的小测试应用程序.我正在使用ActionBarSherlock来实现带有(Sherlock)Fragments的选项卡.

我的代码: TestActivity.java

public class TestActivity extends SherlockFragmentActivity {
    private ActionBar actionBar;

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

    private void setupTabs(Bundle savedInstanceState) {
        actionBar = getSupportActionBar();
        actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS);

        addTab1();
        addTab2();
    }

    private void addTab1() {
        Tab tab1 = actionBar.newTab();
        tab1.setTag("1");
        String tabText = "1";
        tab1.setText(tabText);
        tab1.setTabListener(new TabListener<MyFragment>(TestActivity.this, "1", MyFragment.class));

        actionBar.addTab(tab1);
    }

    private void addTab2() {
        Tab tab1 = actionBar.newTab();
        tab1.setTag("2");
        String tabText = "2";
        tab1.setText(tabText);
        tab1.setTabListener(new TabListener<MyFragment>(TestActivity.this, "2", MyFragment.class));

        actionBar.addTab(tab1);
    }
}

TabListener.java

public class TabListener<T extends SherlockFragment> implements com.actionbarsherlock.app.ActionBar.TabListener {
    private final SherlockFragmentActivity mActivity;
    private final String mTag;
    private final Class<T> mClass;

    public TabListener(SherlockFragmentActivity activity, String tag, Class<T> clz) {
        mActivity = activity;
        mTag = tag;
        mClass = clz;
    }

    /* The following are each of the ActionBar.TabListener callbacks */

    public void onTabSelected(Tab tab, FragmentTransaction ft) {
        SherlockFragment preInitializedFragment = (SherlockFragment) mActivity.getSupportFragmentManager().findFragmentByTag(mTag);

        // Check if the fragment is already initialized
        if (preInitializedFragment == null) {
            // If not, instantiate and add it to the activity
            SherlockFragment mFragment = (SherlockFragment) SherlockFragment.instantiate(mActivity, mClass.getName());
            ft.add(android.R.id.content, mFragment, mTag);
        } else {
            ft.attach(preInitializedFragment);
        }
    }

    public void onTabUnselected(Tab tab, FragmentTransaction ft) {
        SherlockFragment preInitializedFragment = (SherlockFragment) mActivity.getSupportFragmentManager().findFragmentByTag(mTag);

        if (preInitializedFragment != null) {
            // Detach the fragment, because another one is being attached
            ft.detach(preInitializedFragment);
        }
    }

    public void onTabReselected(Tab tab, FragmentTransaction ft) {
        // User selected the already selected tab. Usually do nothing.
    }
}

MyFragment.java

public class MyFragment extends SherlockFragment {

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

        new AsyncTask<Void, Void, Void>() {

            @Override
            protected Void doInBackground(Void... params) {
                try {
                    Thread.sleep(2000);
                } catch (InterruptedException ex) {
                }
                return null;
            }

            @Override
            protected void onPostExecute(Void result){
                getResources().getString(R.string.app_name);
            }

        }.execute();
    }
}

我添加了Thread.sleep部件来模拟下载数据.该代码onPostExecute是为了模拟使用Fragment.

当我在横向和纵向之间快速旋转屏幕时,我在onPostExecute代码中得到一个异常:

java.lang.IllegalStateException:片段MyFragment {410f6060}未附加到Activity

我认为这是因为MyFragment在此期间创建了一个新的,并且在AsyncTask完成之前附加到Activity .onPostExecute调用的代码是单独的MyFragment.

但我该如何解决这个问题呢？

Answer 1

我找到了一个非常简单的答案isAdded():

返回true如果片段正在增加其活性.

@Override
protected void onPostExecute(Void result){
    if(isAdded()){
        getResources().getString(R.string.app_name);
    }
}

为了避免onPostExecute在Fragment没有附加时被调用Activity是取消AsyncTask时暂停或停止Fragment.那就isAdded()没有必要了.但是,建议保持此检查.

Answer 2

问题是您正在尝试使用getResources().getString()来访问资源(在本例中为字符串),它将尝试从Activity获取资源.查看Fragment类的源代码:

 /**
  * Return <code>getActivity().getResources()</code>.
  */
 final public Resources getResources() {
     if (mHost == null) {
         throw new IllegalStateException("Fragment " + this + " not attached to Activity");
     }
     return mHost.getContext().getResources();
 }

mHost 是保存您的Activity的对象.

由于可能未附加Activity,因此getResources()调用将抛出异常.

可接受的解决方案恕我直言,因为你只是隐藏问题.正确的方法是从其他地方获取始终保证存在的资源,例如应用程序上下文:

youApplicationObject.getResources().getString(...)

Answer 3

我在这里遇到了两种不同的情况:

1)当我希望异步任务完成时:假设我的onPostExecute确实存储了收到的数据,然后调用一个监听器来更新视图,为了更高效,我希望任务完成,所以当用户来时我已准备好数据背部.在这种情况下,我通常这样做:

@Override
protected void onPostExecute(void result) {
    // do whatever you do to save data
    if (this.getView() != null) {
        // update views
    }
}

2)当我希望异步任务只在视图可以更新时完成时:你在这里建议的情况下,任务只更新视图,不需要数据存储,所以如果视图是完成的,它就没有完成任务的线索不再被展示.我这样做:

@Override
protected void onStop() {
    // notice here that I keep a reference to the task being executed as a class member:
    if (this.myTask != null && this.myTask.getStatus() == Status.RUNNING) this.myTask.cancel(true);
    super.onStop();
}

我发现没有问题,虽然我也使用(可能)更复杂的方式,包括从活动而不是片段启动任务.

希望这有助于某人!:)

Answer 4

代码的问题在于您使用AsyncTask的方式,因为在睡眠线程期间旋转屏幕时:

Thread.sleep(2000) 

AsyncTask仍在工作,这是因为在片段重建(旋转时)之前没有在onDestroy()中正确取消AsyncTask实例,并且当同一个AsyncTask实例(旋转后)运行onPostExecute()时,这会尝试查找getResources()与旧片段实例(无效实例)的资源:

getResources().getString(R.string.app_name)

这相当于:

MyFragment.this.getResources().getString(R.string.app_name)

所以最终的解决方案是在旋转屏幕时重建片段之前管理AsyncTask实例(如果它仍然有效则取消),如果在转换期间取消,则在重建后借助布尔标志重新启动AsyncTask:

public class MyFragment extends SherlockFragment {

    private MyAsyncTask myAsyncTask = null;
    private boolean myAsyncTaskIsRunning = true;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        if(savedInstanceState!=null) {
            myAsyncTaskIsRunning = savedInstanceState.getBoolean("myAsyncTaskIsRunning");
        }
        if(myAsyncTaskIsRunning) {
            myAsyncTask = new MyAsyncTask();
            myAsyncTask.execute();
        }
    }

    @Override
    public void onSaveInstanceState(Bundle outState) {
        super.onSaveInstanceState(outState);
        outState.putBoolean("myAsyncTaskIsRunning",myAsyncTaskIsRunning);
    }

    @Override
    public void onDestroy() {
        super.onDestroy();
        if(myAsyncTask!=null) myAsyncTask.cancel(true);
        myAsyncTask = null;

    }

    public class MyAsyncTask extends AsyncTask<Void, Void, Void>() {

        public MyAsyncTask(){}

        @Override
        protected void onPreExecute() {
            super.onPreExecute();
            myAsyncTaskIsRunning = true;
        }
        @Override
        protected Void doInBackground(Void... params) {
            try {
                Thread.sleep(2000);
            } catch (InterruptedException ex) {}
            return null;
        }

        @Override
        protected void onPostExecute(Void result){
            getResources().getString(R.string.app_name);
            myAsyncTaskIsRunning = false;
            myAsyncTask = null;
        }

    }
}

Answer 5

它们是非常棘手的解决方案,也可以从活动中泄漏碎片.

因此,对于getResource或依赖于从Fragment访问的活动上下文的任何东西,它总是检查活动状态和片段状态如下

 Activity activity = getActivity(); 
    if(activity != null && isAdded())

         getResources().getString(R.string.no_internet_error_msg);
//Or any other depends on activity context to be live like dailog


        }
    }

Answer 6

if (getActivity() == null) return;

在某些情况下也有效.只需打破代码执行,确保应用程序不会崩溃

Answer 7

我遇到了同样的问题,我只需添加单一实例来获取Erick所引用的资源

MainFragmentActivity.defaultInstance().getResources().getString(R.string.app_name);

你也可以用

getActivity().getResources().getString(R.string.app_name);

我希望这将有所帮助.