Fragment MyFragment 未附加到 Activity [英] Fragment MyFragment not attached to Activity

问题描述

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

I've created a small test app which represents my problem. I'm using ActionBarSherlock to implement tabs with (Sherlock)Fragments.

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 部分来模拟下载数据.onPostExecute中的代码是模拟Fragment的使用.

I've added the Thread.sleep part to simulate downloading data. The code in the onPostExecute is to simulate use of the Fragment.

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

When I rotate the screen very fast between landscape and portrait, I get an Exception at the onPostExecute code:

java.lang.IllegalStateException: Fragment MyFragment{410f6060} 不是附加到活动

java.lang.IllegalStateException: Fragment MyFragment{410f6060} not attached to Activity

我认为这是因为在此期间创建了一个新的 MyFragment，并在 AsyncTask 完成之前附加到 Activity.onPostExecute 中的代码调用未附加的 MyFragment.

I think it's because a new MyFragment has been created in the meantime, and was attached to the Activity before the AsyncTask finished. The code in onPostExecute calls upon a unattached MyFragment.

但是我该如何解决这个问题?

But how can I fix this?

推荐答案

我找到了非常简单的答案:isAdded():

I've found the very simple answer: isAdded():

如果片段当前已添加到其活动中，则返回 true.

Return true if the fragment is currently added to its activity.

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

当Fragment没有附加到Activity时，避免onPostExecute被调用是取消AsyncTask> 暂停或停止 Fragment 时.那么 isAdded() 就不再需要了.但是，建议保留此检查.

To avoid onPostExecute from being called when the Fragment is not attached to the Activity is to cancel the AsyncTask when pausing or stopping the Fragment. Then isAdded() would not be necessary anymore. However, it is advisable to keep this check in place.

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