android - Null FragmentTransaction 被传递给 TabListener.onTabSelected()

Question

我正在使用此处提供的示例代码将导航选项卡添加到操作栏: http://developer.android.com/guide/topics/ui/actionbar.html我正在使用 ActionBarSherlock。

我的 TabListener 是直接从文档中复制而来的。

    public static class TabListener<T extends Fragment> implements ActionBar.TabListener {
    private Fragment mFragment;
    private final Activity mActivity;
    private final String mTag;
    private final Class<T> mClass;

    /** Constructor used each time a new tab is created.
      * @param activity  The host Activity, used to instantiate the fragment
      * @param tag  The identifier tag for the fragment
      * @param clz  The fragment's Class, used to instantiate the fragment
      */
    public TabListener(Activity activity, String tag, Class<T> clz) {
        mActivity = activity;
        mTag = tag;
        mClass = clz;
    }

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

    public void onTabSelected(ActionBar.Tab tab, FragmentTransaction ft) {
        // Check if the fragment is already initialized
        if (mFragment == null) {
            // If not, instantiate and add it to the activity
            mFragment = Fragment.instantiate(mActivity, mClass.getName());
            ft.add(android.R.id.content, mFragment, mTag);
        } else {
            // If it exists, simply attach it in order to show it
            ft.attach(mFragment);
        }
    }

    public void onTabUnselected(ActionBar.Tab tab, FragmentTransaction ft) {
        if (mFragment != null) {
            // Detach the fragment, because another one is being attached
            ft.detach(mFragment);
        }
    }

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

我在我的 Activity 中设置了监听器:

    ActionBar.Tab tab = actionBar.newTab();
    tab.setText(getString(R.string.TAB_CALC));
    tab.setTabListener(new TabListener<StrokeSelectorFragment>(this, "blah", StrokeSelectorFragment.class));
    actionBar.addTab(tab);

但应用程序因空指针异常而崩溃，因为空 FragmentTransaction 正在传递给监听器中的 onTabSelected()。我应该在某个地方创建一个吗？

我一遍又一遍地看文档，我很困惑。你能看到我错过了什么吗？

Answer 1

我发现上面 Ollie 的帖子很有帮助，但不够详细。我设法重构了 ActionBar Guide 中提供的代码如下(注意:您必须对 TabListener 中实现的其他方法进行相同的更改):

public void onTabSelected(Tab tab, FragmentTransaction ignoredFt) {
    FragmentManager fragMgr = ((FragmentActivity)mActivity).getSupportFragmentManager();
    FragmentTransaction ft = fragMgr.beginTransaction();

    // Check if the fragment is already initialized
    if (mFragment == null) {
        // If not, instantiate and add it to the activity
        mFragment = Fragment.instantiate(mActivity, mClass.getName());

        ft.add(android.R.id.content, mFragment, mTag);
    } else {
        // If it exists, simply attach it in order to show it
        ft.attach(mFragment);
    }
}