android - 屏幕旋转后未添加 Android 片段

Question

我使用没有任何布局的片段（它在后台下载文件）。我在第一次创建活动时添加它一次，假设它将在屏幕方向后再次附加，但事实并非如此！
创建活动：

if (savedInstanceState == null) {
    getSupportFragmentManager().beginTransaction()
            .add(new FragmentDownloadApp(), FragmentDownloadApp.TAG)
            .commit();
} else {
    if (savedInstanceState.containsKey(STATE_SELECTED_NAVIGATION_ITEM)) {

片段 onActivityCreated：

@Override
public void onActivityCreated(Bundle savedInstanceState) {
    super.onActivityCreated(savedInstanceState);
    // this is really important in order to save the state across screen
    // configuration changes for example
    setRetainInstance(true);

    // ---- magic lines starting here -----
    // call this to re-connect with an existing
    // loader (after screen configuration changes for e.g!)
    LoaderManager lm = getLoaderManager();
    if (lm.getLoader(LOADER_DOWNLOAD_APP) != null) {
        lm.initLoader(LOADER_DOWNLOAD_APP, null, mDownLoader);
    }
    // ----- end magic lines -----
}

和错误：

public void onPositiveClick() {
            FragmentDownloadApp fragment = (FragmentDownloadApp) getSupportFragmentManager()
                    .findFragmentByTag(FragmentDownloadApp.TAG);
            fragment.startDownload(mAppToDownload);
        }

这里出现 NullPointerException！

Answer 1

找到答案：

@Override
public void onSaveInstanceState(Bundle outState) {
    super.onSaveInstanceState(outState); // NEED THIS!