PreferenceActivity 中的 onBuildHeaders() 调用时机
1. onBuildHeaders(List<PreferenceActivity.Header>)在前一篇博文中（何时使用 Preference Headers）我们讲述了 Preference Headers 的作用和使用场景，onBuildHeaders 这个方法是在 PreferenceActivity 中用于创建和初始化 headers 的，创建 headers 之后我们可以在程序中引用 headers 实例，但需要注意其创建的时机，以免在创建之前就去引用从而导致出错。
2. 调用时机通过分析 PreferenceActivity 的源代码，可知 headers 的创建时机是在 onCreate() 的过程中被调用的，如下所示：
@Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(com.android.internal.R.layout.preference_list_content); mListFooter = (FrameLayout)findViewById(com.android.internal.R.id.list_footer); mPrefsContainer = (ViewGroup) findViewById(com.android.internal.R.id.prefs_frame); boolean hidingHeaders = onIsHidingHeaders(); mSinglePane = hidingHeaders || !onIsMultiPane(); String initialFragment = getIntent().getStringExtra(EXTRA_SHOW_FRAGMENT); Bundle initialArguments = getIntent().getBundleExtra(EXTRA_SHOW_FRAGMENT_ARGUMENTS); int initialTitle = getIntent().getIntExtra(EXTRA_SHOW_FRAGMENT_TITLE, 0); int initialShortTitle = getIntent().getIntExtra(EXTRA_SHOW_FRAGMENT_SHORT_TITLE, 0); if (savedInstanceState != null) { // We are restarting from a previous saved state; used that to // initialize, instead of starting fresh. ArrayList<Header> headers = savedInstanceState.getParcelableArrayList(HEADERS_TAG); if (headers != null) { mHeaders.addAll(headers); int curHeader = savedInstanceState.getInt(CUR_HEADER_TAG, (int) HEADER_ID_UNDEFINED); if (curHeader >= 0 && curHeader < mHeaders.size()) { setSelectedHeader(mHeaders.get(curHeader)); } } } else { if (initialFragment != null && mSinglePane) { // If we are just showing a fragment, we want to run in // new fragment mode, but don't need to compute and show // the headers. switchToHeader(initialFragment, initialArguments); if (initialTitle != 0) { CharSequence initialTitleStr = getText(initialTitle); CharSequence initialShortTitleStr = initialShortTitle != 0 ? getText(initialShortTitle) : null; showBreadCrumbs(initialTitleStr, initialShortTitleStr); } } else { // We need to try to build the headers. onBuildHeaders(mHeaders); // If there are headers, then at this point we need to show // them and, depending on the screen, we may also show in-line // the currently selected preference fragment. if (mHeaders.size() > 0) { if (!mSinglePane) { if (initialFragment == null) { Header h = onGetInitialHeader(); switchToHeader(h); } else { switchToHeader(initialFragment, initialArguments); } } } } }/* 以下省略 */ }