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 */
    }

• 我们在继承的 PreferenceActivity 中实现 onCreate() 的过程中必须认清楚，在调用 super.onCreate() 之后才能引用已经初始化的 headers，否则将出错。