1. onBuildHeaders(List<PreferenceActivity.Header>)
在前一篇博文中(何時使用 Preference Headers)我們講述了 Preference Headers 的作用和使用場景,onBuildHeaders 這個方法是在 PreferenceActivity 中用於創建和初始化 headers
的,創建 headers 之後我們可以在程序中引用 headers 實例,但需要注意其創建的時機,以免在創建之前就去引用從而導致出錯。
2. 調用時機
- 通過分析 PreferenceActivity 的源代碼,可知 headers 的創建時機是在 onCreate() 的過程中被調用的,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 */
}
- 我們在繼承的 PreferenceActivity 中實現 onCreate() 的過程中必須認清楚,在調用 super.onCreate() 之後才能引用已經初始化的 headers,否則將出錯。