转载请注明出处：http://blog.csdn.net/droyon/article/details/17798697

 

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本文主要回答下面这个问题：
Android中，Header是如何被PreferenceActivity进行加载的？
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在Android应用程序中，我们可以在继承自PreferenceActivity的页面中通过两种方式，加载设置项。

其一：loadHeadersFromResource

@Overridepublic void onBuildHeaders(List headers) {loadHeadersFromResource(R.xml.settings_headers, headers);updateHeaderList(headers);mHeaders = headers;}

settings_headers.xml

<preference-headersxmlns:android="http://schemas.android.com/apk/res/android"><!-- WIRELESS and NETWORKS --><header android:title="@string/header_category_wireless_networks" /><!-- Wifi --><headerandroid:id="@+id/wifi_settings"android:fragment="com.android.settings.wifi.WifiSettings"android:title="@string/wifi_settings_title"android:icon="@drawable/ic_settings_wireless" /><!-- Bluetooth --><headerandroid:id="@+id/bluetooth_settings"android:fragment="com.android.settings.bluetooth.BluetoothSettings"android:title="@string/bluetooth_settings_title"android:icon="@drawable/ic_settings_bluetooth2" /></preference-headers>

其二：

 addPreferencesFromResource(R.xml.device_info_settings);

device_info_settings.xml

<PreferenceScreen xmlns:android="http://schemas.android.com/apk/res/android"android:title="@string/about_settings"><!-- System update settings - launches activity --><PreferenceScreen android:key="system_update_settings"android:title="@string/system_update_settings_list_item_title" android:summary="@string/system_update_settings_list_item_summary"><intent android:action="android.settings.SYSTEM_UPDATE_SETTINGS" /></PreferenceScreen><PreferenceScreen android:key="additional_system_update_settings"android:title="@string/additional_system_update_settings_list_item_title"><intent android:action="android.intent.action.MAIN"android:targetPackage="@string/additional_system_update"android:targetClass="@string/additional_system_update_menu" /></PreferenceScreen></PreferenceScreen>

这两种方式都可以满足我们的需求，今天我们主要来看一下第一种方式。

 

使用Header，内部大体经历了两个阶段，第一、解析xml配置文件，构建Header列表。第二、构建adapter进行Header列表的加载以及显示。

 

1、在PreferenceActivity.java中，调用loadHeadersFromResource进行Header的加载。

流程如下：

PreferenceActivity.java

public void loadHeadersFromResource(int resid, List<Header> target) {XmlResourceParser parser = null;try {parser = getResources().getXml(resid);AttributeSet attrs = Xml.asAttributeSet(parser);int type;while ((type=parser.next()) != XmlPullParser.END_DOCUMENT&& type != XmlPullParser.START_TAG) {// Parse next until start tag is found}String nodeName = parser.getName();if (!"preference-headers".equals(nodeName)) {throw new RuntimeException("XML document must start with <preference-headers> tag; found"+ nodeName + " at " + parser.getPositionDescription());}Bundle curBundle = null;final int outerDepth = parser.getDepth();while ((type=parser.next()) != XmlPullParser.END_DOCUMENT&& (type != XmlPullParser.END_TAG || parser.getDepth() > outerDepth)) {if (type == XmlPullParser.END_TAG || type == XmlPullParser.TEXT) {continue;}nodeName = parser.getName();if ("header".equals(nodeName)) {Header header = new Header();TypedArray sa = getResources().obtainAttributes(attrs,com.android.internal.R.styleable.PreferenceHeader);header.id = sa.getResourceId(com.android.internal.R.styleable.PreferenceHeader_id,(int)HEADER_ID_UNDEFINED);TypedValue tv = sa.peekValue(com.android.internal.R.styleable.PreferenceHeader_title);if (tv != null && tv.type == TypedValue.TYPE_STRING) {if (tv.resourceId != 0) {header.titleRes = tv.resourceId;} else {header.title = tv.string;}}tv = sa.peekValue(com.android.internal.R.styleable.PreferenceHeader_summary);if (tv != null && tv.type == TypedValue.TYPE_STRING) {if (tv.resourceId != 0) {header.summaryRes = tv.resourceId;} else {header.summary = tv.string;}}tv = sa.peekValue(com.android.internal.R.styleable.PreferenceHeader_breadCrumbTitle);if (tv != null && tv.type == TypedValue.TYPE_STRING) {if (tv.resourceId != 0) {header.breadCrumbTitleRes = tv.resourceId;} else {header.breadCrumbTitle = tv.string;}}tv = sa.peekValue(com.android.internal.R.styleable.PreferenceHeader_breadCrumbShortTitle);if (tv != null && tv.type == TypedValue.TYPE_STRING) {if (tv.resourceId != 0) {header.breadCrumbShortTitleRes = tv.resourceId;} else {header.breadCrumbShortTitle = tv.string;}}header.iconRes = sa.getResourceId(com.android.internal.R.styleable.PreferenceHeader_icon, 0);header.fragment = sa.getString(com.android.internal.R.styleable.PreferenceHeader_fragment);sa.recycle();if (curBundle == null) {curBundle = new Bundle();}final int innerDepth = parser.getDepth();while ((type=parser.next()) != XmlPullParser.END_DOCUMENT&& (type != XmlPullParser.END_TAG || parser.getDepth() > innerDepth)) {if (type == XmlPullParser.END_TAG || type == XmlPullParser.TEXT) {continue;}String innerNodeName = parser.getName();if (innerNodeName.equals("extra")) {getResources().parseBundleExtra("extra", attrs, curBundle);XmlUtils.skipCurrentTag(parser);} else if (innerNodeName.equals("intent")) {header.intent = Intent.parseIntent(getResources(), parser, attrs);} else {XmlUtils.skipCurrentTag(parser);}}if (curBundle.size() > 0) {header.fragmentArguments = curBundle;curBundle = null;}target.add(header);} else {XmlUtils.skipCurrentTag(parser);}}} catch (XmlPullParserException e) {throw new RuntimeException("Error parsing headers", e);} catch (IOException e) {throw new RuntimeException("Error parsing headers", e);} finally {if (parser != null) parser.close();}}

上面的这段代码是基于android 4.0版本截取的，代码内容大体可以分为三个阶段。

• 阶段一：将resId对应的xml文件，加载成为XmlResourceParser、AttributeSet。即：

 parser = getResources().getXml(resid);AttributeSet attrs = Xml.asAttributeSet(parser);

• 阶段二：进行xml文件解析，解析出来Headers。解析方式为Pull解析，这也是说android内部是使用Pull解析的。（关于Pull解析请移步：http://blog.csdn.net/droyon/article/details/9346885）

if (!"preference-headers".equals(nodeName)) {throw new RuntimeException("XML document must start with <preference-headers> tag; found"+ nodeName + " at " + parser.getPositionDescription());}

首先xml的标题头部必须为“preference-headers”，否则，抛出Runntime异常。

if ("header".equals(nodeName)) {Header header = new Header();TypedArray sa = getResources().obtainAttributes(attrs,com.android.internal.R.styleable.PreferenceHeader);header.id = sa.getResourceId(com.android.internal.R.styleable.PreferenceHeader_id,(int)HEADER_ID_UNDEFINED);TypedValue tv = sa.peekValue(com.android.internal.R.styleable.PreferenceHeader_title);if (tv != null && tv.type == TypedValue.TYPE_STRING) {if (tv.resourceId != 0) {header.titleRes = tv.resourceId;} else {header.title = tv.string;}}tv = sa.peekValue(com.android.internal.R.styleable.PreferenceHeader_summary);if (tv != null && tv.type == TypedValue.TYPE_STRING) {if (tv.resourceId != 0) {header.summaryRes = tv.resourceId;} else {header.summary = tv.string;}}tv = sa.peekValue(com.android.internal.R.styleable.PreferenceHeader_breadCrumbTitle);if (tv != null && tv.type == TypedValue.TYPE_STRING) {if (tv.resourceId != 0) {header.breadCrumbTitleRes = tv.resourceId;} else {header.breadCrumbTitle = tv.string;}}tv = sa.peekValue(com.android.internal.R.styleable.PreferenceHeader_breadCrumbShortTitle);if (tv != null && tv.type == TypedValue.TYPE_STRING) {if (tv.resourceId != 0) {header.breadCrumbShortTitleRes = tv.resourceId;} else {header.breadCrumbShortTitle = tv.string;}}header.iconRes = sa.getResourceId(com.android.internal.R.styleable.PreferenceHeader_icon, 0);header.fragment = sa.getString(com.android.internal.R.styleable.PreferenceHeader_fragment);sa.recycle();

这部分解析出来Header对象，并且对Header对象的title，summary，以及id，frament进行赋值。

while ((type=parser.next()) != XmlPullParser.END_DOCUMENT&& (type != XmlPullParser.END_TAG || parser.getDepth() > innerDepth)) {if (type == XmlPullParser.END_TAG || type == XmlPullParser.TEXT) {continue;}String innerNodeName = parser.getName();if (innerNodeName.equals("extra")) {getResources().parseBundleExtra("extra", attrs, curBundle);XmlUtils.skipCurrentTag(parser);} else if (innerNodeName.equals("intent")) {header.intent = Intent.parseIntent(getResources(), parser, attrs);} else {XmlUtils.skipCurrentTag(parser);}}

这部分解析extra以及Intent。

• 阶段三、将解析出来的Header加入到列表中。

target.add(header);

这里的target，参见上文，target为：PreferenceActivity中的mHeaders。

 

有人可能有疑问：为什么要在PreferenceActivity的继承类的onBuildHeaders方法中进行xml加载，onBuildHeaders什么时候调用？

理解了这个问题，也就明白了为什么上文中的target为PreferenceActivity中的mHeaders了。

答案就是，在PreferenceActivity的onCreate方法中，会调用onBuildHeader方法，同时将成员变量mHeader作为参数。

@Overrideprotected void onCreate(Bundle savedInstanceState) {super.onCreate(savedInstanceState);setContentView(com.android.internal.R.layout.preference_list_content);//........// 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);}}}}}//........  
}

我们在继承自PreferenceActivity的子类中：

public class Settings extends PreferenceActivity{@Overridepublic void onBuildHeaders(List<Header> headers) {loadHeadersFromResource(R.xml.settings_headers, headers);updateHeaderList(headers);mHeaders = headers;}
}

回答完毕。

通过上面的介绍，我们完成了第一步，即：xml配置文件的解析以及Header的列表加载。

第二、adapter的构建。

通过第一步，PreferenceActivity得到了一个Header的List集合，mHeaders。

同样是在PreferenceActivity的onCreate中，会执行如下代码：

setListAdapter(new HeaderAdapter(this, mHeaders));

PreferenceActivity继承自ListActivity，内部拥有ListView的引用。

上段代码就是我们常用的Adapter，ListView形式，不需要再介绍了吧。

附：HeaderAdapter的源码：

private static class HeaderAdapter extends ArrayAdapter<Header> {private static class HeaderViewHolder {ImageView icon;TextView title;TextView summary;}private LayoutInflater mInflater;public HeaderAdapter(Context context, List<Header> objects) {super(context, 0, objects);mInflater = (LayoutInflater)context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);}@Overridepublic View getView(int position, View convertView, ViewGroup parent) {HeaderViewHolder holder;View view;if (convertView == null) {view = mInflater.inflate(com.android.internal.R.layout.preference_header_item,parent, false);holder = new HeaderViewHolder();holder.icon = (ImageView) view.findViewById(com.android.internal.R.id.icon);holder.title = (TextView) view.findViewById(com.android.internal.R.id.title);holder.summary = (TextView) view.findViewById(com.android.internal.R.id.summary);view.setTag(holder);} else {view = convertView;holder = (HeaderViewHolder) view.getTag();}// All view fields must be updated every time, because the view may be recycled Header header = getItem(position);holder.icon.setImageResource(header.iconRes);holder.title.setText(header.getTitle(getContext().getResources()));CharSequence summary = header.getSummary(getContext().getResources());if (!TextUtils.isEmpty(summary)) {holder.summary.setVisibility(View.VISIBLE);holder.summary.setText(summary);} else {holder.summary.setVisibility(View.GONE);}return view;}}

以上就是PreferenceActivity加载Header的整个流程。不当之处，欢迎大家提出宝贵意见，共同学习。