Android 4.0新增WIFI DIRECT的功能，但是在模拟器上以及一些可以升级至4.0的手机或平板，在settings里面仍然没有WIFI DIRECT功能选项。于是出于好奇，所以跟踪了一
下源码。
1. 查找在系统设置包中是否有WIFI DIRECT这部分代码处理。
在Android4.0的源码路径（我的是源码路径为:myandroid_4.0）/packages/apps/Settings/src/com/android/settings/wifi中发现有P2P这个文件夹，哪说明设置包里面具有WIFI DIRECT这部分的代码处理。
2. 查找在系统设置里对WIFI DIRECT是否有过滤。
跟踪Settings包源码，终于发现苗头：在WirelessSettings.java (myandroid_4.0\packages\apps\settings\src\com\android\settings) 文件中
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
..........................................
...........................................
WifiP2pManager p2p = (WifiP2pManager) activity.getSystemService(Context.WIFI_P2P_SERVICE);
if (!getPackageManager().hasSystemFeature(PackageManager.FEATURE_WIFI_DIRECT)) {
getPreferenceScreen().removePreference(wifiP2p);
} else {
mWifiP2pEnabler = new WifiP2pEnabler(activity, wifiP2p);
}
...........................................
...........................................
}
1）分析PackageManager.FEATURE_WIFI_DIRECT：文件PackageManager.java (frameworks\base\core\java\android\content\pm) 中：
@SdkConstant(SdkConstantType.FEATURE)
public static final String FEATURE_WIFI_DIRECT = "android.hardware.wifi.direct";//即为传进去做判断的String。
2）分析getPackageManager()函数
文件SettingsPreferenceFragment.java (packages\apps\settings\src\com\android\settings)中，
...................................
...................................
/**
* Returns the PackageManager from the owning Activity.
*/
protected PackageManager getPackageManager() {
return getActivity().getPackageManager();
}
...................................
...................................
分析getAcivity()返回一个Activity。Activity.java (frameworks\base\core\java\android\app) 中没有 getPackageManager()函数；
由于public class Activity extends ContextThemeWrapper，所以进入文件ContextThemeWrapper.java (frameworks\base\core\java\android\view)中，
也没有getPackageManager（）函数，同理发现public class ContextThemeWrapper extends ContextWrapper，所以进入文件ContextWrapper.java (frameworks\base\core\java\android\content) ，此时终于见到getPackageManager（）踪影：
public class ContextWrapper extends Context {
Context mBase;
....................................
....................................
@Override
public PackageManager getPackageManager() {
return mBase.getPackageManager();
}
.....................................
......................................
}
继续跟踪文件Context.java (frameworks\base\core\java\android\content)，发现getPackageManager（）是一个抽象函数：
/** Return PackageManager instance to find global package information. */
public abstract PackageManager getPackageManager();
分析到这里其实我也不知到该怎么继续跟踪这个函数了，想想是否跟这个文件（因为class ContextImpl extends Context）有关联ContextImpl.java (frameworks\base\core\java\android\app)，的确找到了关心的代码：
@Override
public PackageManager getPackageManager() {
if (mPackageManager != null) {
return mPackageManager;
}
IPackageManager pm = ActivityThread.getPackageManager();
if (pm != null) {
// Doesn't matter if we make more than one instance.
return (mPackageManager = new ApplicationPackageManager(this, pm));
}
return null;
}
因为 final class ApplicationPackageManager extends PackageManager，所以从上面代码分析getPackageManager（）返回一个ApplicationPackageManager.
3) 分析getPackageManager().hasSystemFeature（PackageManager.FEATURE_WIFI_DIRECT）
由上面分析而知，getPackageManager().hasSystemFeature函数应该调到文件ApplicationPackageManager.java (frameworks\base\core\java\android\app) ，
final class ApplicationPackageManager extends PackageManager {
..............................
..............................
@Override
public boolean hasSystemFeature(String name) {
try {
return mPM.hasSystemFeature(name);
} catch (RemoteException e) {
throw new RuntimeException("Package manager has died", e);
}
}
..............................
...............................
}
mPM.hasSystemFeature(name)经过AIDL实际上调用到文件PackageManagerService.java (frameworks\base\services\java\com\android\server\pm)
public class PackageManagerService extends IPackageManager.Stub {
....................................
....................................
public boolean hasSystemFeature(String name) {
synchronized (mPackages) {
return mAvailableFeatures.containsKey(name);
}
}
...................................
...................................
}
mAvailableFeatures里面的内容是通过读取/system/etc/permissions下面的文档。具体代码如下所示：
void readPermissions() {
// Read permissions from .../etc/permission directory.
File libraryDir = new File(Environment.getRootDirectory(), "etc/permissions");
if (!libraryDir.exists() || !libraryDir.isDirectory()) {
Slog.w(TAG, "No directory " + libraryDir + ", skipping");
return;
}
if (!libraryDir.canRead()) {
Slog.w(TAG, "Directory " + libraryDir + " cannot be read");
return;
}
// Iterate over the files in the directory and scan .xml files
for (File f : libraryDir.listFiles()) {
// We'll read platform.xml last
if (f.getPath().endsWith("etc/permissions/platform.xml")) {
continue;
}
if (!f.getPath().endsWith(".xml")) {
Slog.i(TAG, "Non-xml file " + f + " in " + libraryDir + " directory, ignoring");
continue;
}
if (!f.canRead()) {
Slog.w(TAG, "Permissions library file " + f + " cannot be read");
continue;
}
readPermissionsFromXml(f);
}
// Read permissions from .../etc/permissions/platform.xml last so it will take precedence
final File permFile = new File(Environment.getRootDirectory(),
"etc/permissions/platform.xml");
readPermissionsFromXml(permFile);
}
private void readPermissionsFromXml(File permFile) {
FileReader permReader = null;
try {
permReader = new FileReader(permFile);
} catch (FileNotFoundException e) {
Slog.w(TAG, "Couldn't find or open permissions file " + permFile);
return;
}
try {
XmlPullParser parser = Xml.newPullParser();
parser.setInput(permReader);
XmlUtils.beginDocument(parser, "permissions");
while (true) {
XmlUtils.nextElement(parser);
if (parser.getEventType() == XmlPullParser.END_DOCUMENT) {
break;
}
String name = parser.getName();
if ("group".equals(name)) {
String gidStr = parser.getAttributeValue(null, "gid");
if (gidStr != null) {
int gid = Integer.parseInt(gidStr);
mGlobalGids = appendInt(mGlobalGids, gid);
} else {
Slog.w(TAG, "<group> without gid at "
+ parser.getPositionDescription());
}
XmlUtils.skipCurrentTag(parser);
continue;
}
...................
...................
else if ("feature".equals(name)) {
String fname = parser.getAttributeValue(null, "name");
if (fname == null) {
Slog.w(TAG, "<feature> without name at "
+ parser.getPositionDescription());
} else {
//Log.i(TAG, "Got feature " + fname);
FeatureInfo fi = new FeatureInfo();
fi.name = fname;
mAvailableFeatures.put(fname, fi);
}
XmlUtils.skipCurrentTag(parser);
continue;
} else {
XmlUtils.skipCurrentTag(parser);
continue;
}
}
permReader.close();
} catch (XmlPullParserException e) {
Slog.w(TAG, "Got execption parsing permissions.", e);
} catch (IOException e) {
Slog.w(TAG, "Got execption parsing permissions.", e);
}
}
3. 验证上面代码分析的正确性。
1） 启动一个ANDROID 4.0的模拟器，然后通过adb shell进入/system/etc/permissions目录下查看，
# cd /system/etc/permissions
# ls
com.android.location.provider.xml
platform.xml
#
的确没有android.hardware.wifi.direct.xml文件。
2） 因为在源码/frameworks/base/data/etc中有android.hardware.wifi.direct.xml文件，所以我手动拷贝此文件到out/target/product/generic/system/etc/permissions/
目录下，然后编译源码，然后用命令行指定编译完成的system.img,userdata.img,ramdisk.img来启动模拟器：
XXX@XXX:~/Android_code/system_img$ /home/XXX/Android_install/android-sdk-linux_x86/tools/emulator -system system.img -data userdata.img -ramdisk ramdisk.img -partition-size 256 -avd Android4.0.3-APILevel15
然后在模拟器设置中看到了WiFi Direct的设置项了，如图所示：
只是点击时弹出错误框提示“Couldn't start Wi-Fi Direct”,简单的跟踪了一下是WifiP2pService.java中有WifiNative.startP2pSupplicant()的一个判断，跟到JNI层
WifiP2pService.java (frameworks\base\wifi\java\android\net\wifi\p2p)
static jboolean android_net_wifi_startP2pSupplicant(JNIEnv* env, jobject)
{
return (jboolean)(::wifi_start_p2p_supplicant() == 0); //这个应该是硬件方面的判断。
}
没有再继续跟踪下去了，至少关于WIFI P2P设置的过滤过程已经清楚了。