Android4.0中判断WIFI P2P选项是否显示的源码分析

     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, " 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, " 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。