在谷歌云消息推送GCM中，为什么需要在AndroidManifest添加如下权限声明，有什么原理吗？

Question

在Google Cloud Messaging(GCM)中，需要在AndroidManifest文件中添加下面的权限声明，小米推送和个推也有类似的做法，有什么原理吗？

code from cloud-messaging/android/client#manifest

<permission android:name="<your-package-name>.permission.C2D_MESSAGE" android:protectionLevel="signature" />
<uses-permission android:name="<your-package-name>.permission.C2D_MESSAGE" />

Answer 1

Won’t gcm hit a wall in China? How do you use it

Answer 2

These permissions are custom permissions for the security of mutual access calls between applications. Application A defines permission statement permission，应用B想和应用A交互，得声明使用A声明的特定权限，即uses-permission。android:protectionLevel="signature" which means that while it has permission, the applications that access each other must have the same application signature to access each other.