Android Binder

1 files changed, 40 insertions, 0 deletions

diff --git a/docs/coding/android/binder.md b/docs/coding/android/binder.md
index 6bb3af2..4ff1748 100644
--- a/docs/coding/android/binder.md
+++ b/docs/coding/android/binder.md
@@ -1 +1,41 @@
 # Binder
+
+Android Binder is a IPC mechanism that allows one process to invoke a function
+in another process and exchange data. It consists of three parts:
+
+* Kernel Driver (`/dev/binder`)
+* Per-process userspace native code
+* Per-process userspace Java code (e.g., `android.os.Binder`)
+
+From an App perspective, the client can either obtain the server binder handle
+(`IBinder`) in two ways:
+
+1. App -> System Service: ServiceManager. ServiceManager is available in both
+C++ and Java (while the C++ API is mainly intended for system components). It
+allows system_server to register system services, so other apps can get their
+handle.
+2. App -> App: ActivityManagerService. This is mainly used for the `bindService`
+API, which the AMS starts the target service specified by the given
+ComponentName and returns the handle (`android.app.Service#onBind`) to the
+client.
+
+The client can call a function using the `android.os.Binder#transact()` method.
+This specifies an action code, function arguments in Parcel, and optional return
+Parcel. Parcel is Android's IPC serialization / deserialization mechanism.
+
+The server is get called from `android.os.Binder#onTransact()`, with the
+provided data. The server can then do whatever they want and return a value.
+Exceptions are also supported (but limited?).
+
+The server can check the client's UID and PID to enforce permissions. A usual
+design pattern is to call PackageManagerService APIs to query if the given app
+UID has required permissions, and throw exceptions if not.
+
+Peers can also link the binder to depth, such that when the other end get
+killed, the peer gets notified.
+
+Writing binder function codes can be troublesome, so Android uses a AIDL, a
+DSL, to generate Java code for both the client side and server side. AIDL is
+heavily used in both AOSP (for defining system services) and user apps.
+
+For a comprehensive understanding of Binder, take a look at gityuan.com.