Remote procedure calls--遠(yuǎn)程過(guò)程調(diào)用

Android has a lightweight mechanism for remote procedure calls (RPCs) — where a method is called locally, but executed remotely (in another process), with any result returned back to the caller. This entails decomposing the method call and all its attendant data to a level the operating system can understand, transmitting it from the local process and address space to the remote process and address space, and reassembling and reenacting the call there. Return values have to be transmitted in the opposite direction. Android provides all the code to do that work, so that you can concentrate on defining and implementing the RPC interface itself.

翻譯：Android提供了對(duì)遠(yuǎn)程調(diào)用（RPC）的輕量級(jí)支持--RPC從表面上看是一個(gè)本地調(diào)用，但是實(shí)際是在另外一個(gè)進(jìn)程中執(zhí)行遠(yuǎn)程方法，執(zhí)行結(jié)果返回本地調(diào)用者。這個(gè)過(guò)程要求操作系統(tǒng)能夠?qū)Ψ椒ㄕ{(diào)用、方法參數(shù)及返回值具有解析能力；能夠把數(shù)據(jù)對(duì)象從本地進(jìn)程、本地地址空間傳遞到遠(yuǎn)程進(jìn)程、遠(yuǎn)程地址空間；而且返回值能夠逆向傳遞到本地進(jìn)程中。所有這些，Android都提供了支持，作為開(kāi)發(fā)者只需要關(guān)注遠(yuǎn)程過(guò)程調(diào)用（RPC）接口的定義和實(shí)現(xiàn)。

An RPC interface can include only methods. By default, all methods are executed synchronously (the local method blocks until the remote method finishes), even if there is no return value.

翻譯：RPC接口可以只是包含方法定義，接口中的方法默認(rèn)都是同步執(zhí)行（也就是說(shuō)本地方法的執(zhí)行需要等待遠(yuǎn)程方法執(zhí)行完成方可繼續(xù)執(zhí)行，即使遠(yuǎn)程方法沒(méi)有返回值。）

In brief, the mechanism works as follows: You'd begin by declaring the RPC interface you want to implement using a simple IDL (interface definition language). From that declaration, the aidl tool generates a Java interface definition that must be made available to both the local and the remote process. It contains two inner class, as shown in the following diagram:

翻譯：簡(jiǎn)單說(shuō)，實(shí)際過(guò)程是：開(kāi)發(fā)者先聲明RPC接口（使用IDL接口定義語(yǔ)言），然后用aidl 工具生成該接口的Java接口定義，該接口對(duì)于本地和遠(yuǎn)程進(jìn)程都必須是可見(jiàn)的，這個(gè)Java接口中包含兩個(gè)內(nèi)部類，如下圖所示：


The inner classes have all the code needed to administer remote procedure calls for the interface you declared with the IDL. Both inner classes implement the IBinder interface. One of them is used locally and internally by the system; the code you write can ignore it. The other, called Stub, extends the Binder class. In addition to internal code for effectuating the IPC calls, it contains declarations for the methods in the RPC interface you declared. You would subclass Stub to implement those methods, as indicated in the diagram.

翻譯：這兩個(gè)內(nèi)部類中提供了所有必要的代碼，用于基于你所定義的接口的遠(yuǎn)程過(guò)程調(diào)用管理，這兩個(gè)類都實(shí)現(xiàn)了IBinder接口，其中一個(gè)內(nèi)部類用于本地，與操作系統(tǒng)交互。作為遠(yuǎn)程方法的開(kāi)發(fā)者可以不必考慮它，另外一個(gè)內(nèi)部類也叫Stub（存根），是Binder的子類，該類除了負(fù)責(zé)RPC調(diào)用的相關(guān)代碼外，還包含RPC接口中所聲明的方法聲明，開(kāi)發(fā)者應(yīng)該把這個(gè)Stub（存根）類作為基類擴(kuò)展出子類，在子類中實(shí)現(xiàn)接口定義的方法，請(qǐng)參考上圖。

Typically, the remote process would be managed by a service (because a service can inform the system about the process and its connections to other processes). It would have both the interface file generated by the aidl tool and the Stub subclass implementing the RPC methods. Clients of the service would have only the interface file generated by the aidl tool.

翻譯：通常，遠(yuǎn)程進(jìn)程是由一個(gè)服務(wù)來(lái)管理的（因?yàn)榉?wù)可以向系統(tǒng)通知進(jìn)程以及和其他進(jìn)程的連接），這樣的服務(wù)既包含了aidl工具生成的接口文件也包含遠(yuǎn)程方法開(kāi)發(fā)者由Stub類擴(kuò)展出的子類實(shí)現(xiàn)的遠(yuǎn)程方法，使用該服務(wù)的客戶端代碼只需要aidl工具生成的接口文件。

Here's how a connection between a service and its clients is set up:

下面是服務(wù)使用方和服務(wù)本身之間連接創(chuàng)建的過(guò)程:

??? * Clients of the service (on the local side) would implement onServiceConnected() and onServiceDisconnected() methods so they can be notified when a successful connection to the remote service is established, and when it goes away. They would then call bindService() to set up the connection.

????? 翻譯：服務(wù)客戶端 (運(yùn)行在本地) 需要實(shí)現(xiàn) onServiceConnected() 和 onServiceDisconnected() 方法，這兩個(gè)方法在與遠(yuǎn)程服務(wù)連接建立完成后或是連接斷開(kāi)時(shí)被系統(tǒng)調(diào)用。一旦連接建立，onServiceConnected()方法繼續(xù)調(diào)用 bindService() 方法。
???
?? * The service's onBind() method would be implemented to either accept or reject the connection, depending on the intent it receives (the intent passed to bindService()). If the connection is accepted, it returns an instance of the Stub subclass.

????? 翻譯：服務(wù)的bindService()的作用要么是接受要么是拒絕連接，具體取決于方法的參數(shù)intent，如果接受連接，那么該方法返回Stub類的子類實(shí)例對(duì)象（IBinder接口的實(shí)現(xiàn)類）。

??? * If the service accepts the connection, Android calls the client's onServiceConnected() method and passes it an IBinder object, a proxy for the Stub subclass managed by the service. Through the proxy, the client can make calls on the remote service.

????? 翻譯：如果服務(wù)接受了連接，Android將立刻調(diào)用客戶端的onServiceConnected()方法，同時(shí)為該方法傳遞一個(gè)IBinder對(duì)象，這個(gè)IBinder對(duì)象是一個(gè)Stub類的子類代理的概念，由服務(wù)負(fù)責(zé)管理它，借助這個(gè)代理實(shí)例，客戶端就可以對(duì)遠(yuǎn)程服務(wù)做調(diào)用。

This brief description omits some details of the RPC mechanism. For more information, see Designing a Remote Interface Using AIDL and the IBinder class description.

翻譯：以上簡(jiǎn)要描述的過(guò)程中忽視了RPC過(guò)程的一些細(xì)節(jié)，更多信息請(qǐng)查閱Designing a Remote Interface Using AIDL and the IBinder class description.

Application Fundamentals--Remote procedure calls（遠(yuǎn)程過(guò)程調(diào)用）