前言

我在[003]AIDL是什么中介绍的AIDL，但是好像还有朋友不明白问我，那我就来写一个终极版的文章，让你十分钟彻底明白AIDL，以下代码全为手写。

目标

Server进程注册一个Binder服务到SM，该Binder服务提供两个接口：add和minus
Client 进程通过SM获得Binder服务的代理类BinderProxy，并调用两个接口add，minus

1 没有AIDL的世界

1.1 Server进程

    class CommandBinder extends Binder {@Overrideprotected boolean onTransact(int code, @NonNull Parcel data, @Nullable Parcel reply, int flags) throws RemoteException {if (code == 1) {int a = data.readInt();int b = data.readInt();reply.writeInt(a + b);return true;} else if (code == 2) {int a = data.readInt();int b = data.readInt();reply.writeInt(a + b);return true;}return super.onTransact(code, data, reply, flags);}}void main() {//注册binder服务ServiceManager.addService("command", new CommandBinder());}

1.2 Client进程

    int add(int a, int b) {int code = 1;BinderProxy mClient = ServiceManager.getService("command");Parcel data = Parcel.obtain();data.writeInt(a);data.writeInt(b);Parcel reply = Parcel.obtain();mClient.transact(code, data, reply, 0);//flag设置成0,调用这个方法会跨进程调用Binder服务类中onTransact方法int result = reply.readInt();return result;}int minus(int a, int b) {int code = 2;BinderProxy mClient = ServiceManager.getService("command");Parcel data = Parcel.obtain();data.writeInt(a);data.writeInt(b);Parcel reply = Parcel.obtain();mClient.transact(code, data, reply, 0);//flag设置成0,调用这个方法会跨进程调用Binder服务类中onTransact方法int result = reply.readInt();return result;}

2 有AIDL的世界

AIDL文件

    //aidlinterface ICommand {int add(int a, int b);int minus(int a, int b);}

2.1 Server进程

与1.1中代码比较，我们发现，我们不是直接继承Binder，而是继承aidl文件自动生成ICommandBinder，我们只需专注写add和minus接口的实现，不需要在Binder类中的onTransact写一堆if else代码和Parcel.read write.

    class ADILCommandBinder extends ICommandBinder {@Overridepublic int add(int a, int b) {return a + b;}@Overridepublic int minus(int a, int b) {return a - b;}}void main() {//注册binder服务ServiceManager.addService("command", new ADILCommandBinder());}

2.2 Client进程

与1.2中代码比较，我们发现，我们不需要写Parcel.read write和BinderProxy.transact，而是用BinderProxy对象构造一个CommandBinderProxy对象，然后直接调用CommandBinderProxy的add和minus

    int add(int a, int b) {BinderProxy mClient = ServiceManager.getService("command");return new CommandBinderProxy(mClient).add(a, b);}int minus(int a, int b) {BinderProxy mClient = ServiceManager.getService("command");return new CommandBinderProxy(mClient).minus(a, b);}

3 ADIL自动生成了什么代码

生成了三个部分：ICommand，ICommandBinder，CommandBinderProxy

3.1 ICommand

看起来和aidl文件差不多

    interface ICommand {int add(int a, int b);int minus(int a, int b);}

3.2 ICommandBinder

继承于Binder，并实现ICommand，然后在onTransact方法中调用还没有实现的add和minus方法

   abstract public class ICommandBinder extends Binder implements ICommand {int add = 1;int minus = 2;@Overrideprotected boolean onTransact(int code, @NonNull Parcel data, @Nullable Parcel reply, int flags) throws RemoteException {if (code == 1) {int a = data.readInt();int b = data.readInt();reply.writeInt(add(a, b));return true;} else if (code == 2) {int a = data.readInt();int b = data.readInt();reply.writeInt(minus(a, b));return true;}return super.onTransact(code, data, reply, flags);}abstract public int add(int a, int b);abstract public int minus(int a, int b);}

3.3 CommandBinderProxy

对BinderProxy进行封装，把调用CommandBinderProxy的add转化成Parcel.write,read和BinderProxy.transact代码。

    public class CommandBinderProxy implements ICommand {private BinderProxy remote;int add = 1;int minus = 2;public CommandBinderProxy(BinderProxy remote) {this.remote = remote;}@Overridepublic int add(int a, int b) {Parcel data = Parcel.obtain();data.writeInt(a);data.writeInt(b);Parcel reply = Parcel.obtain();remote.transact(add, data, reply, 0);int result = reply.readInt();return result;}@Overridepublic int minus(int a, int b) {int code = 2;Parcel data = Parcel.obtain();data.writeInt(a);data.writeInt(b);Parcel reply = Parcel.obtain();remote.transact(code, data, reply, 0);int result = reply.readInt();return result;}}

总结

看完应该明白了AIDL作用主要有
a.偷懒，少很多代码，尤其是你要写上百个方法的时候，AIDL就派上大用场了。
b.让服务端更专注接口的实现，而减少犯错误的可能性
c.规范client端和server端的接口定义，有助于代码的迭代

当然这是我自己写的伪代码，AIDL真正生成的代码比较复杂，但这些伪代码就是AIDL核心点。

思考

oneway的这个语法对AIDL生成的代码有什么影响，或者你们自己写一下oneway的方法AIDL生成的伪代码

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