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Android AIDL学习笔记

2022年07月19日153tintown

目标：通过这个例子来介绍AIDL

Server进程注册一个Calculator服务到ServiceManager，该Calculator服务提供add和minus两个接口，Client进程通过ServiceManager获得Calculator服务的代理类，通过Binder调用add和minus两个接口。

1、没有AIDL的世界

1.1 Server进程

class Calculator extends Binder { 
    private static final int CODE_ADD = 1; 
    private static final int CODE_MINUS = 2; 
 
    @Override 
    protected boolean onTransact(int code, @NonNull Parcel data, @Nullable Parcel reply, int flags) throws RemoteException { 
        if (code == CODE_ADD) { 
            int a = data.readInt(); 
            int b = data.readInt(); 
            reply.writeInt(a + b); 
            return true; 
        } else if (code == CODE_MINUS) { 
            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("calculator", new Calculator()); 
}

1.2 Client进程

private static final int CODE_ADD = 1; 
private static final int CODE_MINUS = 2; 
 
int add(int a, int b) { 
    IBinder mClient = ServiceManager.getService("calculator");//BinderProxy 
    Parcel data = Parcel.obtain(); 
    Parcel reply = Parcel.obtain(); 
    data.writeInt(a); 
    data.writeInt(b); 
    mClient.transact(CODE_ADD, data, reply, 0); 
    int result = reply.readInt(); 
    return result; 
} 
 
int minus(int a, int b) { 
    IBinder mClient = ServiceManager.getService("calculator");//BinderProxy 
    Parcel data = Parcel.obtain(); 
    Parcel reply = Parcel.obtain(); 
    data.writeInt(a); 
    data.writeInt(b); 
    mClient.transact(CODE_MINUS, data, reply, 0); 
    int result = reply.readInt(); 
    return result; 
}

2、有AIDL的世界

ICalculator.aidl文件

//aidl 
interface ICalculator { 
    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ICalculator.Stub，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和Parcel.read write

class Calculator extends ICalculator.Stub { 
    @Override 
    public int add(int a, int b) { 
        return a + b; 
    } 
 
    @Override 
    public int minus(int a, int b) { 
        return a - b; 
    } 
} 
 
void main() { 
    //注册binder服务 
    ServiceManager.addService("calculator", new Calculator()); 
}

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ICalculator.Stub.Proxy对象，然后直接调用ICalculator.Stub.Proxy的add和minus

int add(int a, int b) { 
    IBinder mClient = ServiceManager.getService("calculator");//BinderProxy 
    return new ICalculator.Stub.Proxy(mClient).add(a, b); 
} 
 
int minus(int a, int b) { 
    IBinder mClient = ServiceManager.getService("calculator");//BinderProxy 
    return new ICalculator.Stub.Proxy(mClient).minus(a, b); 
}

3、AIDL自动生成了什么代码

ICalculator.aidl生成的代码如下，有三个部分组成，我们后面仔细分析

ICalculator接口
ICalculator.Stub类
ICalculator.Stub.Proxy类

interface ICalculator {//看3.1分析 
    int add(int a, int b); 
    int minus(int a, int b); 
 
    abstract static class Stub extends Binder implements ICalculator {//看3.2分析 
        private static final int CODE_ADD = 1; 
        private static final int CODE_MINUS = 2; 
 
        @Override 
        protected boolean onTransact(int code, @NonNull Parcel data, @Nullable Parcel reply, int flags) throws RemoteException { 
            if (code == CODE_ADD) { 
                int a = data.readInt(); 
                int b = data.readInt(); 
                reply.writeInt(add(a, b)); 
                return true; 
            } else if (code == CODE_MINUS) { 
                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); 
 
        public static class Proxy implements ICalculator {//看3.3分析 
            private IBinder remote; 
            public Proxy(IBinder remote) { 
                this.remote = remote; 
            } 
 
            @Override 
            public int add(int a, int b) { 
                Parcel data = Parcel.obtain(); 
                Parcel reply = Parcel.obtain(); 
                data.writeInt(a); 
                data.writeInt(b); 
                remote.transact(CODE_ADD, data, reply, 0); 
                int result = reply.readInt(); 
                return result; 
            } 
 
            @Override 
            public int minus(int a, int b) { 
                Parcel data = Parcel.obtain(); 
                Parcel reply = Parcel.obtain(); 
                data.writeInt(a); 
                data.writeInt(b); 
                remote.transact(CODE_MINUS, data, reply, 0); 
                int result = reply.readInt(); 
                return result; 
            } 
        } 
    } 
}

3.1 ICalculator接口

看起来和aidl文件差不多

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

3.2 ICalculator.Stub类

继承于Binder，实现ICalculator接口，但是是空实现，然后在onTransact方法中调用空实现的add和minus接口

abstract static class Stub extends Binder implements ICalculator { 
    private static final int CODE_ADD = 1; 
    private static final int CODE_MINUS = 2; 
 
    @Override 
    protected boolean onTransact(int code, @NonNull Parcel data, @Nullable Parcel reply, int flags) throws RemoteException { 
        if (code == CODE_ADD) { 
            int a = data.readInt(); 
            int b = data.readInt(); 
            reply.writeInt(add(a, b)); 
            return true; 
        } else if (code == CODE_MINUS) { 
            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 ICalculator.Stub.Proxy类

用 BinderProxy 构造 ICalculator.Stub.Proxy，把调用 ICalculator.Stub.Proxy 的add和minus接口转化成 Parcel.write,read 和 BinderProxy.transact 代码。

public static class Proxy implements ICalculator { 
    private IBinder remote; 
 
    public Proxy(IBinder remote) { 
        this.remote = remote; 
    } 
 
    @Override 
    public int add(int a, int b) { 
        Parcel data = Parcel.obtain(); 
        data.writeInt(a); 
        data.writeInt(b); 
        Parcel reply = Parcel.obtain(); 
        remote.transact(CODE_ADD, data, reply, 0); 
        int result = reply.readInt(); 
        return result; 
    } 
 
    @Override 
    public int minus(int a, int b) { 
        Parcel data = Parcel.obtain(); 
        data.writeInt(a); 
        data.writeInt(b); 
        Parcel reply = Parcel.obtain(); 
        remote.transact(CODE_MINUS, 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真正生成的代码更加复杂一点，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.

参考： 

十分钟让你明白AIDL：https://www.jianshu.com/p/8f9e9147f5ba


本文参考链接：https://www.cnblogs.com/hellokitty2/p/13736415.html
  文章来源：
  https://www.itguest.com/post/efb2b5.html
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