AIDL学习笔记

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