什么是Binder?
从IPC角度：它是Android的一种跨进程通信方式，
从fromwork角度：它是ServiceManager联系各种Manager和相应ManagerService的桥梁
从应用层角度:是客户端和服务端通信的媒介。
在Android开发中，BInder主要用于Service中，包括AIDL和Messenger,其中普通Service中的Binder不涉及跨进程通讯，而Messager的底层是AIDL,所以我么通过AIDL来了解Binder.
注：我们需要使用Binder和Intent传输数据的时候，传输的数据序列化，不熟悉的可以过一遍我之前的笔记
序列化之serializable和parcelable的区别
新建三个文件Book.java、Book.aidl、IBookManager.aidl

• Book.java
  就是个java类，注意实现parcelable就行了。

public class Book  implements Parcelable{
    private int bookId;
    private String bookName;

    public Book(int bookId, String bookName) {
        this.bookId = bookId;
        this.bookName = bookName;
    }

    protected Book(Parcel in) {
        bookId = in.readInt();
        bookName = in.readString();
    }

    public static final Creator<Book> CREATOR = new Creator<Book>() {
        @Override
        public Book createFromParcel(Parcel in) {
            return new Book(in);
        }

        @Override
        public Book[] newArray(int size) {
            return new Book[size];
        }
    };

    @Override
    public int describeContents() {
        return 0;
    }

    @Override
    public void writeToParcel(Parcel dest, int flags) {
        dest.writeInt(bookId);
        dest.writeString(bookName);
    }
}

image.png

• IBookManager.aidl
  声明了两个方法
  ps:新建的aid文件都会默认有个方法，注释/删除掉就行了。

// IBookManager.aidl
package com.demo.nick.ipcstudy;

// Declare any non-default types here with import statements
import com.demo.nick.ipcstudy.Book;
interface IBookManager {
    /**
     * Demonstrates some basic types that you can use as parameters
     * and return values in AIDL.
     */
//    void basicTypes(int anInt, long aLong, boolean aBoolean, float aFloat,
//            double aDouble, String aString);
   List<Book> getBookList();
   void addBook(in Book book);
}

• Book.aidl
  在AIDL中声明parcelable 类Book;
  注意：这里如果你先建了Book.java 在直接new aidl会提示 interface Name must be unique
  解决办法：可以先创建Book.adil文件，也可以先任意文件名，创建好了在改。

// IBook.aidl
package com.example.nick.ipcdemoone.aidl;

// Declare any non-default types here with import statements
parcelable Book;

重行build下，系统会为我们生成一个binder文件

image.png

下面看它内部代码(我改了下代码结构顺序，方便阅读)
首先它声明了两个方法，就是我们之前创建AIDL文件时定义的方法。然后有个内部类Stub，继承了Binder类，Stub自己也有个内部类代理Proxy。具体方法解释我放在注释里。

package com.demo.nick.ipcstudy;

public interface IBookManager extends android.os.IInterface {
    /**
     * Demonstrates some basic types that you can use as parameters
     * and return values in AIDL.
     */
    public java.util.List<com.demo.nick.ipcstudy.Book> getBookList() throws android.os.RemoteException;

    public void addBook(com.demo.nick.ipcstudy.Book book) throws android.os.RemoteException;


    /**
     * Local-side IPC implementation stub class.
     */
    public static abstract class Stub extends android.os.Binder implements com.demo.nick.ipcstudy.IBookManager {
        //binder的唯一标识
        private static final java.lang.String DESCRIPTOR = "com.demo.nick.ipcstudy.IBookManager";
        //方法的标识
        static final int TRANSACTION_getBookList = (android.os.IBinder.FIRST_CALL_TRANSACTION + 0);
        static final int TRANSACTION_addBook = (android.os.IBinder.FIRST_CALL_TRANSACTION + 1);

        /**
         * Construct the stub at attach it to the interface.
         */
        public Stub() {
            this.attachInterface(this, DESCRIPTOR);
        }

        /**
         * Cast an IBinder object into an com.demo.nick.ipcstudy.IBookManager interface,
         * generating a proxy if needed.
         * 用于将服务端的Binder对象转换成客户端所需要的AIDL接口类型的对象，
         * 如果在客户端和服务端在同一进程，则返回的就是stub本身，否则返回封装后的Stub.proxy对象
         */
        public static com.demo.nick.ipcstudy.IBookManager asInterface(android.os.IBinder obj) {
            if ((obj == null)) {
                return null;
            }
            android.os.IInterface iin = obj.queryLocalInterface(DESCRIPTOR);
            if (((iin != null) && (iin instanceof com.demo.nick.ipcstudy.IBookManager))) {
                return ((com.demo.nick.ipcstudy.IBookManager) iin);
            }
            return new com.demo.nick.ipcstudy.IBookManager.Stub.Proxy(obj);
        }

        //返回当前Binder对象
        @Override
        public android.os.IBinder asBinder() {
            return this;
        }

        /**
         * 运行在Binder线程池中，当client发起跨进程请求时（也就是说不跨进程不会调用），会调用该方法，
         *
         * @param code  确定服务端的目标方法
         * @param data  取出目标方法所需要的参数（有参数的话），然后调用该目标方法。
         * @param reply 写入返回值
         * @param flags
         * @return false则请求失败，
         * @throws android.os.RemoteException
         */
        @Override
        public boolean onTransact(int code, android.os.Parcel data, android.os.Parcel reply, int flags) throws android.os.RemoteException {
            switch (code) {
                case INTERFACE_TRANSACTION: {
                    reply.writeString(DESCRIPTOR);
                    return true;
                }
                case TRANSACTION_getBookList: {
                    data.enforceInterface(DESCRIPTOR);
                    java.util.List<com.demo.nick.ipcstudy.Book> _result = this.getBookList();
                    reply.writeNoException();
                    reply.writeTypedList(_result);
                    return true;
                }
                case TRANSACTION_addBook: {
                    data.enforceInterface(DESCRIPTOR);
                    com.demo.nick.ipcstudy.Book _arg0;
                    if ((0 != data.readInt())) {
                        _arg0 = com.demo.nick.ipcstudy.Book.CREATOR.createFromParcel(data);
                    } else {
                        _arg0 = null;
                    }
                    this.addBook(_arg0);
                    reply.writeNoException();
                    return true;
                }
            }
            return super.onTransact(code, data, reply, flags);
        }

        private static class Proxy implements com.demo.nick.ipcstudy.IBookManager {
            private android.os.IBinder mRemote;

            Proxy(android.os.IBinder remote) {
                mRemote = remote;
            }

            @Override
            public android.os.IBinder asBinder() {
                return mRemote;
            }

            public java.lang.String getInterfaceDescriptor() {
                return DESCRIPTOR;
            }

            /**
             * Demonstrates some basic types that you can use as parameters
             * and return values in AIDL.
             * 运行在client
             */
            @Override
            public java.util.List<com.demo.nick.ipcstudy.Book> getBookList() throws android.os.RemoteException {
                //创建两个parcel对象。前者用于输入，后者用于输出
                android.os.Parcel _data = android.os.Parcel.obtain();
                android.os.Parcel _reply = android.os.Parcel.obtain();
                java.util.List<com.demo.nick.ipcstudy.Book> _result;//返回结果
                try {
                    _data.writeInterfaceToken(DESCRIPTOR);
                    //发起请求
                    mRemote.transact(Stub.TRANSACTION_getBookList, _data, _reply, 0);
                    _reply.readException();
                    //读取返回结果---无返回值则不用
                    _result = _reply.createTypedArrayList(com.demo.nick.ipcstudy.Book.CREATOR);
                } finally {
                    _reply.recycle();
                    _data.recycle();
                }
                return _result;
            }

            @Override
            public void addBook(com.demo.nick.ipcstudy.Book book) throws android.os.RemoteException {
                android.os.Parcel _data = android.os.Parcel.obtain();
                android.os.Parcel _reply = android.os.Parcel.obtain();
                try {
                    _data.writeInterfaceToken(DESCRIPTOR);
                    if ((book != null)) {
                        _data.writeInt(1);
                        book.writeToParcel(_data, 0);
                    } else {
                        _data.writeInt(0);
                    }
                    mRemote.transact(Stub.TRANSACTION_addBook, _data, _reply, 0);
                    _reply.readException();
                } finally {
                    _reply.recycle();
                    _data.recycle();
                }
            }

        }

    }


}

一个完成的流程就是这样
客户端调用方法，getBookList(),然后
mRemote.transact(Stub.TRANSACTION_getBookList, _data, _reply, 0);调起服务端。走onTransact方法。
_result = _reply.createTypedArrayList(com.demo.nick.ipcstudy.Book.CREATOR);获得结果

Binder流程