binder中的callback可以用在客户端请求服务one way的情况下，服务器处理完毕之后主动通知客户端处理结果，对客户端来说，就是回调。
以下是演示代码。首先是服务端代码，在服务端处理完毕之后，会通过readStrongBinder方法获取到一个IBinder对象，然后设置一个8888的处理码。

#include <utils/Trace.h>
#include <stdio.h>
#include <pthread.h>
#include <stdlib.h>
#include <unistd.h>
#include <semaphore.h>
#include <iostream>
#include <stdint.h>
#include <sys/types.h>
#include <set>
#include <thread>
#include <unordered_map>
#include <unordered_set>
#include <vector>
#include <inttypes.h>

#include <cutils/properties.h>
#include <utils/Log.h>
#include <utils/SystemClock.h>
#include <android-base/properties.h>
#include <errno.h>
#include <fcntl.h>
#include <fstream>
#include <poll.h>
#include <pthread.h>

#include <binder/IPCThreadState.h>
#include <binder/ProcessState.h>
#include <binder/Binder.h>
#include <binder/IBinder.h>
#include <binder/IPCThreadState.h>
#include <binder/IServiceManager.h>
#include <binder/IServiceManager.h>
#include <sys/wait.h>

#include <private/binder/binder_module.h>
#include <sys/epoll.h>
#include <sys/prctl.h>

using namespace android;
using namespace std;

enum BinderLibTestTranscationCode {
	BINDER_LIB_TEST_NOP_TRANSACTION = IBinder::FIRST_CALL_TRANSACTION,
	BINDER_LIB_TEST_REGISTER_SERVER,
	BINDER_LIB_TEST_ADD_SERVER,
    BINDER_LIB_TEST_ADD_POLL_SERVER,
    BINDER_LIB_TEST_CALL_BACK,
    BINDER_LIB_TEST_CALL_BACK_VERIFY_BUF,
    BINDER_LIB_TEST_DELAYED_CALL_BACK,
    BINDER_LIB_TEST_NOP_CALL_BACK,
    BINDER_LIB_TEST_GET_SELF_TRANSACTION,
    BINDER_LIB_TEST_GET_ID_TRANSACTION,
    BINDER_LIB_TEST_INDIRECT_TRANSACTION,
    BINDER_LIB_TEST_SET_ERROR_TRANSACTION,
    BINDER_LIB_TEST_GET_STATUS_TRANSACTION,
    BINDER_LIB_TEST_ADD_STRONG_REF_TRANSACTION,
    BINDER_LIB_TEST_LINK_DEATH_TRANSACTION,
    BINDER_LIB_TEST_WRITE_FILE_TRANSACTION,
    BINDER_LIB_TEST_WRITE_PARCEL_FILE_DESCRIPTOR_TRANSACTION,
    BINDER_LIB_TEST_EXIT_TRANSACTION,
    BINDER_LIB_TEST_DELAYED_EXIT_TRANSACTION,
    BINDER_LIB_TEST_GET_PTR_SIZE_TRANSACTION,
    BINDER_LIB_TEST_CREATE_BINDER_TRANSACTION,
    BINDER_LIB_TEST_GET_WORK_SOURCE_TRANSACTION,
    BINDER_LIB_TEST_GET_SCHEDULING_POLICY,
    BINDER_LIB_TEST_NOP_TRANSACTION_WAIT,
    BINDER_LIB_TEST_GETPID,
    BINDER_LIB_TEST_ECHO_VECTOR,
    BINDER_LIB_TEST_REJECT_BUF,
};

static String16 binderLibTestServiceName = String16("test.binderLib.0921");

class BinderLibTestService: public BBinder {
public:
	explicit BinderLibTestService(int32_t id)
			 {

	}
	~BinderLibTestService() {
		exit(EXIT_SUCCESS);
	}

	virtual status_t onTransact(uint32_t code, const Parcel &data,
			Parcel *reply, uint32_t flags = 0) override {
		printf("%s: code %d\n", __func__, code);
		switch (code) {
		  case BINDER_LIB_TEST_NOP_CALL_BACK: {
		                Parcel data2, reply2;
		                sp<IBinder> binder;
		                binder = data.readStrongBinder();
		                if (binder == nullptr) {
		                    return BAD_VALUE;
		                }
		                data2.writeInt32(8888);
		                binder->transact(BINDER_LIB_TEST_CALL_BACK, data2, &reply2);
		                return NO_ERROR;
		            }
		default:
			return UNKNOWN_TRANSACTION;
		};
	}
private:
};

int main(int argc, char **argv) {
	sp<IBinder>   m_server;
	ProcessState::self()->startThreadPool();
	printf("10\n");
	status_t ret;
	sp<IServiceManager> sm = defaultServiceManager();
	BinderLibTestService *testServicePtr;
	int index = 0;
	sp<BinderLibTestService> testService = new BinderLibTestService(index);
	printf("addService\n");
	ret = sm->addService(binderLibTestServiceName, testService);
	printf("end\n");
	IPCThreadState::self()->joinThreadPool();
	return 0;
}

下面是客户端的代码：，客户端获取到服务的BInder对象之后，调用 writeStrongBinder注册本地的一个回调。

#include <utils/Trace.h>
#include <stdio.h>
#include <pthread.h>
#include <stdlib.h>
#include <unistd.h>
#include <semaphore.h>
#include <iostream>
#include <stdint.h>
#include <sys/types.h>
#include <set>
#include <thread>
#include <unordered_map>
#include <unordered_set>
#include <vector>
#include <inttypes.h>

#include <cutils/properties.h>
#include <utils/Log.h>
#include <utils/SystemClock.h>
#include <android-base/properties.h>
#include <errno.h>
#include <fcntl.h>
#include <fstream>
#include <poll.h>
#include <pthread.h>

#include <binder/IPCThreadState.h>
#include <binder/ProcessState.h>
#include <binder/Binder.h>
#include <binder/IBinder.h>
#include <binder/IPCThreadState.h>
#include <binder/IServiceManager.h>
#include <binder/IServiceManager.h>
#include <sys/wait.h>

#include <private/binder/binder_module.h>
#include <sys/epoll.h>
#include <sys/prctl.h>

using namespace android;
using namespace std;

enum BinderLibTestTranscationCode {
	BINDER_LIB_TEST_NOP_TRANSACTION = IBinder::FIRST_CALL_TRANSACTION,
	BINDER_LIB_TEST_REGISTER_SERVER,
	BINDER_LIB_TEST_ADD_SERVER,
	BINDER_LIB_TEST_ADD_POLL_SERVER,
	BINDER_LIB_TEST_CALL_BACK,
	BINDER_LIB_TEST_CALL_BACK_VERIFY_BUF,
	BINDER_LIB_TEST_DELAYED_CALL_BACK,
	BINDER_LIB_TEST_NOP_CALL_BACK,
	BINDER_LIB_TEST_GET_SELF_TRANSACTION,
	BINDER_LIB_TEST_GET_ID_TRANSACTION,
	BINDER_LIB_TEST_INDIRECT_TRANSACTION,
	BINDER_LIB_TEST_SET_ERROR_TRANSACTION,
	BINDER_LIB_TEST_GET_STATUS_TRANSACTION,
	BINDER_LIB_TEST_ADD_STRONG_REF_TRANSACTION,
	BINDER_LIB_TEST_LINK_DEATH_TRANSACTION,
	BINDER_LIB_TEST_WRITE_FILE_TRANSACTION,
	BINDER_LIB_TEST_WRITE_PARCEL_FILE_DESCRIPTOR_TRANSACTION,
	BINDER_LIB_TEST_EXIT_TRANSACTION,
	BINDER_LIB_TEST_DELAYED_EXIT_TRANSACTION,
	BINDER_LIB_TEST_GET_PTR_SIZE_TRANSACTION,
	BINDER_LIB_TEST_CREATE_BINDER_TRANSACTION,
	BINDER_LIB_TEST_GET_WORK_SOURCE_TRANSACTION,
	BINDER_LIB_TEST_GET_SCHEDULING_POLICY,
	BINDER_LIB_TEST_NOP_TRANSACTION_WAIT,
	BINDER_LIB_TEST_GETPID,
	BINDER_LIB_TEST_ECHO_VECTOR,
	BINDER_LIB_TEST_REJECT_BUF,
};

static String16 binderLibTestServiceName = String16("test.binderLib.0921");

class BinderLibTestCallBack: public BBinder {
public:
	BinderLibTestCallBack() :
			m_result(0) {
	}
	status_t getResult(void) {
		return m_result;
	}

private:
	virtual status_t onTransact(uint32_t code, const Parcel &data,
			Parcel *reply, uint32_t flags = 0) {
		(void) reply;
		(void) flags;
		switch (code) {
		case BINDER_LIB_TEST_CALL_BACK: {
			status_t status = data.readInt32();
			if (status != NO_ERROR) {
				m_result = status;
			}
			printf("cb: callback called\n");
			std::cout << "cb: " << m_result << std::endl;

			//   triggerEvent();
			return NO_ERROR;
		}

		default:
			return UNKNOWN_TRANSACTION;
		}
	}

	status_t m_result;
	const uint8_t *m_prev_end;
};

int main(int argc, char **argv) {
	sp < IBinder > m_server;
	ProcessState::self()->startThreadPool();
	printf("10\n");
	status_t ret;
	sp < IServiceManager > sm = defaultServiceManager();
	m_server = sm->getService(binderLibTestServiceName);

	Parcel data, reply;
	sp<BinderLibTestCallBack> callBack = new BinderLibTestCallBack();
	data.writeStrongBinder(callBack);
	ret = m_server->transact(BINDER_LIB_TEST_NOP_CALL_BACK, data, &reply,
			TF_ONE_WAY);
	std::cout << ret << std::endl;
	printf("end\n");
	IPCThreadState::self()->joinThreadPool();
	return 0;
}

编译运行，先运行服务端，再运行客户端，如果打印8888，则是成功的。

# binder # java # android
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