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116 lines
3.1 KiB
116 lines
3.1 KiB
5 years ago
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---
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谈谈你对 Binder 的理解？
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---
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1. binder 是干嘛的？
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2. binder 存在的意义是什么？
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3. binder 的架构原理是怎样的？
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#### Binder 是干嘛的？
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进程间通信。
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#### Binder 存在的意义是什么？
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1. 性能
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2. 方便易用
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3. 安全
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#### 架构原理
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通信架构：
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#### 进程如何启用 Binder 机制？
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1. 打开 binder 驱动
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2. 内存映射，分配缓冲区
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3. 启动 binder 线程
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```c++
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// ServiceManager#main
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int main(int argc, char **argv){
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struct binder_state *bs;
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bs = binder_open(128*1024);
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binder_become_context_manager(bs);
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binder_loop(bs, svcmgr_handler);
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return 0;
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}
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void binder_loop(struct binder_state *bs, binder_handler func){
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// 把当前线程注册为 binder 线程，也就是 SM 的主线程
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readbuf[0] = BC_ENTER_LOOPER;
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binder_write(bs, readbuf, sizeof(uint32_t));
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for(;;){
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bwr.read_size = sizeof(readbuf);
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res = ioctl(bs->fd, BINDER_WRITE_READ, &bwr);
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res = binder_parse(bs, 0, (uintptr_t) readbuf, bwr.read_consumed, func);
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}
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}
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```
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以 SurfaceFlinger 注册服务为例：
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```c++
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int main(int, char**){
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// 启动 binder 机制
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sp<ProcessState> ps(ProcessState::self());
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ps->startThreadPool();
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// 初始化服务
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sp<SurfaceFlinger> flinger = new SurfaceFlinger();
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flinger->init();
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// 注册服务
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sp<IServiceManager> sm(defaultServiceManager());
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sm->addService(String16(SurfaceFlinger::getServiceName(), flinger, false));
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flinger->run();
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return 0;
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}
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sp<IServiceManager> defaultServiceManager(){
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while(gDefaultServiceManager==NULL){
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ProcessState::self()->getContextObject(NULL);
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if(gDefaultServiceManager==NULL){
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sleep(1);
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}
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}
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return gDefaultServiceManager;
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}
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status_t addService(const String16& name, const sp<IBinder>& service, ...){
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Parcel data, reply;
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data.writeInterfaceToken(IServiceManager::getInterfaceDescriptior());
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data.writeString16(name);
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data.writeStrongBinder(service);
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data.writeInt32(allowlsolated?1:0);
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status_t err = remote()->transact(ADD_SERVICE_TRANSACTION, data, &reply);
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return err == NO_ERROR?reply.readExceptionCode():err;
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}
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status_t IPCThreadState::transact(int32_t handle, unit32_t code, ...){
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err = writeTransactionData(BC_TRANSACTION, flags, handle, code, data, ...);
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if((flag&TF_ONE_WAY)==0){
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if(reply){
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err=waitForResponse(reply);
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}else{
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Parcel fakeReply;
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err = waitForResponse(&fakeReply);
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}
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}else{
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err = waitForResponse(NULL, NULL);
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}
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return err;
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}
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status_t BnServiceManager::onTransact(uint32_t code, const Parcel& data, Parcel* reply, uint32_t flags){
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switch(code){
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case ADD_SERVICE_TRANSACTION:{
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CHECK_INTERFACE(IServiceManager, data, reply);
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String16 which = data.readString16();
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sp<IBinder> b = data.readStrongBinder();
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status_t err = addService(which, b);
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reply->writeInt32(err);
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return NO_ERROR;
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}break;
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}
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}
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```
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