Android Binder分析三：Natvie Service的获取和调用
来源：互联网 发布：tile python 编辑：程序博客网 时间：2024/05/16 18:56
这一章，我们通过分析如何获取MediaPlayerService，并调用其中的方法。我们以前面分析过的WifiDisplay的代码以起点开始分析。
获取Native Service
这是我们前面在RemoteDisplay中看过的一段代码：
static jint nativeListen(JNIEnv* env, jobject remoteDisplayObj, jstring ifaceStr) { ScopedUtfChars iface(env, ifaceStr); sp<IServiceManager> sm = defaultServiceManager(); sp<IMediaPlayerService> service = interface_cast<IMediaPlayerService>( sm->getService(String16("media.player"))); if (service == NULL) { ALOGE("Could not obtain IMediaPlayerService from service manager"); return 0; } sp<NativeRemoteDisplayClient> client(new NativeRemoteDisplayClient(env, remoteDisplayObj)); sp<IRemoteDisplay> display = service->listenForRemoteDisplay( client, String8(iface.c_str()));这里首先通过defaultServiceManager()获取一个BpServiceManager对象，然后调用它的getService方法，并通过它的返回构造一个BpMeidaPlayerService。前面分析过interface_cast<IMediaPlayerService>，它调用asInterface()函数，展开后如下：
android::sp<IMediaPlayerService> IMediaPlayerService::asInterface( const android::sp<android::IBinder>& obj) { android::sp<IMediaPlayerService> intr; if (obj != NULL) { intr = static_cast<IMediaPlayerService*>( obj->queryLocalInterface( IMediaPlayerService::descriptor).get()); if (intr == NULL) { intr = new BpMediaPlayerService(obj); } } return intr; } 参数obj就是上面调用sm->getService(String16("media.player"))的返回，那首先来看getService：
virtual sp<IBinder> getService(const String16& name) const { unsigned n; for (n = 0; n < 5; n++){ sp<IBinder> svc = checkService(name); if (svc != NULL) return svc; ALOGI("Waiting for service %s...\n", String8(name).string()); sleep(1); } return NULL; } virtual sp<IBinder> checkService( const String16& name) const { Parcel data, reply; data.writeInterfaceToken(IServiceManager::getInterfaceDescriptor()); data.writeString16(name); remote()->transact(CHECK_SERVICE_TRANSACTION, data, &reply); return reply.readStrongBinder(); }getService方法循环5次调用checkService方法来获取MediaPlayerService，以防在客户端调用的时候，MediaPlayerService还没来得及注册。在checkService()中，首先写入向Parcel中写入strict mode和"android.os.IServiceManager"，然后再写入"media.player"。最后调用remote()->transact将数据发送出去。我们前面分析过，这里的remote()就是BpBinder(0)，最终会调用IPCThreadState的transact方法。在transact方法中，首先将上面的data Parcel中的数据写入到mOut中，然后调用talkWithDriver()把数据发往binder驱动，发往binder驱动的数据如下：
interface "android.os.IServiceManager"
name ”media.player" offsets0
binder驱动先调用binder_thread_write来处理写请求，在binder_thread_write函数中，处理BC_TRANSACTION又调用binder_transaction方法：
static void binder_transaction(struct binder_proc *proc, struct binder_thread *thread, struct binder_transaction_data *tr, int reply){ struct binder_transaction *t; struct binder_work *tcomplete; size_t *offp, *off_end; struct binder_proc *target_proc; struct binder_thread *target_thread = NULL; struct binder_node *target_node = NULL; struct list_head *target_list; wait_queue_head_t *target_wait; struct binder_transaction *in_reply_to = NULL; struct binder_transaction_log_entry *e; uint32_t return_error; e = binder_transaction_log_add(&binder_transaction_log); e->call_type = reply ? 2 : !!(tr->flags & TF_ONE_WAY); e->from_proc = proc->pid; e->from_thread = thread->pid; e->target_handle = tr->target.handle; e->data_size = tr->data_size; e->offsets_size = tr->offsets_size; if (reply) { } else { if (tr->target.handle) { } else { target_node = binder_context_mgr_node; if (target_node == NULL) { } } e->to_node = target_node->debug_id; target_proc = target_node->proc; if (target_proc == NULL) { } if (security_binder_transaction(proc->tsk, target_proc->tsk) < 0) { return_error = BR_FAILED_REPLY; goto err_invalid_target_handle; } if (!(tr->flags & TF_ONE_WAY) && thread->transaction_stack) { } } if (target_thread) { } else { target_list = &target_proc->todo; target_wait = &target_proc->wait; } e->to_proc = target_proc->pid; /* TODO: reuse incoming transaction for reply */ t = kzalloc(sizeof(*t), GFP_KERNEL); if (t == NULL) { return_error = BR_FAILED_REPLY; goto err_alloc_t_failed; } binder_stats_created(BINDER_STAT_TRANSACTION); tcomplete = kzalloc(sizeof(*tcomplete), GFP_KERNEL); if (tcomplete == NULL) { return_error = BR_FAILED_REPLY; goto err_alloc_tcomplete_failed; } binder_stats_created(BINDER_STAT_TRANSACTION_COMPLETE); t->debug_id = ++binder_last_id; e->debug_id = t->debug_id; if (!reply && !(tr->flags & TF_ONE_WAY)) t->from = thread; else t->from = NULL; t->sender_euid = proc->tsk->cred->euid; t->to_proc = target_proc; t->to_thread = target_thread; t->code = tr->code; t->flags = tr->flags; t->priority = task_nice(current); trace_binder_transaction(reply, t, target_node); t->buffer = binder_alloc_buf(target_proc, tr->data_size, tr->offsets_size, !reply && (t->flags & TF_ONE_WAY)); if (t->buffer == NULL) { return_error = BR_FAILED_REPLY; goto err_binder_alloc_buf_failed; } t->buffer->allow_user_free = 0; t->buffer->debug_id = t->debug_id; t->buffer->transaction = t; t->buffer->target_node = target_node; trace_binder_transaction_alloc_buf(t->buffer); if (target_node) binder_inc_node(target_node, 1, 0, NULL); offp = (size_t *)(t->buffer->data + ALIGN(tr->data_size, sizeof(void *))); if (copy_from_user(t->buffer->data, tr->data.ptr.buffer, tr->data_size)) { binder_user_error("binder: %d:%d got transaction with invalid " "data ptr\n", proc->pid, thread->pid); return_error = BR_FAILED_REPLY; goto err_copy_data_failed; } if (copy_from_user(offp, tr->data.ptr.offsets, tr->offsets_size)) { binder_user_error("binder: %d:%d got transaction with invalid " "offsets ptr\n", proc->pid, thread->pid); return_error = BR_FAILED_REPLY; goto err_copy_data_failed; } if (!IS_ALIGNED(tr->offsets_size, sizeof(size_t))) { binder_user_error("binder: %d:%d got transaction with " "invalid offsets size, %zd\n", proc->pid, thread->pid, tr->offsets_size); return_error = BR_FAILED_REPLY; goto err_bad_offset; } off_end = (void *)offp + tr->offsets_size; if (reply) { } else if (!(t->flags & TF_ONE_WAY)) { BUG_ON(t->buffer->async_transaction != 0); t->need_reply = 1; t->from_parent = thread->transaction_stack; thread->transaction_stack = t; } else { } t->work.type = BINDER_WORK_TRANSACTION; list_add_tail(&t->work.entry, target_list); tcomplete->type = BINDER_WORK_TRANSACTION_COMPLETE; list_add_tail(&tcomplete->entry, &thread->todo); if (target_wait) wake_up_interruptible(target_wait); return;这里的target_node、target_proc、target_list和target_wait都是ServiceManager所在的进程。然后新建一个binder_transaction的数据结构t，并设置t的to_proc、to_thread为ServiceManager，t->code等于CHECK_SERVICE_TRANSACTION。然后申请binder_buffer，并把参数中tr中的buffer指针所指向的数据拷贝到binder_buffer中。然后设置t的from_parent为当前thread的transaction_stack，并将t加入到当前thread的transaction_stack中。最后将t加入到ServiceManager所在thread的todo链表中，并向当前thread的todo链表中加入tcomplete，我们前面分析过处理tcomplete的流程，就是向用户空间拷贝BR_NOOP和BR_TRANSACTION_COMPLETE两个指令。接着来看ServiceManager处理CHECK_SERVICE_TRANSACTION的流程，还是首先调用binder_parse去解析命令，然后调用svcmgr_handler去处理CHECK_SERVICE_TRANSACTION命令：
int svcmgr_handler(struct binder_state *bs, struct binder_txn *txn, struct binder_io *msg, struct binder_io *reply){ struct svcinfo *si; uint16_t *s; unsigned len; void *ptr; uint32_t strict_policy; int allow_isolated; if (txn->target != svcmgr_handle) return -1; strict_policy = bio_get_uint32(msg); s = bio_get_string16(msg, &len); if ((len != (sizeof(svcmgr_id) / 2)) || memcmp(svcmgr_id, s, sizeof(svcmgr_id))) { fprintf(stderr,"invalid id %s\n", str8(s)); return -1; } switch(txn->code) { case SVC_MGR_GET_SERVICE: case SVC_MGR_CHECK_SERVICE: s = bio_get_string16(msg, &len); ptr = do_find_service(bs, s, len, txn->sender_euid); if (!ptr) break; bio_put_ref(reply, ptr); return 0;这里首先还是做RPC检查，然后调用do_find_service去查找相应的svcinfo：
void *do_find_service(struct binder_state *bs, uint16_t *s, unsigned len, unsigned uid){ struct svcinfo *si; si = find_svc(s, len); if (si && si->ptr) { if (!si->allow_isolated) { unsigned appid = uid % AID_USER; if (appid >= AID_ISOLATED_START && appid <= AID_ISOLATED_END) { return 0; } } return si->ptr; } else { return 0; }}首先通过find_svc找到media.player的svcinfo，然后返回它的ptr指针。通过注册Service的知识，我们知道，这里的ptr指针实际上就是指向前面注册Service的handle id值。然后调用bio_put_ref(reply, ptr)将获得的ptr写入到reply中。
void bio_put_ref(struct binder_io *bio, void *ptr){ struct binder_object *obj; if (ptr) obj = bio_alloc_obj(bio); else obj = bio_alloc(bio, sizeof(*obj)); if (!obj) return; obj->flags = 0x7f | FLAT_BINDER_FLAG_ACCEPTS_FDS; obj->type = BINDER_TYPE_HANDLE; obj->pointer = ptr; obj->cookie = 0;}回到binder_parse中，它接着调用binder_send_reply将获取到的结果返回给binder驱动，binder_send_reply会发送BC_FREE_BUFFER和BC_REPLY两个指令给binder驱动，前面我们已经分析过BC_FREE_BUFFER和BC_REPLY了，这里与前面不同的是，在处理BC_REPLY是，t->buffer->data中储存着上面填充的binder_object对象：
static void binder_transaction(struct binder_proc *proc, struct binder_thread *thread, struct binder_transaction_data *tr, int reply){ struct binder_transaction *t; struct binder_work *tcomplete; size_t *offp, *off_end; struct binder_proc *target_proc; struct binder_thread *target_thread = NULL; struct binder_node *target_node = NULL; struct list_head *target_list; wait_queue_head_t *target_wait; struct binder_transaction *in_reply_to = NULL; struct binder_transaction_log_entry *e; uint32_t return_error; e = binder_transaction_log_add(&binder_transaction_log); e->call_type = reply ? 2 : !!(tr->flags & TF_ONE_WAY); e->from_proc = proc->pid; e->from_thread = thread->pid; e->target_handle = tr->target.handle; e->data_size = tr->data_size; e->offsets_size = tr->offsets_size; if (reply) { in_reply_to = thread->transaction_stack; if (in_reply_to == NULL) { } binder_set_nice(in_reply_to->saved_priority); if (in_reply_to->to_thread != thread) { } thread->transaction_stack = in_reply_to->to_parent; target_thread = in_reply_to->from; if (target_thread == NULL) { return_error = BR_DEAD_REPLY; goto err_dead_binder; } if (target_thread->transaction_stack != in_reply_to) { } target_proc = target_thread->proc; } else { } if (target_thread) { e->to_thread = target_thread->pid; target_list = &target_thread->todo; target_wait = &target_thread->wait; } else { } e->to_proc = target_proc->pid; /* TODO: reuse incoming transaction for reply */ t = kzalloc(sizeof(*t), GFP_KERNEL); if (t == NULL) { return_error = BR_FAILED_REPLY; goto err_alloc_t_failed; } binder_stats_created(BINDER_STAT_TRANSACTION); tcomplete = kzalloc(sizeof(*tcomplete), GFP_KERNEL); if (tcomplete == NULL) { return_error = BR_FAILED_REPLY; goto err_alloc_tcomplete_failed; } binder_stats_created(BINDER_STAT_TRANSACTION_COMPLETE); t->debug_id = ++binder_last_id; e->debug_id = t->debug_id; if (!reply && !(tr->flags & TF_ONE_WAY)) t->from = thread; else t->from = NULL; t->sender_euid = proc->tsk->cred->euid; t->to_proc = target_proc; t->to_thread = target_thread; t->code = tr->code; t->flags = tr->flags; t->priority = task_nice(current); trace_binder_transaction(reply, t, target_node); t->buffer = binder_alloc_buf(target_proc, tr->data_size, tr->offsets_size, !reply && (t->flags & TF_ONE_WAY)); if (t->buffer == NULL) { return_error = BR_FAILED_REPLY; goto err_binder_alloc_buf_failed; } t->buffer->allow_user_free = 0; t->buffer->debug_id = t->debug_id; t->buffer->transaction = t; t->buffer->target_node = target_node; trace_binder_transaction_alloc_buf(t->buffer); if (target_node) binder_inc_node(target_node, 1, 0, NULL); offp = (size_t *)(t->buffer->data + ALIGN(tr->data_size, sizeof(void *))); if (copy_from_user(t->buffer->data, tr->data.ptr.buffer, tr->data_size)) { binder_user_error("binder: %d:%d got transaction with invalid " "data ptr\n", proc->pid, thread->pid); return_error = BR_FAILED_REPLY; goto err_copy_data_failed; } if (copy_from_user(offp, tr->data.ptr.offsets, tr->offsets_size)) { binder_user_error("binder: %d:%d got transaction with invalid " "offsets ptr\n", proc->pid, thread->pid); return_error = BR_FAILED_REPLY; goto err_copy_data_failed; } off_end = (void *)offp + tr->offsets_size; for (; offp < off_end; offp++) { struct flat_binder_object *fp; if (*offp > t->buffer->data_size - sizeof(*fp) || } fp = (struct flat_binder_object *)(t->buffer->data + *offp); switch (fp->type) { case BINDER_TYPE_HANDLE: case BINDER_TYPE_WEAK_HANDLE: { struct binder_ref *ref = binder_get_ref(proc, fp->handle); if (ref == NULL) { binder_user_error("binder: %d:%d got " "transaction with invalid " "handle, %ld\n", proc->pid, thread->pid, fp->handle); return_error = BR_FAILED_REPLY; goto err_binder_get_ref_failed; } if (security_binder_transfer_binder(proc->tsk, target_proc->tsk)) { return_error = BR_FAILED_REPLY; goto err_binder_get_ref_failed; } if (ref->node->proc == target_proc) { if (fp->type == BINDER_TYPE_HANDLE) fp->type = BINDER_TYPE_BINDER; else fp->type = BINDER_TYPE_WEAK_BINDER; fp->binder = ref->node->ptr; fp->cookie = ref->node->cookie; binder_inc_node(ref->node, fp->type == BINDER_TYPE_BINDER, 0, NULL); trace_binder_transaction_ref_to_node(t, ref); binder_debug(BINDER_DEBUG_TRANSACTION, " ref %d desc %d -> node %d u%p\n", ref->debug_id, ref->desc, ref->node->debug_id, ref->node->ptr); } else { struct binder_ref *new_ref; new_ref = binder_get_ref_for_node(target_proc, ref->node); if (new_ref == NULL) { return_error = BR_FAILED_REPLY; goto err_binder_get_ref_for_node_failed; } fp->handle = new_ref->desc; binder_inc_ref(new_ref, fp->type == BINDER_TYPE_HANDLE, NULL); trace_binder_transaction_ref_to_ref(t, ref, new_ref); binder_debug(BINDER_DEBUG_TRANSACTION, " ref %d desc %d -> ref %d desc %d (node %d)\n", ref->debug_id, ref->desc, new_ref->debug_id, new_ref->desc, ref->node->debug_id); } } break; } } if (reply) { BUG_ON(t->buffer->async_transaction != 0); binder_pop_transaction(target_thread, in_reply_to); } else if (!(t->flags & TF_ONE_WAY)) { } else { } t->work.type = BINDER_WORK_TRANSACTION; list_add_tail(&t->work.entry, target_list); tcomplete->type = BINDER_WORK_TRANSACTION_COMPLETE; list_add_tail(&tcomplete->entry, &thread->todo); if (target_wait) wake_up_interruptible(target_wait); return;这里与前面处理BC_REPLY不同的是，在binder_transaction_data中的offsets_size不为0，所以要去处理buffer里面的flat_binder_object结构，其实这里的flat_binder_object结构就是上面的binder_object。这里首先通过handle id找到前面注册的binder_ref结构，然后通过binder_ref的node变量就可以找到binder_node结构，在binder_node中就存在只有我们注册的MediaplayerService对象了。这里首先判断ref->node->proc == target_proc，即注册的进程和现在获取MediaPlayerService的进程是否是同一个，如果是，则改写fp->type为BINDER_TYPE_BINDER，并设置fp->binder为binder->getWeakRefs()，fp->cookie等于binder->local本身。如果不是在同一个进程（这也是大多数case的状况），则首先调用binder_get_ref_for_node为获取MediaPlayerService的进程分配一个新的binder_ref结构，这里的binder id值可能不是之前注册的binder id值了（因为不在同一个进程，desc是往上增长的），然后设置fp->handle为新的desc值。最后返回到用户空间，在waitForResponse中，还是调用Parcel的ipcSetDataReference构造相应的数据结构，并让Parcel中的mData指针指向上面的binder_buffer。然后回到BpServiceMananger中的checkService方法，它会调用Parcel的readStrongBinder返回一个新的BpBinder。
sp<IBinder> Parcel::readStrongBinder() const{ sp<IBinder> val; unflatten_binder(ProcessState::self(), *this, &val); return val;}status_t unflatten_binder(const sp<ProcessState>& proc, const Parcel& in, sp<IBinder>* out){ const flat_binder_object* flat = in.readObject(false); if (flat) { switch (flat->type) { case BINDER_TYPE_BINDER: *out = static_cast<IBinder*>(flat->cookie); return finish_unflatten_binder(NULL, *flat, in); case BINDER_TYPE_HANDLE: *out = proc->getStrongProxyForHandle(flat->handle); return finish_unflatten_binder( static_cast<BpBinder*>(out->get()), *flat, in); } } return BAD_TYPE;}当在不同的进程时，type等于BINDER_TYPE_HANDLE；当在同一个进程中时，type等于BINDER_TYPE_BINDER。这里是不在同一个进程的跨进程间调用，先来看ProcessState的getStrongProxyForHandle方法：
sp<IBinder> ProcessState::getStrongProxyForHandle(int32_t handle){ sp<IBinder> result; AutoMutex _l(mLock); handle_entry* e = lookupHandleLocked(handle); if (e != NULL) { IBinder* b = e->binder; if (b == NULL || !e->refs->attemptIncWeak(this)) { if (handle == 0) { Parcel data; status_t status = IPCThreadState::self()->transact( 0, IBinder::PING_TRANSACTION, data, NULL, 0); if (status == DEAD_OBJECT) return NULL; } b = new BpBinder(handle); e->binder = b; if (b) e->refs = b->getWeakRefs(); result = b; } else { result.force_set(b); e->refs->decWeak(this); } } return result;}这里通过handle id值去mHandleToObject数组中查找，如果前面有调用过MediaPlayerService的方法，就会存在这样一个handle_entry；若是第一次调用，这里会构造一个新的handle_entry，并将其binder设置为NULL，然后返回。这里的handle id肯定是大于0，所以构造一个BpBinder(handle)对象并返回。
回到最开始的IMediaPlayerService的asInterface方法：
android::sp<IMediaPlayerService> IMediaPlayerService::asInterface( const android::sp<android::IBinder>& obj) { android::sp<IMediaPlayerService> intr; if (obj != NULL) { intr = static_cast<IMediaPlayerService*>( obj->queryLocalInterface( IMediaPlayerService::descriptor).get()); if (intr == NULL) { intr = new BpMediaPlayerService(obj); } } return intr; } 这里的obj就是BpBinder(handle)，显然它的queryLocalInterface方法返回NULL，所以这里构造一个BpMediaPlayerService(BpBinder(handle))并返回。所以在nativeListen函数中的service就等于BpMediaPlayerService(BpBinder(handle))。
调用Native Service的方法
当前面调用service->listenForRemoteDisplay方法时，相当于调用的是BpMediaPlayerService(BpBinder(handle))->listenForRemoteDisplay()，我们先来看BpMediaPlayerService的listenForRemoteDisplay方法：
virtual sp<IRemoteDisplay> listenForRemoteDisplay(const sp<IRemoteDisplayClient>& client, const String8& iface) { Parcel data, reply; data.writeInterfaceToken(IMediaPlayerService::getInterfaceDescriptor()); data.writeStrongBinder(client->asBinder()); data.writeString8(iface); remote()->transact(LISTEN_FOR_REMOTE_DISPLAY, data, &reply); return interface_cast<IRemoteDisplay>(reply.readStrongBinder()); }这里首先向Parcel中写入strict mode和"android.media.IMediaPlayerService"字串，然后将两个参数也写入到data中。调用remote()->transact方法其实就是调用BpBinder(handle)->transac方法：
status_t BpBinder::transact( uint32_t code, const Parcel& data, Parcel* reply, uint32_t flags){ // Once a binder has died, it will never come back to life. if (mAlive) { status_t status = IPCThreadState::self()->transact( mHandle, code, data, reply, flags); if (status == DEAD_OBJECT) mAlive = 0; return status; } return DEAD_OBJECT;}这里的mHandle就是上面的handle id（大于0），code就是LISTEN_FOR_REMOTE_DISPLAY。然后调用IPCThreadState的transact方法，这里的流程就不分析了，基本与前面注册AudioFlinger的流程差不多，最终会调用到binder_transaction方法：
static void binder_transaction(struct binder_proc *proc, struct binder_thread *thread, struct binder_transaction_data *tr, int reply){ struct binder_transaction *t; struct binder_work *tcomplete; size_t *offp, *off_end; struct binder_proc *target_proc; struct binder_thread *target_thread = NULL; struct binder_node *target_node = NULL; struct list_head *target_list; wait_queue_head_t *target_wait; struct binder_transaction *in_reply_to = NULL; struct binder_transaction_log_entry *e; uint32_t return_error; e = binder_transaction_log_add(&binder_transaction_log); e->call_type = reply ? 2 : !!(tr->flags & TF_ONE_WAY); e->from_proc = proc->pid; e->from_thread = thread->pid; e->target_handle = tr->target.handle; e->data_size = tr->data_size; e->offsets_size = tr->offsets_size; if (reply) { } else { if (tr->target.handle) { struct binder_ref *ref; ref = binder_get_ref(proc, tr->target.handle); if (ref == NULL) { binder_user_error("binder: %d:%d got " "transaction to invalid handle\n", proc->pid, thread->pid); return_error = BR_FAILED_REPLY; goto err_invalid_target_handle; } target_node = ref->node; } else { } e->to_node = target_node->debug_id; target_proc = target_node->proc; if (target_proc == NULL) { } if (!(tr->flags & TF_ONE_WAY) && thread->transaction_stack) { } } if (target_thread) { } else { target_list = &target_proc->todo; target_wait = &target_proc->wait; }这里的代码大致与注册Sevice的流程差不多，只是tr->target.handle的不同，当注册Service时，这里的handle等于0；当调用MediaPlayerService时，这是的handle值大于0。所以上面的代码先调用binder_get_ref通过handle id值找到对应的binder_ref结构，然后设置target_node = ref->node。接下来的代码与注册service的一样，首先为NativeRemoteDisplayClient申请一个binder_node和binder_ref结构，并将fp->type 改为 BINDER_TYPE_HANDLE，fp->handle改为 ref->desc。最后往MediaPlayerService的todo链表中插入一个binder_transaction结构，当MediaPlayerService所在的线程通过binder_thread_read读出待处理的任务时，它的数据结构如下：
interface "android.os.IServiceManager"
flat_binder_object type BINDER_TYPE_HANDEL
flags 0
binder ref->desc
cookie local
iface ip:port
offsets0
IPCThreadState通过talkWithDriver方法获取上面的数据，并且调用executeCommand中去处理BR_TRANSACTION命令：
case BR_TRANSACTION: { binder_transaction_data tr; result = mIn.read(&tr, sizeof(tr)); Parcel buffer; buffer.ipcSetDataReference( reinterpret_cast<const uint8_t*>(tr.data.ptr.buffer), tr.data_size, reinterpret_cast<const size_t*>(tr.data.ptr.offsets), tr.offsets_size/sizeof(size_t), freeBuffer, this); const pid_t origPid = mCallingPid; const uid_t origUid = mCallingUid; mCallingPid = tr.sender_pid; mCallingUid = tr.sender_euid; int curPrio = getpriority(PRIO_PROCESS, mMyThreadId); if (gDisableBackgroundScheduling) { if (curPrio > ANDROID_PRIORITY_NORMAL) { setpriority(PRIO_PROCESS, mMyThreadId, ANDROID_PRIORITY_NORMAL); } } else { if (curPrio >= ANDROID_PRIORITY_BACKGROUND) { set_sched_policy(mMyThreadId, SP_BACKGROUND); } } Parcel reply; if (tr.target.ptr) { sp<BBinder> b((BBinder*)tr.cookie); const status_t error = b->transact(tr.code, buffer, &reply, tr.flags); if (error < NO_ERROR) reply.setError(error); } else { const status_t error = the_context_object->transact(tr.code, buffer, &reply, tr.flags); if (error < NO_ERROR) reply.setError(error); } //ALOGI("<<<< TRANSACT from pid %d restore pid %d uid %d\n", // mCallingPid, origPid, origUid); if ((tr.flags & TF_ONE_WAY) == 0) { LOG_ONEWAY("Sending reply to %d!", mCallingPid); sendReply(reply, 0); } else { LOG_ONEWAY("NOT sending reply to %d!", mCallingPid); } mCallingPid = origPid; mCallingUid = origUid; } break;这里首先通过binder_transaction_data的buffer构造一个Parcel对象，然后保存mCallingPid和mCallingUid，并设置它们为新的sender_pid和sender_euid。然后获取在tr.cookie保存的MediaPlayerService对象，然后调用它的transact方法，这里的transact实现在BBinder中：
status_t BBinder::transact( uint32_t code, const Parcel& data, Parcel* reply, uint32_t flags){ data.setDataPosition(0); status_t err = NO_ERROR; switch (code) { case PING_TRANSACTION: reply->writeInt32(pingBinder()); break; default: err = onTransact(code, data, reply, flags); break; } if (reply != NULL) { reply->setDataPosition(0); } return err;}onTransact的实现在BnMediaPlayerService中：
status_t BnMediaPlayerService::onTransact( uint32_t code, const Parcel& data, Parcel* reply, uint32_t flags){ switch (code) { case LISTEN_FOR_REMOTE_DISPLAY: { CHECK_INTERFACE(IMediaPlayerService, data, reply); sp<IRemoteDisplayClient> client( interface_cast<IRemoteDisplayClient>(data.readStrongBinder())); String8 iface(data.readString8()); sp<IRemoteDisplay> display(listenForRemoteDisplay(client, iface)); reply->writeStrongBinder(display->asBinder()); return NO_ERROR; } break;通过data中读出记录的RemoteDisplayClient的handle id值构造一个BpRemoteDisplayClient对象，再读出iface值。然后调用MediaPlayerService的listenForRemoteDisplay方法：
sp<IRemoteDisplay> MediaPlayerService::listenForRemoteDisplay( const sp<IRemoteDisplayClient>& client, const String8& iface) { if (!checkPermission("android.permission.CONTROL_WIFI_DISPLAY")) { return NULL; } return new RemoteDisplay(client, iface.string());}将上面创建的RemoteDisplay对象写入到reply这个Parcel中，回到executeCommand中调用sendReply方法将RemoteDisplay对象返回到调用的进程中：
status_t IPCThreadState::sendReply(const Parcel& reply, uint32_t flags){ status_t err; status_t statusBuffer; err = writeTransactionData(BC_REPLY, flags, -1, 0, reply, &statusBuffer); if (err < NO_ERROR) return err; return waitForResponse(NULL, NULL);}这里调用writeTransactionData将reply中的数据写入到mOut中，然后调用waitForResponse将数据发送到binder驱动中。这里的两个参数都是NULL，表示不需要等待BR_REPLY，收到BR_TRANSACTION_COMPLETE后就直接退出。在binder驱动收到上面的数据后，和注册Service类似，首先创建一个binder_transaction的结构，然后将target_thread和target_proc都设置为调用listenForRemoteDisplay方法的线程，然后创建binder_buffer，将上面的RemoteDisplay对象拷贝到binder_buffer中，并改写fp->type 改为 BINDER_TYPE_HANDLE，fp->handle 改为ref->desc。最后将这个binder_transaction放在调用listenForRemoteDisplay线程的todo链表中。当调用listenForRemoteDisplay线程通过waitForResponse获取到BR_REPLY后，通过binder_transaction_data中的buffer数据构造一个reply(Pacel)对象，然后调用interface_cast<IRemoteDisplay>(reply.readStrongBinder())，就会返回一个BpRemoteDisplay(BpBinder(handle id))的对象。这里的RemoteDisplay又是一个binder调用，后面调用它的方法时就像我们调用MediaPlayerService的方法一样，需要进过binder驱动去实现跨进程的调用。
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