一简介

从android 10开始，hwbinder引入了lazy service模式，android R正式引入到binder中。使用lazy方式注册的binder或者hidl servicer在client退出后确认相关service没有client调用后会自动退出，让出系统资源，节省内存占用。比较智能。

二 原理

1） hidl service lazy 注册接口

class LazyServiceRegistrarImpl {
   public:
    LazyServiceRegistrarImpl() : mClientCallback(new ClientCounterCallback) {}

    status_t registerService(const sp<::android::hidl::base::V1_0::IBase>& service,
                             const std::string& name);

   private:
    sp<ClientCounterCallback> mClientCallback;
};

注册hidl service成功后多了个onClients回调，在client连接数为0时退出service

/**
 * onClients is oneway, so no need to worry about multi-threading. Note that this means multiple
 * invocations could occur on different threads however.
 */
Return<void> ClientCounterCallback::onClients(const sp<::android::hidl::base::V1_0::IBase>& service,
                                              bool clients) {
    if (clients) {
        mNumConnectedServices++;
    } else {
        mNumConnectedServices--;
    }

    LOG(INFO) << "Process has " << mNumConnectedServices << " (of " << mRegisteredServices.size()
              << " available) client(s) in use after notification " << getDescriptor(service.get())
              << " has clients: " << clients;

    if (mNumConnectedServices == 0) {
        tryShutdown();
    }

    return Status::ok();
}

在hwservicemanager侧注册的Hidl service会判断当前的client数，如果有client那么就发送sendClientCallbackNotifications(true);通知对应的hidl service。如果client为空的话sendClientCallbackNotifications(false)通知hidl service client为空了。

ssize_t HidlService::forceHandleClientCallbacks(bool isCalledOnInterval) {
    ssize_t count = getNodeStrongRefCount();

    // binder driver doesn't support this feature
    if (count == -1) return count;

    bool hasClients = count > 1; // this process holds a strong count

    if (mGuaranteeClient) {
        // we have no record of this client
        if (!mHasClients && !hasClients) {
            sendClientCallbackNotifications(true);
        }

        // guarantee is temporary
        mGuaranteeClient = false;
    }

    if (hasClients && !mHasClients) {
        // client was retrieved in some other way
        sendClientCallbackNotifications(true);
    }

    // there are no more clients, but the callback has not been called yet
    if (!hasClients && mHasClients && isCalledOnInterval) {
        mNoClientsCounter++;

        if (mNoClientsCounter >= kNoClientRepeatLimit) {
            sendClientCallbackNotifications(false);
        }
    }

    return count;
}

forceHandleClientCallbacks调用的几个时机：
1） 在client get service时通知hidl servie mNumConnectedServices++

2. 取消注册时tryUnregister
   3） ClientCallbackCallback 调用mManager->handleClientCallbacks();interval是5秒钟

三 总结

从android R开始，binder借鉴了hidl也引入了lazy模式，具体见frameworks/native/libs/binder/LazyServiceRegistrar.cpp，原理同HIDL方式