一簡介

從android 10開始，hwbinder引入了lazy service模式，android R正式引入到binder中。使用lazy方式注冊的binder或者hidl servicer在client退出后確認相關service沒有client調用后會自動退出，讓出系統資源，節(jié)省內存占用。比較智能。

二 原理

1） hidl service lazy 注冊接口

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;
};

注冊hidl service成功后多了個onClients回調，在client連接數為0時退出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側注冊的Hidl service會判斷當前的client數，如果有client那么就發(fā)送sendClientCallbackNotifications(true);通知對應的hidl service。如果client為空的話sendClientCallbackNotifications(false)通知hidl service client為空了。

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調用的幾個時機：
1） 在client get service時通知hidl servie mNumConnectedServices++

2. 取消注冊時tryUnregister
   3） ClientCallbackCallback 調用mManager->handleClientCallbacks();interval是5秒鐘

三 總結

從android R開始，binder借鑒了hidl也引入了lazy模式，具體見frameworks/native/libs/binder/LazyServiceRegistrar.cpp，原理同HIDL方式