The Plugin Control Flow

1. The Server loads the plugin’s dynamic library and invokes its entry point function createNxPlugin() (see i_plugin.h), which returns a pointer to the nx::sdk::IPlugin interface instance.
2. The Server creates an Engine class instance by calling IPlugin::createEngine() using the pointer. 
    a. createEngine() calls the  doCreateEngine() virtual method of the interface class (implemented in the helper class Plugin. 
    b. This method then calls the virtual method doObtainEngine(). The  doObtainEngine() is overridden in the Plugin helper class.
    c. The IPlugin::createEngine() returns a pointer to the IEngine interface instance.
3. When a Plugin is enabled for a specific camera in the Desktop Client, the DeviceAgent class instance is created. 
    a. The Desktop Client shows deviceAgentSettingsModel returned by the Engine::manifestString() method. 
    b. When the “OK” or the “Apply” button is pressed in the Desktop Client, the Server calls IEngine::obtainDeviceAgent() virtual method of the interface. The  doObtainDeviceAgent() is overridden in the Engine helper class. 
   c. The IEngine::obtainDeviceAgent() calls the doObtainDeviceAgent() virtual method
4. The Server provides the plugin with data packets (i.e. video frames). 
    a. It invokes the doPushDataPacket() method of the IConsumingDeviceAgent interface on retrieval of every frame. It is the general method for all types of Plugins (analytics, video source, storage) and is the only method the Server can execute without accessing external resources.
    b. In the Metadata SDK,pushDataPacket invokes the doPushDataPacket() virtual method of the interface class (see src/nx/sdk/analytics/i_consuming_device_agent.h). The doPushDataPacket() is overridden in the ConsumingDeviceAgent helper class of the SDK (see src/nx/sdk/analytics/helpers/consuming_device_agent.cpp). The class implements several other methods in order to make plugin development more convenient and efficient: pushCompressedVideoFrame(), pushUncompressedVideoFrame(), pullMetadataPackets(), pushMetadataPacket(), etc.
5. When a new frame is retrieved from the camera, the Server simultaneously calls the following methods:
    a. pushUncompressedFrame() or  pushUncompressedVideoFrame() to deliver the frame to the plugin;
    b. pullMetadataPackets() to receive metadata from the plugin;
6. pushMetadataPacket() can be called at any time by the DeviceAgent.

You can choose either one of the "push" or "pull" paradigms depending on the desired workflow in the Plugin. The differences are:

• pushMetadataPacket() is called by the Plugin, while pullMetadataPackets() is called by the Server.
• pushMetadataPacket() expects one metadata packet, while pullMetadataPackets() expects a std::vector of them.

Currently, the analytics plugin consists of three helper classes, which we recommend using:

• Plugin - implemented in plugin.cpp and plugin.h.
• Engine - implemented in engine.cpp and engine.h.
• DeviceAgent - implemented in device_agent.cpp and device_agent.h. 

We recommend reading this article to understand the plugin lifecycle better.

• In this tutorial, we are going to use the DeviceAgent helper class and its pushUncompressedFrame and pushMetadataPacket methods.