Using Callbacks

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Callbacks are useful when the server needs to communicate with the client while executing a server method. Callbacks allow a JSON parameter to be passed back to the client and the client can return a JSON value back to the server.

In order to pass a callback, the server method needs to have a parameter of type TDBXCallback, like in the following example:

  procedure Callback(const PCallback: TDBXCallback);

In the server implementation, the callback can be invoked at any time:


The invocation is synchronous and it returns a client JSON value.

On the client side, you need to extend a TDBXCallback class and override the Execute method:

  TCallbackClient = class(TDBXCallback)
    function Execute(const Arg: TJSONValue): TJSONValue; override;

The server method needs to be invoked with an instance of the client callback class.

The callback parameters are owned by the proxy instance or server and released upon completion.

See Also