Monday, August 25, 2008

Flex JSON-RPC implementation

So, you're checking out Flex and you're thinking to yourself, "Gee this e4x stuff is pretty rad, but it's 2008 and I could really use a good JSON implementation so I can integrate my bitchin' Flex component into my AJAX page." Amazingly, I was thinking the same thing, so I put together a couple of classes to implement a JSON-RPC v1.1 client.

The application code for this post is available on Google code. In addition to the JSON-RPC classes, the application contains a simple gui with controls for testing and logging JSON-RPC communication. To compile the code, you'll also need as3corelib in your compile path, which handles the JSON serialization/deserialization.

The 1st thing you'll need is a JSON-RPC server. If you don't already have one, it shouldn't be too difficult to put together. Follow this tutorial to create a simple Python JSON-RPC server, or use any 1.1 compliant server.

Next, we need to extend mx.rpc.AbstractOperation. This class will send the method invokation to the server and decode the result. Two important methods are overriden. The first is 'send', which will encode the request according to the JSON-RPC v1.1 spec, create a HTTPRequestMessage, and then call the parent invoke() to send the message to the server. If JSON encoding fails, a FaultEvent must be dispatched. Use the parent method dispatchRpcEvent() to make sure the event is dispatched by both the service and the operation.

The other important method is 'processResult', which decodes the response from the server and checks for any errors defined by the server application. The 'processResult' method is in the mx_internal namespace. If you're not familiar with using mx_internal, you should probably read up on it. The parent class takes care of most of the heavy lifting, but you will need to dispatch a FaultEvent if the JSON decode fails, or if a server application error was defined. Set the _result variable equal to the decoded result, and you're good to go.

Finally, we need to extend mx.rpc.AbstractService. This class is used to instantiate mx.rpc.AbstractOperation instances and invoke remote method calls in our application code. The class's main purpose is to act as a proxy that intercepts method calls, so we will be able to invoke a remote method with the syntax: service.method(arguments) instead of operation.send(method, args). You will need to override the 'getOperation' method to instantiate the correct mx.rpc.AbstractOperation subclass.

Here's how the jsonrpc.JsonRpcService can be used in your application code to communicate with the server:


import jsonrpc.JsonRpcService;

private function callFoo(fooArgs:Object):void
// Create service object and
// call remote method 'foo'.
var jsonService:JsonRpcService = new JsonRpcService();
jsonService.rootUrl = 'http://localhost';
jsonService.url = 'test';
jsonService.addEventListener(ResultEvent.RESULT, gotBar);;

private function gotBar(event:ResultEvent):void
{, gotBar);
var bar:Object = ResultEvent.result;
// do something with bar

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