Intercept GWT RPC Request and Response

Some times you want to do some generic things like setting some header or adding some log or may be even showing a progress bar while rpc call is in progress and hide it once you receive the response. I came across one of such requirement for showing progress bar while service call is in progress. Following is complete end to end solution I found after searching for some solutions. I got some help at following threads https://groups.google.com/forum/?fromgroups=#!msg/google-web-toolkit/VeiJUPBUsi4/Hq2VDMF9W1kJ and http://stackoverflow.com/questions/5309624/how-to-intercept-the-onsuccess-method-in-gwt

I created one complete end  to end working example from that help.

1:  public class LoaderRpcRequestBuilder extends RpcRequestBuilder {  
2:       private Loader     loader;  
3:       private class RequestCallbackWrapper implements RequestCallback {  
4:            private RequestCallback     callback;  
5:            RequestCallbackWrapper(RequestCallback aCallback) {  
6:                 this.callback = aCallback;  
7:            }  
8:            @Override  
9:            public void onResponseReceived(Request request, Response response) {  
10:                 loader.hide();  
11:                 callback.onResponseReceived(request, response);  
12:            }  
13:            @Override  
14:            public void onError(Request request, Throwable exception) {  
15:                 loader.hide();  
16:                 callback.onError(request, exception);  
17:            }  
18:       }  
19:       public LoaderRpcRequestBuilder(Loader loader) {  
20:            this.loader = loader;  
21:       }  
22:       public LoaderRpcRequestBuilder() {  
23:            this(new Loader());  
24:       }  
25:       @Override  
26:       protected RequestBuilder doCreate(String serviceEntryPoint) {  
27:            RequestBuilder rb = super.doCreate(serviceEntryPoint);  
28:            loader.show();  
29:            return rb;  
30:       }  
31:       @Override  
32:       protected void doFinish(RequestBuilder rb) {  
33:            super.doFinish(rb);  
34:            rb.setCallback(new RequestCallbackWrapper(rb.getCallback()));  
35:       }  
36:  }  

Here in above code we are creating our own custom RpcRequestBuilder by extending RpcRequestBuilder and by overriding its doCreate and doFinish() method. We are intercepting the calls that will be made. In doCreate() we are showing loader and then inside doFinish we are setting one general purpose RequestCallback wrapper which we have created to intercept response received. Where we are hiding our loader inside onResponseReceived() and onError.

Note: be sure to call super.doFinish(rb); other wise you will get SecurityException. There are some tokens which is set by doFinish method to avoid XSRF attack. (Though they say one should not just rely on this mechanism to avoid XSRF attack.)

Now that we have created custom RpcRequestBuilder we need to understand how to use it.

1:  ServiceAsync serivce = GWT.create(Service.class);  
2:  ServiceDefTarget target = (ServiceDefTarget) serivce;  
3:  target.setRpcRequestBuilder(new LoaderRpcRequestBuilder());  

When we make rpc call we get async instance before making any service call we need to set our own custom RpcRequestBuilder.