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## com.google.gwt.user.client.rpc
Interface AsyncCallback

    public interface AsyncCallback

The primary interface a caller must implement to receive a response from a remote procedure call.

If an RPC is successful, then `onSuccess(Object)` is called, otherwise `onFailure(Throwable)` is called.

Each callable asynchronous method corresponds to a method in the correlated service interface. The asynchronous method always takes an `AsyncCallback` as its last parameter.

As an example, suppose the service interface defines a method called `getShapes` as follows:

     Shape[] getShapes(String databaseName) throws ShapeException, DbException;

Its asynchronous counterpart method be declared as:

     void getShapes(String databaseName, AsyncCallback callback);

Note that `throws` declaration is not repeated in the async version.

A call with a typical use of `AsyncCallback` might look like this:

     service.getShapes(dbName, new AsyncCallback() {
       public void onSuccess(Object result) {
         // It's always safe to downcast to the known return type.
         Shape[] shapes = (Shape[]) result;
         controller.processShapes(shapes);
       }

       public void onFailure(Throwable caught) {
         // Convenient way to find out which exception was thrown.
         try {
           throw caught;
         } catch (IncompatibleRemoteServiceException e) {
           // this client is not compatible with the server; cleanup and refresh the
           // browser
         } catch (InvocationException e) {
           // the call didn't complete cleanly
         } catch (ShapeException e) {
           // one of the 'throws' from the original method
         } catch (DbException e) {
           // one of the 'throws' from the original method
         } catch (Throwable e) {
           // last resort -- a very unexpected exception
         }
       }
     });

Method Summary
`void` | `onFailure(java.lang.Throwable caught)` Called when an asynchronous call fails to complete normally.
`void` | `onSuccess(java.lang.Object result)` Called when an asynchronous call completes successfully.

Method Detail

### onFailure

    void onFailure(java.lang.Throwable caught)

  Called when an asynchronous call fails to complete normally. `IncompatibleRemoteServiceException`s, `InvocationException`s, or checked exceptions thrown by the service method are examples of the type of failures that can be passed to this method.

  If `caught` is an instance of an `IncompatibleRemoteServiceException` the application should try to get into a state where a browser refresh can be safely done.

  Parameters:
    `caught` - failure encountered while executing a remote procedure call

### onSuccess

    void onSuccess(java.lang.Object result)

  Called when an asynchronous call completes successfully. It is always safe to downcast the parameter (of type `Object`) to the return type of the original method for which this is a callback. Note that if the return type of the synchronous service interface method is a primitive then the parameter will be the boxed version of the primitive (for example, an `int` return type becomes an `Integer`.

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