GwtMockito.java

package com.objectpartners.buesing.sample;

import com.google.gwt.event.dom.client.ClickEvent;
import com.google.gwt.event.dom.client.ClickHandler;
import com.google.gwt.event.shared.SimpleEventBus;
import com.google.gwt.user.client.Element;
import com.google.gwt.user.client.rpc.AsyncCallback;
import com.google.gwt.user.client.ui.Button;
import com.google.gwt.user.client.ui.HTML;
import com.google.gwt.user.client.ui.Label;
import com.google.gwtmockito.GwtMock;
import com.google.gwtmockito.GwtMockito;
import com.google.gwtmockito.GwtMockitoTestRunner;
import com.google.gwtmockito.fakes.FakeProvider;
import com.objectpartners.buesing.sample.client.SampleApplicationServiceAsync;
import com.objectpartners.buesing.sample.client.SampleWidget;
import com.sencha.gxt.widget.core.client.Component;
import org.junit.Before;
import org.junit.Test;
import org.junit.runner.RunWith;
import org.junit.runners.model.InitializationError;
import org.mockito.Mockito;
import org.mockito.invocation.InvocationOnMock;
import org.mockito.stubbing.Answer;

import java.util.Collection;

import static org.mockito.Matchers.any;

@RunWith(GwtMockitoTestRunner.class)
public class SampleWidgetTest {

    private SampleWidget sampleWidget;

    private SimpleEventBus simpleEventBus = new SimpleEventBus();

    @GwtMock
    private Button button;

    @GwtMock
    private Label label;

    @GwtMock
    private Element element;

    @GwtMock
    private HTML html;

    @GwtMock
    private SampleApplicationServiceAsync sampleApplicationService;

    private ClickHandler clickHandler;

    @Before
    public void setUp() {

        GwtMockito.useProviderForType(SimpleEventBus.class, new FakeProvider() {
            @Override
            public Object getFake(Class aClass) {
                return simpleEventBus;
            }
        });

        Mockito.doAnswer(new Answer() {
            public Object answer(InvocationOnMock invocation) throws Throwable {
                String msg = (String)invocation.getArguments()[0];
                AsyncCallback  callback = (AsyncCallback) invocation.getArguments()[1];
                callback.onSuccess("Client said: \"" + msg + "\"<br>Server answered: \"Hi!\"");
                return null;
            }
        }).when(sampleApplicationService)
                .getMessage(Mockito.any(String.class), (AsyncCallback) Mockito.any());

        Mockito.when(button.addClickHandler(Mockito.any(ClickHandler.class))).thenAnswer(new Answer() {
            public Object answer(InvocationOnMock aInvocation) throws Throwable {
                clickHandler = (ClickHandler) aInvocation.getArguments()[0];
                return null;
            }
        });


        Mockito.when(label.getText()).thenReturn("");

        Mockito.when(label.getElement()).thenReturn(element);

        // Mocking the click handler needs to be done BEFORE the constructor is called, so the 'clickHandler' can be extracted from the SampleWidget.
        sampleWidget = new SampleWidget();
    }

    @Test
    public void clickOnButton() {
        clickHandler.onClick(new ClickEvent(){});
        Mockito.verify(element).setInnerHTML("Client said: \"Hello, World!\"<br>Server answered: \"Hi!\"");
        Mockito.verify(html).setHTML("event fired.");
    }
}