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copied!<p>Separation between the presenter (which contains logic) and view (a dumb wrapper around UI controls) allows you to:</p> <ul> <li>write unit tests for the presenters that can run without needing the corresponding environment (desktop, browser, GWT widgets)</li> <li>reuse front-end logic without being tied to a particular set of widgets/UI framework</li> </ul> <p>The latter use case is rare, so let's focus on the MVP model's suitability for automated, programmatic testing. With a team of developers this often takes the form of a continuous build/test cycle using <a href="http://hudson-ci.org/" rel="noreferrer">Hudson</a> (or similar) on a headless server, where it's not practical to open a web browser, create controls, etc. every time a test is run.</p> <p>Typical usage of MVP+GWT is that views implement an interface provided by the presenter, and often this interface is defined in terms of other generic interfaces. Here's a very simple presenter that increments a numeric label when a button is clicked - note that instead of exposing the <a href="http://google-web-toolkit.googlecode.com/svn/javadoc/2.0/com/google/gwt/user/client/ui/TextBox.html" rel="noreferrer">TextBox</a> and <a href="http://google-web-toolkit.googlecode.com/svn/javadoc/2.0/com/google/gwt/user/client/ui/Button.html" rel="noreferrer">Button</a> directly, the view returns generic <a href="http://google-web-toolkit.googlecode.com/svn/javadoc/2.0/com/google/gwt/user/client/ui/HasText.html" rel="noreferrer">HasText</a> and <a href="http://google-web-toolkit.googlecode.com/svn/javadoc/2.0/com/google/gwt/event/dom/client/HasClickHandlers.html" rel="noreferrer">HasClickHandlers</a> instances:</p> <pre><code>public class ButtonClickPresenter { public interface View { HasText currentValue(); HasClickHandlers incrementButton(); } private final View myView; private int currentValue = 0; public ButtonClickPresenter(View myView) { this.myView = myView; this.myView.currentValue().setText("0"); this.bind(); // for the sake of demonstration } public void bind() { this.myView.incrementButton.addClickHandler( @Override new ClickHandler() { void onClick(ClickEvent event) { currentValue ++; myView.currentValue().setText( Integer.toString(currentValue)); } }); } } </code></pre> <p>The "real" view returns UI widgets (created via UiBinder in this example):</p> <pre><code>public class ButtonClickView implements ButtonClickPresenter.View { // ... skipped UiBinder initialisation ... @UiField Label currentValueLabel; @UiField Button incrementButton; @Override public HasText currentValue() { return currentValueLabel; } @Override public HasClickHandlers incrementButton() { return incrementButton; } // ... etc ... } </code></pre> <p>whereas unit tests create a dummy implementation (or use <a href="http://code.google.com/p/mockito/" rel="noreferrer">Mockito</a>, <a href="http://easymock.org/" rel="noreferrer">EasyMock</a>, etc.) and thus don't require any UI components:</p> <pre><code>public class ButtonClickPresenterTest { ButtonClickPresenter presenter; ClickHandler currentHandler; String currentText; @Before public void setUp() { presenter = new ButtonClickPresenter( // dummy view - just stores label text in a String and // keeps track of the Presenter's click handler new ButtonClickPresenter.View() { @Override public HasText currentValue() { return new HasText() { @Override public String getText() { return currentText; } @Override public void setText(String text) { currentText = text; } }; } @Override public HasClickHandlers incrementButton() { return new HasClickHandlers() { @Override public HandlerRegistration addClickHandler(ClickHandler handler) { currentHandler = handler; } }; } }); } @Test public void testIncrement() { // initial value assertEquals("0", currentText); // clicking the button should increment the number currentHandler.onClick(null); assertEquals("1", currentText); } } </code></pre> <p>As for your next paragraph: your views shouldn't be connecting to the database at all! The presenter should request data via Service/ServiceAsync (or an abstraction such as <a href="http://code.google.com/p/gwt-dispatch/" rel="noreferrer">gwt-dispatch</a> or <a href="http://code.google.com/p/gwt-platform/" rel="noreferrer">gwt-platform</a>), then call methods on the view to populate the UI.</p> <p>By the way, those last two links (along with <a href="http://code.google.com/p/gwt-presenter/" rel="noreferrer">gwt-presenter</a>) are a good start if you're looking for GWT MVP code samples - combined with <a href="http://code.google.com/p/google-gin/" rel="noreferrer">Google GIN</a> they provide frameworks for tying all this stuff together.</p> <p>Having said all that, I agree - the combination of GWT+MVP+Java can be hard work and <strong>extremely</strong> verbose (I'm glad I don't have to work with it much these days). The alternative, though, is even less attractive: an untestable, unmaintainable ball of spaghetti...</p>

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