Prevent server handler from firing - google-apps-script

Question

Can we stop a server handler from firing if a client handler (on the same widget) meets a validation condition? UI
I don't want Submit to perform any server function if the text box is empty.
function doGet() {
var app = UiApp.createApplication();
var flex = app.createFlexTable()
.setWidget(0, 0, app.createTextBox().setId('textbox'))
.setWidget(0, 1, app.createButton('Submit').setId('submit'))
.setWidget(0, 2, app.createLabel().setId('status'));
var clientHandler = app.createClientHandler().validateNotMatches(app.getElementById('textbox'), ' ');
var serverHandler = app.createServerHandler('submit').addCallbackElement(flex);
app.getElementById('submit').addClickHandler(clientHandler).addClickHandler(serverHandler);
app.add(flex);
return app;
}
function submit(e) {
var app = UiApp.getActiveApplication();
app.getElementById('status').setText('Server handler fired');
return app;
}

Answer 1

You don't need or want a client handler here, just a validator on the server handler:
function doGet() {
var app = UiApp.createApplication();
var flex = app.createFlexTable()
.setWidget(0, 0, app.createTextBox().setId('textbox'))
.setWidget(0, 1, app.createButton('Submit').setId('submit'))
.setWidget(0, 2, app.createLabel().setId('status'));
var serverHandler = app.createServerHandler('submit')
.validateLength(app.getElementById('textbox'), 1, null)
.addCallbackElement(flex);
app.getElementById('submit').addClickHandler(serverHandler);
app.add(flex);
return app;
}
function submit(e) {
var app = UiApp.getActiveApplication();
app.getElementById('status').setText('Server handler fired');
return app;
}
If you want a message explaining what went wrong, you can add this:
var clientHandler = app.createClientHandler()
.validateNotLength(app.getElementById('textbox'), 1, null)
.forTargets(app.getElementById('status'))
.setText('Cannot be empty');
app.getElementById('submit').addClickHandler(serverHandler)
.addClickHandler(clientHandler);