use return app; without ending current function - google-apps-script

Question

Is there another way to update the gui than return app;?
I want to set the text on a label before doing an url fetch, like started downloading, and after it completes turn the label into download complete.
function EventHandler(e) {
var app = UiApp.getActiveApplication();
var url = e.parameter.URLInput;
var label = app.getElementById("label");
label.setText("Download started");
try{
var file = UrlFetchApp.fetch(url).getBlob();
} catch(err){
label.setText(err);
}
label.setText("Download finished");
return app;
}
The label stays empty until UrlFetchApp is completed, and then the label's content is 'Download finished'. Adding return app; before the fetch ends the function.

Answer 1

You have to use a clientHandler to set Text to your label in the doGet function, the clientHandler executes immediately when you click the button.
Here is a test app that shows how it works: (online test available here with simulated download)
function doGet(){
var app = UiApp.createApplication();
var label = app.createLabel('---empty---').setId('label');
app.add(label)
var handler = app.createServerHandler('EventHandler');
var cHandler = app.createClientHandler().forTargets(label).setText('starting download');
var btn = app.createButton('start',handler).addClickHandler(cHandler);
app.add(btn);
return app;
}
function EventHandler(e) {
var app = UiApp.getActiveApplication();
var url = e.parameter.URLInput;
var ulabel = app.getElementById("label");
ulabel.setText("Download started");
try{
//var file = UrlFetchApp.fetch(url).getBlob();
} catch(err){
label.setText(err);
}
ulabel.setText("Download finished");
return app;
}
note : you can use the same client handler to do lots of other usefull things : disable the button, show a spinner... whatever you like that must happen in the doGet function without delay.
EDIT following your comment
Have you tried using 2 server handlers in parallel ? in the displayHandler you could setup any condition you want, I left it simple in the following example :
function doGet(){
var app = UiApp.createApplication();
var label = app.createLabel('---empty---').setId('label');
app.add(label)
var handler = app.createServerHandler('EventHandler');
var displayHandler = app.createServerHandler('displayHandler');
var btn = app.createButton('start',handler).addClickHandler(displayHandler);
// you can add other handlers (keypress, hover... whatever) they will all execute at the same time
app.add(btn);
return app;
}
function displayHandler(e) {
var app = UiApp.getActiveApplication();
var ulabel = app.getElementById("label");
ulabel.setText("Download started");
return app;
}
function EventHandler(e) {
var app = UiApp.getActiveApplication();
var url = e.parameter.URLInput;
var ulabel = app.getElementById("label");
try{
Utilities.sleep(2000);// simulating download
} catch(err){
label.setText(err);
}
ulabel.setText("Download finished");
return app;
}