refreshing a button's text is not working with GAS - google-apps-script

Question

I have the following code which creates a simple app to allow the user to enter two values and click a button:
function start() {
var ss = SpreadsheetApp.getActiveSpreadsheet();
var app = UiApp.createApplication();
app.setTitle("Appraisals Analysis");
app.setHeight(100);
app.setWidth(500);
var grid = app.createGrid(3, 2);
grid.setId("grid");
grid.setCellSpacing(2);
grid.setCellPadding(2);
var uafLabel = app.createLabel("Unprocessed apparaisals folder name: ");
uafLabel.setStyleAttributes({"font-weight": "bold"});
var uafTextBox = app.createTextBox();
uafTextBox.setName('uafTextBox').setId('uafTextBox');
uafTextBox.setText('Unprocessed Appraisals');
grid.setWidget(0, 0, uafLabel);
grid.setWidget(0, 1, uafTextBox);
var pafLabel = app.createLabel("Processed apparaisals folder name: ");
pafLabel.setStyleAttributes({"font-weight": "bold"});
var pafTextBox = app.createTextBox();
pafTextBox.setName('pafTextBox').setId('pafTextBox');
pafTextBox.setText('Processed Appraisals');
grid.setWidget(1, 0, pafLabel);
grid.setWidget(1, 1, pafTextBox);
var button = app.createButton('Submit').setId("submitButton");
grid.setWidget(2, 0, button);
var mypanel = app.createVerticalPanel();
mypanel.add(grid);
app.add(mypanel);
var clickHandler = app.createServerClickHandler("parseFiles");
button.addClickHandler(clickHandler);
clickHandler.addCallbackElement(grid);
ss.show(app);
}
I then have the parseFiles function which can take up to 2 minutes to do its job as follow
function parseFiles(e) {
var app = UiApp.getActiveApplication();
var processedFolder = DocsList.getFolder(e.parameter.pafTextBox);
var workingFolder = DocsList.getFolder(e.parameter.uafTextBox);
var appraisals = workingFolder.find('Performance Appraisal');
app.getElementById("submitButton").setText("Parsing Files...");
for (var i in appraisals) {
var doc = DocumentApp.openById(appraisals[i].getId());
parseDocument(doc, getEmpName(doc.getName()));
}
return app;
}
My problem is that when I click the button, the work gets done, but the button text stays as "Submit" instead of changing to "Parsing Files...". Once the work is over, then the button changes.
Any idea what I may be doing wrong?
Regards
Crouz

Answer 1

You got nothing wrong, just the concept. Think like this: all Apps Script code you write runs on a google server (server-side), but the interface is (obviously) shown on your computer (client-side). The Apps Script "environment" has a client-side script (that we do not have access or control of) that receives the information on how to build the interface you defined in your code (server-side).
So, everything you do in your code gets updated at once, in a bundle, after your function finishes. And that's why we need to return app, so that our UiApp definition gets sent/returned to the client-side that have triggered the script.
For very simple situations, like disabling or setting the text on a button or label, there's a clientHandler that can perform basic operations on directly the client-side without requiring a network trip to the server-side to run your custom code. Since these operations are done on the client-side they're done "instantly". Note that this is not for generic code, but only predefined operations. clientHandlers are really meant just for simple stuff. It's difficult (if not impossible) to do complex operations.
Here's my suggestion using a clientHandler:
function start() {
//your current code...
clickHandler.addCallbackElement(grid);
var clientHandler = app.createClientHandler().forEventSource().setText('Parsing Files...').setEnabled(false);
button.addClickHandler(clientHandler);
ss.show(app);
}
function parseFiles(e) {
//...
app.getElementById("submitButton").setText("Submit again").setEnabled(true);
//...
return app;
}
Note that you can add multiple handlers, client or server, to a button (or any other widget that accept handlers) and all of them will run concurrently.
Also, it's very important to notice that we're talking about UiApp here, when using HtmlService the approach is significantly different.