Question

I am running a lengthly task in an Action and I would like to have a display of where I am at. For that I created a Text Field and I tried it with setStringValue:

[textField setStingValue: [NSSting stringWithFormat:@"%ld",currentValue]]

The code works but unfortunately it is not updating the NSTextField after every iteration but rather when the whole Action is done.

What am I doing wrong?

Answer 1

This is because applications with the Cocoa framework use an event loop to perform operations, and events occur in a completely serial fashion.

An event is basically any kind of action that the framework designer could not predict or found convenient to have run in a delayed manner. Since you can’t predict when clicks will be performed, they need to be considered events; and for efficiency reasons (since you don’t want to repaint a component multiple times if you don’t need to), the repaint actions are events too.

Your action runs in response to a user event (for instance, a click on a button is an event) and therefore blocks all other events waiting in the queue until it’s complete. However, components are repainted in response to a different, framework-triggered event, and as such the text field must wait until your action completes to repaint itself. This is why you cannot visually change the value of a text field from inside an action.

In order to notify your user of the progress of your task, you’ll need to run it on a different thread. There’s a lot to say about threads, so you should probably read some about them. I’m also sure that there are plenty of examples of how to run a long action in a background thread and update the UI accordingly for Cocoa all over the Internet.