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In Java, the Swing library implements the Observer/Subscriber-Publisher design pattern to design the way the program responds to events on the GUI (button clicks, etc).

The programmer registers ActionListener objects (the observers) to a GUI element (the subject/observable/publisher), and these objects get notified by the GUI element when it's state is changed, and do something accordingly. This seems a reasonable way to design a GUI system, and I can't think of any other way a framework could do this.

Is there any other way to design an event-response system in a GUI? Are there any imperative-language frameworks or libraries with a GUI event-response system not designed this way? Does the event-response system in the most common C# GUI library implement event-response issues using Observer, or something else? What about libraries for Python, etc.

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There's functional reactive programming. The idea is to turn time-varying entities into first-class objects. Suppose we're dealing with a mouse cursor, and we want to draw an icon 10 pixels to the right of the cursor. Imagine that there was a type TimeVarying<T> that represents a value that changes over time. TimeVarying always magically contains the current value of that quantity. So instead of registering a callback and doing a bunch of imperative things to grab the current mouse position and update the position of the icon, imagine the mouse position was a TimeVarying<Point>. Then, we could write something like...

iconX = mousePosition.x + 10;

And the iconX variable would also be a time-varying quantity. Whenever the mousePosition changes, iconX would change, much like cells in a spreadsheet. Naturally, there needs to be something that's keeping all these values current and propagating the changes in a safe and efficient way.

For more details, see the paper Deprecating the Observer Pattern.

I know functional reactive programming frameworks exist, but I don't know if there's any mature frameworks for Java.

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  • Thanks for your answer. What you describe (correct me if I'm wrong), will still need to have some sort of 'observer pattern' in the underlying implementation. The difference is it will be transparent to the client and programmer. Still, underlying Observer. Are there any frameworks for imperative programming languages like C# or Python, that don't implement a variation of the Observer? Is it possible?
    – Aviv Cohn
    Mar 28, 2014 at 15:54
  • @Prog The underlying implementation may or may not use observers. For example, instead of registering observers directly and updating all values any time something changes, it might keep a dependency tree and only update values on demand. It might also only update values at certain time intervals. Plus, I believe there's some synchronization issues that need to be taken care of to prevent race conditions. E.g. value A gets updated, B depends on A, C depends on both B and A. You have to make sure B gets updated before computing C. It's not a simple thing to implement.
    – Doval
    Mar 28, 2014 at 16:27
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Swing also allows you to inherit the panels and override the Process*Event methods from the core Container class , you need to enable the events first though.

This is only a good idea if you are creating a new complex widget though

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