Why GUI’s Lock Upby Rob Galanakis on 8/04/2012
I’m going to tell a story of a mouse click event. It is born when a user clicks her mouse button. This mouse click goes from the hardware, to the OS. The OS determines that the user wants to interact with your GUI, so it sends the mouse click event to your GUI’s process. Here, the mouse click sits in a queue of other similar events, just hanging out. At some point, usually almost immediately, your process (actually the main thread in the process) goes through and, one by one, dispatches the items (messages) in the queue. It has determined our mouse click is over a button and then tells the button it’s been clicked. The button then usually repaints itself so it looks pressed, and then invokes any callbacks that are hooked up. The process (main thread) goes into one such callback you hooked up, that will look at 1000 files on disk. This takes a while. In the mean time, the user is clicking, but the messages are just piling up in the queue. And then someone drags a window over your GUI, because they’re tired of their clicks not doing anything and want to see what’s new on G+. The OS sends a message to your UI that it needs to repaint itself, but that message, too, just sits in the queue. At some point, your OS may even realize your window is not responding, and fade it out and change the title bar. Finally your on-button-click callback finishes, the process (thread) is done processing our initial mouse click, and then goes back to processing the messages that may have accumulated in the queue, and your UI will refresh and start responding again.
All this happens because the thread that processes messages to draw the UI was also responsible for looking at 1000 files on disk, so it wasn’t around to respond to the paint and click messages. A few pieces of info:
- You can’t just ‘update the UI’ from the middle of your code. In addition to being terrible form code-wise, clearing the message queue would just cause other things to block the main thread, and it’d all get into one giant asynchronous mess. Some programs may have their own UI framework that supports this. Don’t trust it. You really just need the main/GUI thread clear as much as possible to respond to events.
- Your GUI process has a single ‘main thread.’ A thread roughly corresponds to, and I’m being not nuanced here, the software concept of a hardware CPU core. Your GUI objects can only be created and manipulated by the main thread.
This means, you want to keep your main thread free so it can act on GUI stuff (paint events, mouse clicks) only. The processing, such as your callback that looks at 1000 files, should happen on another thread (a background thread). When the processing is complete, it can tell the GUI thread that it is finished, and the GUI thread can update the UI. Your background thread can also fire events or invoke a callback that will be picked up by the GUI thread, so the GUI can update a progress bar or whatever.
How you actually do this varies with each UI framework. .NET, including WinForms and WPF, is quite easy to use (look at the BackgroundWorker class, but the Tasks Parallel Library and Async CTP make that less necessary). Python GUI frameworks are a bit worse off- multithreading in python in general is worse off- so it’ll be different for each one, and probably not as simple as .NET. There’s no excuse for python GUI’s to lock up, it just takes a little more effort to get it completely right (like callbacks to update a UI are a bit tricky).
There is one other vital thing to keep in mind- DCC programs generally require you to interact with the API or run all their script on the main thread, which as discussed should also be kept clear. Bummer! So the best thing we can do is block while we get our data from the scene, put the processing on a background thread, and report back to the main thread when done, applying the new data back to the scene if necessary. Unfortunately, if your processing interacts with the API in any way, you probably need to put it in the main thread as well. So, right now, your GUI’s in DCC apps may need to lock up, by design. There are, in theory, ways to avoid this, but they’re well outside of the scope of what you can handle if you’re learning anything from this article.
Whatever your language and program, those are the essentials of why your GUI locks up.