In Chapter 5, we learned about threads and the queue mechanism that threads typically use to communicate with each other. We also described the application of those ideas to GUIs in the abstract. Now that we've become fully functional GUI programmers, we can finally see what these ideas translate to in terms of code. If you skipped the related material in Chapter 5, you should probably go back and take a look first; we won't be repeating the thread or queue background material here.
The application to GUIs, however, is straightforward. Recall that long-running operations must generally be run in parallel threads, to avoid blocking the GUI from updating itself. In our packing and unpacking examples earlier in this chapter, for instance, we noted that the calls to run the actual file processing should generally run in threads so that the main GUI thread is not blocked until they finish.
In the general case, if a GUI waits for anything to finish, it will be completely unresponsive during the wait—it can't be resized, it can't be minimized, and it won't even redraw itself if it is covered and uncovered by other windows. To avoid being blocked this way, the GUI must run long-running tasks in parallel, usually with threads. That way, the main GUI thread is freed up to update the display while threads do other work.
Because only the main thread should generally update a GUI's display, though, threads you start to handle long-running tasks or to avoid blocking input/output ...