Give me a real-life, non-trivial use of the State Pattern.

Question

I'm looking for an example of where the State design pattern has been used to solve or simplify interesting or complicated state transitions. There are plenty of examples with three or four simple states. But what about code from real life projects that have sub-states and more than a handful of transitions? The kind of code that actually motivates use of the pattern. Bonus points pointers to code!

Answer 1

One real-life use of State Pattern I have seen so far is in a Video Player capable of playing online video.

You have to handle playing, paused, buffering, connecting, seeking and even other states.

When the player is in playing or paused states, it is responsive to user interaction events.
When the player is in "connecting" state, it might not have enough data to know the video duration and so the seek bar should be disabled.
Once the player is connected, it will move to buffering state. In buffering state, the user can seek, or stop the video. But if he tries to pause or play the video, the command is saved for later, so that when the buffering is done, the video is either paused or start playing. etc.