Let me start from the beginning, in the last iteration of the project I made two collections for fleets, one for stationary and the other for moving fleets. Reasons were difference in data structure, moving ones have current, start and destination positions while stationary only have current position; in visualization, stationary fleet marker should appear next to the star instead in the center of it; and in some internal logic. In the current iteration I wanted to unify those two collections for easier handling and I did it by introducing the concept of fleet missions. I started by giving stationary fleets a "stationary" mission and moving fleets a "move" missions and while most parts got simpler, some were not in the best shape. With only two things fleet could do there was no need for spending more time on the topic.
That changed when implementation of colonization started. At first I started by making separate collection for colony ships but then I've realized it won't make stuff simpler. It worked well for colonizing within a star system but when it came to moving ships to remote stars I had to either duplicate the code of normal fleet movement or unify colonizers with regular fleets. Guess which path did I choose, more complicated one with better scalability. And on top of that I've pulled out visitor pattern. It was like climbing uphill a bit be rewarded with relaxing bike ride downhill. I've changed fleet data from having a single mission to having a list of missions which combined with visitor pattern led to simpler code, believe it or not. Missions themselves were changed, descriptor for "stationary" mission was removed, now fleets with empty mission list are considered stationary, "move" missions have reference to star lane they are using instead of starting star (which was used to deduce whether star lane was in use or not), "move" missions also have reference to only one destination instead of all waypoints (not yet implemented on GUI but planned for future) and "colonize" and "skip turn" missions were introduced. "Colonize" missions does what you'd expect, marks ships ready for landing and establishing a colony. "Skip turn" also does what the name says but the reason for introducing it was to prevent colony ships from having unfair advantage over normal ships by taking action on the same turn when they are built.
In the Stareater I aim for having strict rules about what happens when. Construction is an action that takes a whole turn so when a ship is built it should act as if it (or should I say "she", I find it weird to refer to ship as she, partly because it's inanimate genderless object and partly because in my first language and in German the ship is "he") has spent it's turn. Since colonies and stellarises (made up word for star system wide government) are processed before fleets and colony ships are built with non-empty list of missions, colony ship could colonize a planet within a system or move toward destination on the same turn it was built. Explicit "skip turn" mission was a way to prevent that.
Finishing fleet movement allowed for tackling more features. As you might have figured out colonization was next but it's worthy to mention that also brought me one step closer to implementing space combat. And I've decided to make it more interesting then it was in last iteration.