XNA phase management - xna-4.0

Question

I am making a tactical RPG game in XNA 4.0 and was wondering what the best way to go about "phases" is? What I mean by phases is creating a phase letting the player place his soldiers on the map, creating a phase for the player's turn, and another phase for the enemy's turn.
I was thinking I could create some sort of enum and set the code in the upgrade/draw methods to run accordingly, but I want to make sure this is the best way to go about it first.
Thanks!
Edit: To anaximander below:
I should have mentioned this before, but I already have something implemented in my application that is similar to what you mentioned. Mine is called ScreenManager and Screen but it works exactly in the same way. I think the problem is that I am treating screen, phase, state, etc, to be different things but in reality they are the same thing.
Basically what I really want is a way to manage different "phases" in a single screen. One of my screens called map will basically represent all of the possible maps in the game. This is where the fighting takes place etc. I want to know what is the best way to go about this:
Either creating an enum called FightStage that holds values like
PlacementPhase,PlayerPhase, etc, and then split the Draw and
Update method according to the enum
Or create an external class to manage this.
Sorry for the confusion!

Answer 1

An approach I often take with states or phases is to have a manager class. Essentially, you need a GamePhase object which has Initialise(), Update(), Draw() and Dispose() methods, and possibly Pause() and Resume() as well. Also often worth having is some sort of method to handle the handover. More on that later. Once you have this class, inherit from it to create a class for each phase; SoldierPlacementPhase, MovementPhase, AttackPhase, etc.
Then you have a GamePhaseManager class, which has Initialise(), Update(), Draw() and Dispose() methods, and probably a SetCurrentPhase() method of some kind. You'll also need an Add() method to add states to the manager - it'll need a way to store them. I recommend a Dictionary<> using either an int/enum or string as the key. Your SetCurrentPhase() method will take that key as a parameter.
Basically, what you do is to set up an instance of the GamePhaseManager in your game, and then create and initialise each phase object and add it to the manager. Then your game's update loop will call GamePhaseManager.Update(), which simply calls through to the current state's Update method, passing the parameters along.
Your phases will need some way of telling when it's time for them to end, and some way of handling that. I find that the easiest way is to set up your GamePhase objects, and then have a method like GamePhase.SetNextPhase(GamePhase next) which gives each phase a reference to the one that comes next. Then all they need is a boolean Exiting with a protected setter and a public getter, so that they can set Exiting = true in their Update() when their internal logic decides that phase is over, and then in the GamePhaseManager.Update() you can do this:
public void Update(TimeSpan elapsed)
{
if (CurrentPhase.Exiting)
{
CurrentPhase.HandOver();
CurrentPhase = CurrentPhase.NextPhase;
}
CurrentPhase.Update(elapsed);
}
You'll notice I change phase before the update. That's so that the exiting phase can finish its cycle; you get odd behaviour otherwise. The CurrentPhase.HandOver() method basically gets the current phase to pass on anything the next phase needs to know to carry on from the right point. This is probably done by having it call NextPhase.Resume() internally, passing it any info it needs as parameters. Remember to also set Exiting = false in here, or else it'll keep handing over after only one update loop.
The Draw() methods are handled in the same way - your game loop calls GamePhaseManager.Draw(), which just calls CurrentPhase.Draw(), passing the parameters through.
If you have anything that isn't dependent on phase - the map, for example - you can either have it stored in the GamePhaseManager and call its methods in GamePhaseManager's methods, you can have the phases pass it around and have them call its methods, or you can keep it up at the top level and call it's methods alongsideGamePhaseManager's. It depends how much access the phases need to it.
EDIT
Your edit shows that a fair portion of what's above is known to you, but I'm leaving it there to help anyone who comes across this question in future.
If already you have a manager to handle stages of the game, my immediate instinct would be to nest it. You saw that your game has stages, and built a class to handle them. You have a stage that has its own stages, so why not use the stage-handling code you already wrote? Inherit from your Screen object to have a SubdividedScreen class, or whatever you feel like calling it. This new class is mostly the same as the parent, but it also contains its own instance of the ScreenManager class. Replace the Screen object you're calling map with one of these SubdividedScreen objects, and fill its ScreenManager with Screen instances to represent the various stages (PlacementPhase, PlayerPhase, etc). You might need a few tweaks to the ScreenManager code to make sure the right info can get to the methods that need it, but it's much neater than having a messy Update() method subdivided by switch cases.