Detect which key was pressed last each frame

Question

Why hello there, im developing a 2D game using XNA. My character movment is currently based on the keyboard state. Example (in the right order):

if (state.IsKeyDown(Keys.W))
    {
          //Do stuff
    }
else if (state.IsKeyDown(Keys.D))
    {
          //Do Stuff
    }               
else if (state.IsKeyDown(Keys.A))
    {
          //Do Stuff
    }
else if (state.IsKeyDown(Keys.S))
    {
          //Do Stuff
    }

That way I am able to deny diagonal movment. So my problem is, if "W" is pressed, meaning the character is moving up, And a frame after that while W is still pressed and I press "S" for example, It will keep moving up. I've been looking for a solution for a while and didn't come up with anything good enough. What I need help with is ideas about how to detect if a different key was pressed after W (because it is the first one to check, it doesn't check the others), while denying diagonal movment. Any help would be sincerely appreciated, Thanks in advance.

Answer 1

I believe you want something like the below. It will keep track of the most recently pressed key in the variable _movementKey and set the movement key to none when the most recent key is released.

So the end result should be _movementKey contains the key that dictates direction, it will always be most recently pressed key and will be reset to Key.None when the most recent key is released.

// ###################### 
// non local variables 
// ######################

private KeyboardState _oldState;
private Key _movementKey; //This keeps track of the key that was pressed last.
                          //It is reset to Key.None id released no matter what
                          //other keys are pressed.

// ######################
// function to update input
// ######################

var newState = Keyboard.GetState();

if (!newState.IsKeyDown(_movementKey) && _oldState.IsKeyDown(_movementKey)) 
{
    //If the current key that dictates the moment was released (down last 
    //frame and up this frame) then set the moment key to none. 
    _movementKey = Key.None;
} else {
    //Else use this if else chain to detect when a button is pressed (definition 
    //of pressed being it was up last frame and is down this frame). When it 
    //detects a button being pushed it sets the value of _movementKey to
    //that key;
    if (newState.IsKeyDown(Keys.W) && !_oldState.IsKeyDown(Keys.W))
    {
        _movementKey = Keys.W;
    }
    else if (newState.IsKeyDown(Keys.D) && !_oldState.IsKeyDown(Keys.D))
    {
        _movementKey = Keys.D;
    }
    else if (newState.IsKeyDown(Keys.A) && !_oldState.IsKeyDown(Keys.A))
    {
        _movementKey = Keys.A;
    }
    else if (newState.IsKeyDown(Keys.S) && !_oldState.IsKeyDown(Keys.S))
    {
        _movementKey = Keys.S;
    }
}

_oldState = newState;

// ######################
// using the input to dictate direction
// ######################

//The above code simply keeps track of which key should be dictating the
//movement. Each frame check the '_movementKey' and use it to dictate the 
//movement. You could do this in the above code but I like to separate 
//handling the the input into 2 parts: interpreting it and applying it.

//This applies it while the above function interprets it. 
switch (_movementKey) {
    case Keys.W:
        //move up
        break;
    case Keys.D:
        //move right
        break;
    case Keys.S:
        //move down
        break;
    case Keys.A:
        //move left
        break;
    case Keys.None:
        break;
}