Checking for alternate keys with XNA IsKeyDown

Question

I'm working on picking up XNA and this was a confusing point for me.

KeyboardState keyState = Keyboard.GetState();
if (keyState.IsKeyDown(Keys.Left) || keyState.IsKeyDown(Keys.A))
{
    //Do stuff...
}

The book I'm using (Learning XNA 4.0, O'Rielly) says that this method accepts a bitwise OR series of keys, which I think should look like this...

KeyboardState keyState = Keyboard.GetState();
if (keyState.IsKeyDown(Keys.Left | Keys.A))
{
    //Do stuff...
}

But I can't get it work. I also tried using !IsKeyUp(... | ...) as it said that all keys had to be down for it to be true, but had no luck with that either. Ideas? Thanks.

Answer 1

The XNA Keys struct is not marked with the Flags Attribute which means that it's not legal to | a load of keys together like that.

Keys keyCombo = Keys.Left | Keys.A;
Console.WriteLine(keyCombo);

Will print NumPad5, which is clearly not what you want here! You've never been able to do this in any version of XNA, I think the book is just wrong here :O

Copying from one of my own games, this is how I solved it:

#if WINDOWS
public static bool Any(this KeyboardState state, params Keys[] keys)
{
    return keys.Any(state.IsKeyDown);
}
#endif

And then you can just do:

KeyboardState k = Keyboard.GetState();

if (k.Any(Keys.Left, Keys.A))
    DoStuff();

However, as someone in the comments pointed out this method is bad practice on the xbox due to it allocating (params array allocation, and LINQ allocating an Enumerator). An alternative system for xbox is this:

Keys[] doStuffKeys; //Allocate an array of keys for this particular action

Initialize()
{
    doStuffKeyBinding = new[] { Keys.Left, Keys.A };
}

Update()
{
    KeyboardState k = Keyboard.GetState();

    if (k.Any(doStuffKeys)) //No allocations here!
        DoStuff();
}

The Any method needs modifying too:

#if XBOX
public static bool Any(this KeyboardState state, Keys[] keys)
{
    for (int i = 0; i < keys.Length; i++)
        if (state.IsKeyDown(keys[i]))
            return true;
    return false;
}
#endif

It's a while since I've done any work for the xbox, but this now shouldn't allocate.

Answer 2

IsKeyDown takes an enum Key for its type. I don't see from the documentation that it can take multiple emums ORed together. Perhaps your book is wrong?

You could create your own method if you wanted the shortcut:

boolean anyOfTheseDown(KeyboardState keyState, params Keys[] keys) {
   boolean result = false;
   for (int i = 0; i < keys.Length; i++)
   {
       result |= keyState.IsKeyDown(keys[i]);
   }    
   return result;
}

Then use it like:

KeyboardState keyState = Keyboard.GetState();
if (anyOfTheseDown(keyState, Keys.Left, Keys.A))
{
    //Do stuff...
}

Code is untested.