Keyboard input is a surprisingly complicated topic after you step outside of your usual country and language (and sometimes before you do, too).
Almost any game, no matter what approach it is taking, should probably offer a means for users to configure key bindings; not only will this solve concerns about what kind of keyboard a user is typing on, it will also make your game flexible to whatever is most comfortable for a user. This is not just a question of keyboard layouts, but being kind to those that don't have full motion of their hands and would benefit from moving keys to accessible locations that don't make obvious sense to an outside observer. At least offer a config file, if not a user interface; people you've never met will thank you for it!
There are, as far as we can tell, four common ways that apps want to use keyboard input. Sometimes they want different approaches at different moments, too.
Many games just want to treat a keyboard not as a way to input text, but just as a joystick that has a lot of buttons. The well-known "WASD" key pattern for FPS games is a fine example of this: you want the physical location of a key, regardless of what symbol is printed on the key. After all, on a French keyboard, instead of "WASD", you'd press "ZQSD", and on a Hiragana keyboard "てちとし". Same locations on the keyboard, totally different symbols.
For these, you want SDL_Scancodes: these are guaranteed to reference the physical location on the keyboard and not what is printed on it.
Specifically, they assume a US English QWERTY keyboard layout, no matter what the keyboard in use actually has at that location, but that's okay because here we just want physical location of the key, not its meaning.
Simply grab the scancode field from SDL_EVENT_KEY_DOWN and SDL_EVENT_KEY_UP events.
/* returns 1 if moving forward with this keypress, -1 if moving backward, 0 if not moving. */
int direction_user_should_move(const SDL_Event *e)
{
SDL_assert(e->type == SDL_EVENT_KEY_DOWN); /* just checking key presses here... */
if (e->key.scancode == SDL_SCANCODE_W) {
return 1; /* pressed what would be "W" on a US QWERTY keyboard. Move forward! */
} else if (e->key.scancode == SDL_SCANCODE_S) {
return -1; /* pressed what would be "S" on a US QWERTY keyboard. Move backward! */
}
/* (We aren't doing it here, but extra credit if your code sees both keys are pressed and cancel each other out!) */
return 0; /* wasn't key in W or S location, don't move. */
}Some games might want to know the symbol on a key. This tends to be a "press 'I' to open your inventory" thing, and you don't really care where the 'I' key is on the keyboard.
(But, again: offer keybindings, because some keyboards don't have an 'I' key!)
This is also useful for looking for the ESC key to cancel an operation, or Enter to confirm, etc; it doesn't matter where the key is, you still want that key.
These are SDL_Keycodes. They name specific keys and don't care where on the user's keyboard they actually are. Like scancodes, you also get these from SDL_EVENT_KEY_DOWN and SDL_EVENT_KEY_UP events.
/* sit in a loop forever until the user presses Escape. */
SDL_bool quit_the_app = SDL_FALSE;
while (!quit_the_app) {
SDL_Event e;
while (SDL_WaitEvent(&e)) {
/* user has pressed a key? */
if (e.type == SDL_EVENT_KEY_DOWN) {
/* the pressed key was Escape? */
if (e.key.key == SDLK_ESCAPE) {
quit_the_app = SDL_TRUE;
}
}
}
}Unicode is hard! If you are composing text a string at a time ("Enter your name, adventurer!" screens or accepting sentences for a chat interface, etc) you should not be using key press events! This will never do the right thing across various keyboards and human languages around the world.
One should instead call SDL_StartTextInput, and listen for SDL_EVENT_TEXT_INPUT events. When done accepting input, call SDL_StopTextInput. This approach will let the system provide input interfaces that are familiar to the user (including popping up a virtual keyboard on mobile devices, and other UI for composing in various languages). Then the event will provide Unicode strings in UTF-8 format, which might be complete lines of text or a single character, depending on the system. You will not be able to replicate these interfaces in your application for everyone in the world, do not try to build this yourself on top of individual keypress events.
The downside of this is that a virtual keyboard, etc, might be disruptive to your game, so you need to design accordingly.
/* Set the text area and start text input */
SDL_bool text_input_complete = SDL_FALSE;
char text[1024] = { 0 };
SDL_Rect area = { textfield.x, textfield.y, textfield.w, textfield.h };
int cursor = 0;
SDL_SetTextInputArea(window, &area, cursor);
SDL_StartTextInput(window);
while (!text_input_complete) {
SDL_Event e;
while (SDL_PollEvent(&e)) {
/* user has pressed a key? */
if (e.type == SDL_EVENT_KEY_DOWN) {
/* the pressed key was Escape or Return? */
if (e.key.key == SDLK_ESCAPE || e.key.key == SDLK_RETURN) {
SDL_StopTextInput(window);
text_input_complete = SDL_TRUE;
}
/* Handle arrow keys, etc. */
} else if (e.type == SDL_EVENT_TEXT_INPUT) {
SDL_strlcat(text, e.text.text, sizeof(text));
}
}
/* Render the text, adjusting cursor to the offset in pixels from the left edge of the textfield */
...
/* Update the text input area, adjusting the cursor so IME UI shows up at the correct location. */
SDL_SetTextInputArea(window, &area, cursor);
}If you're writing a fullscreen game, you might want to render IME UI yourself, so you'd set SDL_HINT_IME_IMPLEMENTED_UI appropriately and handle the SDL_EVENT_TEXT_EDITING and SDL_EVENT_TEXT_EDITING_CANDIDATES events. See testime.c for an example of this.
This is a special case, and if you think your game fits here, you should think very hard about how to change that before thinking about how to go this route, because often times you are just on a path to build The Chat Box, incorrectly, from scratch.
If you were writing an SDL frontend for Vim, you would need to know what keypresses-plus-modifiers produce: hitting shift-Z twice produces a different result than pressing z twice, but also you want arrow keys to do the right thing on the numpad, unless NumLock is pressed, when they should produce numbers. On top of all this, it's difficult to correlate between keypress and proper text input events and untangle them to get correct results.
In this case, you would need to handle both key events and input events as above, in addition you might want to know what the modified keycode for the event is:
SDL_bool quit_the_app = SDL_FALSE;
while (!quit_the_app) {
SDL_Event e;
while (SDL_WaitEvent(&e)) {
/* user has pressed a key? */
if (e.type == SDL_EVENT_KEY_DOWN) {
/* Was the pressed key '$', generated by Shift+4 on a US keyboard or the '$' key on the French keyboard? */
SDL_Keycode keycode = SDL_GetKeyFromScancode(e.key.scancode, e.key.mod, SDL_FALSE);
if (keycode == '$') {
/* Show me the money! */
}
}
}
}Obviously there are many keys that don't generate a character, or characters that are composed by pressing multiple keys (or navigating through IME interfaces that don't map to specific keypresses at all), so this is niche functionality and not how one should accept input in a general sense.
So you've made your game, and now you're taking the original advice about adding user-configurable keybindings, and you need to know how to show the user the key's name in your config UI, and maybe also in a "press [current keybinding] to jump" tutorial message.
For this, use SDL_GetKeyName with the SDL_Keycode you get from an event:
SDL_bool quit_the_app = SDL_FALSE;
while (!quit_the_app) {
SDL_Event e;
while (SDL_WaitEvent(&e)) {
/* user has pressed a key? */
if (e.type == SDL_EVENT_KEY_DOWN) {
SDL_Log("Wow, you just pressed the %s key!", SDL_GetKeyName(e.key.key));
}
}
}