key_events: A way to hook into the event handling

This adds a new `handle_user_key_event` function, that will get called
before anything else, and can override what happens. If it returns true,
the event will not be processed further by the firmware. By default,
this function returns false, and is a no-op.

It is defined with a `weak` attribute, to let the linker know that any
other function with the same name should override his one. This makes it
possible to have another version of this function in a firmware Sketch,
and override the behaviour, to extend the event handling.

This is the foundation that allows it to use external libraries, and tap
into the firmware's event handler, to add new stuff. (We can already
hook into the main loop by changing the top `loop` function in the
Sketch)

This addresses #21 for the most part.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>

KeyboardioFirmware.ino

@@ -10,6 +10,11 @@
 uint8_t primary_keymap = 0;
 uint8_t temporary_keymap = 0;
 
+bool handle_user_key_event(byte row, byte col, uint8_t currentState, uint8_t previousState) {
+  //Serial.print ("user_key_event");
+  return false;
+}
+
 void setup() {
     wdt_disable();
     delay(100);

key_events.cpp

@@ -46,10 +46,20 @@ void handle_synthetic_key_event(Key mappedKey, uint8_t currentState, uint8_t pre
 
     }
 }
+
+__attribute__((weak))
+bool handle_user_key_event(byte row, byte col, uint8_t currentState, uint8_t previousState) {
+  return false;
+}
+
 void handle_key_event(byte row, byte col, uint8_t currentState, uint8_t previousState) {
     //for every newly pressed button, figure out what logical key it is and send a key down event
     // for every newly released button, figure out what logical key it is and send a key up event
 
+    if (handle_user_key_event(row, col, currentState, previousState)) {
+        return;
+    }
+
    Key mappedKey;
    mappedKey.raw=  pgm_read_word(&(keymaps[temporary_keymap][row][col]));
    Key baseKey;

key_events.h

@@ -20,3 +20,4 @@ void press_key(Key mappedKey);
 void handle_keymap_key_event(Key keymapEntry, uint8_t currentState, uint8_t previousState);
 void handle_mouse_key_event(Key mappedKey, uint8_t currentState, uint8_t previousState);
 
+bool handle_user_key_event(byte row, byte col, uint8_t currentState, uint8_t previousState);