Extensions to PebbleSDK to provide missing support and extended user interface elements.

Timer based eventing is available in the PebbleSDK, but there is only a single timer callback function and your app needs to utilized timer cookies to know which scheduled timer fired. It would be much better if apps were able to schedule timers with a timeout period and have a handler method for each scheduled timer. Timer (timer.h) provides this facility, to a limited extent, making working with custom timers far easier.

Timer supports two additional features, one being automatic rescheduling of expired timers - repeating timers. A timer context is optionally supported and passed to the timer hanlder callback function. Contexts allow for all sorts of enhancements, some of which are custom animation timing support. In fact ActivityLayer utilizes Timer to perform its animations.

Timers utilize the timer_handler function pointer in PebbleAppHandlers. In order for Timers to work your watch app must set timer_handler to TIMER_APP_TIMER_HANDLER in the watch app's main function.

#include "pebble_ex.h"

void pbl_main(AppTaskContextRef ctx) {
	PebbleAppHandlers handlers = {
		.init_handler = &handle_init,
		...
		.timer_handler = TIMER_APP_TIMER_HANDLER,
	};

	app_event_loop(ctx, &handlers);
}

If you must, you can always implement your own PebbleAppTimerHandler and call timer_handler() when you do not recognize the passed timer handler cookie.

static Layer custom_layer;
static Timer repeating_timer;
static bool stop_flipping = no;

// Forward declaration
void repeating_timer_callback(AppContextRef app_ctx, void *ctx);

// Main appplication initialization
void handle_init(AppContextRef ctx) {
	...
	// Initialized a Timer using a timer hander callback function.
	// The 10 represents 10 milliseconds until the timer fires
	timer_init(&repeating_timer, repeating_timer_callback, 10);
	
	// Designate the timer to reschedule itself automatically, creating
	// a repeating timer.
	timer_set_repeat(&repeating_timer, true);
	
	// Finally, schedule the timer
	timer_schedule(&repeating_timer, &custom_layer);
	...
}

// Timer handler as reference in handle_init()
void repeating_timer_callback(AppContextRef app_ctx, void *ctx) {
	if (stop_flipping) {
		// Cancel the timer, preventing any more repeats scheduling.
		// This is only required to be called when the timer has not
		// fired or when the timer is a repeat timer.
		timer_cancel(&repeating_timer, app_ctx);
	}
	else {
		static bool flipped = false;
		
		// Use the layer context parameter, set when initializing the timer
		Layer *layer = (Layer *)ctx;
		layer_set_hidden(layer, flipped);
		
		flipped = !flipped;
	}
}

There is a limit on the number of timers that can be actively scheduled concurrently, currently capped at 5 concurrent timers. This can be adjusted by altering TIMER_MAX_TIMERS in timer.c. The cap was chosen to limit the amount of memory Timers use, which is negligible. This does not imply a limit on the number of Timers initialized only those scheduled.

If you are using libpebble to monitor logs, Timer will log an error when the maximum number of timers scheduled has been reached.