Prevent a closure from running until another has completed - ios

Question

Here is code for two closures in two different IBAction button presses. The desired outcome is for the button press to turn on/off an LED, then to access a light sensor and read the light value after the change in LED status.
What happens is a race condition where the function getVariable runs and returns before the callFunction has implemented the change. The result is that the value displayed in getLightLabel.text is that of the prior condition, not the current condition.
My question is how to rewrite the code below so that myPhoton!.getVariable does not execute until after the myPhoton!.callFunction has returned (completed its task).
I have tried placing getVariable inside callFunction, both before and after the } closing if (error == nil), but the result was identical to the code shown here.
#IBAction func lightOn(sender: AnyObject) {
let funcArgs = [1]
myPhoton!.callFunction("lightLed0", withArguments: funcArgs) { (resultCode : NSNumber!, error : NSError!) -> Void in
if (error == nil) {
self.lightStateLabel.text = "LED is on"
}
}
myPhoton!.getVariable("Light", completion: { (result:AnyObject!, error:NSError!) -> Void in
if let e = error {
self.getLightLabel.text = "Failed reading light"
}
else {
if let res = result as? Float {
self.getLightLabel.text = "Light level is \(res) lumens"
}
}
})
}
#IBAction func lightOff(sender: AnyObject) {
let funcArgs = [0]
myPhoton!.callFunction("lightLed0", withArguments: funcArgs) { (resultCode : NSNumber!, error : NSError!) -> Void in
if (error == nil) {
self.lightStateLabel.text = "LED is off"
}
}
myPhoton!.getVariable("Light", completion: { (result:AnyObject!, error:NSError!) -> Void in
if let e = error {
self.getLightLabel.text = "Failed reading light"
}
else {
if let res = result as? Float {
self.getLightLabel.text = "Light level is \(res) lumens"
}
}
})
}
Here is the callFunction comments and code from the .h file. This SDK is written in Objective C. I am using it in Swift with a bridging header file.
/**
* Call a function on the device
*
* #param functionName Function name
* #param args Array of arguments to pass to the function on the device. Arguments will be converted to string maximum length 63 chars.
* #param completion Completion block will be called when function was invoked on device. First argument of block is the integer return value of the function, second is NSError object in case of an error invoking the function
*/
-(void)callFunction:(NSString *)functionName withArguments:(NSArray *)args completion:(void (^)(NSNumber *, NSError *))completion;
/*
-(void)addEventHandler:(NSString *)eventName handler:(void(^)(void))handler;
-(void)removeEventHandler:(NSString *)eventName;
*/
Here is the .m file code
-(void)callFunction:(NSString *)functionName withArguments:(NSArray *)args completion:(void (^)(NSNumber *, NSError *))completion
{
// TODO: check function name exists in list
NSURL *url = [self.baseURL URLByAppendingPathComponent:[NSString stringWithFormat:#"v1/devices/%#/%#", self.id, functionName]];
NSMutableDictionary *params = [NSMutableDictionary new]; //[self defaultParams];
// TODO: check response of calling a non existant function
if (args) {
NSMutableArray *argsStr = [[NSMutableArray alloc] initWithCapacity:args.count];
for (id arg in args)
{
[argsStr addObject:[arg description]];
}
NSString *argsValue = [argsStr componentsJoinedByString:#","];
if (argsValue.length > MAX_SPARK_FUNCTION_ARG_LENGTH)
{
// TODO: arrange user error/codes in a list
NSError *err = [self makeErrorWithDescription:[NSString stringWithFormat:#"Maximum argument length cannot exceed %d",MAX_SPARK_FUNCTION_ARG_LENGTH] code:1000];
if (completion)
completion(nil,err);
return;
}
params[#"args"] = argsValue;
}
[self setAuthHeaderWithAccessToken];
[self.manager POST:[url description] parameters:params success:^(AFHTTPRequestOperation *operation, id responseObject) {
if (completion)
{
NSDictionary *responseDict = responseObject;
if ([responseDict[#"connected"] boolValue]==NO)
{
NSError *err = [self makeErrorWithDescription:#"Device is not connected" code:1001];
completion(nil,err);
}
else
{
// check
NSNumber *result = responseDict[#"return_value"];
completion(result,nil);
}
}
} failure:^(AFHTTPRequestOperation *operation, NSError *error)
{
if (completion)
completion(nil,error);
}];
}

Answer 1

One solution is to put the second closure inside the first, where the first returns and provides and Error value. If no error,then execuet the second closure. That is one way to tightly couple the two closures without resorting to semaphores or other messaging schemes.
In this application, the problem I was encountering cannot be solved on the IOS/Swift side of the stack. The cloud API and embedded uP are not tightly coupled, so the cloud returns to the IOS with a completion before the full function code has run on the Particle uP.
The solution to this overall problem actually lies in either modifying the cloud API or adding some additional code to the uP firmware to tightly couple the process to the IOS app with additional communication.