Stopwatch using NSTimer incorrectly includes paused time in display - objective-c

Question

This is my code for an iPhone stopwatch. It works as expected and stops and resumes when the buttons are clicked.
When I hit "Stop", however, the timer won't stop running in the background, and when I hit "Start" to resume it, it will update the time and skip to where it is currently instead of resuming from the stopped time.
How can I stop the NSTimer? What is causing this to occur?
#implementation FirstViewController;
#synthesize stopWatchLabel;
NSDate *startDate;
NSTimer *stopWatchTimer;
int touchCount;
-(void)showActivity {
NSDate *currentDate = [NSDate date];
NSTimeInterval timeInterval = [currentDate timeIntervalSinceDate:startDate];
NSDate *timerDate = [NSDate dateWithTimeIntervalSince1970:timeInterval];
NSDateFormatter *dateFormatter = [[NSDateFormatter alloc] init];
[dateFormatter setDateFormat:#"mm:ss.SS"];
[dateFormatter setTimeZone:[NSTimeZone timeZoneForSecondsFromGMT:0.0]];
NSString *timeString=[dateFormatter stringFromDate:timerDate];
stopWatchLabel.text = timeString;
[dateFormatter release];
}
- (IBAction)onStartPressed:(id)sender {
stopWatchTimer = [NSTimer scheduledTimerWithTimeInterval:1/10 target:self selector:#selector(showActivity) userInfo:nil repeats:YES];
touchCount += 1;
if (touchCount > 1)
{
[stopWatchTimer fire];
}
else
{
startDate = [[NSDate date]retain];
[stopWatchTimer fire];
}
}
- (IBAction)onStopPressed:(id)sender {
[stopWatchTimer invalidate];
stopWatchTimer = nil;
[self showActivity];
}
- (IBAction)reset:(id)sender; {
touchCount = 0;
stopWatchLabel.text = #"00:00.00";
}

Answer 1

Your calculation of the current display always uses the original start time of the timer, so the display after pausing includes the interval that the timer was paused.
The easiest thing to do would be to store another NSTimeInterval, say secondsAlreadyRun, when the timer is paused, and add that to the time interval you calculate when you resume. You'll want to update the timer's startDate every time the timer starts counting. In reset:, you would also clear out that secondsAlreadyRun interval.
-(void)showActivity:(NSTimer *)tim {
NSDate *currentDate = [NSDate date];
NSTimeInterval timeInterval = [currentDate timeIntervalSinceDate:startDate];
// Add the saved interval
timeInterval += secondsAlreadyRun;
NSDate *timerDate = [NSDate dateWithTimeIntervalSince1970:timeInterval];
NSDateFormatter *dateFormatter = [[NSDateFormatter alloc] init];
[dateFormatter setDateFormat:#"mm:ss.SS"];
[dateFormatter setTimeZone:[NSTimeZone timeZoneForSecondsFromGMT:0.0]];
NSString *timeString=[dateFormatter stringFromDate:timerDate];
stopWatchLabel.text = timeString;
[dateFormatter release];
}
- (IBAction)onStartPressed:(id)sender {
stopWatchTimer = [NSTimer scheduledTimerWithTimeInterval:1/10
target:self
selector:#selector(showActivity:)
userInfo:nil
repeats:YES];
// Save the new start date every time
startDate = [[NSDate alloc] init]; // equivalent to [[NSDate date] retain];
[stopWatchTimer fire];
}
- (IBAction)onStopPressed:(id)sender {
// _Increment_ secondsAlreadyRun to allow for multiple pauses and restarts
secondsAlreadyRun += [[NSDate date] timeIntervalSinceDate:startDate];
[stopWatchTimer invalidate];
stopWatchTimer = nil;
[startDate release];
[self showActivity];
}
- (IBAction)reset:(id)sender; {
secondsAlreadyRun = 0;
stopWatchLabel.text = #"00:00.00";
}
Don't forget to release that startDate somewhere appropriate! Also keep in mind that the documented NSTimer interface is for the method you give it to accept one argument, which will be the timer itself. It seems to work without that, but why tempt fate?
Finally, since you're using that NSDateFormatter so much, you might want to consider making it an ivar or put it in static storage in showActivity:, like so:
static NSDateFormatter * dateFormatter = nil;
if( !dateFormatter ){
dateFormatter = [[NSDateFormatter alloc] init];
[dateFormatter setDateFormat:#"mm:ss.SS"];
[dateFormatter setTimeZone:[NSTimeZone timeZoneForSecondsFromGMT:0.0]];
}

Answer 2

So, when the user presses stop, and then start again, you aren't resetting the start time. But when you update the label, you are basing that on the total elapsed time from the original start time to the current time.
So if you run the timer for 10 seconds, stop, wait 10 seconds, and then start again, the timer will show 00:20.00 and start counting again from there.
What you want to do is reset the start time each time the user starts the clock, but then add the elapsed times of all previous runs as well. Or something similar.
BTW, you are leaking the start time every time you reset it now. Minor bug.
EDIT: looks like #Josh Caswell was thinking the same thing, but he types a LOT faster. :)

Answer 3

Are you using ARC or not?
If you are using ARC, it looks like you arent using a _strong reference. If you aren't using ARC, it doesn't looking you are retaining a reference to the timer.
I'm posting this from mobile so might be missing something.
EDIT: just noticed you were using release elsewhere, so I'll assume no ARC. You need to retain the timer after setting it to be able to access it later and invalidate.

Answer 4

You can use NSTimeInterval instead of timer. I have a functional code to pause and stop the timer.
#interface PerformBenchmarksViewController () {
int currMinute;
int currSecond;
int currHour;
int mins;
NSDate *startDate;
NSTimeInterval secondsAlreadyRun;
}
#end
- (void)viewDidLoad
{
[super viewDidLoad];
running = false;
}
- (IBAction)StartTimer:(id)sender {
if(running == false) {
//start timer
running = true;
startDate = [[NSDate alloc] init];
startTime = [NSDate timeIntervalSinceReferenceDate];
[sender setTitle:#"Pause" forState:UIControlStateNormal];
[self updateTime];
}
else {
//pause timer
secondsAlreadyRun += [[NSDate date] timeIntervalSinceDate:startDate];
startDate = [[NSDate alloc] init];
[sender setTitle:#"Start" forState:UIControlStateNormal];
running = false;
}
}
- (void)updateTime {
if(running == false) return;
//calculate elapsed time
NSTimeInterval currentTime = [NSDate timeIntervalSinceReferenceDate];
NSTimeInterval elapsed = secondsAlreadyRun + currentTime - startTime;
// extract out the minutes, seconds, and hours of seconds from elapsed time:
int hours = (int)(mins / 60.0);
elapsed -= hours * 60;
mins = (int)(elapsed / 60.0);
elapsed -= mins * 60;
int secs = (int) (elapsed);
//update our lable using the format of 00:00:00
timerLabel.text = [NSString stringWithFormat:#"%02u:%02u:%02u", hours, mins, secs];
//call uptadeTime again after 1 second
[self performSelector:#selector(updateTime) withObject:self afterDelay:1];
}
Hope this will help. Thanks

Answer 5

A timer class I created in Swift for a timer program in which a counter is updated every second from a set time. Answered to illustrate the Swift solution and the NSTimer function.
The timer can be stopped and restarted; it will resume from where it stopped. Events can be intercepted by the delegate for start, stop, reset, end and second events. Just check the code.
import Foundation
protocol TimerDelegate {
func didStart()
func didStop()
func didReset()
func didEnd()
func updateSecond(timeToGo: NSTimeInterval)
}
// Inherit from NSObject to workaround Selector bug
class Timer : NSObject {
var delegate: TimerDelegate?
var defaultDuration: NSTimeInterval?
var isRunning: Bool {
get {
return self.timer != nil && timer!.valid
}
}
private var secondsToGo: NSTimeInterval = 0
private var timer: NSTimer?
init(defaultDuration: NSTimeInterval, delegate: TimerDelegate? = nil) {
self.defaultDuration = defaultDuration
self.delegate = delegate
super.init()
}
func start() {
self.timer = NSTimer.scheduledTimerWithTimeInterval(1, target: self, selector: "updateTimer", userInfo: nil, repeats: true)
self.timer!.tolerance = 0.05
if delegate != nil { delegate!.didStart() }
}
func stop () {
self.timer?.invalidate()
self.timer = nil
if delegate != nil { delegate!.didStop() }
}
func reset() {
self.secondsToGo = self.defaultDuration!
if delegate != nil { delegate!.didReset() }
}
func updateTimer() {
--self.secondsToGo
if delegate != nil { delegate!.updateSecond(self.secondsToGo) }
if self.secondsToGo == 0 {
self.stop()
if delegate != nil { delegate!.didEnd() }
}
}
}