Saving NSTimer's Time to Disk on Entering Background and Retrieving on Foreground

Saving NSTimer’s Time to Disk on Entering Background and Retrieving on Foreground

As a developer, we often find ourselves in situations where we need to keep track of some aspect of our application across multiple states. In this article, we will explore how to save the time of an NSTimer on entering background and retrieve it when the app enters foreground.

Understanding the Issue with Direct Countdown

When using a direct countdown like the one described in the question, there are several issues that can arise:

The NSTimer will not fire on exact one-second intervals. Instead, delays will accumulate due to various factors such as system load and other processes competing for resources.
Using a direct countdown can lead to inaccuracies in timing.

A Better Approach: Calculating Time Elapsed

A better way to handle the timer is to create an NSDate when the timer starts and calculate the time elapsed since then at each tick. This approach provides more accurate results:

- (IBAction)startTimer:(id)sender {
    if (timer == nil) {
        [startButton setTitle:@"Pause" forState:UIControlStateNormal];
        startDate = [NSDate date]; // Store the start time
        timer = [NSTimer scheduledTimerWithTimeInterval:1.0 target:self selector:@selector(timerAction:) userInfo:nil repeats:YES];
    } else {
        [startButton setTitle:@"Resume" forState:UIControlStateNormal];
        [timer invalidate];
        timer = nil;
    }
}

Registering for Application Entering Background Notification

To save the time when the app enters background, we need to register any object to receive the UIApplicationDidEnterBackgroundNotification. This notification is posted at the same time that the app delegate gets applicationDidEnterBackground:.

- (void)application:(UIApplication *)application didEnterBackground:(NitrogenApplicationDidEnterBackground *)notification {
    NSDate *dateEnteredBackground = [NSDate date];
    // Save the entered background time to disk or a database
}

Retrieving Time on Foreground

To retrieve the saved time when the app enters foreground, we can use the applicationDidBecomeActive: notification:

- (void)application:(UIApplication *)application didBecomeActive:(NitrogenApplicationDidBecomeActive *)notification {
    NSDate *dateReturnedToForeground = [NSDate date];
    // Retrieve the entered background time from disk or database
    NSTimeInterval idleTime = [dateReturnedToForeground timeIntervalSinceDate:dateEnteredBackground];
    NSTimeInterval elapsedTime = [[NSDate date] timeIntervalSinceDate:startDate];
    elapsedTime -= idleTime;
    
    // Update the timer with the elapsed time
}

Calculating Active Time Elapsed

The active time elapsed is calculated by subtracting the idleTime from the total elapsed time:

NSTimeInterval idleTime = [dateReturnedToForeground timeIntervalSinceDate:dateEnteredBackground];
NSTimeInterval elapsedTime = [[NSDate date] timeIntervalSinceDate:startDate];
elapsedTime -= idleTime;

Conclusion

Saving the time of an NSTimer on entering background and retrieving it when the app enters foreground requires a more accurate approach than using a direct countdown. By calculating the time elapsed since the timer started, storing the entered background time, and retrieving it when the app becomes active, we can provide a more reliable timing solution.

Example Use Case

Here is an example of how to implement this approach in a real-world scenario:

#import <Foundation/Foundation.h>
#import <UIKit/UIKit.h>

@interface NitrogenViewController : UIViewController

@property (nonatomic) NSTimer *timer;
@property (nonatomic, strong) NSDate *startDate;

@end

@implementation NitrogenViewController

- (IBAction)startTimer:(id)sender {
    if (self.timer == nil) {
        [self.startButton setTitle:@"Pause" forState:UIControlStateNormal];
        self.startDate = [NSDate date];
        self.timer = [NSTimer scheduledTimerWithTimeInterval:1.0 target:self selector:@selector(timerAction:) userInfo:nil repeats:YES];
    } else {
        [self.startButton setTitle:@"Resume" forState:UIControlStateNormal];
        [self.timer invalidate];
        self.timer = nil;
    }
}

- (void)timerAction:(NSTimer *)t {
    if (self.timer) {
        // Update the timer label with the elapsed time
        NSUInteger seconds = (NSUInteger)round([NSDate date] timeIntervalSinceDate:self.startDate);
        NSString *string = [NSString stringWithFormat:@"%02u:%02u:%02u",
                            seconds / 3600, (seconds / 60) % 60, seconds % 60];
        self.timerLabel.text = string;
        
        // Save the entered background time to disk or a database
        NSDate *dateEnteredBackground = [NSDate date];
        // ...
    }
}

- (void)application:(UIApplication *)application didEnterBackground:(NitrogenApplicationDidEnterBackground *)notification {
    NSDate *dateEnteredBackground = [NSDate date];
    // Store the entered background time to disk or a database
}

- (void)application:(UIApplication *)application didBecomeActive:(NitrogenApplicationDidBecomeActive *)notification {
    NSDate *dateReturnedToForeground = [NSDate date];
    // Retrieve the entered background time from disk or database
    NSTimeInterval idleTime = [dateReturnedToForeground timeIntervalSinceDate:dateEnteredBackground];
    NSTimeInterval elapsedTime = [[NSDate date] timeIntervalSinceDate:self.startDate];
    elapsedTime -= idleTime;
    
    // Update the timer with the elapsed time
}

@end

This implementation demonstrates how to save and retrieve the entered background time, calculate the active time elapsed, and update the timer label accordingly.

Last modified on 2025-02-01