Watchdog

Watchdog.h

@interface SCWatchdog : NSObject

- (instancetype)initWithThreashold:(NSTimeInterval)threshold;

@end

Watchdog.m

#import "SCWatchdog.h"

typedef void(^SCWatchdogHandler)(void);

@interface SCPingThread : NSThread

@property (atomic) BOOL pingTaskIsRunning;
@property (nonatomic) dispatch_semaphore_t semaphore;
@property (nonatomic, readonly) NSTimeInterval threshold;
@property (nonatomic, readonly, copy) SCWatchdogHandler handler;

- (instancetype)initWithThreashold:(NSTimeInterval)threshold handler:(SCWatchdogHandler)handler;

@end

@implementation SCPingThread

- (instancetype)initWithThreashold:(NSTimeInterval)threshold handler:(SCWatchdogHandler)handler
{
    self = [super init];
    if (self) {
        _threshold = threshold;
        _handler = [handler copy];
        _semaphore = dispatch_semaphore_create(0);
        self.name = @"Watch🐶";
    }
    
    return self;
}

- (void)main
{
    while (!self.isCancelled) {
        self.pingTaskIsRunning = YES;
        
        dispatch_async(dispatch_get_main_queue(), ^{
            self.pingTaskIsRunning = NO;
            dispatch_semaphore_signal(self.semaphore);
        });
        
        [NSThread sleepForTimeInterval:self.threshold];

        if (self.pingTaskIsRunning) {
            self.handler();
        }
        
        dispatch_semaphore_wait(self.semaphore, DISPATCH_TIME_FOREVER);
    }
}

@end

@implementation SCWatchdog {
    SCPingThread* _pingThread;
}

- (instancetype)initWithThreashold:(NSTimeInterval)threshold
{
    self = [super init];
    if (self) {
        _pingThread = [[SCPingThread alloc] initWithThreashold:threshold handler:^{
            // !!! Never call NSAsset() because it captures `self` implicitly
            NSLog(@"🐶 Main thread was blocked for %.2f seconds 🐶", threshold);
        }];
        [_pingThread start];
    }
    
    return self;
}

- (void)dealloc
{
    [_pingThread cancel];
}

@end