gistfile1.txt

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

int main(int argc, const char * argv[]) {
    @autoreleasepool {

        printf("[Main Thread] Application started.\n");
        printf("[Main Thread] Initiating a heavy task on a background queue...\n");

        dispatch_queue_t backgroundQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);

        dispatch_async(backgroundQueue, ^{
            // runs on background 
            printf("[Background Thread] Starting simulated heavy computation...\n");

            // simulate long-running work  
            for (int i = 1; i <= 3; i++) {
                [NSThread sleepForTimeInterval:1.0]; 
                printf("[Background Thread] ...processing step %d of 3...\n", i);
            }
            printf("[Background Thread] Heavy computation finished.\n");

            dispatch_async(dispatch_get_main_queue(), ^{
                printf("[Main Thread] UI update: Background task completed successfully!\n");
                // update UI from main queue 
            });
        });

        printf("[Main Thread] Main thread is free and can continue doing other work.\n");

        // Keep the main thread's run loop alive for a few seconds.
        // In a full Cocoa application (e.g., iOS or macOS app), the main application
        // run loop would automatically keep the program alive. For a command-line tool,
        // we manually run it to allow the asynchronous tasks time to complete.
        [[NSRunLoop currentRunLoop] runUntilDate:[NSDate dateWithTimeIntervalSinceNow:4.0]];

        printf("[Main Thread] Application ending.\n");
    }
    return 0;
}