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NSObject Category Question

If you extend a Category on the NSObject class, will the following code work?

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#import <Foundation/Foundation.h>

@interface NSObject (Test)

+ (void)test;

@end

@implementation NSObject (Test)

- (void)test {
    NSLog(@"11111");
}

@end

The calling code

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- (void)viewDidLoad {
    [super viewDidLoad];
    [NSObject test];
}

The printed result

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2022-07-14 09:03:41.039406+0800 UIViewTest[1700:16744] 11111

Answer

Because an object’s isa pointer ultimately points to the metaclass, and the metaclass is its own instance, the instance method gets called.

Block Question

What does the following print, and which address pointers are the same?

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__block int a = 10;
NSLog(@"begin %d, %p",a,&a);
dispatch_async(dispatch_get_main_queue(), ^{
    NSLog(@"in block %d, %p",a,&a);
});
a = 20;
NSLog(@"end %d, %p",a,&a);

Results

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2022-07-14 09:09:04.562941+0800 UIViewTest[2035:23455] begin 10, 0x30998ff08
2022-07-14 09:09:04.563160+0800 UIViewTest[2035:23455] end 20, 0x600003973538
2022-07-14 09:09:04.652175+0800 UIViewTest[2035:23455] in block 20, 0x600003973538

Thread Question

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- (void)viewDidLoad {
    [super viewDidLoad];
    dispatch_queue_t queue = dispatch_queue_create("test", DISPATCH_QUEUE_SERIAL);
    dispatch_async(queue, ^{
        sleep(5);
        NSLog(@"1");
    });
    dispatch_sync(queue, ^{
        sleep(3);
        NSLog(@"2");
    });
    sleep(1);
    NSLog(@"3");
}

Printed result

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2022-07-14 09:12:46.798861+0800 UIViewTest[2179:26391] 1
2022-07-14 09:12:49.801871+0800 UIViewTest[2179:26296] 2
2022-07-14 09:12:50.804045+0800 UIViewTest[2179:26296] 3

Explanation

The order of synchronous tasks depends on which thread the code line containing dispatch_sync() runs on. If it’s the main thread, the main thread will be blocked waiting for the task to complete. So 1 and 2 are printed first. 3 is printed last because the preceding code block is blocked by the task in the synchronous queue.

Summary

For detail issues encountered in development, you should be good at investigating their roots and causes. Being good at learning and accumulating knowledge is a necessary condition for becoming an excellent developer.