未使用信号量控制并发

代码

#import "ViewController.h"@interface ViewController ()@end@implementation ViewController- (void)viewDidLoad {    [super viewDidLoad];        dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{        for (NSUInteger i = 0;  i < 10; ++i) {            NSLog(@"Task - 1");        }    });        dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{        for (NSUInteger i = 0;  i < 10; ++i) {            NSLog(@"Task - 2");        }    });}@end复制代码

输出

Task - 1 Task - 2 Task - 1 Task - 2 Task - 1 Task - 2 Task - 1 Task - 2 Task - 1 Task - 2 Task - 1 Task - 2 Task - 1 Task - 2 Task - 1 Task - 2 Task - 1 Task - 2 Task - 1 Task - 2 复制代码

使用信号量控制并发

代码

#import "ViewController.h"@interface ViewController ()@property (nonatomic, strong) dispatch_semaphore_t semaphore;@end@implementation ViewController- (void)viewDidLoad {    [super viewDidLoad];        self.semaphore = dispatch_semaphore_create(1);    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{        dispatch_semaphore_wait(self.semaphore, DISPATCH_TIME_FOREVER);        for (NSUInteger i = 0;  i < 10; ++i) {            NSLog(@"Task - 1");        }        dispatch_semaphore_signal(self.semaphore);    });        dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{        dispatch_semaphore_wait(self.semaphore, DISPATCH_TIME_FOREVER);        for (NSUInteger i = 0;  i < 10; ++i) {            NSLog(@"Task - 2");        }        dispatch_semaphore_signal(self.semaphore);    });}@end复制代码

输出

Task - 1 Task - 1 Task - 1 Task - 1 Task - 1 Task - 1 Task - 1 Task - 1 Task - 1 Task - 1 Task - 2 Task - 2 Task - 2 Task - 2 Task - 2 Task - 2 Task - 2 Task - 2 Task - 2 Task - 2 复制代码