#import "ViewController.h"@interface ViewController ()@end@implementation ViewController- (void)viewDidAppear:(BOOL)animated{ [super viewDidAppear:animated]; UIButton *button = [UIButton buttonWithType:UIButtonTypeCustom]; button.frame = CGRectMake(10, 10, 100, 100); button.backgroundColor = [UIColor brownColor]; [self.view addSubview:button]; dispatch_queue_t concurrentQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0); dispatch_async(concurrentQueue, ^{ NSLog(@"111111111111111 %@",[NSThread currentThread]); __block UIImage *image = nil; dispatch_sync(concurrentQueue, ^{ //@1dispatch_sync NSLog(@"22222222222222 %@",[NSThread currentThread]); //download image NSString *urlString = @"http://images.apple.com/v/macbook-pro/f/images/loupe.png"; NSURL *url = [NSURL URLWithString:urlString]; NSURLRequest *urlRequest = [NSURLRequest requestWithURL:url]; NSError *downloadError = nil; NSData *imageData = [NSURLConnection sendSynchronousRequest:urlRequest returningResponse:nil error:&downloadError]; if (downloadError == nil && imageData != nil) { image = [UIImage imageWithData:imageData]; }else if(downloadError != nil){ NSLog(@"Error Happened = %@",downloadError); }else{ NSLog(@"No data could get download from URL."); } }); dispatch_sync(concurrentQueue, ^{ //@2//dispatch_sync //show image here NSLog(@"33333333333 %@",[NSThread currentThread]); if (image != nil) { UIImageView *imageView = [[UIImageView alloc] initWithFrame:self.view.bounds]; [imageView setImage:image]; // [imageView setContentMode:UIViewContentModeScaleAspectFill]; [self.view addSubview:imageView]; }else{ NSLog(@"Image isn't downloaded. Nothing to display."); } }); });}@end 输出如下：
2015-03-31 15:13:59.848 iOSCookbook 7.7[21442:599814] 111111111111111 <NSThread: 0x7ff57ae02480>{number = 2, name = (null)}
2015-03-31 15:13:59.851 iOSCookbook 7.7[21442:599814] 22222222222222 <NSThread: 0x7ff57ae02480>{number = 2, name = (null)} 2015-03-31 15:13:59.992 iOSCookbook 7.7[21442:599814] 33333333333 <NSThread: 0x7ff57ae02480>{number = 2, name = (null)}
第二种测试：
#import "ViewController.h"@interface ViewController ()@end@implementation ViewController- (void)viewDidAppear:(BOOL)animated{ [super viewDidAppear:animated]; UIButton *button = [UIButton buttonWithType:UIButtonTypeCustom]; button.frame = CGRectMake(10, 10, 100, 100); button.backgroundColor = [UIColor brownColor]; [self.view addSubview:button]; dispatch_queue_t concurrentQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0); dispatch_async(concurrentQueue, ^{ NSLog(@"111111111111111 %@",[NSThread currentThread]); __block UIImage *image = nil; dispatch_async(concurrentQueue, ^{ //@1dispatch_async NSLog(@"22222222222222 %@",[NSThread currentThread]); //download image NSString *urlString = @"http://images.apple.com/v/macbook-pro/f/images/loupe.png"; NSURL *url = [NSURL URLWithString:urlString]; NSURLRequest *urlRequest = [NSURLRequest requestWithURL:url]; NSError *downloadError = nil; NSData *imageData = [NSURLConnection sendSynchronousRequest:urlRequest returningResponse:nil error:&downloadError]; if (downloadError == nil && imageData != nil) { image = [UIImage imageWithData:imageData]; }else if(downloadError != nil){ NSLog(@"Error Happened = %@",downloadError); }else{ NSLog(@"No data could get download from URL."); } }); dispatch_sync(concurrentQueue, ^{ //@2//dispatch_sync //show image here NSLog(@"33333333333 %@",[NSThread currentThread]); if (image != nil) { UIImageView *imageView = [[UIImageView alloc] initWithFrame:self.view.bounds]; [imageView setImage:image]; // [imageView setContentMode:UIViewContentModeScaleAspectFill]; [self.view addSubview:imageView]; }else{ NSLog(@"Image isn't downloaded. Nothing to display."); } }); });}@end
将1@处改为dispatch_async
输出如下：
2015-03-31 15:16:13.143 iOSCookbook 7.7[21471:600996] 111111111111111 <NSThread: 0x7f8769f01670>{number = 2, name = (null)}
2015-03-31 15:16:13.145 iOSCookbook 7.7[21471:600996] 33333333333 <NSThread: 0x7f8769f01670>{number = 2, name = (null)} 2015-03-31 15:16:13.145 iOSCookbook 7.7[21471:600996] Image isn't downloaded. Nothing to display. 2015-03-31 15:16:13.145 iOSCookbook 7.7[21471:600997] 22222222222222 <NSThread: 0x7f8769e29ee0>{number = 3, name = (null)}#import "ViewController.h"@interface ViewController ()@end@implementation ViewController- (void)viewDidAppear:(BOOL)animated{ [super viewDidAppear:animated]; UIButton *button = [UIButton buttonWithType:UIButtonTypeCustom]; button.frame = CGRectMake(10, 10, 100, 100); button.backgroundColor = [UIColor brownColor]; [self.view addSubview:button]; dispatch_queue_t concurrentQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0); dispatch_async(concurrentQueue, ^{ NSLog(@"111111111111111 %@",[NSThread currentThread]); __block UIImage *image = nil; dispatch_sync(concurrentQueue, ^{ //@1dispatch_sync NSLog(@"22222222222222 %@",[NSThread currentThread]); //download image NSString *urlString = @"http://images.apple.com/v/macbook-pro/f/images/loupe.png"; NSURL *url = [NSURL URLWithString:urlString]; NSURLRequest *urlRequest = [NSURLRequest requestWithURL:url]; NSError *downloadError = nil; NSData *imageData = [NSURLConnection sendSynchronousRequest:urlRequest returningResponse:nil error:&downloadError]; if (downloadError == nil && imageData != nil) { image = [UIImage imageWithData:imageData]; }else if(downloadError != nil){ NSLog(@"Error Happened = %@",downloadError); }else{ NSLog(@"No data could get download from URL."); } }); dispatch_sync(dispatch_get_main_queue(), ^{ //@2//dispatch_sync // dispatch_get_main_queue() //show image here NSLog(@"33333333333 %@",[NSThread currentThread]); if (image != nil) { UIImageView *imageView = [[UIImageView alloc] initWithFrame:self.view.bounds]; [imageView setImage:image]; // [imageView setContentMode:UIViewContentModeScaleAspectFill]; [self.view addSubview:imageView]; }else{ NSLog(@"Image isn't downloaded. Nothing to display."); } }); });}@end 不知道为什么使用dispatch_get_main_queue() 比concurrentQueue要快很多！
一般的使用原则如下：
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{ // 耗时的操作 dispatch_async(dispatch_get_main_queue(), ^{ // 更新界面 });});