GCD 全局队列+异步执行 获取网络图片

选择: 全局队列+异步执行 
UIButton + UIImageView 

#import "ViewController.h"

@interface ViewController ()

@property (weak, nonatomic) 

IBOutlet UIImageView *imageView;

@end

@implementation ViewController

- (void)viewDidLoad {

[super viewDidLoad];

}

- (IBAction)downloadImageByGCD:(id)sender {  

//全局队列+异步执行  

dispatch_queue_t queue = dispatch_get_global_queue(0, 0);  

dispatch_async(queue, ^{  

//下载图片(子线程)  

//方式一: NSString -> NSURL -> NSData -> UIImage  

NSString *imageStr = @"http://www.egouz.com/uploadfile/2015/0305/20150305103626911.jpg";  

NSURL *imageURL = [NSURL URLWithString:imageStr];  

//真正下载图片(耗时操作)  

NSData *imageData = [NSData dataWithContentsOfURL:imageURL];  

UIImage *image = [UIImage imageWithData:imageData];  

//显示已经下载好的图片  

if (imageData != nil) {

//self.imageView.image = image;  

//方式一: 回到主线程将image赋值给imageView(原因:使得任务优先级最高)  

dispatch_async(dispatch_get_main_queue(), ^{  

//主线程做赋值操作  

self.imageView.image = image;  

});  

//方式二: 回到主线程(频率没有方式一高)

// [self performSelectorOnMainThread:@selector(showImage:) withObject:image waitUntilDone:YES];  

}  

});  

NSLog(@"下载完毕");

}

- (void)showImage:(UIImage *)image {  

//主线程([NSThread currentThread])  

self.imageView.image = image;

}