GCD中的网络图片下载

#import "ViewController.h"
/*
 如果下载地址不是https 则将以下步骤进行，允许xcode下载任意路径的文件。
1.在Info.plist中添加NSAppTransportSecurity类型Dictionary。

2.在NSAppTransportSecurity下添加NSAllowsArbitraryLoads类型Boolean,值设为YES
*/
@interface ViewController ()
@property (nonatomic,strong)UIButton * downBtn;
@property (nonatomic,strong)UIImageView * imageView;
@end

@implementation ViewController

- (UIButton *)downBtn{
    if (!_downBtn) {
        _downBtn = [UIButton buttonWithType:UIButtonTypeSystem];
        _downBtn.frame =CGRectMake(110, 350, 100, 40);
        [_downBtn setTitle:@"下载图片" forState:UIControlStateNormal];
        [_downBtn addTarget:self action:@selector(download) forControlEvents:UIControlEventTouchUpInside];
    }
    return _downBtn;
}

- (UIImageView *)imageView{
    if (!_imageView) {
        _imageView = [[UIImageView alloc]initWithFrame:CGRectMake(0, 0, 320, 300)];
        _imageView.contentMode = UIViewContentModeScaleAspectFit;
    }
    return _imageView;
}

-(void)download{
    NSString * imagePath = @"http://pic64.nipic.com/file/20150418/20717244_162150469000_2.jpg";
    NSURL * imageURL = [NSURL URLWithString:imagePath];
    //把网络数据下载到NSData中，是耗时操作，时间由网速决定
    dispatch_queue_t queue = dispatch_get_global_queue(0, 0);
    //0 是默认优先级的值 default级别
    dispatch_async(queue, ^{
        NSData *data = [NSData dataWithContentsOfURL:imageURL];
        UIImage *image = [UIImage imageWithData:data];
        //把图片放入到图片控件中，因为图片控件的更新必须在主线程中执行，所以我们需要主队列异步
        dispatch_async(dispatch_get_main_queue(), ^{
            self.imageView.image =image;
        });
    });
    
}

- (void)viewDidLoad {
    [super viewDidLoad];
    [self.view addSubview:self.imageView];
    [self.view addSubview:self.downBtn];
    
}


@end