使用delegate的方式:

@interface ViewController () <NSURLConnectionDataDelegate>
/** 文件数据 */
@property (nonatomic, strong) NSMutableData *fileData;
/** 文件的总长度 */
@property (nonatomic, assign) NSInteger contentLength;
@end

<NSURLConnectionDataDelegate>相关代理方法实现:

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    
    NSURL *url = [NSURL URLWithString:@"http://www.example.com:8080/resources/videos/minion_15.mp4"];
    [NSURLConnection connectionWithRequest:[NSURLRequest requestWithURL:url] delegate:self];
}

#pragma mark - <NSURLConnectionDataDelegate>
- (void)connection:(NSURLConnection *)connection didReceiveResponse:(NSHTTPURLResponse *)response{
    self.contentLength = [response.allHeaderFields[@"Content-Length"] integerValue];
    self.fileData = [NSMutableData data];
}

- (void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)data{
    [self.fileData appendData:data];
    CGFloat progress = 1.0 * self.fileData.length / self.contentLength;
    NSLog(@"已下载：%.2f%%", (progress) * 100);
}

- (void)connectionDidFinishLoading:(NSURLConnection *)connection{
    NSLog(@"下载完毕");
    // 将文件写入沙盒中
    // 缓存文件夹
    NSString *caches = [NSSearchPathForDirectoriesInDomains(NSCachesDirectory, NSUserDomainMask, YES) lastObject];
    // 文件路径
    NSString *file = [caches stringByAppendingPathComponent:@"box_minion_15.mp4"];
    NSLog(@"%@",file);
    // 写入数据
    [self.fileData writeToFile:file atomically:YES];
    self.fileData = nil;
}

如果要下载的文件足够小:

- (void)dataDownlaod{
    NSURL *url = [NSURL URLWithString:@"http://www.example.com:8080/resources/images/minion_15.png"];
    NSData *data = [NSData dataWithContentsOfURL:url];
    NSString *caches = [NSSearchPathForDirectoriesInDomains(NSCachesDirectory, NSUserDomainMask, YES) lastObject];
    NSString *file = [caches stringByAppendingPathComponent:@"box_minion_15.png"];
    [data writeToFile:file atomically:YES];
}