作品链接：
http://www.jianshu.com/users/1e0f5e6f73f6/top_articles

• 1.第一种

NSURL *url = [NSURL URLWithString:@"图片请求路径"];
    NSData *data = [NSData dataWithContentsOfURL:url];
    self.imagView.image = [UIImage imageWithData:data];

• 2.第二种 NSURLConnection

 NSURL *url = [NSURL URLWithString:@"图片请求路径"];
    
    NSURLRequest *request = [NSURLRequest requestWithURL:url];
    
    [NSURLConnection sendAsynchronousRequest:request queue:[NSOperationQueue mainQueue] completionHandler:^(NSURLResponse * _Nullable response, NSData * _Nullable data, NSError * _Nullable connectionError) {
        
        self.imagView.image = [UIImage imageWithData:data];

• 3.下载视频 代理方法

- (void)viewDidLoad {
    [super viewDidLoad];
    NSURL *url = [NSURL URLWithString:@"请求路径"];
    //2.创建请求对象
    NSURLRequest *request = [NSURLRequest requestWithURL:url];
    //3.设置代理,发送请求
    [NSURLConnection connectionWithRequest:request delegate:self];
}


pragma mark  NSURLConnectionDataDelegate 

声明文件数据 已下载大小和总文件大小
// 文件数据
@property (nonatomic, strong) NSMutableData *fileData;
//当前已经下载文件的大小
@property (nonatomic, assign) NSInteger currentLength;
//下载文件的总大小
@property (nonatomic, assign) NSInteger totalLength;
/*
 1.接收到服务器响应的适合调用 1次
 */
-(void)connection:(NSURLConnection *)connection didReceiveResponse:(NSURLResponse *)response
{
    // 创建data
    self.fileData = [NSMutableData data];
    
    //拿到文件的总大小
    self.totalLength = response.expectedContentLength;
    
    NSLog(@"%zd",self.totalLength);
}

/*
 2.接收到服务器返回的数据,会调用多次
 */
-(void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)data
{
    // 拼接数据
    [self.fileData appendData:data];
   
    
    self.currentLength = self.fileData.length;
//     NSLog(@"%zd---%zd",data.length,self.currentLength);
    
    NSLog(@"%f",1.0 * self.currentLength / self.totalLength);// 此时数据是在内存中，直接退出的话，再次启动数据消失
}

/*
 3.当请求完成之后调用该方法
 */
-(void)connectionDidFinishLoading:(NSURLConnection *)connection
{
    NSLog(@"connectionDidFinishLoading");
    
    //保存下载的文件到沙河
    NSString *caches = [NSSearchPathForDirectoriesInDomains(NSCachesDirectory, NSUserDomainMask, YES) lastObject];
    
    //拼接文件全路径
    NSString *fullPath = [caches stringByAppendingPathComponent:@"XXOO.mp4"];
    
    //写入数据到文件
    [self.fileData writeToFile:fullPath atomically:YES];
    NSLog(@"%@",fullPath);
//    self.imagView.image = [UIImage imageWithData:self.fileData];
}
/*
 4.当请求失败的适合调用该方法,如果失败那么error有值
 */
-(void)connection:(NSURLConnection *)connection didFailWithError:(NSError *)error
{
    NSLog(@"didFailWithError");
}