1.下载图片
注意：这是同步，在当前线程执行

//1.图片的url
    NSURL *url = [NSURL URLWithString:@"imageURL"];
    
    //2.下载二进制数据
   NSData *data = [NSData dataWithContentsOfURL:url];
    
    //3.转换
    UIImage *image = [UIImage imageWithData:data];

2.使用系统的网络请求方法

//1.url
    NSURL *url = [NSURL URLWithString:@"http://120.25.226.186:32812/resources/videos/minion_01.mp4"];
    
    //2.创建请求对象
    NSURLRequest *request = [NSURLRequest requestWithURL:url];
    
    //3.发送请求
    [NSURLConnection sendAsynchronousRequest:request queue:[NSOperationQueue mainQueue] completionHandler:^(NSURLResponse * _Nullable response, NSData * _Nullable data, NSError * _Nullable connectionError) {
       
        //4.写数据到沙盒中
        NSString *fullPath = [[NSSearchPathForDirectoriesInDomains(NSCachesDirectory, NSUserDomainMask, YES) lastObject]stringByAppendingPathComponent:@"123.mp4"];
        [data writeToFile:fullPath atomically:YES];
    }];

//存在的问题：
//1.无法监听进度
//2.内存飙升

3.使用代理下载小文件

// 添加属性，记录进度
@property (nonatomic, strong) NSMutableData *fileData;
@property (nonatomic, assign) NSInteger totalSize;

//注意初始化可变data，可使用懒加载
-(NSMutableData *)fileData
{
    if (_fileData == nil) {
        _fileData = [NSMutableData data];
    }
    return _fileData;
}


-(void)download3
{
    //1.url
    // NSURL *url = [NSURL URLWithString:@"http://imgsrc.baidu.com/forum/w%3D580/sign=54a8cc6f728b4710ce2ffdc4f3cec3b2/d143ad4bd11373f06c0b5bd1a40f4bfbfbed0443.jpg"];
    
    NSURL *url = [NSURL URLWithString:@"http://120.25.226.186:32812/resources/videos/minion_01.mp4"];
    
    //2.创建请求对象
    NSURLRequest *request = [NSURLRequest requestWithURL:url];
    
    //3.发送请求 遵守代理 NSURLConnectionDataDelegate
    [[NSURLConnection alloc]initWithRequest:request delegate:self];
}

#pragma mark ----------------------
#pragma mark NSURLConnectionDataDelegate
-(void)connection:(NSURLConnection *)connection didReceiveResponse:(NSURLResponse *)response
{
    NSLog(@"didReceiveResponse");
    
    //得到文件的总大小(本次请求的文件数据的总大小)
    self.totalSize = response.expectedContentLength;
}

-(void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)data
{
   // NSLog(@"%zd",data.length);
    [self.fileData appendData:data];
    
    //进度=已经下载/文件的总大小
    NSLog(@"%f",1.0 * self.fileData.length /self.totalSize);
}
-(void)connection:(NSURLConnection *)connection didFailWithError:(NSError *)error
{
}

-(void)connectionDidFinishLoading:(NSURLConnection *)connection
{
    NSLog(@"connectionDidFinishLoading");
    //4.写数据到沙盒中
    NSString *fullPath = [[NSSearchPathForDirectoriesInDomains(NSCachesDirectory, NSUserDomainMask, YES) lastObject]stringByAppendingPathComponent:@"123.mp4"];
    
    [self.fileData writeToFile:fullPath atomically:YES];
    NSLog(@"%@",fullPath);
}