一，小文件下載：

1，NSData直接下載：（內部默認發送GET請求，但是只會在子線程，that‘s bad）

注意：這兩種方法如果在下載過程中斷網終止下載，那么重新下載需要從零開始，所以不適合大文件
不能監聽下載進度

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
    NSURL *url = [NSURL URLWithString:@"https://ss0.bdstatic.com/5aV1bjqh_Q23odCf/static/superman/img/logo/bd_logo1_31bdc765.png"];
    //下載到內存中
    NSData *data = [NSData dataWithContentsOfURL:url];
    NSLog(@"%lu",(unsigned long)data.length);
});

2，向服務器發送請求下載：（手動發送GET請求）

NSURL *url = [NSURL URLWithString:@"http://192.168.1.100:8080/MJServer/resources/images/minion_01.png"];
NSURLRequest *request = [NSURLRequest requestWithURL:url];

[NSURLConnection sendAsynchronousRequest:request queue:[NSOperationQueue mainQueue] completionHandler:^(NSURLResponse * _Nullable response, NSData * _Nullable data, NSError * _Nullable connectionError) {
    NSLog(@"%lu",data.length);
}];

二，單線程大文件下載：（可以監聽下載進度）

1，非斷點下載單線程下載，如下：

主要的API：（NSURLConnection異步方法，會自動開啟異步線程，設置代理后可以通過代理方法監聽事件并作出相應的處理）
[NSURLConnection connectionWithRequest:request delegate:self];

如下：

//1，發送請求
- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event{
    NSURL *url = [NSURL URLWithString:@"http://w.x.baidu.com/alading/anquan_soft_down_all/14247"];
    NSURLRequest *request = [NSURLRequest requestWithURL:url];
    [NSURLConnection connectionWithRequest:request delegate:self];
}
//2，如果出錯來到這里
- (void)connection:(NSURLConnection *)connection didFailWithError:(NSError *)error{
    NSLog(@"didFailWithError");
}
//3，接受到響應，只會調用一次
- (void)connection:(NSURLConnection *)connection didReceiveResponse:(NSURLResponse *)response{
    NSString *cachesPath = [NSSearchPathForDirectoriesInDomains(NSCachesDirectory, NSUserDomainMask, YES) lastObject];
    NSString *file = [cachesPath stringByAppendingPathComponent:@"QQ影音.exe"];
    
    NSFileManager *mgr = [NSFileManager defaultManager];
    [mgr createFileAtPath:file contents:nil attributes:nil];
    
    NSFileHandle *handle = [NSFileHandle fileHandleForWritingAtPath:file];
    self.handle = handle;
    self.totalLength = response.expectedContentLength;
}
//4，開始接受數據，調用多次，這里創建句柄對象對數據分段存儲（如果只有一個子線程可以用下面seekToEndOfFile方法，如果是多線程那么需要先創建一個等同整個原始文件大小的文件，然后在用[self.handle seekToFileOffset:offset]方法存儲數據）
- (void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)data{
    [self.handle seekToEndOfFile];
    [self.handle writeData:data];
    self.currentLength += data.length;
    NSLog(@"下載進度是：%f",(double)self.currentLength/self.totalLength);
}
//5，存儲完畢后關閉句柄，調用一次
- (void)connectionDidFinishLoading:(NSURLConnection *)connection{
    [self.handle closeFile];
    self.handle ＝ nil;
    self.currentLength = 0;
    self.totalLength = 0;
}

三，單線程的斷點下載：

• 單線程斷點下載.gif

- (IBAction)btnClicked:(UIButton *)sender {
    sender.selected = !sender.isSelected;
    
    if (sender.selected) {//下載或者重新下載
        //注意：這里的請求需要改成可變請求，因為后面要設置請求的下載位置
        NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:[NSURL URLWithString:@"http://w.x.baidu.com/alading/anquan_soft_down_all/14247"]];
        //需要一個全局的request以方便我暫停之后重新下載的時候設置下載的位置
        self.request = request;
        
        //設置請求頭，從當前位置下載
        NSString *range = [NSString stringWithFormat:@"bytes=%lld-",self.currentLength];
        [request setValue:range forHTTPHeaderField:@"Range"];
        
        //單線程下載（非斷點下載）
        self.connection = [NSURLConnection connectionWithRequest:request delegate:self];
    }else{//按鈕未被選中，暫停下載
        //NSURLConnection不支持暫停，只能取消，重新開始任務時候從當前位置下載等同于取消操作
        [self.connection cancel];
        self.connection = nil;
    }
}

//如果請求失敗調用
- (void)connection:(NSURLConnection *)connection didFailWithError:(NSError *)error{
    NSLog(@"didFailWithError---%@",error);
}

//建立連接獲得相應的時候調用，只會調用一次,這里因為要實現斷點續傳，所以這個方法會調用多次，每次暫停一次就會重新調用一次
- (void)connection:(NSURLConnection *)connection didReceiveResponse:(NSURLResponse *)response{
    NSLog(@"didReceiveResponse--接收到響應，開始下載--%@",response);
    //下面這句非常重要，如果沒有這一句，會重新創建新的文件，會出問題
    if (self.currentLength) return;
    
    NSString *filePath = @"/users/feng/desktop/QQ影音.exe";
    NSFileManager *manager = [NSFileManager defaultManager];
    [manager createFileAtPath:filePath contents:nil attributes:nil];
    
    //建立句柄以方便后面每接受到一段數據可以插入到文件后面
    NSFileHandle *handle = [NSFileHandle fileHandleForWritingAtPath:filePath];
    self.handle = handle;
    //統計該數據的總大小
    self.totalLength = response.expectedContentLength;
}

//每次獲取一段數據調用過一次，會調用多次
- (void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)data{
    //    NSLog(@"didReceiveData----%@",data);
    NSLog(@"didReceiveData------");
    [self.handle seekToEndOfFile];
    [self.handle writeData:data];
    
    self.currentLength += data.length;
    self.progressView.progress = (double)self.currentLength/self.totalLength;
}

//完成下載的時候調用一次
- (void)connectionDidFinishLoading:(NSURLConnection *)connection{
    NSLog(@"connectionDidFinishLoading");
    
    [self.handle closeFile];
    self.handle = nil;
    
    self.totalLength = 0;
    self.currentLength = 0;
    self.btn.selected = NO;
}