Question

我不明白为什么这不起作用：

try
    {
     //Give Message of Command
       NewMessageEventArgs e = new NewMessageEventArgs("COMMAND", "Download File", "RETR");
                        OnNewMessageReceived(this, e);
                        byte[] buffer = new byte[2048];
                        int read = 0;
                        Int64 TotalBytesRead = 0;
                        Int64 FileSize = this.GetFileSize(sourceFilename);
                        DownloadCanceled = false;
                        do
                        {
                            if (DownloadCanceled)
                            {
                                NewMessageEventArgs CancelMessage = new NewMessageEventArgs("RESPONSE", "Download Canceled.", "CANCEL");

                                DownloadCanceled = false;
                                OnNewMessageReceived(this, CancelMessage);
                                return false;
                            }

                            read = DownloadResponseStream.Read(buffer, 0, buffer.Length);
                            DownloadFileStream.Write(buffer, 0, read);
                            TotalBytesRead += read;
                            //Declare Event
                            DownloadProgressChangedArgs DownloadProgress = new DownloadProgressChangedArgs(TotalBytesRead, FileSize);

                            //Progress changed, Raise the event.
                            OnDownloadProgressChanged(this, DownloadProgress);

                            System.Windows.Forms.Application.DoEvents();

                        } while (!(read == 0));


                        //Get Message and Raise Event
                        NewMessageEventArgs NewMessageArgs = new NewMessageEventArgs("RESPONSE", DownloadResponse.StatusDescription, DownloadResponse.StatusCode.ToString());
                        OnNewMessageReceived(this, NewMessageArgs);

                        //Declare Event
                        DownloadCompletedArgs Args = new DownloadCompletedArgs("Successful", true);
                        //Raise Event
                        OnDownloadCompleted(this, Args);

                        DownloadResponseStream.Close();
                        DownloadFileStream.Flush();
                        DownloadFileStream.Close();
                        DownloadFileStream = null;
                        DownloadResponseStream = null;
                    }
                    catch (Exception ex)
                    {
                        //catch error and delete file only partially downloaded
                        DownloadFileStream.Close();
                        //delete target file as it's incomplete
                        targetFI.Delete();

                        //Decalre Event for Error
                        DownloadCompletedArgs DownloadCompleted = new DownloadCompletedArgs("Error: " + ex.Message, false);
                        //Raise Event
                        OnDownloadCompleted(this, DownloadCompleted);
                    }
                }

结果我得到了不确定的结果。

我一直在鼓励自己： JavaScript closure inside loops – simple practical example

我知道这是一个范围问题，所以我正在尝试创建一个闭包，我不能在这里找到什么？

提前致谢！

Answer 1

你要返回的最后一个函数是你正在调用的最后一个函数，并且它只用()调用，这意味着没有参数，但是你已经在内部函数中指定了一个参数，并且它是{{ 1}}。

undefined

只是不要在内部函数中指定一个参数，而外部函数的参数将在范围内，并且是封闭的（闭包）

function returnVerse(x){
                 //  ^ outer argument

   return function(x) { 
                // ^ inner argument overwrites outer argument

       console.log(x); // undefined
   };

}

for(var x = 9; x>=0; x--){
   return returnVerse(x)();
                //    ↑  ^ innner argument not passed, "undefined"
                      outer argument   
}

此外，从function returnVerse(x){ return function() {console.log(x);}; } for(var x=9; x>=0; x--){ returnVerse(x)(); }循环返回毫无意义，并且意味着它只运行一次

关闭for循环

1 个答案: