WebClient.DownloadDataAsync正在冻结我的UI [英] WebClient.DownloadDataAsync is freezing my UI

问题描述

在Form构造函数中，在InitializeComponent之后有以下代码:

I have in my Form constructor, after the InitializeComponent the following code:

using (WebClient client = new WebClient())
{
    client.DownloadDataCompleted += new DownloadDataCompletedEventHandler(client_DownloadDataCompleted);
    client.DownloadDataAsync("http://example.com/version.txt");
}

启动表单时，只有在引发client_DownloadDataCompleted之前，UI才会出现.client_DownloadDataCompleted方法为空，因此没有问题.

When I start my form, the UI doesn't appears till client_DownloadDataCompleted is raised. The client_DownloadDataCompleted method is empty, so there's no problem there.

我做错了什么?应该如何在不冻结UI的情况下做到这一点?

What I'm doing wrong? How is supposed to do this without freezing the UI?

感谢您的时间.
最好的问候.

Thanks for your time.
Best regards.

完整代码:

Program.cs

using System;
using System.Windows.Forms;

namespace Lala
{
    static class Program
    {
        /// <summary>
        /// The main entry point for the application.
        /// </summary>
        [STAThread]
        static void Main()
        {
            Application.EnableVisualStyles();
            Application.SetCompatibleTextRenderingDefault(false);
            Application.Run(new Form1());
        }
    }
}

Form1.cs

using System;
using System.Net;
using System.Windows.Forms;

namespace Lala
{
    public partial class Form1 : Form
    {
        WebClient client = new WebClient();

        public Form1()
        {
            client.DownloadDataCompleted += new DownloadDataCompletedEventHandler(client_DownloadDataCompleted);
            client.DownloadDataAsync(new Uri("http://www.google.com"));
            InitializeComponent();
        }

        void client_DownloadDataCompleted(object sender, DownloadDataCompletedEventArgs e)
        {
            textBox1.Text += "A";
        }
    }

    partial class Form1
    {
        /// <summary>
        /// Required designer variable.
        /// </summary>
        private System.ComponentModel.IContainer components = null;

        /// <summary>
        /// Clean up any resources being used.
        /// </summary>
        /// <param name="disposing">true if managed resources should be disposed; otherwise, false.</param>
        protected override void Dispose(bool disposing)
        {
            if (disposing && (components != null))
            {
                components.Dispose();
            }
            base.Dispose(disposing);
        }

        #region Windows Form Designer generated code

        /// <summary>
        /// Required method for Designer support - do not modify
        /// the contents of this method with the code editor.
        /// </summary>
        private void InitializeComponent()
        {
            this.button1 = new System.Windows.Forms.Button();
            this.textBox1 = new System.Windows.Forms.TextBox();
            this.SuspendLayout();
            // 
            // button1
            // 
            this.button1.Location = new System.Drawing.Point(12, 12);
            this.button1.Name = "button1";
            this.button1.Size = new System.Drawing.Size(75, 23);
            this.button1.TabIndex = 0;
            this.button1.Text = "button1";
            this.button1.UseVisualStyleBackColor = true;
            // 
            // textBox1
            // 
            this.textBox1.Location = new System.Drawing.Point(12, 41);
            this.textBox1.Multiline = true;
            this.textBox1.Name = "textBox1";
            this.textBox1.Size = new System.Drawing.Size(468, 213);
            this.textBox1.TabIndex = 1;
            // 
            // Form1
            // 
            this.AutoScaleDimensions = new System.Drawing.SizeF(6F, 13F);
            this.AutoScaleMode = System.Windows.Forms.AutoScaleMode.Font;
            this.ClientSize = new System.Drawing.Size(492, 266);
            this.Controls.Add(this.textBox1);
            this.Controls.Add(this.button1);
            this.Name = "Form1";
            this.Text = "Form1";
            this.ResumeLayout(false);
            this.PerformLayout();

        }

        #endregion

        private System.Windows.Forms.Button button1;
        private System.Windows.Forms.TextBox textBox1;
    }
}

推荐答案

现在我们已经有了完整的代码，我可以肯定地说我绝对没有看到问题-还是不完全像描述的那样.

Now that we've got full code, I can say I'm definitely not seeing the problem - not quite as described, anyway.

我有一些日志记录来指示在DownloadDataAsync调用之前和之后以及何时触发完成的处理程序.如果我通过3G下载大文件，则在之前"和之后"之间会有一个 暂停，但是UI会在文件完成下载之前出现一段时间.

I've got a bit of logging to indicate just before and after the DownloadDataAsync calls, and when the completed handler is fired. If I download a large file over 3G, there is a pause between "before" and "after" but the UI comes up ages before the file completes downloading.

我怀疑 connect 是同步完成的，但实际下载是异步的.当然，这仍然是不幸的-可能将所有方法都插入另一个线程是要走的路-但是，如果我是对的，那至少是值得了解的.

I have a suspicion that the connect is done synchronously, but the actual download is asynchronous. That's still unfortunate, of course - and possibly punting all of that into a different thread is the way to go - but if I'm right it's at least worth knowing about.

这篇关于WebClient.DownloadDataAsync正在冻结我的UI的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！