并行处理导致 UI 卡顿是否正常

Question

我有一个 Windows 窗体应用程序，它可以复制 GEDCOM 系谱文件中的图像引用并重新调整其大小。用户从主窗体中选择文件和输出目录以及调整大小选项，然后打开另一个窗体作为包含标签、进度条和按钮的对话框。我正在更新应用程序以使用 .NET 4.5 中的新异步功能，并对其进行修改以利用并行处理。一切正常，除了我注意到 UI 响应有点不稳定（口吃）；如果我不使用百分比更新消息标签，那么它会更顺畅。此外，当我取消任务时，UI 将挂起 1 到 15 秒。该应用程序仅供我个人使用，所以没什么大不了的，但我很好奇可能导致问题的原因以及处理它的推荐方法是什么。并行处理是否只是因为有太多线程需要处理而使 CPU 过载？我尝试在每个循环迭代中添加一个 Thread.Sleep(100)，它似乎有点帮助。

这是仍然会导致问题的应用程序的最小版本。重现：

1. 使用以下表单创建一个新的 Windows 窗体应用程序。
2. 用一堆jpeg图片（50+图片）创建一个目录
3. 将 _SourceDirectoryPath 和 _DestinationDirectoryPath 变量替换为您的目录。
4. 运行应用程序

Designer.cs：

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.lblMessage = new System.Windows.Forms.Label();
        this.pgProgressBar = new System.Windows.Forms.ProgressBar();
        this.btnStart = new System.Windows.Forms.Button();
        this.btnCancel = new System.Windows.Forms.Button();
        this.SuspendLayout();
        // 
        // lblMessage
        // 
        this.lblMessage.AutoSize = true;
        this.lblMessage.Location = new System.Drawing.Point(32, 25);
        this.lblMessage.Name = "lblMessage";
        this.lblMessage.Size = new System.Drawing.Size(0, 13);
        this.lblMessage.TabIndex = 0;
        // 
        // pgProgressBar
        // 
        this.pgProgressBar.Location = new System.Drawing.Point(35, 51);
        this.pgProgressBar.Name = "pgProgressBar";
        this.pgProgressBar.Size = new System.Drawing.Size(253, 23);
        this.pgProgressBar.TabIndex = 1;
        // 
        // btnStart
        // 
        this.btnStart.Location = new System.Drawing.Point(132, 97);
        this.btnStart.Name = "btnStart";
        this.btnStart.Size = new System.Drawing.Size(75, 23);
        this.btnStart.TabIndex = 2;
        this.btnStart.Text = "Start";
        this.btnStart.UseVisualStyleBackColor = true;
        this.btnStart.Click += new System.EventHandler(this.btnStart_Click);
        // 
        // btnCancel
        // 
        this.btnCancel.Location = new System.Drawing.Point(213, 97);
        this.btnCancel.Name = "btnCancel";
        this.btnCancel.Size = new System.Drawing.Size(75, 23);
        this.btnCancel.TabIndex = 3;
        this.btnCancel.Text = "Cancel";
        this.btnCancel.UseVisualStyleBackColor = true;
        this.btnCancel.Click += new System.EventHandler(this.btnCancel_Click);
        // 
        // Form1
        // 
        this.AutoScaleDimensions = new System.Drawing.SizeF(6F, 13F);
        this.AutoScaleMode = System.Windows.Forms.AutoScaleMode.Font;
        this.ClientSize = new System.Drawing.Size(315, 149);
        this.Controls.Add(this.btnCancel);
        this.Controls.Add(this.btnStart);
        this.Controls.Add(this.pgProgressBar);
        this.Controls.Add(this.lblMessage);
        this.Name = "Form1";
        this.Text = "Form1";
        this.ResumeLayout(false);
        this.PerformLayout();

    }

    #endregion

    private System.Windows.Forms.Label lblMessage;
    private System.Windows.Forms.ProgressBar pgProgressBar;
    private System.Windows.Forms.Button btnStart;
    private System.Windows.Forms.Button btnCancel;
}

代码：

using System;
using System.Collections.Generic;
using System.Drawing;
using System.IO;
using System.Linq;
using System.Threading;
using System.Threading.Tasks;
using System.Windows.Forms;

public partial class Form1 : Form
{
    private CancellationTokenSource _CancelSource;
    private string _SourceDirectoryPath = @"Your\Source\Directory";
    private string _DestinationDirectoryPath = @"Your\Destination\Directory";

    public Form1()
    {
        InitializeComponent();
        lblMessage.Text = "Click Start to begin extracting images";
        btnCancel.Enabled = false;
        _CancelSource = new CancellationTokenSource();
    }

    private async void btnStart_Click(object sender, EventArgs e)
    {
        btnStart.Enabled = false;
        btnCancel.Enabled = true;

        List<string> files = await Task.Run(() => Directory.GetFiles(_SourceDirectoryPath, "*.jpg").ToList());

        // scan/extract files
        Progress<int> progress = new Progress<int>(UpdateProgress);
        int result = await Task.Run(() => ExtractFiles(files, progress, _CancelSource.Token));

        if (_CancelSource.IsCancellationRequested)
        {
            lblMessage.Text = "Extraction cancelled by user.";
        }
        else
        {
            lblMessage.Text = string.Format("Extraction Complete: {0} files extracted.", result);
        }
        btnStart.Enabled = true;
        btnCancel.Enabled = false;
    }

    private void btnCancel_Click(object sender, EventArgs e)
    {
        lblMessage.Text = "Cancelling...";
        btnCancel.Enabled = false;
        _CancelSource.Cancel();
    }

    private void UpdateProgress(int value)
    {
        lblMessage.Text = string.Format("Extracting files: {0}%", value);
        pgProgressBar.Value = value;
    }

    public int ExtractFiles(List<string> fileReferences, IProgress<int> progress, CancellationToken cancelToken)
    {
        double totalFiles = fileReferences.Count;
        int processedCount = 0;
        int extractedCount = 0;
        int previousPercent = 0;
        Directory.CreateDirectory(_DestinationDirectoryPath);

        Parallel.ForEach(fileReferences, (reference, state) =>
        {
            if (cancelToken.IsCancellationRequested)
            {
                state.Break();
            }

            string fileName = Path.GetFileName(reference);
            string filePath = Path.Combine(_DestinationDirectoryPath, fileName);

            using (Image image = Image.FromFile(reference))
            {
                using (Image newImage = ResizeImage(image, 1000, 1000))
                {
                    newImage.Save(filePath);
                    Interlocked.Increment(ref extractedCount);
                }
            }

            Interlocked.Increment(ref processedCount);
            int percent = (int)(processedCount / totalFiles * 100);
            if (percent > previousPercent)
            {
                progress.Report(percent);
                Interlocked.Exchange(ref previousPercent, percent);
            }
        });

        return extractedCount;
    }

    public Image ResizeImage(Image image, int maxWidth, int maxHeight)
    {
        Image newImage = null;

        if (image.Width > maxWidth || image.Height > maxHeight)
        {
            double widthRatio = (double)maxWidth / (double)image.Width;
            double heightRatio = (double)maxHeight / (double)image.Height;
            double ratio = Math.Min(widthRatio, heightRatio);
            int newWidth = (int)(image.Width * ratio);
            int newHeight = (int)(image.Height * ratio);

            newImage = new Bitmap(newWidth, newHeight);
            using (Graphics graphic = Graphics.FromImage(newImage))
            {
                graphic.CompositingQuality = System.Drawing.Drawing2D.CompositingQuality.HighQuality;
                graphic.InterpolationMode = System.Drawing.Drawing2D.InterpolationMode.HighQualityBicubic;
                graphic.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.HighQuality;
                graphic.DrawImage(image, 0, 0, newWidth, newHeight);
            }
        }

        return newImage;
    }
}

Answer 1

我相信我找到了问题所在。在后台线程中调用 Graphics.DrawImage() 时，GDI+ 在 UI 线程中被阻塞。见Why do Graphics operations in background thread block Graphics operations in main UI thread?

一个明显的解决方案是使用多个进程（参见：Parallelizing GDI+ Image Resizing .net）

Answer 2

我可以在这里看到两个潜在的问题：

1. 您正在循环体的开头检查取消，这不允许在操作完成时中断每个循环迭代。取消后的卡顿可能是由于图像大小调整仍在执行。中止线程可能会更好（不建议这样做，但在这种情况下它可能会运行得更快）。
2. _CancelSource.Cancel() 正在阻塞 UI 线程。您可以将取消作为异步任务进行。查看相关帖子：Why does cancellation block for so long when cancelling a lot of HTTP requests?。

至于 CPU 过载，这也是可能的。您可以使用分析器来检查 CPU 使用率。 Visual Studio 有一个集成的profiler，可以很好地与 C# 配合使用。