解决卡顿——在WPF中使用多线程更新UI

2017-04-01 21:23:36来源:CSDN作者:amanda_zhang2010人点击



解决卡顿——在WPF中使用多线程更新UI

有经验的程序员们都知道:不能在UI线程上进行耗时操作,那样会造成界面卡顿,如下就是一个简单的示例:

    publicpartial class MainWindow :Window
    {
        public MainWindow()
        {
            InitializeComponent();
            this.Dispatcher.Invoke(newAction(()=> { }));
            this.Loaded += MainWindow_Loaded;
        }

        private void MainWindow_Loaded(object sender,RoutedEventArgs e)
        {
            this.Content = new UserControl1();
        }
    }

    classUserControl1 : UserControl
    {
        TextBlock textBlock;

        public UserControl1()
        {
            textBlock = new TextBlock();
            this.Content = textBlock;

            this.Dispatcher.BeginInvoke(newAction(updateTime),null);
        }

        private async void updateTime()
        {
            while (true)
            {
                Thread.Sleep(900);            //
模拟耗时操作

                textBlock.Text = DateTime.Now.ToString();
                await Task.Delay(100);
            }
        }
    }

当我们运行这个程序的时候,就会发现:由于主线程大部分的时间片被占用,无法及时处理系统事件(如鼠标,键盘等输入),导致程序变得非常卡顿,连拖动窗口都变得不流畅;

如何解决这个问题呢,初学者可能想到的第一个方法就是新启一个线程,在线程中执行更新:

    public UserControl1()
    {
        textBlock = new TextBlock();
        this.Content = textBlock;

        
ThreadPool.QueueUserWorkItem(_ => updateTime());
    }

但很快就会发现此路不通,因为WPF不允许跨线程访问程序,此时我们会得到一个:"The calling thread cannot access this object because a different thread owns it."的InvalidOperationException异常

    

那么该如何解决这一问题呢?通常的做法是把耗时的函数放在线程池执行,然后切回主线程更新UI显示。前面的updateTime函数改写如下:

    privateasync void updateTime()
    {
        while (true)
        {
            
awaitTask.Run(() => Thread.Sleep(900));
            textBlock.Text = DateTime.Now.ToString();
            await Task.Delay(100);
        }
    }

这种方式能满足我们的大部分需求。但是,有的操作是比较耗时间的。例如,在多窗口实时监控的时候,我们就需要同时多十来个屏幕每秒钟各进行几十次的刷新,更新图像这个操作必须在UI线程上进行,并且它有非常耗时间,此时又会回到最开始的卡顿的情况。

看起来这个问题无法解决,实际上,WPF只是不允许跨线程访问程序,并非不允许多线程更新界面。我们大可以对每个视频监控窗口单独其一个独立的线程,在那个线程中进行更新操作,此时就不会影响到主线程。MSDN上有篇文章介绍了详细的操作:Multithreaded UI: HostVisual。用这种方式将原来的程序改写如下:

    privatevoid MainWindow_Loaded(object sender,RoutedEventArgs e)
    {
        HostVisual hostVisual =new HostVisual();

        UIElement content =new VisualHost(hostVisual);
        this.Content = content;

        Thread thread =new Thread(newThreadStart(() =>
        {
            VisualTarget visualTarget =new VisualTarget(hostVisual);
            var control = new UserControl1();
            control.Arrange(newRect(newPoint(), content.RenderSize));
            visualTarget.RootVisual = control;

            System.Windows.Threading.Dispatcher.Run();

        }));

        thread.SetApartmentState(ApartmentState.STA);
        thread.IsBackground = true;
        thread.Start();
    }

    public class VisualHost :FrameworkElement
    {
        Visual child;

        public VisualHost(Visual child)
        {
            if (child == null)
                throw new ArgumentException("child");

            this.child = child;
            AddVisualChild(child);
        }

        protected override Visual GetVisualChild(int index)
        {
            return (index == 0) ? child :null;
        }

        protected override int VisualChildrenCount
        {
            get { return 1; }
        }
    }

这个里面用来了两个新的类:HostVisual、VisualTarget。以及自己写的一个VisualHost。MSDN上相关的解释,也不算难理解,这里就不多介绍了。最后,再来重构一下代码,把在新线程中创建控件的方式改写如下:

    privatevoid MainWindow_Loaded(object sender,RoutedEventArgs e)
    {
        createChildInNewThread<UserControl1>(this);
    }

    void createChildInNewThread<T>(ContentControl container)
        where T : UIElement ,new()
    {
        HostVisual hostVisual =new HostVisual();

        UIElement content =new VisualHost(hostVisual);
        container.Content = content;

        Thread thread =new Thread(newThreadStart(() =>
        {
            VisualTarget visualTarget =new VisualTarget(hostVisual);

            var control = new T();
            control.Arrange(newRect(newPoint(), content.RenderSize));

            visualTarget.RootVisual = control;
            System.Windows.Threading.Dispatcher.Run();

        }));

        thread.SetApartmentState(ApartmentState.STA);
        thread.IsBackground = true;
        thread.Start();
    }

当然,我这个函数多了一些不必要的的限制:容器必须是ContentControl,子元素必须是UIElement。可以根据实际需要进行相关修改。这里有一个完整的示例,也可以参考一下。


转自:http://www.cnblogs.com/TianFang/p/3969430.html

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