WPF实现类似ChatGPT的逐字打印效果
背景前一段时间ChatGPT类的应用十分火爆，这类应用在回答用户的问题时逐字打印输出，像极了真人打字回复消息。出于对这个效果的兴趣，决定用WPF模拟这个效果。
真实的ChatGPT逐字输出效果涉及其语言生成模型原理以及服务端与前端通信机制，本文不做过多阐述，重点是如何用WPF模拟这个效果。
技术要点与实现
对于这个逐字输出的效果，我想到了两种实现方法:
[*]方法一：根据字符串长度n，添加n个关键帧DiscreteStringKeyFrame，第一帧的Value为字符串的第一个字符，紧接着的关键帧都比上一帧的Value多一个字符，直到最后一帧的Value是完整的目标字符串。实现效果如下所示：
[*]方法二：首先把TextBlock的字体颜色设置为透明，然后通过TextEffect的PositionStart和PositionCount属性控制应用动画效果的子字符串的起始位置以及长度，同时使用ColorAnimation设置TextEffect的Foreground属性由透明变为目标颜色（假定是黑色）。实现效果如下所示：
由于方案二的思路与WPF实现跳动的字符效果中的效果实现思路非常类似，具体实现不再详述。接下来我们看一下方案一通过关键帧动画拼接字符串的具体实现。
public class TypingCharAnimationBehavior : Behavior{ private Storyboard _storyboard; protected override void OnAttached() { base.OnAttached(); this.AssociatedObject.Loaded += AssociatedObject_Loaded; ; this.AssociatedObject.Unloaded += AssociatedObject_Unloaded; BindingOperations.SetBinding(this, TypingCharAnimationBehavior.InternalTextProperty, new Binding("Tag") { Source = this.AssociatedObject }); } private void AssociatedObject_Unloaded(object sender, RoutedEventArgs e) { StopEffect(); } private void AssociatedObject_Loaded(object sender, RoutedEventArgs e) { if (IsEnabled) BeginEffect(InternalText); } protected override void OnDetaching() { base.OnDetaching(); this.AssociatedObject.Loaded -= AssociatedObject_Loaded; this.AssociatedObject.Unloaded -= AssociatedObject_Unloaded; this.ClearValue(TypingCharAnimationBehavior.InternalTextProperty); if (_storyboard != null) { _storyboard.Remove(this.AssociatedObject); _storyboard.Children.Clear(); } } private string InternalText { get { return (string)GetValue(InternalTextProperty); } set { SetValue(InternalTextProperty, value); } } private static readonly DependencyProperty InternalTextProperty = DependencyProperty.Register("InternalText", typeof(string), typeof(TypingCharAnimationBehavior), new PropertyMetadata(OnInternalTextChanged)); private static void OnInternalTextChanged(DependencyObject d, DependencyPropertyChangedEventArgs e) { var source = d as TypingCharAnimationBehavior; if (source._storyboard != null) { source._storyboard.Stop(source.AssociatedObject); source._storyboard.Children.Clear(); } source.SetEffect(e.NewValue == null ? string.Empty : e.NewValue.ToString()); } public bool IsEnabled { get { return (bool)GetValue(IsEnabledProperty); } set { SetValue(IsEnabledProperty, value); } } public static readonly DependencyProperty IsEnabledProperty = DependencyProperty.Register("IsEnabled", typeof(bool), typeof(TypingCharAnimationBehavior), new PropertyMetadata(true, (d, e) => { bool b = (bool)e.NewValue; var source = d as TypingCharAnimationBehavior; source.SetEffect(source.InternalText); })); private void SetEffect(string text) { if (string.IsNullOrEmpty(text) || this.AssociatedObject.IsLoaded == false) { StopEffect(); return; } BeginEffect(text); } private void StopEffect() { if (_storyboard != null) { _storyboard.Stop(this.AssociatedObject); } } private void BeginEffect(string text) { StopEffect(); int textLength = text.Length; if (textLength < 1|| IsEnabled == false) return; if (_storyboard == null) _storyboard = new Storyboard(); double duration = 0.15d; StringAnimationUsingKeyFrames frames = new StringAnimationUsingKeyFrames(); Storyboard.SetTargetProperty(frames, new PropertyPath(TextBlock.TextProperty)); frames.Duration = TimeSpan.FromSeconds(textLength * duration); for(int i=0;i
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