本文将介绍一个好玩但实际作用可能不太大的动画效果：跳动的字符。为了提高动画效果的可重用性以及调用的灵活性，通过Behavior实现跳动的字符动画。先看下效果：

技术要点与实现

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TranslateTransform设置字符纵坐标的移动变换，以实现跳动的效果。主要步骤如下：

• 在OnAttached方法中，注册Loaded事件，在Load事件中为TextBlock添加TextEffect效果，其中PositionCount设置为1，每次只跳动一个字符。
• 添加启动动画效果的BeginEffect方法，并创建控制子字符纵向移动变换的线性动画。然后根据字符串（剔除空字符）的长度n，创建n个关键帧，每个关键帧中把PositionStart设置为要跳动的字符在字符串中的索引
• 在开启动画属性IsEnabled=true和TextBlock内容变化时,启动动画效果

在创建关键帧设置跳动字符位置时剔除了空字符，是为了是动画效果显得连贯

public class DanceCharEffectBehavior : Behavior<TextBlock>
    {
        private TextEffect _textEffect;
        private string _textEffectName;
        private TranslateTransform _translateTransform = null;
        private string _translateTransformName;
        private Storyboard _storyboard;

        protected override void OnAttached()
        {
            base.OnAttached();

            this.AssociatedObject.Loaded += AssociatedObject_Loaded;
            this.AssociatedObject.Unloaded += AssociatedObject_Unloaded;
            this.AssociatedObject.IsVisibleChanged += AssociatedObject_IsVisibleChanged;
            BindingOperations.SetBinding(this, DanceCharEffectBehavior.InternalTextProperty, new Binding("Text") { Source = this.AssociatedObject });
        }

        protected override void OnDetaching()
        {
            base.OnDetaching();

            this.AssociatedObject.Loaded -= AssociatedObject_Loaded;
            this.AssociatedObject.Unloaded -= AssociatedObject_Unloaded;
            this.AssociatedObject.IsVisibleChanged -= AssociatedObject_IsVisibleChanged;
            this.ClearValue(DanceCharEffectBehavior.InternalTextProperty);

            if (_storyboard != null)
            {
                _storyboard.Remove(this.AssociatedObject);
                _storyboard.Children.Clear();
            }
            if (_textEffect != null)
                this.AssociatedObject.TextEffects.Remove(_textEffect);
        }

        private void AssociatedObject_IsVisibleChanged(object sender, DependencyPropertyChangedEventArgs e)
        {
            if ((bool)e.NewValue == false)
            {
                if (_storyboard != null)
                    _storyboard.Stop(this.AssociatedObject);
            }
            else
            {
                BeginEffect(this.AssociatedObject.Text);
            }
        }

        private void AssociatedObject_Loaded(object sender, RoutedEventArgs e)
        {
            if (_textEffect == null)
            {
                this.AssociatedObject.TextEffects.Add(_textEffect = new TextEffect()
                {
                    PositionCount = 1,
                    Transform = _translateTransform = new TranslateTransform(),
                });
                NameScope.SetNameScope(this.AssociatedObject, new NameScope());
                this.AssociatedObject.RegisterName(_textEffectName = "n" + Guid.NewGuid().ToString("N"), _textEffect);
                this.AssociatedObject.RegisterName(_translateTransformName = "n" + Guid.NewGuid().ToString("N"), _translateTransform);
                if (IsEnabled)
                    BeginEffect(this.AssociatedObject.Text);
            }
        }

        private void AssociatedObject_Unloaded(object sender, RoutedEventArgs e)
        {
            StopEffect();
        }

        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 || _translateTransformName == null || IsEnabled == false) return;

            if (_storyboard == null)
                _storyboard = new Storyboard();
            double duration = 0.5d;
            DoubleAnimation da = new DoubleAnimation();

            Storyboard.SetTargetName(da, _translateTransformName);
            Storyboard.SetTargetProperty(da, new PropertyPath(TranslateTransform.YProperty));
            da.From = 0d;
            da.To = 10d;
            da.Duration = TimeSpan.FromSeconds(duration / 2d);
            da.RepeatBehavior = RepeatBehavior.Forever;
            da.AutoReverse = true;

            char emptyChar = ' ';
            List<int> lsb = new List<int>();
            for (int i = 0; i < textLength; ++i)
            {
                if (text[i] != emptyChar)
                {
                    lsb.Add(i);
                }
            }

            Int32AnimationUsingKeyFrames frames = new Int32AnimationUsingKeyFrames();
            Storyboard.SetTargetName(frames, _textEffectName);
            Storyboard.SetTargetProperty(frames, new PropertyPath(TextEffect.PositionStartProperty));
            frames.Duration = TimeSpan.FromSeconds((lsb.Count) * duration);
            frames.RepeatBehavior = RepeatBehavior.Forever;
            frames.AutoReverse = true;

            int ii = 0;
            foreach (int index in lsb)
            {
                frames.KeyFrames.Add(new DiscreteInt32KeyFrame()
                {
                    Value = index,
                    KeyTime = TimeSpan.FromSeconds(ii * duration),
                });
                ++ii;
            }

            _storyboard.Children.Add(da);
            _storyboard.Children.Add(frames);
            _storyboard.Begin(this.AssociatedObject, true);
        }

        private string InternalText
        {
            get { return (string)GetValue(InternalTextProperty); }
            set { SetValue(InternalTextProperty, value); }
        }

        private static readonly DependencyProperty InternalTextProperty =
        DependencyProperty.Register("InternalText", typeof(string), typeof(DanceCharEffectBehavior),
        new PropertyMetadata(OnInternalTextChanged));

        private static void OnInternalTextChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
        {
            var source = d as DanceCharEffectBehavior;
            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(DanceCharEffectBehavior), new PropertyMetadata(true, (d, e) =>
            {
                bool b = (bool)e.NewValue;
                var source = d as DanceCharEffectBehavior;
                source.SetEffect(source.InternalText);
            }));

    }

调用的时候只需要在TextBlock添加Behavior即可，代码如下

<TextBlock FontSize="20" Text="Hello">
    <i:Interaction.Behaviors>
        <local:DanceCharEffectBehavior x:Name="titleEffect" IsEnabled="True" />
    </i:Interaction.Behaviors>
</TextBlock>

结尾

本例中还有许多可以完善的地方，比如字符跳动的幅度可以根据实际的FontSize来设置，或者增加依赖属性来控制；动画是否倒退播放，是否循环播放，以及动画的速度都可以通过增加依赖属性在调用时灵活设置。