WPF实现跳动的字符效果

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。