Silverlight & Blend动画设计系列十三：三角函数（Trigonometry）动画之飘落的雪花（Falling Snow）

平时我们所看到的雪花（Falling Snow）飘飘的效果实际上也是一个动画，是由许多的动画对象共同完成的一个界面效果。对于不同大小的雪片可以通过缩放变换（ScaleTransform）功能特性确定，雪片飘落是存在于一个空间之中，通过不同的透明度值可使雪花看上去具有一定的空间视觉，雪花的飘落过程是由三角函数的原理实现的左右滑落效果，并可根据随机生成数字作为雪花飘落的速度。

雪花UserControl

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<Path.RenderTransform>
<TransformGroup>
<ScaleTransform x:Name="ScaleFlake" ScaleX="1" ScaleY="1"/>
<RotateTransform x:Name="RotateFlake" Angle="0"/>
</TransformGroup>
</Path.RenderTransform>
</Path>
</Canvas>

　　通过路径（Path）可绘制雪花界面，为此只需要提供一个根据不同参数构造不同的雪花界面效果的接口，既构造方法：

//根据不同的参数构造雪花对象
public Snowflake(double Left, double Top, double Opacity)
{
    InitializeComponent();

    //随机速度
    Speed = Rand.Next(5);
    if (Speed < 1)
    {
        Speed = 1;
    }

    //随机位置、弧度、角度
    DriftPosition = Left;
    DriftRange = Rand.Next(50);
    DriftAngle = Rand.Next(270);

    Spin = Rand.Next(5);

    ScaleFlake.ScaleX = ScaleFlake.ScaleY = Rand.Next(25, 100) / 100.0;

    Canvas.SetLeft(this, Left);
    Canvas.SetTop(this, Top);
    this.Opacity = Opacity;
    Position.X = Left;
    Position.Y = Top;
}

　　出了根据不同参数生成雪花对象外，还需提供一个动态改变雪花位置的接口，也就是动态改变雪花的X,Y的坐标值。

//根据三角函数计算雪片下落过程（Y坐标）中的左右（X方向）浮动效果
public void MoveFlake()
{
    Position.Y += Speed;
    Position.X = DriftPosition + Math.Cos(DriftAngle) * DriftRange;

    RotateFlake.Angle += Spin;

    if (Position.Y > AppHeight)
    {
        Position.Y = -this.Height;
    }

    Canvas.SetLeft(this, Position.X);
    Canvas.SetTop(this, Position.Y);

    DriftAngle += DriftSpeed;
}

　　在主程序中需要定义一个动画，用于控制雪花下落和角度旋转：

<UserControl.Resources>
    <Storyboard x:Name="Snowfall" Duration="00:00:00"/>
</UserControl.Resources>

 　　通过主程序动态的随机构造雪花对象并添加到界面中，然后统一启动动画就实现了飘落的雪花效果，其完整的主控程序如下代码块：

public partial class MainPage : UserControl
{
    private List<Snowflake> Flake;
    private List<Point> XYStart;
    private List<double> OpacityValue;
    private int MaxFlakes = 250;
    private Random Rand = new Random();

    public MainPage()
    {
        InitializeComponent();

        //雪花集合--初始化250个雪花对象并同事进行动画处理
        Flake = new List<Snowflake>(MaxFlakes);
        //雪花坐标集合
        XYStart = new List<Point>(MaxFlakes);
        //不透明度集合
        OpacityValue = new List<double>(MaxFlakes);

        for (int i = 0; i < MaxFlakes; i++)
        {
            //不同的起止坐标
            Point newPoint = new Point(Rand.Next((int)LayoutRoot.Width), Rand.Next((int)LayoutRoot.Height));
            XYStart.Add(newPoint);
            //不同透明度值
            OpacityValue.Add(Rand.NextDouble());
        }

        InitFlakes();
        Snowfall.Completed += new EventHandler(Snowfall_Completed);
        Snowfall.Begin();
    }

    private void InitFlakes()
    {
        //循环生成雪花冰添加到界面
        for (int i = 0; i < MaxFlakes; i++)
        {
            Snowflake flake = new Snowflake(XYStart[i].X, XYStart[i].Y, OpacityValue[i]);
            Flake.Add(flake);
            flake.AppHeight = LayoutRoot.Height;
            LayoutRoot.Children.Add(flake);
        }
    }

    //动画完成后继续开始动画的执行
    private void Snowfall_Completed(object sender, EventArgs e)
    {
        foreach (Snowflake flake in Flake)
        {
            flake.MoveFlake();
        }

        Snowfall.Begin();
    }
}