因此，MandelbrotViewModel类具有许多属性，但如果您不考虑用户界面，则可能与您定义的属性不同。 CurrentCenter属性是程序当前显示的图像中心的复数，CurrentMagnification也适用于该图像。 但TargetMagnification绑定到Stepper的当前设置，它将应用于下一个计算的图像。 RealOffset和ImaginaryOffset属性绑定到两个Slider元素，范围从0到1.从CurrentCenter，CurrentMagnification，RealOffset和ImaginaryOffset属性，ViewModel可以计算TargetCenter属性。 这是下一个计算图像的中心。 如您所见，该TargetCenter属性用于显示两个滑块下方的复数：

namespace MandelbrotXF{    class MandelbrotViewModel : ViewModelBase    {        // Set via constructor arguments.        readonly double baseWidth;        readonly double baseHeight;        // Backing fields for properties.        Complex currentCenter, targetCenter;        int pixelWidth, pixelHeight;        double currentMagnification, targetMagnification;        int iterations;        double realOffset, imaginaryOffset;        bool isBusy;        double progress;        BitmapInfo bitmapInfo;        public MandelbrotViewModel(double baseWidth, double baseHeight)        {            this.baseWidth = baseWidth;            this.baseHeight = baseHeight;            // Create MandelbrotModel object.            MandelbrotModel model = new MandelbrotModel();            // Progress reporter            Progress
    
      progressReporter = new Progress
     
      ((double progress) =>            {                Progress = progress;            });            CancellationTokenSource cancelTokenSource = null;            // Define CalculateCommand and CancelCommand.            CalculateCommand = new Command(                execute: async () =>                    {                        // Disable this button and enable Cancel button.                        IsBusy = true;                        ((Command)CalculateCommand).ChangeCanExecute();                        ((Command)CancelCommand).ChangeCanExecute();                        // Create CancellationToken.                        cancelTokenSource = new CancellationTokenSource();                        CancellationToken cancelToken = cancelTokenSource.Token;                        try                        {                            // Perform the calculation.                            BitmapInfo = await model.CalculateAsync(TargetCenter,                                                             baseWidth / TargetMagnification,                                                             baseHeight / TargetMagnification,                                                                PixelWidth, PixelHeight,                                                                 Iterations,                                                                progressReporter,                                                                 cancelToken);                            // Processing only for a successful completion.                            CurrentCenter = TargetCenter;                            CurrentMagnification = TargetMagnification;                            RealOffset = 0.5;                            ImaginaryOffset = 0.5;                        }                        catch (OperationCanceledException)                        {                            // Operation cancelled!                        }                        catch                        {                            // Another type of exception? This should not occur.                        }                        // Processing regardless of success or cancellation.                        Progress = 0;                        IsBusy = false;                        // Disable Cancel button and enable this button.                        ((Command)CalculateCommand).ChangeCanExecute();                        ((Command)CancelCommand).ChangeCanExecute();                    },                 canExecute: () =>                    {                        return !IsBusy;                    });            CancelCommand = new Command(                execute: () =>                    {                        cancelTokenSource.Cancel();                    },                canExecute: () =>                    {                        return IsBusy;                    });         }        public int PixelWidth        {            set { SetProperty(ref pixelWidth, value); }            get { return pixelWidth; }        }        public int PixelHeight        {            set { SetProperty(ref pixelHeight, value); }            get { return pixelHeight; }        }        public Complex CurrentCenter        {            set            {                 if (SetProperty(ref currentCenter, value))                    CalculateTargetCenter();            }            get { return currentCenter; }        }        public Complex TargetCenter        {            private set { SetProperty(ref targetCenter, value); }            get { return targetCenter; }        }        public double CurrentMagnification        {            set { SetProperty(ref currentMagnification, value); }            get { return currentMagnification; }        }        public double TargetMagnification        {            set { SetProperty(ref targetMagnification, value); }            get { return targetMagnification; }        }        public int Iterations        {            set { SetProperty(ref iterations, value); }            get { return iterations; }        }        // These two properties range from 0 to 1.        // They indicate a new center relative to the         // current width and height, which is the baseWidth        // and baseHeight divided by CurrentMagnification.        public double RealOffset        {            set            {                if (SetProperty(ref realOffset, value))                    CalculateTargetCenter();            }            get { return realOffset; }        }        public double ImaginaryOffset        {            set            {                if (SetProperty(ref imaginaryOffset, value))                   CalculateTargetCenter();            }            get { return imaginaryOffset; }        }        void CalculateTargetCenter()        {            double width = baseWidth / CurrentMagnification;            double height = baseHeight / CurrentMagnification;            TargetCenter = new Complex(CurrentCenter.Real + (RealOffset - 0.5) * width,                                       CurrentCenter.Imaginary + (ImaginaryOffset - 0.5) *                                       height);        }        public bool IsBusy         {            private set { SetProperty(ref isBusy, value); }            get { return isBusy; }        }        public double Progress        {            private set { SetProperty(ref progress, value); }            get { return progress; }        }        public BitmapInfo BitmapInfo        {            private set { SetProperty(ref bitmapInfo, value); }            get { return bitmapInfo; }        }        public ICommand CalculateCommand { private set; get; }        public ICommand CancelCommand { private set; get; }    }}
     
    

MandelbrotViewModel还为Calculate和Cancel按钮，Progress属性和IsBusy属性定义了ICommand类型的两个属性。 正如您将看到的，IsBusy属性用于显示这两个按钮中的一个并隐藏另一个按钮，并在计算过程中禁用其余的用户界面。 这两个ICommand属性是在类的构造函数中使用lambda函数实现的。

XAML文件中的数据绑定到MandelbrotViewModel中的属性需要Xamarin.FormsBook.Toolkit库中的两个新绑定转换器。 第一个简单地否定了bool值：

namespace Xamarin.FormsBook.Toolkit{    public class BooleanNegationConverter : IValueConverter    {        public object Convert(object value, Type targetType,                               object parameter, CultureInfo culture)        {            return !(bool)value;        }        public object ConvertBack(object value, Type targetType,                                   object parameter, CultureInfo culture)        {            return !(bool)value;        }    }}

它与ViewModel的IsBusy属性一起使用。 当IsBusy为true时，需要将几个元素的IsEnabled属性和Go按钮的IsVisible属性设置为false。

两个Stepper元素实际上控制ViewModel中值的指数。 例如，Stepper值为8，对应于Iterations或TargetMagnification值256.该转换需要base-2对数转换器：

namespace Xamarin.FormsBook.Toolkit{    public class BaseTwoLogConverter : IValueConverter    {        public object Convert(object value, Type targetType,                               object parameter, CultureInfo culture)        {            if (value is int)            {                return Math.Log((int)value) / Math.Log(2);            }            return Math.Log((double)value) / Math.Log(2);        }        public object ConvertBack(object value, Type targetType,                                   object parameter, CultureInfo culture)        {            double returnValue = Math.Pow(2, (double)value);            if (targetType == typeof(int))            {                return (int) returnValue;            }            return returnValue;        }    }}

这是XAML文件，绑定到ViewModel的Progress，RealOffset，ImaginaryOffset，TargetCenter，TargetMagnification，Iterations，IsBusy，CalculateCommand和CancelCommand属性：

此XAML文件仅安装三个事件处理程序，它们都是SizeChanged处理程序。