WPF CommandManager.RequerySuggested add remove

 //xaml
<Window x:Class="WpfApp110.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:WpfApp110"
        mc:Ignorable="d" WindowState="Maximized" 
        Title="MainWindow" Height="450" Width="800">
    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition Height="50"/>
            <RowDefinition />
        </Grid.RowDefinitions>
        <StackPanel Grid.Row="0" Orientation="Horizontal">
            <Button Command="{Binding OpenImageFileCommand}" Content="Open" Margin="6" />
            <Button Command="{Binding ZoomCommand}" CommandParameter="ZoomIn" Content="Zoom In" Margin="6"/>
            <Button Command="{Binding ZoomCommand}" CommandParameter="ZoomOut" Content="Zoom Out" Margin="6"/>
            <Button Command="{Binding ZoomCommand}" CommandParameter="ZoomNormal" Content="Normal" Margin="6"/>
        </StackPanel>
        <Image Grid.Row="1" Source="{Binding ImagePath,Mode=TwoWay,UpdateSourceTrigger=PropertyChanged}">
            <Image.RenderTransform>
                <ScaleTransform ScaleX="{Binding Zoom,Mode=TwoWay,UpdateSourceTrigger=PropertyChanged}"
                                ScaleY="{Binding Zoom,Mode=TwoWay,UpdateSourceTrigger=PropertyChanged}"/>
            </Image.RenderTransform>
        </Image>
    </Grid>
</Window>


//App.xaml.cs
using System;
using System.Collections.Generic;
using System.Configuration;
using System.Data;
using System.Linq;
using System.Threading.Tasks;
using System.Windows;

namespace WpfApp110
{
    /// <summary>
    /// Interaction logic for App.xaml
    /// </summary>
    public partial class App : Application
    {
        protected override void OnStartup(StartupEventArgs e)
        {
            base.OnStartup(e);
            var mainWin=new MainWindow();
            mainWin.DataContext = new ImageData();
            mainWin.Show();
        }
    }
}



//vm
using Microsoft.Win32;
using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Input; 

namespace WpfApp110
{
    class ImageData : INotifyPropertyChanged
    {
        ICommand _openImageFileCmd, _zoomCmd;
        public ImageData()
        {
            _openImageFileCmd=new OpenImageFileCommand(this);
            _zoomCmd=new ZoomCommand(this);
        }

        public ICommand OpenImageFileCommand
        {
            get
            {
                return _openImageFileCmd;
            }
        }

        public ICommand ZoomCommand
        {
            get
            {
                return _zoomCmd;
            }
        }

        public event PropertyChangedEventHandler PropertyChanged;
        protected virtual void OnPropertyChanged(string propName)
        {
            var handler= PropertyChanged; 
            if(handler!=null)
            {
                handler?.Invoke(this, new PropertyChangedEventArgs(propName));
            }
        }

        string _imagePath;
        double _zoom = 1.0;
        public double Zoom
        {
            get
            {
                return _zoom;
            }
            set
            {
                _zoom = value;
                OnPropertyChanged(nameof(Zoom));
            }
        }

        public string ImagePath
        {
            get
            {
                return _imagePath;
            }
            set
            {
                _imagePath = value;
                OnPropertyChanged(nameof(ImagePath));
            }
        }
    }

    enum ZoomType
    {
        ZoomIn,
        ZoomOut,
        ZoomNormal
    }

    class ZoomCommand : ICommand
    {
        private ImageData _imgData;
        public ZoomCommand(ImageData imgDataValue)
        {
            _imgData = imgDataValue;
        }

        public event EventHandler CanExecuteChanged
        {
            add
            {
                CommandManager.RequerySuggested += value;
            }
            remove
            {
                CommandManager.RequerySuggested -= value;
            }
        }


        public bool CanExecute(object parameter)
        {
            return !string.IsNullOrWhiteSpace(_imgData.ImagePath) && System.IO.File.Exists(_imgData.ImagePath);
        }

        public void Execute(object parameter)
        {
            var zoomType = (ZoomType)Enum.Parse(typeof(ZoomType), (string)parameter, true);
            switch (zoomType)
            {
                case ZoomType.ZoomIn:
                    _imgData.Zoom *= 1.2;
                    break;
                case ZoomType.ZoomOut:
                    _imgData.Zoom /= 1.2;
                    break;
                case ZoomType.ZoomNormal:
                    _imgData.Zoom = 1;
                    break;
            }
        }
    }

    class OpenImageFileCommand : ICommand
    {
        ImageData _imgData;
        public OpenImageFileCommand(ImageData imgDataValue)
        {
            _imgData = imgDataValue;
        }

        public event EventHandler CanExecuteChanged;

        public bool CanExecute(object parameter)
        {
            return true;
        }

        public void Execute(object parameter)
        {
            var dlg = new OpenFileDialog()
            {
                Filter = "Image Files|*.jpg;*.png;*.bmp;*.gif"
            };
            if (dlg.ShowDialog() == true)
            {
                _imgData.ImagePath = dlg.FileName;
            }
        }
    }
}