当前上下文中不存在名称"CommandManager"(Visual Studio 2015)

Question

尝试使用下面的RelayCommand类我收到错误消息:"名称"CommandManager"在当前上下文中不存在".根据这篇文章,类库无法识别CommandManager类,我试图通过在"添加引用"上添加一个名为PresentationCore的程序集来解决问题.不幸的是"机器上没有找到框架组件".我卸载了Microsoft.NETCore.UniversalWindowsPlatform并按照一些帖子中的建议重新安装它,我修复了Visual Studio,但这并没有改变任何东西.这是我的失败还是一个错误？作为一个新手,我总是倾向于认为我忽略了一些东西;-)

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Input;

namespace MyApplication.Viewmodels
{
public class RelayCommand : ICommand
{
    private readonly Action<object> _Execute;
    private readonly Func<object, bool> _CanExecute;

    public RelayCommand(Action<object> execute)
        : this(execute, null)
    {

    }

    public RelayCommand(Action<object> execute, Func<object, bool> canExecute)
    {
        if (execute == null)
        {
            throw new ArgumentNullException("execute", "Execute cannot be null.");
        }

        _Execute = execute;
        _CanExecute = canExecute;
    }

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

    public void Execute(object parameter)
    {
        _Execute(parameter);
    }

    public bool CanExecute(object parameter)
    {
        if (_CanExecute == null)
        {
            return true;
        }

        return _CanExecute(parameter);
    }
}
}

Answer 1

在.NETCore中不存在CommandManager.我写了自己的解释,因为我和你现在的情况完全相同.(使用MVVM设计模式开发UWP应用程序)

RelayCommand.cs:

using System;
using System.Windows.Input;

namespace MVVMBoostUniversalWindowsApp
{
    public class RelayCommand : ICommand
    {
        private readonly Action execute;
        private readonly Func<bool> canExecute;

        public RelayCommand(Action execute)
            : this(execute, null)
        { }

        public RelayCommand(Action execute, Func<bool> canExecute)
        {
            if (execute == null)
                throw new ArgumentNullException("execute is null.");

            this.execute = execute;
            this.canExecute = canExecute;
            this.RaiseCanExecuteChangedAction = RaiseCanExecuteChanged;
            SimpleCommandManager.AddRaiseCanExecuteChangedAction(ref RaiseCanExecuteChangedAction);
        }

        ~RelayCommand()
        {
            RemoveCommand();
        }

        public void RemoveCommand()
        {
            SimpleCommandManager.RemoveRaiseCanExecuteChangedAction(RaiseCanExecuteChangedAction);
        }

        bool ICommand.CanExecute(object parameter)
        {
            return CanExecute;
        }

        public void Execute(object parameter)
        {
            execute();
            SimpleCommandManager.RefreshCommandStates();
        }

        public bool CanExecute
        {
            get { return canExecute == null || canExecute(); }
        }

        public void RaiseCanExecuteChanged()
        {
            var handler = CanExecuteChanged;
            if (handler != null)
            {
                handler(this, new EventArgs());
            }
        }

        private readonly Action RaiseCanExecuteChangedAction;

        public event EventHandler CanExecuteChanged;
    }
}

在这里我的SimpleCommandManager.cs:

using System;
using System.Collections.Generic;
using System.ComponentModel;

namespace MVVMBoostUniversalWindowsApp
{
    public static class SimpleCommandManager
    {
        private static List<Action> _raiseCanExecuteChangedActions = new List<Action>();

        public static void AddRaiseCanExecuteChangedAction(ref Action raiseCanExecuteChangedAction)
        {
            _raiseCanExecuteChangedActions.Add(raiseCanExecuteChangedAction);
        }

        public static void RemoveRaiseCanExecuteChangedAction(Action raiseCanExecuteChangedAction)
        {
            _raiseCanExecuteChangedActions.Remove(raiseCanExecuteChangedAction);
        }

        public static void AssignOnPropertyChanged(ref PropertyChangedEventHandler propertyEventHandler)
        {
            propertyEventHandler += OnPropertyChanged;
        }

        private static void OnPropertyChanged(object sender, PropertyChangedEventArgs e)
        {
            // this if clause is to prevent an infinity loop
            if (e.PropertyName != "CanExecute")
            {
                RefreshCommandStates();
            }
        }

        public static void RefreshCommandStates()
        {
            for (var i = 0; i < _raiseCanExecuteChangedActions.Count; i++)
            {
                var raiseCanExecuteChangedAction = _raiseCanExecuteChangedActions[i];
                if (raiseCanExecuteChangedAction != null)
                {
                    raiseCanExecuteChangedAction.Invoke();
                }
            }
        }
    }
}

不要问我为什么.NETCore中没有CommandManager我也不知道/取消它.如果有人可以提供有关此的一些信息,那将很高兴知道.

但是你应该关心用法:如果你离开一个页面,你应该销毁SimpleCommandManager中的剩余命令,这些命令不再使用但是被引用.我也有一个解决方案,如果有人需要它,我可以编辑我的帖子.

它实际上只是一个非常简单/原始的解决方案而且没什么特别的.但它的确有效.

编辑:

为了提供更好的使用示例和基础,这里也是我的基本ViewModel.如上所述,在离开当前页面时应该调用"RemoveCommands".否则这些将永远不会被GC释放.

ViewModel.cs

using System.ComponentModel;
using System;
using System.Runtime.CompilerServices;
using Windows.UI.Xaml.Controls;
using System.Collections.Generic;
using Windows.UI.Core;

namespace MVVMBoostUniversalWindowsApp
{
    public abstract class ViewModel : INotifyPropertyChanged
    {
        protected ViewModel()
        {
            DispatcherObject = CoreWindow.GetForCurrentThread().Dispatcher;
            SimpleCommandManager.AssignOnPropertyChanged(ref this.PropertyChanged);
            _commandsList = new List<RelayCommand>();
        }

        private List<RelayCommand> _commandsList;

        protected RelayCommand CreateCommand(Action execute)
        {
            return CreateCommand(execute, null);
        }

        protected RelayCommand CreateCommand(Action execute, Func<bool> canExecute)
        {
            var tempCmd = new RelayCommand(execute, canExecute);
            if (_commandsList.Contains(tempCmd))
            {
                return _commandsList[_commandsList.IndexOf(tempCmd)];
            }
            else
            {
                _commandsList.Add(tempCmd);
                return tempCmd;
            }
        }

        public void RemoveCommands()
        {
            for (var i = 0; i < _commandsList.Count; i++)
            {
                _commandsList[i].RemoveCommand();
            }
        }

        public virtual CoreDispatcher DispatcherObject { get; protected set; }

        public event PropertyChangedEventHandler PropertyChanged;

        protected void ChangeProperty<T>(ref T property, T value, [CallerMemberName] string propertyName = null)
        {
            if (Object.Equals(property, value))
            {
                return;
            }
            else
            {
                property = value;

                OnPropertyChanged(propertyName);
            }
        }

        protected void OnPropertyChangedEmpty()
        {
            OnPropertyChanged(String.Empty);
        }

        protected void OnPropertyChanged(string propertyName)
        {
            var handler = PropertyChanged;
            if (handler != null)
            {
                handler(this, new PropertyChangedEventArgs(propertyName));
            }
        }
    }
}

Answer 2

您正在混合标准.NET FrameworkWPF ( ) 和用于 UWP 开发的CommandManager较小版本。.NET Core Framework

不幸的是，它们并非在所有领域都 100% 兼容。您不能使用该类RelayCommand，因为它是在那里编写的 - 您将需要重写依赖于CommandManager.