如何从命令对象中访问整个ViewModel

时间:2013-11-20 16:16:03

标签: c# android xamarin model-binding mvvmcross

基于@ stuart的blog post,我可以看到如何将文字字符串绑定为CommandParameter。

local:MvxBind="Click FindUserCommand, CommandParameter=foo"
public class FindUserViewModel: ViewModelBase
{
    public FindUserViewModel(IFindUserCommand findUserCommand)
    {
        _findUserCommand = findUserCommand;
    }

    // class truncated for brevity

    private IFindUserCommand _findUserCommand;
    public IFindUserCommand FindUserCommand
    {
        get { return _findUserCommand; }
        set { _findUserCommand = value; RaisePropertyChanged(() = > FindUserCommand); }
    }
}
public class FindUserCommand: IFindUserCommand
{
    // class truncated for brevity
    public void Execute(object parameter)
    {
        // RIGHT HERE:
        // value of parameter == foo
    }
}

当命令存在于自己的命令对象而不是ViewModel中时,如何将整个ViewModel绑定为Button的CommandParame?

public void Execute(object parameter)
{
    var vm = (FindUserViewModel)parameter;
    var firstName = vm.FirstName;
    var lastName = vm.LastName;
    // etc...
}

注意:如果我必须以某种方式扩展MvvmCross以类似于WPF的统一方式制作此功能,我就可以了......我只是不知道从哪里开始。


我找到的所有示例都直接在ViewModel中声明了Command,如下所示:

MvvmCross示例

private Cirrious.MvvmCross.ViewModels.MvxCommand _insertCommand;
public System.Windows.Input.ICommand InsertCommand
{
    get
    {
        _insertCommand = _insertCommand ? ? new Cirrious.MvvmCross.ViewModels.MvxCommand(DoInsert);
        return _insertCommand;
    }
}

private int _countAdded = 0;

private void DoInsert()
{
    _countAdded++;
    var kitten = _kittenGenesisService.CreateNewKitten(_countAdded.ToString());
    _dataService.Insert(kitten);
    RefreshDataCount();
}

然后将按钮绑定到View,如下所示:

<Button
    android:text="Insert"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:id="@+id/button1"
    local:MvxBind="Click InsertCommand" />

但我想在自己的类文件中声明Command对象,以便它更可测试,然后将其注入ViewModel。

在MVVMCross for Android中是否可以绑定CommandParameter,类似于WPF如何进行命令参数绑定?


WPF示例:

查看按钮定义:

<Button 
    Command="{Binding SearchCommand}"
    CommandParameter="{Binding}"/>

ViewModel命令属性:

private ICommand _findUserCommand;
public ICommand FindUserCommand
{
    get { return _findUserCommand; }
    set { _findUserCommand = value; RaisePropertyChanged( () => FindUserCommand ); }
}

命令对象定义:

public class FindUserCommand : ICommand
{
    private readonly IUserService _userService;

    public FindUserCommand(IUserService userService)
    {
        _userService = userService;
    }

    public bool CanExecute ( object parameter )
    {
        var findUserViewModel = ( FindUserViewModel )parameter;
        return findUserViewModel.NameSearch.Length > 0;
    }

    public void Execute ( object parameter )
    {
        FindUserViewModel.Users = _userService.Find( FindUserViewModel.NameSearch );
    }

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

1 个答案:

答案 0 :(得分:2)

Android小部件(如按钮)默认情况下没有Command和CommandParameter成员 - 因此无法使用它们&#34;开箱即用&#34;。

但是,您可以轻松地从Button继承以生成MyButton,然后您可以根据需要提供CommandCommandParameter成员,例如类似的东西:

public class MyButton : Button
{
    public MyButton(Context c, IAttributeSet a) : base(c,a) 
    {
        Click += (s,e) => {
            if (Command == null) return;
            if (!Command.CanExecute(CommandParameter)) return;
            Command.Execute(CommandParameter);
        };
    }

    public ICommand Command { get;set; }
    public object CommandParameter { get;set; }
}

使用此功能,您可以在axml中使用MyButton代替Button

 <MyButton
     local:MvxBind="Command SearchCommand;CommandParameter ." />

有关数据绑定的更多信息,请参阅https://github.com/MvvmCross/MvvmCross/wiki/Databinding


或者,另一种方法是使用基于ValueConverter的方法 - 类似于MvvmCross提供的CommandParameter功能 - 请参阅http://slodge.blogspot.co.uk/2013/06/commandparameter-binding.html