How to pass data from view to viewmodel

Question

I am new to mvvm, I want to pass textbox.text as input to service method, that method should be called in button click. For this I have wrote the below code, but I am unable to retrieve the textbox value. Commands.cs:

 private Action<object> _action;
    public Commands(Action<object> action)
    {
        _action = action;
    }

    public event EventHandler CanExecuteChanged;

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



public void Execute(object parameter)
{
    if (parameter != null)
    {
        _action(parameter);

    }
    else
    {
        _action("Hello World");
    }

}

CommentsandRatingViewModel:

   private ICommand _submitButtonCommand;
        CommentesAndRating model;
        private string Comments;
        public  string _Comments
        {
            get
            {
               // return model._Comments;
                return Comments;
            }
            set
            {
                //model._Comments = value;
                Comments = value;
            }
        }

        public string Rating;
        #region Methods

        //place this command name in the any of view control's commnad
        //like  under submitbutton click of Submit Button in Comment.xaml
        public ICommand SubmitButtonCommand
        {
            get
            {

                return _submitButtonCommand;
            }
            set
            {
                _submitButtonCommand = value;
            }
        }

        public CommentsandRatingViewModel()
        {
           // this.model = new CommentesAndRating();
            SubmitButtonCommand = new Commands(new Action<object>(SubmitCommentsAndRatings));
        }

        public void SubmitCommentsAndRatings(object obj)
        {
            var commnets = _Comments;
            MessageBox.Show("hello");
            //have to call service method here.
        }

     xaml:

     <Grid x:Name="LayoutRoot" Background="{StaticResource PhoneChromeBrush}">
    <TextBox HorizontalAlignment="Left" Name="txt" Text="{Binding Source= {StaticResource viewmodel},Mode=TwoWay,Path=_Comments}"  Height="72" Margin="14,96,0,0" TextWrapping="Wrap" VerticalAlignment="Top" Width="456"/>
    <Button Command="{Binding SubmitButtonCommand}"  Content="Button" HorizontalAlignment="Left" Margin="119,249,0,0" VerticalAlignment="Top"/>
</Grid>

Answer 1

You need to implement the INotifyPropertyChanged interface to make your binding work. After that you can use the property _Comment to get the value needed for your service method.

As a side-note: Do not put an underscore in front of your property name. This will confuse a lot of people reading your code. Better is to use the case sensitivity in C#:

    private string comments;
    public  string Comments
    {
       ...
    }

But if you really feel the need to use an underscore then use it for your backing field, not your property.

Answer 2

在TextBox.Text属性和Comments属性之间进行TwoWay数据绑定，以便允许UI将值下推到ViewModel。

<TextBox Text="{Binding Comments, Mode=TwoWay}"/>