IsEnabled button never changes

Question

This is the declaration of my button, written in a .xaml file:

<dxlc:LayoutGroup Orientation="Horizontal"  Margin="5,15,0,5">
    <Grid MinWidth="100">
        <Grid.RowDefinitions>
            <RowDefinition Height="Auto"/>
            <RowDefinition Height="Auto"/>
        </Grid.RowDefinitions>
        <Button 
            IsEnabled="{Binding IsSearchCriteriaHasValue}" 
            Content="Search" 
            MaxHeight="25" 
            MaxWidth="70" 
            ClipToBounds="True"  
            VerticalAlignment="Center"
            HorizontalAlignment="Center" 
            HorizontalContentAlignment="Center"                  
            VerticalContentAlignment="Center" 
            Command="{Binding SearchCommand}"/>
    </Grid>
</dxlc:LayoutGroup>

This is a function that returns true / false whether the user enters any search text in the search field next to the search button. The function is in another .cs file:

public bool isButtonEnabled
{
    return (SearchBox.Selection.Count > 0);
}

The problem is that the value of isEnabled never changes, it remains true, i.e. the button remains on all the time, or if I change the> sign, the button remains off all the time. Any suggestions?

Answer 1

IsSearchCriteriaHasValueyou need to raise the event that it changed, you can do this using the interface INotifyPropertyChanged:

public class Customer : INotifyPropertyChanged
{
    // INotifyPropertyChanged members
    public event PropertyChangedEventHandler PropertyChanged;

    public void OnPropertyChanged(PropertyChangedEventArgs e)
    {
        if (PropertyChanged != null)
        {
            PropertyChanged(this, e);
        }
    }

    // Your property
    private string _Name;

    public string Name
    {
        get
        {
            return _Name;
        }
        set
        {
           _Name = value;
           OnPropertyChanged(new PropertyChangedEventArgs("Name"));
        }
    }
}   

Answer 2

You can write a trigger for a button (SearchBox looks like a user interface)

<Style TargetType="Button">
  <Setter Property="IsEnabled" Value="True" />
  <Style.Triggers>       
    <DataTrigger Binding="{Binding Selection.Count, ElementName=SearchBox}" Value="0">   !--  You can use convertor also
     <Setter Property="IsEnabled" Value="False" />
    </DataTrigger>
  </Style.Triggers>
</Style>   </Button.Style> </Button>

Answer 3

XAML, . , INotifyProperty NotifyPropertyChanged Event. ViewModel, .

SearchBox.Selection , IsSearchCriteriaHasValue, , .

, SearchBox.Selection , , /, NotifyPropertyChanged("IsSearchCriteriaHasValue"); .

Answer 4

. ViewModel INotifyPropertyChanged.

https://msdn.microsoft.com/en-us/library/system.componentmodel.inotifypropertychanged(v=vs.110).aspx

:

// This form demonstrates using a BindingSource to bind 
// a list to a DataGridView control. The list does not 
// raise change notifications. However the DemoCustomer type  
// in the list does. 
public partial class Form1 : Form
{
    // This button causes the value of a list element to be changed. 
    private Button changeItemBtn = new Button();

    // This DataGridView control displays the contents of the list. 
    private DataGridView customersDataGridView = new DataGridView();

    // This BindingSource binds the list to the DataGridView control. 
    private BindingSource customersBindingSource = new BindingSource();

    public Form1()
    {
        InitializeComponent();

        // Set up the "Change Item" button.
        this.changeItemBtn.Text = "Change Item";
        this.changeItemBtn.Dock = DockStyle.Bottom;
        this.changeItemBtn.Click +=
            new EventHandler(changeItemBtn_Click);
        this.Controls.Add(this.changeItemBtn);

        // Set up the DataGridView.
        customersDataGridView.Dock = DockStyle.Top;
        this.Controls.Add(customersDataGridView);

        this.Size = new Size(400, 200);
    }

    private void Form1_Load(object sender, EventArgs e)
    {
        // Create and populate the list of DemoCustomer objects 
        // which will supply data to the DataGridView.
        BindingList<DemoCustomer> customerList = new BindingList<DemoCustomer>();
        customerList.Add(DemoCustomer.CreateNewCustomer());
        customerList.Add(DemoCustomer.CreateNewCustomer());
        customerList.Add(DemoCustomer.CreateNewCustomer());

        // Bind the list to the BindingSource. 
        this.customersBindingSource.DataSource = customerList;

        // Attach the BindingSource to the DataGridView. 
        this.customersDataGridView.DataSource =
            this.customersBindingSource;

    }

    // Change the value of the CompanyName property for the first  
    // item in the list when the "Change Item" button is clicked.
    void changeItemBtn_Click(object sender, EventArgs e)
    {
        // Get a reference to the list from the BindingSource.
        BindingList<DemoCustomer> customerList =
            this.customersBindingSource.DataSource as BindingList<DemoCustomer>;

        // Change the value of the CompanyName property for the  
        // first item in the list.
        customerList[0].CustomerName = "Tailspin Toys";
        customerList[0].PhoneNumber = "[phone]";
    }

}

// This is a simple customer class that  
// implements the IPropertyChange interface. 
public class DemoCustomer : INotifyPropertyChanged
{
    // These fields hold the values for the public properties. 
    private Guid idValue = Guid.NewGuid();
    private string customerNameValue = String.Empty;
    private string phoneNumberValue = String.Empty;

    public event PropertyChangedEventHandler PropertyChanged;

    // This method is called by the Set accessor of each property. 
    // The CallerMemberName attribute that is applied to the optional propertyName 
    // parameter causes the property name of the caller to be substituted as an argument. 
    private void NotifyPropertyChanged([CallerMemberName] String propertyName = "")
    {
        if (PropertyChanged != null)
        {
            PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
        }
    }

    // The constructor is private to enforce the factory pattern. 
    private DemoCustomer()
    {
        customerNameValue = "Customer";
        phoneNumberValue = "[phone]";
    }

    // This is the public factory method. 
    public static DemoCustomer CreateNewCustomer()
    {
        return new DemoCustomer();
    }

    // This property represents an ID, suitable 
    // for use as a primary key in a database. 
    public Guid ID
    {
        get
        {
            return this.idValue;
        }
    }

    public string CustomerName
    {
        get
        {
            return this.customerNameValue;
        }

        set
        {
            if (value != this.customerNameValue)
            {
                this.customerNameValue = value;
                NotifyPropertyChanged();
            }
        }
    }

    public string PhoneNumber
    {
        get
        {
            return this.phoneNumberValue;
        }

        set
        {
            if (value != this.phoneNumberValue)
            {
                this.phoneNumberValue = value;
                NotifyPropertyChanged();
            }
        }
    }
}

}

Answer 5

. ViewModel INotifyPropertyChanged. . delegate command CanExecute. , http://www.codeproject.com/Questions/179129/MVVM-Enable-Disable-button-via-a-PropertyChanged-e