在应用程序运行后运行命令

Question

I noticed that it happends not only in the one project but on multiple too so I will provide simple example. I've got such xaml:

<Page
x:Class="TestApp.MainPage"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:local="using:TestApp"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
mc:Ignorable="d"
Background="{ThemeResource ApplicationPageBackgroundThemeBrush}">

<Grid>
    <Button Content="Button" Command="{Binding PressedButton}" HorizontalAlignment="Left" Margin="0,-10,0,-9" VerticalAlignment="Top" Height="659" Width="400"/>
</Grid>
</Page>

my classes to binding data:

public abstract class ObservableObject : INotifyPropertyChanged
{
    public event PropertyChangedEventHandler PropertyChanged;

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

public class Command : ICommand
{
    private Action<object> action;

    public Command(Action<object> action)
    {
        this.action = action;
    }

    public bool CanExecute(object parameter)
    {
        if (action != null)
        {
            return true;
        }
        else
        {
            return false;
        }
    }

    public event EventHandler CanExecuteChanged;

    public void Execute(object parameter)
    {
        if (action != null)
        {
            action((string)parameter);
        }
    }
}

public class TestViewModel : ObservableObject
{
    public ICommand PressedButton
    {
        get
        {
            return new Command((param) => { });
        }
    }
}

and main page:

    public MainPage()
    {
        this.InitializeComponent();

        this.NavigationCacheMode = NavigationCacheMode.Required;
        DataContext = new TestViewModel();
    }

It's weird but PressedButton runs only on application start(isn't that weird it runs on start?). After that, even after button click there is nothing triggered. I can't figure out what's wrong.

Answer 1

I think you may be causing binding issues by returning a new command each time the "getter" is called. Try setting the command once, in your constructor (for example).

public MainPage()
{
    PressedAdd = new Command(param => SaveNote());
}

public ICommand PressedAdd { get; private set; }

In the SaveNote() method, you could test the values and either save (or not save) them:

private void SaveNote()
{
    if (NoteTitle == null || NoteContent == null)
        return;

    // Do something with NoteTitle and NoteContent
}