Gum Tool ViewModel Conventions

Base Classes

• ViewModel

  (

  Gum/Mvvm/ViewModel.cs

  ) — base for all view models. Provides

  Get<T>()

  /

  Set()

  property storage,

  NotifyPropertyChanged

  , and

  [DependsOn]

  propagation.

• DialogViewModel

  (

  Gum/Services/Dialogs/DialogViewModel.cs

  ) — extends

  ViewModel

  for dialogs. Adds

  AffirmativeCommand

  /

  NegativeCommand

  ,

  RequestClose

  event, and

  AffirmativeText

  /

  NegativeText

  .

Property Patterns

Stored properties use

Get<T>()

/

Set()

:

public string Name
{
    get => Get<string>() ?? string.Empty;
    set => Set(value);
}

Derived properties must use

[DependsOn]

so changes to the source property automatically raise

PropertyChanged

for the derived property. Without this, the UI will not update.

[DependsOn(nameof(SourceType))]
public bool IsLocalFile => SourceType == SourceType.LocalFile;

Multiple dependencies are expressed with multiple attributes:

[DependsOn(nameof(IsPreviewLoaded))]
[DependsOn(nameof(IsLoading))]
public bool CanImport => IsPreviewLoaded && !IsLoading;

Visibility: ViewModel Properties, Not Converters

Do not use

IValueConverter

in XAML for visibility or other transformations. Instead, expose a

System.Windows.Visibility

property on the ViewModel with

[DependsOn]

:

[DependsOn(nameof(ErrorMessage))]
public Visibility ErrorMessageVisibility =>
    string.IsNullOrEmpty(ErrorMessage) ? Visibility.Collapsed : Visibility.Visible;

XAML then binds directly:

<TextBlock Visibility="{Binding ErrorMessageVisibility}" />

This keeps XAML simple and makes the logic unit-testable.

Common Pitfalls

Missing

[DependsOn]

: If a getter computes from another property but lacks the attribute, the UI will show stale values. The

ViewModel

constructor scans for

[DependsOn]

via reflection at construction time — it only works if the attribute is present.

Two-way derived properties: Properties like

IsLocalFile

that both read from and write to a backing property need

[DependsOn]

for the read direction. The write direction (setter updating

SourceType

) works normally through

Set()

.