Adds IsLoading bindable property to pages and fixes SelectViewModelCommand (#45)

In Maui, the `IsBusy` property on pages now shows a loading indicator
with no way to override that behavior. Since implementing apps may want
their own loading indicator, this creates a new `IsLoading` property
which the implementing app can choose to forward to `IsBusy` if it
wants.

Also fixes the SelectViewModelCommand so it passes the model instead of
the view model to the `CanExecute` callback.

Author: mediabounds

Float.Core/Commands/SelectViewModelCommand.cs

@@ -52,7 +52,7 @@ public event EventHandler CanExecuteChanged
         /// <inheritdoc />
         public bool CanExecute(object parameter)
         {
-            return parameter is ViewModel<T> && command.CanExecute(parameter);
+            return parameter is ViewModel<T> vm && command.CanExecute(vm.UnderlyingModel);
         }
 
         /// <summary>

Float.Core/UI/BaseContentPage.cs

@@ -21,6 +21,21 @@ public abstract class BaseContentPage : ContentPage
         /// </summary>
         public static readonly BindableProperty ErrorProperty = BindableProperty.Create(nameof(Error), typeof(Exception), typeof(ContentPage), null);
 
+        /// <summary>
+        /// Binding for the IsLoading property.
+        /// </summary>
+        public static readonly BindableProperty IsLoadingProperty = BindableProperty.Create(nameof(IsLoading), typeof(bool), typeof(BaseContentPage));
+
+        /// <summary>
+        /// Gets or sets a value indicating whether the page should show a loading indicator.
+        /// </summary>
+        /// <value>Whether the page should show the loading indicator.</value>
+        public bool IsLoading
+        {
+            get => (bool)GetValue(IsLoadingProperty);
+            set => SetValue(IsLoadingProperty, value);
+        }
+
         /// <summary>
         /// Gets or sets the current error on the page.
         /// </summary>
@@ -52,7 +67,7 @@ protected override void OnBindingContextChanged()
             if (BindingContext is ViewModels.IPageViewModel viewModel)
             {
                 this.SetBinding(TitleProperty, nameof(viewModel.Title));
-                this.SetBinding(IsBusyProperty, nameof(viewModel.IsLoading));
+                this.SetBinding(IsLoadingProperty, nameof(viewModel.IsLoading));
                 this.SetBinding(ErrorProperty, nameof(viewModel.Error), BindingMode.TwoWay);
             }
         }
@@ -73,8 +88,8 @@ protected override void OnPropertyChanged(string propertyName = null)
             base.OnPropertyChanged(propertyName);
             switch (propertyName)
             {
-                case nameof(IsBusy):
-                    if (IsBusy)
+                case nameof(IsLoading):
+                    if (IsLoading)
                     {
                         OnStartLoading();
                     }