Merge pull request #464 from AvaloniaUI/watermark-support

Add watermark support

src/AvaloniaEdit.Demo/MainWindow.xaml

@@ -51,7 +51,8 @@
                                HorizontalScrollBarVisibility="Auto"
                                VerticalScrollBarVisibility="Visible"
                                FontWeight="Light"
-                               FontSize="14" >
+                               FontSize="14"
+                               Watermark="Start typing to bring your ideas to life...">
         <AvalonEdit:TextEditor.ContextFlyout>
             <MenuFlyout>
                 <MenuItem Header="Copy" InputGesture="ctrl+C" Command="{Binding CopyMouseCommmand}" CommandParameter="{Binding #Editor.TextArea}"></MenuItem>

src/AvaloniaEdit/Editing/TextArea.cs

@@ -142,6 +142,24 @@ namespace AvaloniaEdit.Editing
 
         #endregion
 
+        #region Watermark
+        /// <summary>
+        /// Defines the <see cref="Watermark"/> property
+        /// </summary>
+        public static readonly StyledProperty<string> WatermarkProperty =
+            AvaloniaProperty.Register<TextArea, string>(nameof(Watermark));
+
+        /// <summary>
+        /// Gets or sets the placeholder or descriptive text that is displayed even if the <see cref="Text"/>
+        /// property is not yet set.
+        /// </summary>
+        public string Watermark
+        {
+            get => GetValue(WatermarkProperty);
+            set => SetValue(WatermarkProperty, value);
+        }
+        #endregion
+
         /// <summary>
         ///     Defines the <see cref="IScrollable.Offset" /> property.
         /// </summary>

src/AvaloniaEdit/Editing/TextArea.xaml

@@ -20,8 +20,20 @@
               </ItemsPanelTemplate>
             </ItemsControl.ItemsPanel>
           </ItemsControl>
-          <ContentPresenter Name="PART_CP" Cursor="IBeam"
-                            Focusable="False" Background="{TemplateBinding Background}" />
+          <Panel>
+            <ContentPresenter Name="PART_CP"
+                              Cursor="IBeam"
+                              Focusable="False"
+                              Background="{TemplateBinding Background}" />
+            <TextBlock Name="PART_Watermark"
+                       Opacity="0.5"
+                       Text="{TemplateBinding Watermark}"
+                       Foreground="{TemplateBinding Foreground}">
+              <TextBlock.IsVisible>
+                <Binding RelativeSource="{RelativeSource TemplatedParent}" Path="Document.Text" Converter="{x:Static StringConverters.IsNullOrEmpty}"/>
+              </TextBlock.IsVisible>
+            </TextBlock>
+          </Panel>
         </DockPanel>
       </ControlTemplate>
     </Setter>

src/AvaloniaEdit/TextEditor.cs

@@ -241,6 +241,16 @@ namespace AvaloniaEdit
         #endregion
 
         #region Text property
+        /// <summary>
+        /// Gets or sets the placeholder or descriptive text that is displayed even if the <see cref="Text"/>
+        /// property is not yet set.
+        /// </summary>
+        public string Watermark
+        {
+            get => textArea.Watermark;
+            set => textArea.Watermark = value;
+        }
+
         /// <summary>
         /// Gets/Sets the text of the current document.
         /// </summary>