Force Redraw of Xamarin.Forms View with Custom Renderer
Introduction
In Xamarin.Forms, custom renderers allow developers to control the native appearance of UI elements. Sometimes, after modifying properties of a custom renderer, the view doesn’t immediately update on the screen. This happens because the native platform might not be aware of the changes. To force a redraw, several techniques can be employed.
Techniques for Forcing Redraw
1. InvalidateMeasure() and InvalidateArrange()
These methods are part of the Xamarin.Forms view hierarchy and are typically used to signal that the layout of the view needs to be recalculated.
* `InvalidateMeasure()`: Indicates that the view’s dimensions need to be recalculated.
* `InvalidateArrange()`: Indicates that the view’s arrangement within its parent needs to be recalculated.
“`C#
public class MyCustomRenderer : ViewRenderer
{
protected override void OnElementChanged(ElementChangedEventArgs
{
base.OnElementChanged(e);
// Update properties of the native view
// …
// Force redraw
Control.InvalidateMeasure();
Control.InvalidateArrange();
}
}
“`
2. SetNeedsDisplay()
This method, specific to native iOS and Android views, forces a redraw of the view’s content.
“`C#
// iOS
Control.SetNeedsDisplay();
// Android
Control.Invalidate();
“`
3. UpdateLayout()
On Android, the `UpdateLayout()` method can be used to request a layout pass. It forces the view to remeasure, relayout, and redraw itself.
“`C#
Control.UpdateLayout();
“`
4. ForceLayout()
In Xamarin.Forms, the `ForceLayout()` method can be used to trigger a layout pass on the view. This forces the view to recalculate its size and position.
“`C#
// In the custom view
this.ForceLayout();
“`
Example: Custom Button Renderer
Let’s consider an example of a custom button renderer where we want to change the background color dynamically.
Custom View: MyCustomButton
“`C#
public class MyCustomButton : Button
{
public Color ButtonBackgroundColor { get; set; }
}
“`
Custom Renderer: MyCustomButtonRenderer
“`C#
public class MyCustomButtonRenderer : ButtonRenderer
{
protected override void OnElementChanged(ElementChangedEventArgs
if (e.NewElement != null)
{
Control.BackgroundColor = ((MyCustomButton)e.NewElement).ButtonBackgroundColor.ToUIColor();
Control.SetNeedsDisplay(); // Force redraw
}
}
}
“`
In this example, we set the `ButtonBackgroundColor` property of the `MyCustomButton` view. In the custom renderer, we update the `BackgroundColor` of the native button and call `SetNeedsDisplay()` to force a redraw.
Comparison of Techniques
| Technique | Platform | Description |
|—|—|—|
| `InvalidateMeasure()` and `InvalidateArrange()` | Xamarin.Forms | Forces layout recalculation |
| `SetNeedsDisplay()` | iOS, Android | Forces content redraw |
| `UpdateLayout()` | Android | Requests a layout pass |
| `ForceLayout()` | Xamarin.Forms | Triggers a layout pass |
Considerations
* Use the appropriate technique based on the platform and the specific requirement.
* Avoid excessive calls to redraw methods, as they can impact performance.
* Consider using binding mechanisms to update view properties automatically, reducing the need for manual redraw triggers.
Conclusion
Forcing redraws in custom Xamarin.Forms renderers is essential for ensuring that view updates are reflected on the screen. By utilizing the appropriate techniques, developers can effectively control the appearance and behavior of custom UI elements.