How to Use Flutter's BackdropFilter for Stunning UI Backgrounds

Flutter, an open-source UI software development kit created by Google, has revolutionized how developers build cross-platform applications. One powerful tool at your disposal is the BackdropFilter widget, which allows for adding dynamic visual effects to the existing painted content of your app elements by filtering the background.

When working with Flutter to design an application, understanding how to manipulate various widgets is crucial for crafting a visually appealing and functional UI. In decorative and visual effects, the BackdropFilter class holds a unique position.

What is the BackdropFilter Widget?

The BackdropFilter widget does something remarkable; it applies a filter, such as a blur effect, to the existing painted content beneath it. This effect is only used to the area within the ancestor widget's clip, which means it is contained and does not spill out to affect any other part of your app screen, ensuring that you have pinpoint control over how your effects influence the UI.

How to Use BackdropFilter?

To use the BackdropFilter, you must ensure it’s wrapped with a widget that provides an ancestor widget's clip, such as ClipRect, ClipRRect, ClipOval, etc. The clipping behavior is essential because the filter effect could apply to the entire screen without it. The BackdropFilter widget requires an image filter - an object that, for example, can specify a blur effect with a particular radius.

Before we proceed, it’s important to understand that the BackdropFilter class behaves like a regular widget. This means the widget replaces one widget with another within the Flutter framework’s tree structure. And like any other widget, it leverages a BuildContext context object to know where it stands within the tree.

Creating a Blur Effect

Creating a blur effect using the BackdropFilter widget is one of the most common use cases. You can use an ImageFilter.blur to achieve this. Now, let's see an example where we want to blur a specific part of our Flutter application’s background:

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@override
Widget build(BuildContext context) {
 return Stack(
   children: <Widget>[
     Image.network(
       'https://example.com/assets/images/background.jpg', // Replace with an actual image URL
       width: MediaQuery.of(context).size.width,
       height: MediaQuery.of(context).size.height,
       fit: BoxFit.cover,
     ),
     Center(
       child: ClipRect( // Clip widget to contain the blur to one widget
         child: BackdropFilter(
           filter: ImageFilter.blur(sigmaX: 5, sigmaY: 5), // The filter
           child: Container( // Child Container
             alignment: Alignment.center,
             width: 200.0, // Width
             height: 200.0,
             child: Text(
               'Hello World',
               style: TextStyle(
                 fontSize: 24,
                 color: Colors.black,
               ),
             ),
           ),
         ),
       ),
     ),
   ],
 );
} 

In the above snippet, the BackdropFilter widget blurs an image in the background with a sigmaX and sigmaY of 5.0, providing a medium-strength blur. The blurred effect only applies to the area within the ClipRect widget, the one widget that uses the filter.

Coordinating with Other Widgets

In Flutter, every widget works in pairs with others to construct the interface. The BackdropFilter is often used with other widgets, such as the Container widget or a Stack.

Aligning with a Child Container

To properly demonstrate the capabilities of the BackdropFilter widget, we might house a child container within it, using the child attribute. The child of BackdropFilter can be any Widget, but commonly one would use a semi-transparent container with either text, such as a const Text object, or minimal widgets.

Here's how you can nest a child container within a backdropfilter widget:

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BackdropFilter(
 filter: ImageFilter.blur(sigmaX: 10, sigmaY: 10),
 child: Container(
   color: Colors.black.withOpacity(0.1), // Semi-transparent Container
   child: Center(
     child: Text(
       'Blurry Hello World', // Const Text
       style: TextStyle(color: Colors.white),
     ),
   )
 ),
) 

The Container in this scenario affects how the blur is perceived as it carries its color, even if it’s semi-opaque. The Text stands out on the blurred background.

Embedding within a Full Screen or Specific Area

You have the flexibility to apply the BackdropFilter either full screen or confine it to specific areas:

Applying Full Screen

For a full-screen blur that affects the entire background while respecting the ancestor widget's clip, the BackdropFilter can be wrapped around a widget that fills the screen. As previously mentioned, one widget that fills the entire space will amplify the impressive, immersive feel of the filter over the UI.

To create a full-screen blur, it is commonplace to have the BackdropFilter as a sibling to a full-screen element such as an Image in a Stack. Here’s an illustration of this concept:

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Stack(
 fit: StackFit.expand, // Ensures the stack fills the full screen
 children: <Widget>[
   Image.asset('assets/images/full_screen_image.jpg', fit: BoxFit.cover), // Background image
   BackdropFilter(
     filter: ImageFilter.blur(sigmaX: 3, sigmaY: 3), // Blur filter
     child: Container(
       color: Colors.black.withOpacity(0), // Non-local child container with a transparent overlay
     ),
   ),
 ],
) 

In this example, the Stack widget replaces one widget with another, allowing the BackdropFilter to layer over the entire Image. The color property of the child Container is set to transparent (opacity 0), so the blur effect becomes the highlight without any additional color overlay interrupting it.

Confined Effect Area

Alternatively, if the goal is to blur a specific part of the screen (not full screen), you may place the BackdropFilter widget within a constrained container or wrap it within a clip widget. This technique is beneficial when you want to draw attention to a particular widget or create a focal point surrounded by a blurred background.

For instance, applying the effect to a dialog-style widget might look like this:

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Center(
 child: ClipRRect(
   borderRadius: BorderRadius.circular(10.0), // Ancestor widget's clip with rounded corners
   child: BackdropFilter(
     filter: ImageFilter.blur(sigmaX: 10, sigmaY: 10),
     child: Container(
       width: MediaQuery.of(context).size.width / 2, // Width of half the screen width
       height: MediaQuery.of(context).size.height / 3, // Height
       decoration: BoxDecoration(
         color: Colors.white.withOpacity(0.7)
       ),
       child: Center(
         child: Text('Focused Dialog'), // Text within the focused area
       ),
     ),
   ),
 ),
) 

Here, the BackdropFilter is contained within a ClipRRect to achieve rounded corners, thus limiting the extent of the blur effect to the specific area defined by the Container width and height. This creates a floating dialog box feel on the screen.

Balancing Performance and Effectiveness

The BackdropFilter widget impacts performance because it applies potentially complex image processing algorithms in real time. This is an important consideration, especially when targeting low-end devices. Therefore, using this widget sparingly and only when the visual effect significantly enhances the user experience is advisable.

How the filter attribute is being leveraged has a direct correlation with performance. For example, a high blur radius could be more resource-intensive than a lower one. Additionally, consideration should be given to the size of the backdropfiltered area—the smaller the area, the less impact it will have on performance.

Conclusion

The BackdropFilter widget is a powerful class in the Flutter framework that can significantly elevate the visual aesthetics of your application. By understanding how to use the BackdropFilter in conjunction with other widgets such as Container, ClipRect, and leveraging properties like ImageFilter.blur, you can create stunning UIs that backdrop filter specific areas or even full screen. Whether aiming to highlight one widget with a crisp blur or craft captivating full-screen backdrops, Flutter’s BackdropFilter offers you a versatile tool to bring your creative visions to life.