How We Reduced Flutter Memory Usage by 375mb: Image Optimization Strategies
This article presents a technical deep dive into image optimization techniques for Flutter, focusing on practical implementations and performance metrics to enhance app responsiveness and memory management.
1. Introduction
In mobile application development, images play a crucial role in user experience, but they can also introduce significant performance challenges. Flutter, as a modern UI toolkit, provides powerful capabilities for rendering images efficiently; however, improper handling of image assets leads to slow loading times, high memory usage, and an overall degraded user experience.
We will explore strategies for caching, resizing, and optimizing images while maintaining quality. By understanding and applying these techniques, developers can ensure their applications remain performant and responsive across various devices.
This is our target workflow:
+-------------------------------------------------+
| Image Source |
| (Network/File) |
+-------------------------------------------------+
|
v
+-------------------------------------------------+
| Choose File Type |
| (e.g., WebP/PNG) |
+-------------------------------------------------+
|
v
+-------------------------------------------------+
| Set Quality |
| (Low/Medium/High) |
+-------------------------------------------------+
|
v
+-------------------------------------------------+
| Resize Image |
| (memCacheWidth/memCacheHeight) |
+-------------------------------------------------+
|
v
+-------------------------------------------------+
| Caching |
| (cached_network_image package) |
+-------------------------------------------------+
|
v
+-------------------------------------------------+
| Show Placeholder |
| (FadeShimmer) |
+-------------------------------------------------+
|
v
+-------------------------------------------------+
| Lazy Loading |
| (Visibility Detection) |
+-------------------------------------------------+
|
v
+-------------------------------------------------+
| Display Image |
+-------------------------------------------------+2. Image Performance Challenges
Image-related performance issues in Flutter applications typically manifest in three primary areas:
- Memory Consumption:
Large or numerous images can quickly consume available memory, potentially leading to app crashes on devices with limited resources. Flutter’s default image caching mechanism, while beneficial for performance, can exacerbate this issue if not properly managed. - Render Time:
High-resolution images require more processing power to decode and render, which can cause frame drops and UI jank, especially on lower-end devices or during complex animations. - Network Performance:
For network-loaded images, large file sizes increase load times and data usage, negatively impacting user experience, particularly on slower connections.
These challenges are often interrelated. For instance, a large network image will not only slow down initial load times but also consume more memory once loaded and potentially cause render delays.
The core of the problem lies in finding the optimal balance between image quality and performance. High-quality images provide a better visual experience but at the cost of increased resource usage. Conversely, overly compressed or low-resolution images may load quickly and use less memory, but can detract from the app’s visual appeal.
3. Diagnosing Image Issues
Flutter provides several tools to identify and analyze image-related performance issues:
Flutter DevTools
A suite of performance and debugging tools that includes a performance overlay for real-time UI and GPU statistics, a memory tab for tracking memory allocation (including images), and a network tab for monitoring image download times.
Widget Inspector
A tool for examining the widget tree that provides size and position information for images, along with configuration details to help locate specific widgets in the source code.
“Highlight Oversized Images” feature
A tool in the Flutter inspector that visually identifies oversized images by inverting their colors and flipping them vertically, providing console warnings with details on the actual versus display size.
- Activated in the Flutter inspector to visually identify images that are larger than their display size.
- Inverts colors and flips oversized images vertically
- Provides console warnings with details on the image’s actual size versus its display size.
4. Strategies for Image Optimization
Effective image performance in Flutter applications can be achieved through strategies that focus on caching, resizing, and optimizing images.
4.1 Image Format
Use WebP instead of PNG for images in your Flutter mobile applications. WebP provides smaller file sizes and supports both lossy and lossless compression, allowing for high-quality images with reduced loading times. Unlike PNG, which only offers lossless compression and larger files, WebP also supports transparency and animation. For converting existing images to WebP, utilize online tools and compressors such as TinyPNG or CloudConvert.
These tools can help streamline the conversion process while maintaining image quality. Ensure to check browser compatibility, as while most modern browsers support WebP, some older versions may not. Implementing a fallback to PNG for unsupported browsers can ensure a seamless user experience.
4.2 Caching
Caching images is essential for improving performance in Flutter applications. It allows images to be stored locally after the first load, enabling faster access and reducing data usage for subsequent requests.
The cached_network_image package provides a straightforward way to implement image caching in Flutter. It automatically caches images retrieved from the network, allowing for efficient reuse without repeated network calls.
Using cacheWidth and cacheHeight stores images at a size suitable for display, reducing memory usage. You can use it in your widget tree as follows:
CachedNetworkImage(
// URL of the image to be loaded
imageUrl: "https://example.com/image.jpg",
// Widget displayed while the image is loading
placeholder: (context, url) => CircularProgressIndicator(),
// Widget displayed if there is an error loading the image
errorWidget: (context, url, error) => Icon(Icons.error),
// Desired width
cacheWidth: 300,
// Desired height
cacheHeight: 200,
)Resizing images in memory reduces memory usage, preventing crashes and performance issues when dealing with large or multiple images.
Use the memCacheWidth and memCacheHeight parameters in the CachedNetworkImage widget to specify the dimensions of images stored in memory. Adjust these values based on the device’s pixel ratio to ensure images appear sharp on all screens.
CachedNetworkImage(
imageUrl: "https://example.com/image.jpg",
// Desired width and height for in-memory caching, adjusting for pixel density
memCacheWidth: (300 * MediaQuery.of(context).devicePixelRatio).round(),
memCacheHeight: (200 * MediaQuery.of(context).devicePixelRatio).round(),
)4.3 Placeholders
Use placeholders while images are loading to improve the perception of speed, maintaining visual continuity, and allowing for customizable appearances.
The fade_shimmer package (and similar) creates a pleasant shimmer effect:
Implement as a placeholder in your image widget.
CachedNetworkImage(
// target image
imageUrl: "https://example.com/image.jpg",
placeholder: (context, url) => FadeShimmer(
// Set width as needed
width: double.infinity,
// Set height as needed
height: 200,
// Corner radius for rounded edges
radius: 8,
// Customize
highlightColor: Colors.white,
baseColor: Colors.grey[300],
)
)4.4 Lazy Loading
Implement lazy loading to improve performance and reduce memory usage by loading images only when they are needed. This technique is particularly useful for applications that display many images, such as galleries or lists.
- Scroll Detection: Use Flutter’s
ListView(orGridView) widgets, which automatically handle lazy loading, as they only build items that are visible in the viewport. This ensures that images are loaded as the user scrolls.
ListView.builder(
itemCount: imageUrls.length,
itemBuilder: (context, index) {
return CachedNetworkImage(
imageUrl: imageUrls[index],
placeholder: (context, url) => CircularProgressIndicator(),
errorWidget: (context, url, error) => Icon(Icons.error),
);
},
)- Visibility Detection: The
VisibilityDetectorwidget allows you to monitor the visibility of a child widget and execute a callback when its visibility changes, optimizing performance by loading resources only when they are visible on the screen. It reports visibility changes based on its bounding box, triggering callbacks at most once per specified update interval to reduce unnecessary updates.
Note: Use VisibilityDetectorController.notifyNow() for immediate visibility checks, set updateInterval to Duration.zero during tests to avoid pending timer assertions, and be aware that it does not account for widget opacity or overlapping elements.
bool _isVisible = false; // Track visibility state
VisibilityDetector(
key: Key('context-${imageUrl}'), // Unique key for the detector
onVisibilityChanged: (visibilityInfo) {
// Update visibility state when the widget becomes visible
if (visibilityInfo.visibleFraction > 0 && !_isVisible) {
setState(() {
_isVisible = true; // Mark as visible
});
}
},
child: _isVisible
? CachedNetworkImage(
imageUrl: imageUrl, // this is the final image
placeholder: (context, url) => FadeShimmer(
width: double.infinity,
height: 200,
radius: 8,
highlightColor: Colors.white,
baseColor: Colors.grey[300],
),
errorWidget: (context, url, error) => Icon(Icons.error),
)
: FadeShimmer(
width: double.infinity,
height: 200,
radius: 8,
highlightColor: Colors.white,
baseColor: Colors.grey[300],
),
);5. Quality Levels for Image Caching
Choosing the appropriate quality level is subjective and depends on various factors, including individual user perception, display quality, and context of use. Not all users will notice differences in image quality equally; some may be sensitive to compression artifacts, while others may not.
Use online image comparison tools to visually assess differences and determine the best approach based on their audience’s needs.
Choosing Compression and Size
- Low Quality: Background images or thumbs, where detail is not critical.
- Medium Quality: Sufficient quality without major perceptive differences.
- High Quality: Sharp and clear visuals, which can significantly influence user engagement and purchasing decisions.
Conclusion
In mobile application development, effective image optimization is critical for enhancing user experience and maintaining performance.
By implementing strategies such as using appropriate image formats, caching, resizing, and lazy loading, we can significantly reduce memory usage and improve load times. Understanding the impact of different image quality levels allows for tailored solutions based on user needs and device capabilities.
Continuous optimization is essential in delivering high-quality applications that meet user expectations while managing resource constraints effectively.
The Real World
In Saropa Contacts, we successfully reduced the app’s memory usage from 1.5 MB per image to under 250 KB. This was particularly impactful given that the app displays over 300 images simultaneously. The savings in memory consumption were significant and allowed the application to run more efficiently without crashing or slowing down on devices with limited resources.
Further Discussions
- SVG handling in Flutter
- Network image loading strategies (e.g., progressive loading)
- Image caching for offline use
- Considerations for animated images (GIFs, animated WebP)
- Avoiding common pitfalls (e.g., overuse of Opacity widget)
- Responsive image loading based on device capabilities
- Accessibility in image optimization
- Automated image optimization in CI/CD pipelines
- Server-side image optimization techniques
- Comparison of image libraries and their performance impacts
- Image optimization for varied screen sizes and orientations
About Saropa
With a 30-year journey in tech, I’ve worn many hats, from coding to managing industry-leading and international projects. I’m passionate about sparking curiosity and deepening our understanding of complex topics.
If you have any suggestions or thoughts on this article, I welcome your feedback.
Learn more at saropa.com
