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}
<code_end>
<topic_end>
<topic_start>
highlight repaints
this option draws a border around all render boxes
that changes color every time that box repaints.
this rotating rainbow of colors is useful for finding parts of your app
that are repainting too often and potentially harming performance.
for example, one small animation could be causing an entire page
to repaint on every frame.
wrapping the animation in a RepaintBoundary widget limits
the repainting to just the animation.
here the progress indicator causes its container to repaint:
<code_start>
class EverythingRepaintsPage extends StatelessWidget {
const EverythingRepaintsPage({super.key});
@override
widget build(BuildContext context) {
return scaffold(
appBar: AppBar(title: const Text('Repaint example')),
body: const center(
child: CircularProgressIndicator(),
),
);
}
}
<code_end>
wrapping the progress indicator in a RepaintBoundary causes
only that section of the screen to repaint:
<code_start>
class AreaRepaintsPage extends StatelessWidget {
const AreaRepaintsPage({super.key});
@override
widget build(BuildContext context) {
return scaffold(
appBar: AppBar(title: const Text('Repaint example')),
body: const center(
child: RepaintBoundary(
child: CircularProgressIndicator(),
),
),
);
}
}
<code_end>
RepaintBoundary widgets have tradeoffs. they can help with performance,
but they also have an overhead of creating a new canvas,
which uses additional memory.
you can also enable this option in code:
<code_start>
import 'package:flutter/rendering.dart';
void highlightRepaints() {
debugRepaintRainbowEnabled = true;
}
<code_end>
<topic_end>
<topic_start>
highlight oversized images
this option highlights images that are too large by both inverting their colors
and flipping them vertically:
the highlighted images use more memory than is required;
for example, a large 5mb image displayed at 100 by 100 pixels.
such images can cause poor performance, especially on lower-end devices
and when you have many images, as in a list view,
this performance hit can add up.
information about each image is printed in the debug console:
images are deemed too large if they use at least 128kb more than required.
<topic_end>
<topic_start>
fixing images
wherever possible, the best way to fix this problem is resizing
the image asset file so it’s smaller.
if this isn’t possible, you can use the cacheHeight and cacheWidth
parameters on the image constructor:
<code_start>
class ResizedImage extends StatelessWidget {
const ResizedImage({super.key});
@override
widget build(BuildContext context) {
return image.asset(
'dash.png',
cacheHeight: 213,
cacheWidth: 392,
);
}
}
<code_end>
this makes the engine decode this image at the specified size,
and reduces memory usage (decoding and storage is still more expensive
than if the image asset itself was shrunk).
the image is rendered to the constraints of the layout or width and height
regardless of these parameters.
this property can also be set in code:
<code_start>
void showOversizedImages() {
debugInvertOversizedImages = true;
}
<code_end>
<topic_end>