Using the Flutter inspector

What is it?

The Flutter widget inspector is a powerful tool for visualizing and exploring Flutter widget trees. The Flutter framework uses widgets as the core building block for anything from controls (such as text, buttons, and toggles), to layout (such as centering, padding, rows, and columns). The inspector helps you visualize and explore Flutter widget trees, and can be used for the following:

• understanding existing layouts
• diagnosing layout issues

Get started

To debug a layout issue, run the app in debug mode and open the inspector by clicking the Flutter Inspector tab on the DevTools toolbar.

Debugging layout issues visually

The following is a guide to the features available in the inspector’s toolbar. When space is limited, the icon is used as the visual version of the label.

Select widget mode

Enable this button in order to select a widget on the device to inspect it. For more information, see Inspecting a widget.

Refresh tree

Reload the current widget info.

Slow animations

Run animations 5 times slower to help fine-tune them.

Show guidelines

Overlay guidelines to assist with fixing layout issues.

Show baselines

Show baselines, which are used for aligning text. Can be useful for checking if text is aligned.

Highlight repaints

Show borders that change color when elements repaint. Useful for finding unnecessary repaints.

Highlight oversized images

Highlights images that are using too much memory by inverting colors and flipping them.

Inspecting a widget

You can browse the interactive widget tree to view nearby widgets and see their field values.

To locate individual UI elements in the widget tree, click the Select Widget Mode button in the toolbar. This puts the app on the device into a “widget select” mode. Click any widget in the app’s UI; this selects the widget on the app’s screen, and scrolls the widget tree to the corresponding node. Toggle the Select Widget Mode button again to exit widget select mode.

When debugging layout issues, the key fields to look at are the size and constraints fields. The constraints flow down the tree, and the sizes flow back up. For more information on how this works, see Understanding constraints.

Flutter Layout Explorer

The Flutter Layout Explorer helps you to better understand Flutter layouts.

For an overview of what you can do with this tool, see the Flutter Explorer video:

You might also find the following step-by-step article useful:

• How to debug layout issues with the Flutter Inspector

Using the Layout Explorer

From the Flutter Inspector, select a widget. The Layout Explorer supports both flex layouts and fixed size layouts, and has specific tooling for both kinds.

Flex layouts

When you select a flex widget (for example, Row, Column, Flex) or a direct child of a flex widget, the flex layout tool will appear in the Layout Explorer.

The Layout Explorer visualizes how Flex widgets and their children are laid out. The explorer identifies the main axis and cross axis, as well as the current alignment for each (for example, start, end, and spaceBetween). It also shows details like flex factor, flex fit, and layout constraints.

Additionally, the explorer shows layout constraint violations and render overflow errors. Violated layout constraints are colored red, and overflow errors are presented in the standard “yellow-tape” pattern, as you might see on a running device. These visualizations aim to improve understanding of why overflow errors occur as well as how to fix them.

Clicking a widget in the layout explorer mirrors the selection on the on-device inspector. Select Widget Mode needs to be enabled for this. To enable it, click on the Select Widget Mode button in the inspector.

For some properties, like flex factor, flex fit, and alignment, you can modify the value via dropdown lists in the explorer. When modifying a widget property, you see the new value reflected not only in the Layout Explorer, but also on the device running your Flutter app. The explorer animates on property changes so that the effect of the change is clear. Widget property changes made from the layout explorer don’t modify your source code and are reverted on hot reload.

Interactive Properties

Layout Explorer supports modifying mainAxisAlignment, crossAxisAlignment, and FlexParentData.flex. In the future, we may add support for additional properties such as mainAxisSize, textDirection, and FlexParentData.fit.

mainAxisAlignment

Supported values:

• MainAxisAlignment.start
• MainAxisAlignment.end
• MainAxisAlignment.center
• MainAxisAlignment.spaceBetween
• MainAxisAlignment.spaceAround
• MainAxisAlignment.spaceEvenly

crossAxisAlignment

Supported values:

• CrossAxisAlignment.start
• CrossAxisAlignment.center
• CrossAxisAlignment.end
• CrossAxisAlignment.stretch

FlexParentData.flex

Layout Explorer supports 7 flex options in the UI (null, 0, 1, 2, 3, 4, 5), but technically the flex factor of a flex widget’s child can be any int.

Flexible.fit

Layout Explorer supports the two different types of FlexFit: loose and tight.

Fixed size layouts

When you select a fixed size widget that is not a child of a flex widget, fixed size layout information will appear in the Layout Explorer. You can see size, constraint, and padding information for both the selected widget and its nearest upstream RenderObject.

Visual debugging

The Flutter Inspector provides several options for visually debugging your app. These are the options available from the inspector within Flutter DevTools.

Slow animations

When enabled, this option runs animations 5 times slower for easier visual inspection. This can be useful if you want to carefully observe and tweak an animation that doesn’t look quite right.

This can also be set in code:

import 'package:flutter/scheduler.dart';

void setSlowAnimations() {
  timeDilation = 5.0;
}

This slows the animations by 5x.

See also

The following links provide more info.

• Flutter documentation: timeDilation property

The following screen recordings show before and after slowing an animation.

Show guidelines

This feature draws guidelines over your app that display render boxes, alignments, paddings, scroll views, clippings and spacers.

This tool can be used for better understanding your layout. For instance, by finding unwanted padding or understanding widget alignment.

You can also enable this in code:

import 'package:flutter/rendering.dart';

void showLayoutGuidelines() {
  debugPaintSizeEnabled = true;
}

Render boxes

Widgets that draw to the screen create a render box, the building blocks of Flutter layouts. They’re shown with a bright blue border:

Alignments

Alignments are shown with yellow arrows. These arrows show the vertical and horizontal offsets of a widget relative to its parent. For example, this button’s icon is shown as being centered by the four arrows:

Padding

Padding is shown with a semi-transparent blue background:

Scroll views

Widgets with scrolling contents (such as list views) are shown with green arrows:

Clipping

Clipping, for example when using the ClipRect widget, are shown with a dashed pink line with a scissors icon:

Spacers

Spacer widgets are shown with a grey background, such as this SizedBox without a child:

Show baselines

This option makes all baselines visible. Baselines are horizontal lines used to position text.

This can be useful for checking whether text is precisely aligned vertically. For example, the text baselines in the following screenshot are slightly misaligned:

The Baseline widget can be used to adjust baselines.

A line is drawn on any render box that has a baseline set; alphabetic baselines are shown as green and ideographic as yellow.

You can also enable this in code:

import 'package:flutter/rendering.dart';

void showBaselines() {
  debugPaintBaselinesEnabled = true;
}

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:

class EverythingRepaintsPage extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: Text('Repaint Example')),
      body: Center(
        child: CircularProgressIndicator(),
      ),
    );
  }
}

Wrapping the progress indicator in a RepaintBoundary causes only that section of the screen to repaint:

class AreaRepaintsPage extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: Text('Repaint Example')),
      body: Center(
        child: RepaintBoundary(
          child: CircularProgressIndicator(),
        ),
      ),
    );
  }
}

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:

import 'package:flutter/rendering.dart';

void highlightRepaints() {
  debugRepaintRainbowEnabled = true;
}

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:

dash.png has a display size of 213×392 but a decode size of 2130×392, which uses an additional 2542KB.

Images are deemed too large if they use at least 128KB more than required.

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:

class ResizedImage extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Image.asset(
      'dash.png',
      cacheHeight: 213,
      cacheWidth: 392,
    );
  }
}

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:

import 'package:flutter/painting.dart';

void showOversizedImages() {
  debugInvertOversizedImages = true;
}

More information

You can learn more at the following link:

• Flutter documentation: debugInvertOversizedImages

Details Tree

Select the Details Tree tab to display the details tree for the selected widget.

From the details tree, you can gather useful information about a widget’s properties, render object, and children.

Track widget creation

Part of the functionality of the Flutter inspector is based on instrumenting the application code in order to better understand the source locations where widgets are created. The source instrumentation allows the Flutter inspector to present the widget tree in a manner similar to how the UI was defined in your source code. Without it, the tree of nodes in the widget tree are much deeper, and it can be more difficult to understand how the runtime widget hierarchy corresponds to your application’s UI.

You can disable this feature by passing --no-track-widget-creation to the flutter run command.

Here are examples of what your widget tree might look like with and without track widget creation enabled.

Track widget creation enabled (default):

Track widget creation disabled (not recommended):

This feature prevents otherwise-identical const Widgets from being considered equal in debug builds. For more details, see the discussion on common problems when debugging.

Other resources

For a demonstration of what’s generally possible with the inspector, see the DartConf 2018 talk demonstrating the IntelliJ version of the Flutter inspector.