Adding assets and images

Flutter apps can include both code and assets (sometimes called resources). An asset is a file that is bundled and deployed with your app, and is accessible at runtime. Common types of assets include static data (for example, JSON files), configuration files, icons, and images (JPEG, WebP, GIF, animated WebP/GIF, PNG, BMP, and WBMP).

Flutter uses the pubspec.yaml file, located at the root of your project, to identify assets required by an app.

Here is an example:

flutter:
  assets:
    - assets/my_icon.png
    - assets/background.png

To include all assets under a directory, specify the directory name with the / character at the end:

flutter:
  assets:
    - directory/
    - directory/subdirectory/

The assets subsection of the flutter section specifies files that should be included with the app. Each asset is identified by an explicit path (relative to the pubspec.yaml file) where the asset file is located. The order in which the assets are declared doesn't matter. The actual directory name used (assets in first example or directory in the above example) doesn't matter.

During a build, Flutter places assets into a special archive called the asset bundle that apps read from at runtime.

Flutter supports using a Dart package to transform asset files when building your app. To do this, specify the asset files and transformer package in your pubspec file. To learn how to do this and write your own asset-transforming packages, see Transforming assets at build time.

If your project utilizes the flavors feature, you can configure individual assets to be bundled only in certain flavors of your app. For more information, check out Conditionally bundling assets based on flavor.

Your app can access its assets through an AssetBundle object.

The two main methods on an asset bundle allow you to load a string/text asset (loadString()) or an image/binary asset (load()) out of the bundle, given a logical key. The logical key maps to the path to the asset specified in the pubspec.yaml file at build time.

Each Flutter app has a rootBundle object for easy access to the main asset bundle. It is possible to load assets directly using the rootBundle global static from package:flutter/services.dart.

However, it's recommended to obtain the AssetBundle for the current BuildContext using DefaultAssetBundle, rather than the default asset bundle that was built with the app; this approach enables a parent widget to substitute a different AssetBundle at run time, which can be useful for localization or testing scenarios.

Typically, you'll use DefaultAssetBundle.of() to indirectly load an asset, for example a JSON file, from the app's runtime rootBundle.

Outside of a Widget context, or when a handle to an AssetBundle is not available, you can use rootBundle to directly load such assets. For example:

import 'package:flutter/services.dart' show rootBundle;

Future<String> loadAsset() async {
  return await rootBundle.loadString('assets/config.json');
}

To load an image, use the AssetImage class in a widget's build() method.

For example, your app can load the background image from the asset declarations in the previous example:

return const Image(image: AssetImage('assets/background.png'));

Flutter can load resolution-appropriate images for the current device pixel ratio.

AssetImage will map a logical requested asset onto one that most closely matches the current device pixel ratio.

For this mapping to work, assets should be arranged according to a particular directory structure:

.../image.png
.../Mx/image.png
.../Nx/image.png
...etc.

Where M and N are numeric identifiers that correspond to the nominal resolution of the images contained within. In other words, they specify the device pixel ratio that the images are intended for.

In this example, image.png is considered the main asset, while Mx/image.png and Nx/image.png are considered to be variants.

The main asset is assumed to correspond to a resolution of 1.0. For example, consider the following asset layout for an image named my_icon.png:

.../my_icon.png       (mdpi baseline)
.../1.5x/my_icon.png  (hdpi)
.../2.0x/my_icon.png  (xhdpi)
.../3.0x/my_icon.png  (xxhdpi)
.../4.0x/my_icon.png  (xxxhdpi)

On devices with a device pixel ratio of 1.8, the asset .../2.0x/my_icon.png is chosen. For a device pixel ratio of 2.7, the asset .../3.0x/my_icon.png is chosen.

If the width and height of the rendered image are not specified on the Image widget, the nominal resolution is used to scale the asset so that it occupies the same amount of screen space as the main asset would have, just with a higher resolution. That is, if .../my_icon.png is 72px by 72px, then .../3.0x/my_icon.png should be 216px by 216px; but they both render into 72px by 72px (in logical pixels), if width and height are not specified.

You only need to specify the main asset or its parent directory in the assets section of pubspec.yaml. Flutter bundles the variants for you. Each entry should correspond to a real file, with the exception of the main asset entry. If the main asset entry doesn't correspond to a real file, then the asset with the lowest resolution is used as the fallback for devices with device pixel ratios below that resolution. The entry should still be included in the pubspec.yaml manifest, however.

Anything using the default asset bundle inherits resolution awareness when loading images. (If you work with some of the lower level classes, like ImageStream or ImageCache, you'll also notice parameters related to scale.)

To load an image from a package dependency, the package argument must be provided to AssetImage.

For instance, suppose your application depends on a package called my_icons, which has the following directory structure:

.../pubspec.yaml
.../icons/heart.png
.../icons/1.5x/heart.png
.../icons/2.0x/heart.png
...etc.

To load the image, use:

return const AssetImage('icons/heart.png', package: 'my_icons');

Assets used by the package itself should also be fetched using the package argument as above.

If the desired asset is specified in the pubspec.yaml file of the package, it's bundled automatically with the application. In particular, assets used by the package itself must be specified in its pubspec.yaml.

A package can also choose to have assets in its lib/ folder that are not specified in its pubspec.yaml file. In this case, for those images to be bundled, the application has to specify which ones to include in its pubspec.yaml. For instance, a package named fancy_backgrounds could have the following files:

.../lib/backgrounds/background1.png
.../lib/backgrounds/background2.png
.../lib/backgrounds/background3.png

To include, say, the first image, the pubspec.yaml of the application should specify it in the assets section:

flutter:
  assets:
    - packages/fancy_backgrounds/backgrounds/background1.png

The lib/ is implied, so it should not be included in the asset path.

If you are developing a package, to load an asset within the package, specify it in the pubspec.yaml of the package:

flutter:
  assets:
    - assets/images/

To load the image within your package, use:

return const AssetImage('packages/fancy_backgrounds/backgrounds/background1.png');

Flutter assets are readily available to platform code using the AssetManager on Android and NSBundle on iOS.

On Android the assets are available through the AssetManager API. The lookup key used in, for instance openFd, is obtained from lookupKeyForAsset on PluginRegistry.Registrar or getLookupKeyForAsset on FlutterView. PluginRegistry.Registrar is available when developing a plugin while FlutterView would be the choice when developing an app including a platform view.

As an example, suppose you have specified the following in your pubspec.yaml

flutter:
  assets:
    - icons/heart.png

This reflects the following structure in your Flutter app.

.../pubspec.yaml
.../icons/heart.png
...etc.

To access icons/heart.png from your Java plugin code, do the following:

AssetManager assetManager = registrar.context().getAssets();
String key = registrar.lookupKeyForAsset("icons/heart.png");
AssetFileDescriptor fd = assetManager.openFd(key);

On iOS the assets are available through the mainBundle. The lookup key used in, for instance pathForResource:ofType:, is obtained from lookupKeyForAsset or lookupKeyForAsset:fromPackage: on FlutterPluginRegistrar, or lookupKeyForAsset: or lookupKeyForAsset:fromPackage: on FlutterViewController. FlutterPluginRegistrar is available when developing a plugin while FlutterViewController would be the choice when developing an app including a platform view.

As an example, suppose you have the Flutter setting from above.

To access icons/heart.png from your Objective-C plugin code you would do the following:

NSString* key = [registrar lookupKeyForAsset:@"icons/heart.png"];
NSString* path = [[NSBundle mainBundle] pathForResource:key ofType:nil];

To access icons/heart.png from your Swift app you would do the following:

let key = controller.lookupKey(forAsset: "icons/heart.png")
let mainBundle = Bundle.main
let path = mainBundle.path(forResource: key, ofType: nil)

For a more complete example, see the implementation of the Flutter video_player plugin on pub.dev.

The ios_platform_images plugin on pub.dev wraps up this logic in a convenient category. You fetch an image as follows:

Objective-C:

[UIImage flutterImageWithName:@"icons/heart.png"];

Swift:

UIImage.flutterImageNamed("icons/heart.png")

When implementing Flutter by adding it to an existing iOS app, you might have images hosted in iOS that you want to use in Flutter. To accomplish that, use the ios_platform_images plugin available on pub.dev.

There are other occasions to work with assets in the platform projects directly. Below are two common cases where assets are used before the Flutter framework is loaded and running.

Updating a Flutter application's launch icon works the same way as updating launch icons in native Android or iOS applications.

In your Flutter project's root directory, navigate to .../android/app/src/main/res. The various bitmap resource folders such as mipmap-hdpi already contain placeholder images named ic_launcher.png. Replace them with your desired assets respecting the recommended icon size per screen density as indicated by the Android Developer Guide.

In your Flutter project's root directory, navigate to .../ios/Runner. The Assets.xcassets/AppIcon.appiconset directory already contains placeholder images. Replace them with the appropriately sized images as indicated by their filename as dictated by the Apple Human Interface Guidelines. Keep the original file names.

Flutter also uses native platform mechanisms to draw transitional launch screens to your Flutter app while the Flutter framework loads. This launch screen persists until Flutter renders the first frame of your application.

To add a launch screen (also known as "splash screen") to your Flutter application, navigate to .../android/app/src/main. In res/drawable/launch_background.xml, use this layer list drawable XML to customize the look of your launch screen. The existing template provides an example of adding an image to the middle of a white splash screen in commented code. You can uncomment it or use other drawables to achieve the intended effect.

For more details, see Adding a splash screen to your Android app.

To add an image to the center of your "splash screen", navigate to .../ios/Runner. In Assets.xcassets/LaunchImage.imageset, drop in images named LaunchImage.png, [email protected], [email protected]. If you use different filenames, update the Contents.json file in the same directory.

You can also fully customize your launch screen storyboard in Xcode by opening .../ios/Runner.xcworkspace. Navigate to Runner/Runner in the Project Navigator and drop in images by opening Assets.xcassets or do any customization using the Interface Builder in LaunchScreen.storyboard.

For more details, see Adding a splash screen to your iOS app.

Unless stated otherwise, the documentation on this site reflects the latest stable version of Flutter. Page last updated on 2024-09-04. View source or report an issue.