添加资源和图片

Flutter 应用程序包含代码和 assets（有时称为资源）。资源是被打包到应用程序安装包中，可以在运行时访问的一种文件。常见的资源类型包括静态数据（例如 JSON 文件），配置文件，图标和图片（JPEG，WebP，GIF，动画 WebP / GIF，PNG，BMP 和 WBMP）。

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).

指定资源

Specifying assets

Flutter 使用 pubspec.yaml 文件，位于项目根目录, 来识别应用程序所需的资源。

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

下面举个例子:

Here is an example:

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

如果要包含一个目录下的所有 assets，需要在目录名称的结尾加上 /：

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

flutter:
  assets:
    - assets/

注意只包含目录下根节点的所有文件，如果要添加子目录下的文件，需要给每个目录创建节点。

Note that only files located directly in the directory are included. To add files located in subdirectories, create an entry per directory.

资源打包

Asset bundling

该 assets 部分的 flutter 部分需要指定包含在应用程序中的文件。每个资源都通过相对于 pubspec.yaml 文件所在位置的路径进行标识。资源的声明顺序是无关紧要的。资源的实际目录可以是任意文件夹（在本示例中是 assets）

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 does not matter. The actual directory used (assets in this case) does not matter.

在一次构建中，Flutter 将 assets 放到 asset bundle 的特殊归档中，以便应用程序在运行时读取它们。

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

资源变体

Asset variants

构建过程支持 asset 变体：不同版本的资源可能会显示在不同的上下文中。在 pubspec.yaml 的 assets 部分中指定的资源路径，会在构建过程中，查找同级子目录中相同名称的任何文件。这些文件会与指定的资源一起被打包在资源 bundle 中。

The build process supports the notion of asset variants: different versions of an asset that might be displayed in different contexts. When an asset’s path is specified in the assets section of pubspec.yaml, the build process looks for any files with the same name in adjacent subdirectories. Such files are then included in the asset bundle along with the specified asset.

例如，你的应用程序目录中有以下文件：

For example, if you have the following files in your application directory:

  .../pubspec.yaml
  .../graphics/my_icon.png
  .../graphics/background.png
  .../graphics/dark/background.png
  ...etc.

…同时 pubspec.yaml 文件包含：

And your pubspec.yaml file contains the following:

flutter:
  assets:
    - graphics/background.png

…那么这两个图片， graphics/background.png 和 graphics/dark/background.png 将被打包在你的资源 bundle 中。前者被称为是 main asset，后者被称为是一种变体（ variant）。

Then both graphics/background.png and graphics/dark/background.png are included in your asset bundle. The former is considered the main asset, while the latter is considered a variant.

如果指定的是 graphics 目录：

If, on the other hand, the graphics directory is specified:

flutter:
  assets:
    - graphics/

… 那么 graphics/my_icon.png， graphics/background.png 和 graphics/dark/background.png 同时被包含。

Then the graphics/my_icon.png, graphics/background.png and graphics/dark/background.png files are also included.

在选择当前设备分辨率的图片时，Flutter 会使用资源变体；见下文。将来，这种机制可能会扩展到本地化、阅读提示等方面。

Flutter uses asset variants when choosing resolution-appropriate images. In the future, this mechanism might be extended to include variants for different locales or regions, reading directions, and so on.

加载 assets

Loading assets

你的应用程序可以通过 AssetBundle 对象访问其资源。

Your app can access its assets through an AssetBundle object.

Asset bundle 通过指定一个逻辑键（key），允许你读取 string/text（loadString）和 image/binary（load）。在编译期间，这个逻辑键（key）会映射在 pubspec.yaml 中指定的资源路径。

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.

加载文本 assets

Loading text assets

每个 Flutter 应用程序都有一个 rootBundle 对象，可以轻松访问主资源 bundle 。还可以直接使用 package:flutter/services.dart 中全局静态的 rootBundle 来加载资源。

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.

但是，如果获取当前 BuildContext 的 AssetBundle，建议使用 DefaultAssetBundle。这种方式不是使用应用程序构建的默认资源 bundle，而是让父级 widget 在运行时替换的不同的 AssetBundle，这对于本地化或测试场景很有用。

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.

通常，你可以从应用程序运行时的 rootBundle 中，间接使用 DefaultAssetBundle.of() 来加载资源（例如JSON文件）。

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

在 Widget 上下文之外，或 AssetBundle 的句柄不可用时，你可以使用 rootBundle 直接加载这些 assets，例如：

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

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

加载 images

Loading images

Flutter 可以为当前设备加载适合其分辨率的图像。

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

声明分辨率相关的图片 assets

Declaring resolution-aware image assets

AssetImage 可以将逻辑请求资源映射到最接近当前设备 device pixel ratio的资源。为了使这种映射起作用，应该根据特定的目录结构来保存资源：

AssetImage understands how to map a logical requested asset onto one that most closely matches the current device pixel ratio. In order for this mapping to work, assets should be arranged according to a particular directory structure:

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

…其中 M 和 N 是数字标识符，对应于其中包含的图像的分辨率，换句话说，它们指定不同设备像素比例的图片。

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.

主资源默认对应于 1.0 倍的分辨率图片。比如下面的图片 my_icon.png：

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
  .../2.0x/my_icon.png
  .../3.0x/my_icon.png

而在设备像素比率为 1.8 的设备上，对应是 .../2.0x/my_icon.png 。如果是 2.7 的设备像素比，对应是 .../3.0x/my_icon.png 。

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

如果在 Image widget 上未指定渲染图像的宽度和高度，通常会扩展资源来保证与主资源相同的屏幕空间量，并不是相同的物理像素，只是分辨率更高。换句话说，.../my_icon.png 是 72 px 乘 72 px，那么 .../3.0x/my_icon.png 应该是 216 px 乘 216 px；但如果未指定宽度和高度，它们都将渲染为 72 px 乘 72 px（以逻辑像素为单位）。

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.

在 pubspec.yaml 中资源部分的每一项都应与实际文件相对应，除过主资源节点。当主资源缺少某个文件时，会按分辨率从低到高的顺序去选择，也就是说 1x 中没有的话会在 2x 中找，2x 中还没有的话就在 3x 中找。该条目需要在 pubspec.yaml 中指定。

Each entry in the asset section of the pubspec.yaml should correspond to a real file, with the exception of the main asset entry. If the main asset entry does not 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.

加载 images

Loading images

加载图片，请在 widget 的 build 方法中使用 AssetImage 类。

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

例如，你的应用程序可以从上面的资源声明中加载背景图片：

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

Widget build(BuildContext context) {
  return Image(image: AssetImage('graphics/background.png'));
}

使用默认的资源 bundle 加载资源时，系统会自动处理分辨率等。（如果你使用一些更低级别的类，如 ImageStream 或 ImageCache，你需要注意 scale 相关的参数)。

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.)

依赖包中的资源图片

Asset images in package dependencies

加载依赖 package 中的图像，必须给 AssetImage 提供 package 参数。

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

例如，你的应用程序依赖于一个名为 my_icons 的 package，它的目录结构如下：

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.

然后加载 image, 使用：

To load the image, use:

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

package 使用的本身的 Assets 也需要加上 package 参数来获取。

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

打包 assets

Bundling of package assets

如果在 pubspec.yaml 文件中声明了资源文件，它将会打包到响应的 package 中。特别是，package 本身使用的资源必须在 pubspec.yaml 中指定。

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.

package 也可以选择在其 lib/ 文件夹中包含未在 pubspec.yaml 文件中声明的资源。在这种情况下，对于要打包的图片，应用程序必须在 pubspec.yaml 中指定包含哪些图像。例如，一个名为 fancy_backgrounds 的包，可能包含以下文件：

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

总而言之，要包含第一张图像，必须在 pubspec.yaml 的 assets 部分中声明它：

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

lib/ 是隐含的，所以它不应该包含在资源路径中。

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

平台共享 assets

Sharing assets with the underlying platform

在不同平台读取 Flutter assets，Android 是通过 AssetManager，iOS 是 NSBundle。

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

Android

在 Android 平台上，assets 通过 AssetManager API 读取。通过 PluginRegistry.Registrar 的 lookupKeyForAsset 方法，或者 FlutterView 的 getLookupKeyForAsset 方法来获取文件路径，然后 AssetManager API 的 openFd 根据文件路径得到文件描述符。开发插件时可以使用 PluginRegistry.Registrar，而开发应用程序使用平台视图时，FlutterView 是最好的选择。

On Android the assets are available via 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.

举个例子，假设你在 pubspec.yaml 中这样指定：

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

flutter:
  assets:
    - icons/heart.png

在你的 Flutter 应用程序对应以下结构。

This reflects the following structure in your Flutter app.

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

想要在 Java 插件中访问 icons/heart.png；

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);

iOS

在 iOS 平台上，assets 通过 mainBundle 读取。通过 FlutterPluginRegistrar 的 lookupKeyForAsset 或者 lookupKeyForAsset:fromPackage: 方法获取文件路径，同样 FlutterViewController的 lookupKeyForAsset: 或者 lookupKeyForAsset:fromPackage: 方法也可以获取文件路径，然后 AssetManager API 的 openFd 根据文件路径得到文件描述符。开发插件时可以使用 FlutterPluginRegistrar，而开发应用程序使用平台视图时， FlutterViewController 是最好的选择。

On iOS the assets are available via 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.

举个例子，假设你的 Flutter 配置和上面一样。

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

要在 Objective-C 插件中访问 icons/heart.png：

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];

这有一个更完整的实例可以理解 Flutter 的应用：video_player plugin。

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

平台 assets

Platform assets

某些场景可以直接在平台项目中使用 assets。以下是在 Flutter 框架加载并运行之前使用资源的两种常见情况。

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.

更新桌面icon

Updating the app icon

更新你的 Flutter 应用程序启动图标，和原生 Android 或 iOS 应用程序中更新启动图标的方法相同。

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

Android

在 Flutter 项目的根目录中，导航到 .../android/app/src/main/res 路径。各种位图资源文件夹，比如 mipmap-hdpi，已包含占位符图像 ic_launcher.png。只需按照 Android 开发指南中的说明，将其替换为所需的资源，并遵守每种屏幕分辨率的建议图标大小标准。

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.

iOS

在你的 Flutter 项目的根目录中，导航到 .../ios/Runner 路径。该目录中 Assets.xcassets/AppIcon.appiconset已经包含占位符图片，只需将它们替换为适当大小的图片，并且根据 iOS 开发指南，文件名称保持不变。

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.

更新启动图

Updating the launch screen

在 Flutter 框架加载时，Flutter 会使用原生平台机制绘制启动页。此启动页将持续到 Flutter 渲染应用程序的第一帧。

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.

Android

将启动屏幕「splash screen」添加到你的 Flutter 应用程序，请导航至 .../android/app/src/main 路径。在 res/drawable/launch_background.xml 文件中，通过使用 图层列表 XML 来实现自定义启动页。现有模板提供了一个示例，用于将图片添加到白色启动页的中间（注释代码中）。你也可以取消注释使用 可绘制对象资源 来实现预期效果。

To add a “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.

iOS

将图片添加到启动屏幕「splash screen」的中心，请导航至 .../ios/Runner 路径。在 Assets.xcassets/LaunchImage.imageset ，拖入图片，并命名为 LaunchImage.png， LaunchImage@2x.png，LaunchImage@3x.png。如果你使用不同的文件名，那你还必须更新同一目录中的 Contents.json 文件中对应的名称。

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, LaunchImage@2x.png, LaunchImage@3x.png. If you use different filenames, update the Contents.json file in the same directory.

你也可以通过打开 .../ios/Runner.xcworkspace ，完全自定义 storyboard。在 Project Navigator 中导航到 Runner/Runner ，然后打开 Assets.xcassets 拖入图片，或者在 LaunchScreen.storyboard 中使用 Interface Builder 进行自定义。

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.