android端在执行flutter build aar命令时，有时候会遇到如下错误（问题版本1.17.1，目前最新的flutter版本1.22.4已不存在此问题）
FAILURE: Build failed with an exception.
- Where:
Initialization script '....\packages\flutter_tools\gradle\aar_init_script.gradle' line: 15
aar_init_script.gradle中修改相关代码如下
void configureProject(Project project, String outputDir) {
if (!project.hasProperty("android")) {
throw new GradleException(project.toString())
// throw new GradleException("Android property not found.")
}
...
}
再次执行flutter build aar，可定位到具体出错插件项目。如：
引入path_provider:1.6.8的项目，执行以上命令时会有如下错误信息提示
- What went wrong:
project ':path_provider_macos'
由此猜测可能是某些插件依赖了macos相关，查看path_provider-1.6.8中的pubspec.yaml，可发现其依赖了path_provider_macos
dependencies:
flutter:
sdk: flutter
path_provider_platform_interface: ^1.0.1
path_provider_macos: ^0.0.4
目前我们的项目中这三个插件path_provider:1.6.8、shared_preferences: 0.5.7+2、webview_flutter: 0.3.22+1
都会有同样的"Android property not found."问题。
在网上搜了一下，大部分解决办法都是说临时删除掉.pub-cache/hosted/pub.flutter-io.cn下出错的相关包，
这个的确可以解决，但下次执行flutter clean, flutter pub get后，再次执行flutter build aar时还会有同样的问题，
于是想看能否看下源码了解下有没有好的解决办法。
在flutter的packages/flutter_tools/gradle/flutter.gradle中，看到如下项目配置的代码
/**
* Configures the Flutter plugin dependencies.
*
* The plugins are added to pubspec.yaml. Then, upon running `flutter pub get`,
* the tool generates a `.flutter-plugins` file, which contains a 1:1 map to each plugin location.
* Finally, the project's `settings.gradle` loads each plugin's android directory as a subproject.
*/
private void configurePlugins() {
if (!buildPluginAsAar()) {
getPluginList().each this.&configurePluginProject
getPluginDependencies().each this.&configurePluginDependencies
return
}
project.repositories {
maven {
url "${getPluginBuildDir()}/outputs/repo"
}
}
getPluginList().each { pluginName, pluginPath ->
configurePluginAar(pluginName, pluginPath, project)
}
}
配置插件依赖部分的代码如下，其会过滤掉不支持android platform的依赖插件
// Add the dependencies on other plugin projects to the plugin project.
// A plugin A can depend on plugin B. As a result, this dependency must be surfaced by
// making the Gradle plugin project A depend on the Gradle plugin project B.
private void configurePluginDependencies(Object dependencyObject) {
...
dependencyObject.dependencies.each { pluginDependencyName ->
...
Project dependencyProject = project.rootProject.findProject(":$pluginDependencyName")
if (dependencyProject == null ||
!doesSupportAndroidPlatform(dependencyProject.projectDir.parentFile.path)) {
return
}
// Wait for the Android plugin to load and add the dependency to the plugin project.
pluginProject.afterEvaluate {
pluginProject.dependencies {
implementation dependencyProject
}
}
}
}
关键代码doesSupportAndroidPlatform如下，主要是判断插件项目中是否有android目录和android/build.gradle文件
// Returns `true` if the given path contains an `android/build.gradle` file.
//
// TODO(egarciad): Fix https://github.com/flutter/flutter/issues/39657.
// Android Studio may create empty android directories due to the logic in /android/settings.gradle,
// which imports all plugins regardless of whether they support the android platform.
private Boolean doesSupportAndroidPlatform(String path) {
File editableAndroidProject = new File(path, 'android' + File.separator + 'build.gradle')
return editableAndroidProject.exists()
}
根据之前代码configurePlugins中的注释，我们大概了解项目中插件配置流程如下
1.首先在pubspec.yaml中添加相关插件
2.执行flutter pub get命令生成.flutter-plugins文件
3.项目的settings.gradle加载每一个插件的android目录作为子项目
在.android项目中，我们先查看生成的settings.gradle文件
// Generated file. Do not edit.
include ':app'
rootProject.name = 'android_generated'
setBinding(new Binding([gradle: this]))
evaluate(new File(settingsDir, 'include_flutter.groovy'))
再查看生成的文件include_flutter.groovy，部分代码如下
def plugins = new Properties()
def pluginsFile = new File(flutterProjectRoot, '.flutter-plugins')
if (pluginsFile.exists()) {
pluginsFile.withReader('UTF-8') { reader -> plugins.load(reader) }
}
plugins.each { name, path ->
def pluginDirectory = flutterProjectRoot.toPath().resolve(path).resolve('android').toFile()
gradle.include ":$name"
gradle.project(":$name").projectDir = pluginDirectory
}
其先从.flutter-plugins文件中加载插件，然后配置插件项目作为subproject
其中可以看到，在添加插件的android目录作为子目录时，并未判断插件是否含有android目录，
这样上面的path_provider_macos就会被添加到子项目中作为依赖项目，然后在构建时校验失败。
projectsEvaluated {
...
// Gets the plugin subprojects.
Set modulePlugins = rootProject.subprojects.findAll {
it.name != "flutter" && it.name != "app"
}
// When a module is built as a Maven artifacts, plugins must also be built this way
// because the module POM's file will include a dependency on the plugin Maven artifact.
// This is due to the Android Gradle Plugin expecting all library subprojects to be published
// as Maven artifacts.
modulePlugins.each { pluginProject ->
configureProject(pluginProject, moduleProject.property("output-dir"))
moduleProject.android.libraryVariants.all { variant ->
// Configure the `assembleAar` task for each plugin's projects and make
// the module's equivalent task depend on the plugin's task.
String variantName = variant.name.capitalize()
moduleProject.tasks.findByPath("assembleAar$variantName")
.dependsOn(pluginProject.tasks.findByPath("assembleAar$variantName"))
}
}
}
大概了解依赖配置流程后，我们可以想到如下解决办法
1.修改include_flutter.groovy中的插件项目配置规则，加上android目录的校验
2.修改aar_init_script.gradle中的配置规则
具体修改如下：
1.include_flutter.groovy是通过模版生成的，修改的话可修改flutter下packages/flutter_tools/templates/module/android/library_new_embedding/include_flutter.groovy.copy.tmpl
plugins.each { name, path ->
def pluginDirectory = flutterProjectRoot.toPath().resolve(path).resolve('android').toFile()
if (pluginDirectory.exists()) {
gradle.include ":$name"
gradle.project(":$name").projectDir = pluginDirectory
} else {
println("plugins " + name + " pluginDirectory : " + pluginDirectory + " does not exist")
}
}
2.修改flutter下文件packages/flutter_tools/gradle/aar_init_script.gradle中projectsEvaluated配置部分代码：
Set modulePlugins = rootProject.subprojects.findAll {
it.name != "flutter" && it.name != "app" && it.hasProperty("android")
}
以上两个方式都需要修改flutter中的代码，暂时没找到更好的解决方法，有了解其他解决办法的，也希望分享下。
追记：
flutter 1.22.4版本生成的include_flutter.groovy中代码变更
gradle.apply from: "$flutterSdkPath/packages/flutter_tools/gradle/module_plugin_loader.gradle"
查看module_plugin_loader.gradle代码可见其中添加插件依赖时，检查了是否支持android
def pluginsFile = new File(moduleProjectRoot, '.flutter-plugins-dependencies')
if (pluginsFile.exists()) {
def object = new JsonSlurper().parseText(pluginsFile.text)
assert object instanceof Map
assert object.plugins instanceof Map
assert object.plugins.android instanceof List
// Includes the Flutter plugins that support the Android platform.
object.plugins.android.each { androidPlugin ->
assert androidPlugin.name instanceof String
assert androidPlugin.path instanceof String
def pluginDirectory = new File(androidPlugin.path, 'android')
assert pluginDirectory.exists()
include ":${androidPlugin.name}"
project(":${androidPlugin.name}").projectDir = pluginDirectory
}
}