@Managed interface Item extends Named {}
@Managed interface FileItem extends Item {
    void setSize(int size)
    int getSize()
}
@Managed interface DirectoryItem extends Item {
    ModelMap<Item> getChildren()
}

class PluginRules extends RuleSource {
    @Defaults void setDefaultFileSize(@Each FileItem file) {
        file.size = 1024
    }

    @Rules void applyValidateRules(ValidateRules rules, @Each DirectoryItem directory)  {}
}
apply plugin: PluginRules

abstract class ValidateRules extends RuleSource {
    @Validate
    void validateSizeIsPositive(ModelMap<FileItem> files) {
        files.each { file ->
            assert file.size > 0
        }
    }

    @Validate
    void validateSizeDivisibleBySixteen(ModelMap<FileItem> files) {
        files.each { file ->
            assert file.size % 16 == 0
        }
    }
}

model {
    root(DirectoryItem) {
        children {
            dir(DirectoryItem) {
                children {
                    file1(FileItem)
                    file2(FileItem) { size = 2048 }
                }
            }
            file3(FileItem)
        }
    }
}