Skip to `generate.rb` to see the good stuff, and the output follows.

android_xml.rb

class AndroidTag
  attr_accessor :is_root

  def initialize(tag, *args, &block)
    @buffer = []
    @attrs = {}
    @raw_tag = tag.to_s
    if rename = AndroidXml.tags[tag.to_s][:rename]
      @tag = rename
    else
      @tag = tag.to_s.gsub('_', '-')
    end

    args.each do |arg|
      if arg.is_a?(Hash)
        @attrs.merge!(arg)
      else
        raise ArgumentError.new("Unknown argument #{arg.inspect} in #{self.class}#new")
      end
    end

    @generate = block
  end

  def method_missing(method_name, *args, &block)
    tag = AndroidTag.new(method_name, *args, &block)
    @buffer << tag
    tag
  end

  def attrs(whitespace)
    attrs = {}
    if is_root
      attrs.merge!(AndroidXml.root_tag[:defaults])
    end

    attrs.merge!(AndroidXml.tags[@raw_tag][:defaults])
    attrs.merge!(@attrs)

    output = ''
    is_first = true
    attrs.each do |key, value|
      next if value.nil?

      key = key.to_s
      if AndroidXml.tags[@tag][:attrs].key?(key)
        xml_key = AndroidXml.tags[@tag][:attrs][key]
      elsif key.to_s.include?(':')
        xml_key = key.to_s
      else
        xml_key = "android:#{key}"
      end

      if is_first
        output << " #{xml_key}=\"#{value}\""
        is_first = false
      else
        output << "\n#{whitespace}#{xml_key}=\"#{value}\""
      end
    end
    output
  end

  def generate(tab='')
    whitespace = "#{tab}  #{' ' * @tag.length}"
    output = "#{tab}<#{@tag}#{attrs(whitespace)}"
    if @generate
      inside = generate_block(tab + AndroidXml.tab)
      if !inside || inside.strip.empty?
        output << " />\n"
      else
        output << ">"
        if inside.lstrip.start_with?('<')
          output << "\n" << inside << "#{tab}</#{@tag}>\n"
        else
          output << inside << "</#{@tag}>\n"
        end
      end
    else
      output <<" />\n"
    end

    output
  end

  def generate_block(tab='')
    return @block_output if @block_output

    output = ''
    if @generate
      text = instance_exec(&@generate)
      @buffer.each do |tag|
        output << tag.generate(tab)
      end
      if text.is_a?(String)
        output << text
      end
    end

    @block_output = output
  end

  def to_s
    generate
  end

  def to_ary
    [to_s]
  end

  def out
    puts to_s
  end

end


class AndroidXml < AndroidTag
  ROOT = '__root_node_with_a_long_special_name__'

  class << self
    attr_writer :tab

    def files
      @files ||= []
    end

    def write_all
      files.each do |xml_file|
        File.open(xml_file.filename, 'w') do |f|
          f.write(xml_file.to_s)
        end
        puts "✓ #{xml_file.filename}"
      end
    end

    def output_all
      files.each do |xml_file|
        xml_file.out
      end
    end

    def tags
      @tags ||= Hash.new do |hash, key|
        hash[key] = {
          attrs: {},
          defaults: {},
          rename: nil,
        }
      end
    end

    def tab
      @tab ||= '    '
    end

    def setup(&block)
      instance_exec(&block)
    end

    def root_tag
      tags[AndroidXml::ROOT]
    end

    def tag(name, &block)
      context_was = @context

      if name.is_a?(Hash)
        @context = nil
        name.each do |shortcut, tag_name|
          raise "There can be only one key-value pair" if @context

          @context = shortcut.to_s
          tags[@context][:rename] = tag_name.to_s
        end
      else
        @context = name.to_s
      end

      yield

      @context = context_was
    end

    def rename(attrs)
      if attrs.is_a?(Hash)
        attrs.each do |attr_name, attr_rename|
          tags[@context][:attrs][attr_name.to_s] = attr_rename.to_s
        end
      else
        tags[@context][:attrs][attrs.to_s] = attrs.to_s
      end
    end

    def defaults(attrs)
      tags[@context][:defaults].merge!(attrs)
    end

    def root(&block)
      tag(ROOT, &block)
    end

  end

  attr :filename

  def initialize(filename, &block)
    @filename = filename
    super(nil, &block)
    AndroidXml.files << self
  end

  def method_missing(method_name, *args, &block)
    raise "There can be only one (new: #{method_name}, old: #{@root})" if @root

    @root = super
    @root.is_root = true
    @root
  end

  def generate
    output = "<?xml version=\"1.0\" encoding=\"utf-8\"?>\n"
    output << "<!-- Do not edit this file. It was generated by AndroidXml -->\n"
    output << generate_block
  end

  def out
    puts "---- filename: #{@filename} ----"
    super
    puts "---------------#{'-' * @filename.to_s.length}-----"
  end

end

generate.rb

require_relative './android_xml'

AndroidXml.setup do
  # assign the xmlns to all root nodes
  root do
    defaults 'xmlns:android' => 'http://schemas.android.com/apk/res/android'
  end
  tag :resources do
    # disable the xmlns attribute on the resource node
    defaults 'xmlns:android' => nil
  end
  tag :string do
    rename :name  # suppress 'android:' prefix
  end
  tag :manifest do
    rename :package
  end
  # creates a "tag shortcut"
  tag :main_action => 'action' do
    defaults name: 'android.intent.action.MAIN'
  end
  tag :launcher_category => 'category' do
    defaults name: 'android.intent.category.LAUNCHER'
  end
end


AndroidXml.new('AndroidManifest.xml') do
  manifest package: 'com.colinta.MyFirstAndroidApp', versionCode: 1, versionName: 1.0 do
    uses_sdk minSdkVersion: 11

    application label: '@string/app_name', icon: '@drawable/ic_launcher' do
      activity name: 'MainActivity', label: '@string/app_name' do
        intent_filter do
          main_action
          launcher_category
        end
      end

      activity name: 'DisplayMessageActivity', label: '@string/app_name', parentActivityName: 'com.colinta.MyFirstAndroidApp.MainActivity'
    end
  end
end


AndroidXml.new('res/values/strings.xml') do
  resources do
    string(name: 'app_name') { 'Harroo' }
    string(name: 'hello') { 'Well, Harroo!' }
    string(name: 'portrait') { 'portrait' }
    string(name: 'landscape') { 'landscape' }
    string(name: 'button_send') { 'Send' }
    string(name: 'action_search') { 'Search' }
    string(name: 'action_settings') { 'Settings' }
    string(name: 'title_activity_main') { 'MainActivity' }
  end
end


AndroidXml.new('res/menu/main_activity_actions.xml') do
  menu do
    item id: '@+id/action_search',
      icon: '@drawable/ic_action_search',
      title: '@string/action_search',
      showAsAction: "ifRoom"

    item id: '@+id/action_settings',
      title: '@string/action_settings',
      showAsAction: "never"
  end
end

# AndroidXml.write_all
AndroidXml.output_all

output.xml

---- filename: AndroidManifest.xml ----
<?xml version="1.0" encoding="utf-8"?>
<!-- Do not edit this file. It was generated by AndroidXml -->
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
          package="com.colinta.MyFirstAndroidApp"
          android:versionCode="1"
          android:versionName="1.0">
    <uses-sdk android:minSdkVersion="11" />
    <application android:label="@string/app_name"
                 android:icon="@drawable/ic_launcher">
        <activity android:name="MainActivity"
                  android:label="@string/app_name">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />
                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
        <activity android:name="DisplayMessageActivity"
                  android:label="@string/app_name"
                  android:parentActivityName="com.colinta.MyFirstAndroidApp.MainActivity" />
    </application>
</manifest>
---------------------------------------

---- filename: res/values/strings.xml ----
<?xml version="1.0" encoding="utf-8"?>
<!-- Do not edit this file. It was generated by AndroidXml -->
<resources>
    <string name="app_name">Harroo</string>
    <string name="hello">Well, Harroo!</string>
    <string name="portrait">portrait</string>
    <string name="landscape">landscape</string>
    <string name="button_send">Send</string>
    <string name="action_search">Search</string>
    <string name="action_settings">Settings</string>
    <string name="title_activity_main">MainActivity</string>
</resources>
------------------------------------------

---- filename: res/menu/main_activity_actions.xml ----
<?xml version="1.0" encoding="utf-8"?>
<!-- Do not edit this file. It was generated by AndroidXml -->
<menu xmlns:android="http://schemas.android.com/apk/res/android">
    <item android:id="@+id/action_search"
          android:icon="@drawable/ic_action_search"
          android:title="@string/action_search"
          android:showAsAction="ifRoom" />
    <item android:id="@+id/action_settings"
          android:title="@string/action_settings"
          android:showAsAction="never" />
</menu>
------------------------------------------------------

And yes, this will probably be my first rm-android gem, once the beta is out :-)

Looks really good, Colin!

Thanks man! http://rubygems.org/gems/android-xml 😃