Actually support :mobile and :sign_in options for Foursquare strategy.

oa-oauth/lib/omniauth/strategies/foursquare.rb

@@ -9,13 +9,18 @@ module OmniAuth
       # @option options [Boolean, true] :sign_in When true, use a sign-in flow instead of the authorization flow.
       # @option options [Boolean, false] :mobile When true, use the mobile sign-in interface.
       def initialize(app, client_id = nil, client_secret = nil, options = {}, &block)
+        options[:sign_in] ||= true
         super(app, :foursquare, client_id, client_secret, {
           :site => "https://api.foursquare.com/v2",
-          :authorize_url      => "https://foursquare.com/oauth2/authenticate",
+          :authorize_url      => authorize_url(options),
           :access_token_url   => "https://foursquare.com/oauth2/access_token"
         }, options, &block)
       end
 
+      def authorize_url(options)
+        "https://foursquare.com/#{'mobile/' if options[:mobile]}oauth2/#{options[:sign_in] ? 'authenticate' : 'authorize'}"
+      end
+
       def user_data
         @data ||= MultiJson.decode(@access_token.get(client.site+'/users/self', {'oauth_token' => @access_token.token}))
       end

oa-oauth/spec/omniauth/strategies/foursquare_spec.rb

@@ -1,4 +1,18 @@
 require File.expand_path('../../../spec_helper', __FILE__)
 
 describe OmniAuth::Strategies::Foursquare do
+  it_should_behave_like "an oauth2 strategy"
+  subject{ OmniAuth::Strategies::Foursquare.new(lambda{|env|[200,{},[""]]} , 'abc', 'def')}
+
+  it 'should use the mobile authorize url when :mobile is true' do
+    subject.authorize_url(:mobile => true).should be_include("/mobile/")
+  end
+
+  it 'should use the authorize endpoint if :sign_in is false' do
+    subject.authorize_url(:sign_in => false).should be_include("/authorize")
+  end
+
+  it 'should default to the authenticate endpoint' do
+    subject.client.authorize_url.should be_include('/authenticate')
+  end
 end