add ability to use listeners

lib/google_visualr/base_chart.rb

@@ -4,10 +4,11 @@ class BaseChart
     include GoogleVisualr::Packages
     include GoogleVisualr::ParamHelpers
 
-    attr_accessor :data_table
+    attr_accessor :data_table, :listeners
 
     def initialize(data_table, options={})
       @data_table = data_table
+      @listeners = []
       send(:options=, options)
     end
 
@@ -19,6 +20,10 @@ def options=(options)
       @options = stringify_keys!(options)
     end
 
+    def add_listener(event, callback)
+      @listeners << { :event => event.to_s, :callback => callback }
+    end
+
     # Generates JavaScript and renders the Google Chart in the final HTML output.
     #
     # Parameters:
@@ -29,6 +34,9 @@ def to_js(element_id)
       js << "\n    #{@data_table.to_js}"
       js << "\n    var chart = new google.visualization.#{class_name}(document.getElementById('#{element_id}'));"
       js << "\n    chart.draw(data_table, #{js_parameters(@options)});"
+      @listeners.each do |listener|
+        js << "\n    google.visualization.events.addListener(chart, '#{listener[:event]}', #{listener[:callback]});"
+      end
       js << "\n  }});"
       js << "\n</script>"