var prefillField = new Class.create();

prefillField.prototype = {

    field:null,
    value:null,
    className:null,

    initialize: function(field, value, className){
      if($(field)){
        this.field = $(field);
        this.value = value;
        this.className = className || 'field_default_value';
        this.unselect();

        this.field.observe('blur', function(event){
          this.unselect();
        }.bind(this));
        this.field.observe('focus', function(event){
          this.select();
        }.bind(this));
        this.field.observe('widget:submit', function(event){
          this.submit();
        }.bind(this));
      }
    },

    unselect: function(){
      if(this.field.value == ""){
        this.field.value = this.value;
        this.field.addClassName(this.className);
      }
    },

    select: function(){
      if(this.field.value == this.value){
        this.field.value = "";
        this.field.removeClassName(this.className);
      }
    },

    submit: function(){
      this.field.stopObserving('blur');
      this.select();
    }
};