ColdFusion Encapsulation Gotcha

Good object design dictates that we encapsulate our objects, hiding direct access to object properties from outside code. Access to the values of these properties is often granted to outside code with the use of 'getter' methods.

The following, useless, component appears to encapsulate its 'foo' property using a getter method but, as we shall see shortly, does nothing of the kind:

<cfcomponent output="false" displayname="foobar">
   <cfset variables.foo = StructNew()>
   <cfset variables.foo.bar = "Hello world">

   <cffunction name="GetFoo">
      <cfreturn variables.foo>
   </cffunction>
</cfcomponent>

Why is that not encapsulated?

The reason the 'foo' property is not encapsulated here is that ColdFusion passes certain variable types by reference rather than value. So when we return the variables.foo structure we are actually returning a reference to the original data in memory rather than the data itself.

The following code demonstrates what effect this has:

<h1>Encapsulation gotcha example</h1>
<cfset oFoo = CreateObject('component', 'foobar')><!--- the component defined above --->

<h2> Before:</h2>
<cfdump var="#oFoo.GetFoo()#">

<cfset fooReference = oFoo.GetFoo()>
<cfset fooReference.foo = "Wayne's world, excellent">

<h2> After:</h2>
<cfdump var="#oFoo.GetFoo()#">

If the 'foo' property were properly encapsulated here, both dumps would show the same thing. However, because we have a reference to the original data in the variables 'fooReference', we can make changes to the data directly (breaking encapsulation).

What to do about it?

It is important to note here that passing variables by reference saves memory - it is certainly NOT a bad thing! However, if encapsulation is more important to you, you can use the Duplicate() method to return a reference to a copy of the data which has the effect of returning the data itself:

<cffunction name="GetFoo">
   <cfreturn Duplicate(variables.foo)>
</cffunction>

Final note

ColdFusion passes queries, structures and external objects such as COM objects and CFC instances by reference.

Strings (including numeric values) and arrays are passed by value and do not need to be 'duplicated' in this way.

Here is a link to an Adobe livedocs page that talks about passing variables to and from functions in detail:

http://livedocs.adobe.com/coldfusion/7/htmldocs/wwhelp/wwhimpl/common/html/wwhelp.htm?context=ColdFusion_Documentation&file=00001008.htm

Scatting with ColdSpring

I really like ColdSpring - quiet, clean and powerful - mmmm (forgive me, I'm very tired). There is one thing that I have found that bugs me a little however; the verbosity of defining aspects and advisors.

Here's a quick xml sample of creating a really simple aspect and applying its advice to a bean:

<beans>
 <!-- state checker advice and advisor -->
 <bean id="stateChecker" class="aopxml.stateChecker" />
 <bean id="stateCheckerAdvisor" class="coldspring.aop.support.NamedMethodPointcutAdvisor">
  <property name="advice">
   <ref bean="stateChecker" />
  </property>
  <property name="mappedNames">
   <value>*</value>
  </property>
 </bean>

 <!-- test component with proxy -->
 <bean id="testerTarget" class="aopxml.tester" singleton="false" />
 <bean id="tester" class="coldspring.aop.framework.ProxyFactoryBean" singleton="false">
  <property name="target">
   <ref bean="testerTarget" />
  </property>
  <property name="interceptorNames">
   <list>
    <value>stateCheckerAdvisor</value>
   </list>
  </property>
 </bean>

</beans>

This is example is from my blogpost http://fusion.dominicwatson.co.uk/2008/03/stricter-oop-using-aop.html.

Worse still, if I want to apply my aspect to any another beans, I would have to explicitly do so by defining a proxy for each bean.

So what's the solution?

The first thing I thought was to have specific AOP xml tags for the configuration that would allow you to define aspects and instruct ColdSpring to automagically create proxies for you based on the component name rules you supply it.

I posted this thought on the ColdSpring google group and the reply made me look to the Spring framework (on which ColdSpring is based) - what does the Spring framework do? Well, it does pretty much exactly as I suggested:

http://static.springframework.org/spring/docs/2.5.x/reference/aop.html#aop-schema

So, with a minor adaption for ColdFusion (though not budging from the Spring Xml schema), here is how the above Xml example could look:

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans" xmlns:aop="http://www.springframework.org/schema/aop">
 <!-- 'new' aop tags -->
 <aop:config>
  <aop:aspect id="stateCheckerAspect" ref="stateChecker">
   <aop:around pointcut="*.*" method="invokeMethod"/>
  </aop:aspect>
 </aop:config>

 <!-- 'aspect' beans -->
 <bean id="stateChecker" class="temp.stateChecker" />

 <!-- 'regular' beans -->
 <bean id="tester" class="temp.tester"/>
</beans>

The above XML defines an Around advice that will be applied to every bean and every method in the factory. Of course, different rules could be setup and I think the benefit is quite graphical.

This got me all excited and sent me on an all night coding spree. The result was a new ColdSpring bean factory that would take the Xml above and make it work. Its just a proof of concept and a little rough around the edges but it shows what can be done with reasonable ease (thanks to the excellent codebase already in ColdSpring).

The extended factory will indeed parse most of the aop elements defined in the Spring schema. I won't go into detail as they are very well documented in the Spring documentation that I linked to. However, here is a quick example of an aop:config that the extended factory will do stuff with:

<aop:config>
 <aop:pointcut id="aopxmlPackagePC" expression="aopxml.*.Add*"/>
 <aop:aspect id="stateCheckerAspect" ref="stateChecker">
  <aop:around pointcut-ref="aopxmlPackagePC" method="invokeMethod"/>
 </aop:aspect>
 <aop:aspect id="someErrorCatchingAspect" ref="someAdviceBean">
  <aop:pointcut id="thisAppServices" expression="com.myCo.thisApp.services.*.*"/>
  <aop:after-throwing pointcut-ref="thisAppServices" method="afterThrowing"/>
 </aop:aspect>
</aop:config>

You can download the simple working example here. You may need to create a mapping to whereever you unzip it (mapping called 'aopxml').

A disclaimer: this is completely unofficial and just a proof of concept - I'm just scatting. Enjoy :)

Stricter OOP without AOP! Sort of ;)

I've been playing around with the idea of forcing my components not to add or remove variables from their 'variables' scope, other than when instantiated. Its something I'd like not to be possible and I'd like a ColdFusion error if it happens. I blogged my first effort at cracking it here:

Stricter OOP using AOP!

The biggest downside to this was complexity of implementation. While I don't mind hidden complexity, using AOP meant that I would have to explicitly declare that each of my objects was wanting to use this feature/'aspect' - the effort in doing so using ColdSpring far outweighed any benefit. This was a 'global aspect' that would be best placed in a global base class if possible.

So, how to wrap all of a component's methods automatically using a base class that it extended? The concept is similar to AOP but it would lack the hassle of having to apply AOP to my model. Of course, the hassle would be figuring out how to do it!

A very long and steep-learning-curved night later and I have a working version. It borrows from the concept used in ColdSpring's AOP implementation of creating a temporary file in order to create component methods on the fly (mixins). The use is slightly different however. The following is an attempt to explain the base class constructor as concicely as possible:

1. Store copy of self in a state variable.


2. Create temporary cfc file with methods of the same name as those in this component. The body of these methods have a single line that calls a 'CallMethod()' method, passing the method name and any args as arguments.


3. Instantiate temporary cfc and delete the file.


4. Overwrite the original component methods with those in the temporary component


5. Removed methods used in this process from the component

The result is a component that appears unchanged (from the outside) but that has each of its methods replaced with a wrapper that invokes the samed named method on a copy of the original component.

Once this component has been written, implementation couldn't be much easier. Simply extend the component and call it's constructor from within the extended component and you're done.

Rather than have all that code be used specifically for monitoring a component's variables, I first created an uber-base class which for now I have called selfproxy. This does what I have described above but does nothing useful in the 'self proxied' methods. To do something useful (such as monitor the variables scope), I just extend the component and override its method interceptor.

This 'self proxying' mullarky lacks the precision and flexibility of AOP, but if you're painting with a broad brush it might be a useful thing.

A working example (tested on ColdFusion 8 running on Windows XP).

I'm just scatting on a theme here and would be interested to hear any constructive criticism ;)

Anyways, time for bed!

A better autosuggesting widget

Following on from my previous post about extending the cfinput auto-suggest functionality (a tidy autosuggesting solution), I decided that my solution was too narrow. I have now written a custom tag that allows access to all the useful properties of the auto-suggest widget.

The tag can be found @ http://betterautosuggest.riaforge.org/

Here is a quick and slim example of how it can be used:

<cfimport taglib="myCustomTagsFolder" prefix="custom">

<cfform action="" method="post">
   <custom:betterautosuggest
           name="fruit"
           autosuggest="apple,banana,lemon,lime,mango,orange,peach,pear"
           delimchar=";"/>
</cfform>

Enjoy :)

A tidy autosuggesting solution

A question was asked on the CF-Talk list for which I did not know the answer. I chimed in anyhow and got investigating. In the process, I learned a whole load more javascript as well as all sorts of interesting things about CF8's ajax form fun and, in the end, I solved the problem consicely :)

The problem

"I have a form field that autosuggests an email address, however it only works with one email address at a time. How would I go about setting it up so that when a user starts typing the autosuggest will append the emailaddress with a semicolon and then allow the user to search for anotheraddress in the same field?"

The bottom line, unless I am misguided, is that it can't be done with the ColdFusion form tags. An autosuggestdelimiters attribute would be a really useful addition to the cfinput tag but for now, it doesn't exist.

Hunting for the solution

I had a look at the javascript that an autosuggest box produces and, after going crosseyed, found myself looking at the YAHOO.widget.autocomplete documentation (on which ColdFusion bases its autocomplete functionality). A quick look down the list of properties uncovered the delimChar property; 'surely not' I thought - surely. Set the delimChar property to the list delimiter you want and the auto suggest will work exactly as asked.

The solution

After some experimentation I came up with a single, tiny, javascript function to add in the delimiter to the autosuggest control:

SetDelimiter = function(elId, delim){
   if(!ColdFusion.objectCache[elId])
      alert("Error adding delimiter: Auto suggest item, '" + elId + "', could not be found");
   else
      ColdFusion.objectCache[elId].delimChar = delim;
}

This function needs to be run at some point after the page and the ajax objects have been loaded. I have it working at the moment by calling it when the input receives focus which clearly isn't the best but isn't going to hurt anyone (can someone tell me where it should go?)

Here is some CF code to demonstrate:

<script type="text/javascript" src="SetAutoSuggestDelimiter.js"></script>
<cfform action="" method="post">
 <cfinput name="person" autosuggest="cfc:foo.bar({cfautosuggestvalue})" onfocus="SetDelimiter('person',';')"/>
</cfform>

Time for bed!