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Class Annotations in Flex

Class annotations, also known as metadata in Flex, are extremely valuable as they allow developers to provide additional information about classes, properties and methods which may not be appropriate to convey through implementation details such as Marker interfaces or some other means. An annotation can be viewed as a comment of sorts that provides a facility which can be utilized to convey the intent of a class, property or method, however unlike comments annotations are compiled into byte code with the class, thus allowing inspection at runtime via object introspection / reflection. Annotations do not directly affect code semantics themselves, however they can be inspected at runtime which in turn may affect the semantics of the running application.

Annotations are also very useful for providing pre-compiler instructions for generating boilerplate code. Although custom annotations can not be used in this way directly in Flex, this usage can be found throughout the Flex Framework. A perfect example of how the Flex framework uses annotations to generate boiler plate code is the [Bindable] meta-data tag, which itself is an annotation as are all meta-data tags in Flex. When a class or property is defined as [Bindable] the pre-compiler in turn reads this attribute and generates the code which facilitates actual data binding; i.e. PropertyChangeEvent, IEventDispatcher etc.

In order to use custom annotations in Flex you first need to instruct the compiler to keep Actionscript 3 metadata. This is achieved by using the keep-as3-metadata compiler argument. An example of setting two custom metadata attributes named “Foo” and “Bar” in the Flex IDE under project > compiler options is as follows.

Additionally annotations can be specified in a flex configuration file as follows:

When you create custom metadata in Actionscript you do so by first declaring the name of the annotation followed by arbitrary properties specified as name/value pairs. For example, you are most likely familiar with the [Event] metadata tag in Flex. The name of the annotation is “Event” and the valid properties for the annotation are “name” and “type”, as can be seen below:

Likewise you create your own custom metadata following the same format. The example which follows defines a custom metadata attribute utilized for annotating a class with version information. The name of the annotation is “Version”, which contains three properties; major, minor and revision.

Accessing custom annotations in Flex is accomplished via the flash.utils reflection APIs; describeType, getQualifiedClassName and getDefinitionByName.

To help simplify the process of accessing custom annotations in Flex I have developed a simple API: MetadataUtils which is an all static class that provides utility operations from which class annotations can be located and inspected, and Metadata which provides a strongly typed implementation of a metadata entry.

As you begin experimenting with your own custom annotations you will quickly find that there are numerous applications where they can be utilized, the most significant of which (IMHO) is to help facilitate IoC / DI solutions. It would be great if at some point Adobe would provide an APT implementation for mxmlc as part of a future release of Flex.

September 13, 2008 Software Engineering
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Let design guide, not dictate

A good design should intend to guide implementation, not dictate it; and for good reason as, the dynamic nature of requirements and systems is often too complex to view for a technical design to be considered as anything more than a basic prescription intended to convey the basis for implementation. Yet far too often people seem to believe that once a detailed design has been completed and approved implementation should be a breeze; however, this is just not a very realistic expectation.

For instance, one of my core job responsibilities is to review technical design documents and provide feedback and direction. This is an iterative process which typically has between 1-3 iterations depending on the complexity of the system. Initially myself and an engineer are given requirements for review. He or she then begins an initial draft of the design and once completed passes it on to me for review. I then review the document and provide feedback where applicable, either via annotations to the document itself or by reviewing with the developer (which is by far my preferred process). Should modifications be required the developer will then make revisions as needed. This process is repeated (within practicality) until final design has been approved.

At first it may appear as if only a single design iteration and review would be needed, however more often than not, requirements may not be completely understood during the beginning stages of design, nor are they typically ever set in stone so it is very common that a design will need to change during the early stages of a project or even throughout the entire development stage. Once final design has been completed an engineer then begins implementing the design. Theoretically this may appear to be a quite simple process: create a great design which contains as much detail as possible, review it, make revisions and approve it, then just pass it off to any developer for implementation and that’s it, done, right? – wrong!

There are a number of problems to this approach. Below I have outlined the three I feel are most significant and the solutions I have found to address each.

Creativity

The first problem is that a design which goes into too much detail completely limits or even worse, kills creativity – which in my opinion is the single most important trait a developer can possess, especially when designing. The developer is now merely a typist and will undoubtedly become very bored when implementing the design, especially if it is not even his/her design to begin with! Because of this lack of creativity the final code will ultimately suffer and bugs can be expected. Keeping design on a higher level allows developers to have the creative freedom needed to provide quality implementations and work they can feel is their own.

Flexibility

The second problem is that the more detailed and precise the design the less flexibility there is when requirements change and modifications need to be made to the design and thus implementation. For example, if a design contains very low level details, such as method signatures and other implementation specific details the ability to change the design now becomes increasingly complex and will result in much of the design needing to be reworked significantly. In addition the more detail there is the harder it is to write unit tests against the design as the actual implementation has already been defined. Designs need to be very high level and should not go beyond identifying class names, their responsibilities, relationships and dependencies.

Tools

The third problem is that far too often developers get caught up in all the details of UML notation and related tools. Again, this negates creativity and results in the developer concentrating more on making the design look technically correct rather than concentrating on designing towards a great solution which addresses the problem at hand. In addition, this also results in unnecessary time being spent to complete the design – time which otherwise could have been much better spent on something that produces a better pay off for the project. Now this is not to say that UML shouldn’t be used, actually quite the contrary as I feel a final design should be in UML (or some other format) as a shared language is very helpful in allowing readers to easily understand the design. I always suggest a technique where developers draw out their design in any way that makes sense to them without having to give much thought to anything other than the solution itself. This could be anything from drawing / scribbling thoughts on a pad, to building out a vision from legos – seriously! Only once the design has been envisioned would I recommend bringing it to realization through the use of a formal design tool, such as Visio or other UML tools to be used.

The above illustrates the three most common design issues I have encountered, most of which pertain to over-detailed designs, as well as the approach I take to address each. If you have not encountered any of these issues in your own work than that is generally a good sign, however try to keep them in mind when designing as it will pay off in the end. The important thing to remember when designing is to design for flexibility and simplicity. Less is usually more and the KISS principle, especially when applied to software design, will always pay off in the end.

June 29, 2008 Agile / Code Review / Design Patterns / Object Oriented Design / Software Engineering
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Package-level function closures in ActionScript

Package-level function closures are very useful for creating generalized functionality which does not require a class (static methods) or instance of a class (instance methods).

Unlike static and instance methods package-level function closures are not associated with a class or instance of a class but rather with a package. There are no syntactical differences between package-level functions and static or instance methods.

Package-level functions are for the most part utility functions; for instance the flash.utils package contains a number of package-level functions, the most common of which are describeType(), getDefintionByName(), getTimer() and so forth.

Package-level function closures are created by defining a function directly inside the body of a package (where class and interfaces are defined), as can be seen in the following example:

Calling a package level function is straightforward, simply import the function just as you would a class or interface and then invoke the function directly…

Typically you will find that most functionality can be grouped to a Class or an instance object, however on occasion you may identify specific functionality which is common to packaged functionality as opposed to a specific object, and in these cases utilizing package-level functions is a great option.

May 6, 2008 Object Oriented Design / Software Engineering
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Web-Based UML Sequence Diagram Generator

If you need to create sequence diagrams quickly and do not have the time to use the more traditional Software Modeling tools; Together, Enterprise Architect, Visio etc. you should take a look at www.websequencediagrams.com.

This handy little tool is pretty capable for a free web based utility and is very easy to use. It took me just seconds to create the simple sequence diagram below…

Web-Based UML Sequence Diagram Generator

So the next time you need to create UML sequence diagrams in a hurry make sure to check out this very useful tool.

April 14, 2008 Software Engineering
2 Comments

Principle of Least Knowledge

One very important (yet often overlooked) design guideline which I advocate is the Principle of least knowledge.

The Principle of Least knowledge, also known as The law of Demeter, or more precisely, the Law of Demeter for Functions/Methods (LoD-F) is a design principle which provides guidelines for designing a system with minimal dependencies. It is typically summarized as “Only talk to your immediate friends.”

What this means is a client should only have knowledge of an objects members, and not have access to properties and methods of other objects via the members. To put it in simple terms you should only have access to the members of the object, and nothing beyond that. Think if it like this: if you use more than 1 dot you are violating the principle.

Consider the following: We have three classes: ClassA, ClassB and ClassC. ClassA has an instance member of type ClassB. ClassB has an instance member of type ClassC. This can be designed in such a way which allows direct access all the way down the dependency chain to ClassC or beyond, as in the following example:

The above example is quite common, however it violates The Principle of Least Knowledge as it creates multiple dependencies, thus reducing maintainability as should the internal structure of ClassA need to change so would all instances of ClassA.

Now keep in mind that in all software development there are trade-offs to some degree. Sometimes performance trumps scalability or vice-versa, other times readability trumps both. A perfect example of where you would not want to use The Principle of Least Knowledge is in a Cairngorm ModelLocator implementation. The Cairngorm ModelLocator violates the Principle of least knowledge for good reason – it simply would not be practical to write wrapper methods for every object on the ModelLocator. This is the main drawback of the Principle of least Knowledge; the need to create wrapper methods for each object, which are more formally known as Demeter Transmogrifiers.

The goal of good software design is to minimize dependencies, and by carefully following the guidelines provided by The Principle of Least Knowledge this becomes much easier to accomplish.

February 2, 2008 Agile / Code Review / Design Patterns / News / Object Oriented Design / Software Engineering
1 Comment

Flex Unit TestSuiteHelper

Test Driven Development is an essential part of any software project. Leveraging Flex Unit in your Flex and AIR projects is a well known best practice and will inevitably result in less unknowns and provide a much more reliable codebase, however like most things in life that provide value, unit testing comes at a cost: Time.

In general, writing Unit tests is often an overlooked process as it ultimately equates to an additional level of effort (time) in a project plan. However, writing unit tests should not be over looked for a number of reasons, and in my opinion most importantly because it forces developers to understand what problems the code they write are intended to solve, thus resulting in a better understanding of the problem domain.

Now without getting into a lengthy discussion about the benefits of Test Driven Development, as there is plenty of information available on the subject, I am going to focus on how to make it easier in Flex.

Typically when I write a unit test I stub it out and then run the test runner to make sure it fails. I pretty much do this immediately after I write the test and then I see that my test did not come up and realize I never added it to the test suite. Obviously something like adding tests to a test suite can be an automated process which would allow unit tests to be developed a little faster as all that would be needed was to write the test.

So in an effort to simplify the process of adding tests to a test suite I have developed a TestSuiteHelper which is a simple utility class that automates the process of adding unit tests to a test suite. Rather than manually adding each test the TestSuiteHelper will automate the process simply by invoking a static method and passing in the test class from which you need to extract all of the tests.

So if you would like to simplify your team’s unit testing workflow feel free to utilize the TestSuiteHelper.

January 18, 2008 Software Engineering / TDD / BDD
4 Comments
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