Flex code annotations

Today I was talking with a friend of mine that said to me that the only way to find something related to code annotations in Flex is a book he has… that’s definitely not true, the Flex documentation contain a lot of info on this topic and the web is full of resources.

In this short post I want to summarize the building blocks of code annotation in Flex.

In computing, the programmer often adds annotations to source code in the form of comments. These do not affect the working of the program but give explanations (for other programmers, or potential readers of the code principally, but also as a reminder for the author),  hints or plans for improvement, etc.
Further Annotations can also be added by a compiler or programmer in the form of metadata, which is then made available in later stages of building or executing a program. For example, a compiler may use metadata to make decisions about what warnings to issue, or a linker can use metadata to connect multiple object files into a single executable.

In computer programming, a Java annotation is a way of adding metadata to Java source code that can also be available to the programmer at run-time.
Java annotations can be added to program elements such as classes, methods, fields, parameters, local variables, and packages.
Unlike tags added to Java documentation and processed with tools such as XDoclet, Java annotations are completely accessible to the programmer while the software is running using reflection.

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.

You can 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.

ArrayElementType metadata tag can be easily associated to code annotations in fact when you define an Array variable in ActionScript, you specify Array as the data type of the variable but you cannot specify the data type of the elements of the Array.
To allow the Flex MXML compiler to perform type checking on Array elements, you can use the [ArrayElementType] metadata tag to specify the allowed data type of the Array elements, as the following example shows:

In a more complex environment you can use java beans in Flex as the datatype of the ArrayElementType tag specifyng the fully qualified name of the class or get inspiration from this article http://www.flexpasta.com/index.php/2008/05/19/blazeds-with-annotations-for-remote-objects/ that is strictly related to Flex and Blaze DS (i.e. java based enterprise applications).

This tag only works for Arrays – there is no way to set the type of elements in ArrayCollection, to have typed array collection, you can take existing ArrayCollection.as rename it to YourArrayCollection.as and modify source property and add ArrayElementType tag with your prefered type, (I have not tried it but it should work) or even better extend the ArrayCollection in order to create your custom collection with the support for the ArrayElementType metatag.

In order to check the generated code add the -keep parameters to the flex builder compiler.



3 Responses to “Flex code annotations”
  1. Alessandro Ronchi October 14, 2008
  2. Java Programming October 14, 2008
  3. Pydrorytons January 13, 2010

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