Tutorial 7 - Properties

One of the goals of Qi4j is to give you domain modeling tools that allow you to more concisely use domain concepts in code. One of the things we do rather often is model Properties of objects as getters and setters. But this is a very weak model, and does not give you any access to metadata about the property, and also makes common tasks like UI binding non-trivial. There is also a lot of repetition of code, which is unnecessary. Using JavaBeans conventions one typically have to have code in five places for one property, whereas in Qi4j the same thing can be achieved with one line of code.

But lets start out easy. To declare a property you have to make a method in a mixin type that returns a value of the type Property, and which does not take any parameters. Here's a simple example:

    Property<String> name();

This declares a Property of type String with the name "name". The Property interface has methods "get" and "set" to access and mutate the value, respectively.

For now you will be responsible for implementing these methods, but later on these will be handled automatically, thus reducing Properties to one-liners!

In the Mixin implementation of the interface with the Property declaration you should have an injection of the Property, which is created for you by Qi4j. The injection can be done in a field like this:

    @PropertyField Property<String> name;

The PropertyField dependency injection annotation means that Qi4j will inject the Property for you. The field has the name "name", which matches the name in the interface, and therefore that Property is injected. You can then implement the method trivially by just returning the "name" field.

Properties can have Constraints just like method parameters. Simply set them on the Property method instead, and they will be applied just as before when you call "set".

Steps for this tutorial:

1. Remove JavaBeans properties from HelloWorldState
2. Add Property methods with the correct type and the @NotNull annotation
3. Update the state mixin to inject and return the properties as described above

Solution

If you have successfully completed the task, you should end up with the following artifacts;

HelloWorldBehaviour.java
HelloWorldBehaviourMixin.java
HelloWorldComposite.java
HelloWorldState.java
HelloWorldStateMixin.java