Gradle support the definition of so called live collections. These collections are mostly created based on criteria like with a filter() or matching() method. The collection content can change if the content of the source collection changes. For example the org.gradle.api.DomainObjectCollection interface has a matching method that returns a live collection. The list of tasks in a project implements this interface so we can have a live collection of tasks that match a certain criteria.

The nice thing about the live collection of tasks is that we can add tasks to the project after we have defined the task collection and they will be included in the collection. Normally if we would access the list of tasks in a project and use for example a findAll method than the returned list of tasks will not change. So if we add a new task to the project it will not be added to the list of task that apply to the condition of the findAll method.

Now if we use the matching() method on a list of tasks the result will be a live list of tasks. Even if we add tasks to the project after the definition of the list of tasks, they will be added to the collection.

Let‘s see the following Gradle build file that defines the tasks allCompile and allCopy. Each task has dependencies on other tasks. We use the dependsOn() method of a task to set those dependencies. The dependsOn() method accepts a collection of other tasks. For the allCompile task we don‘t use a live collection and for the allCopy task we use a live collection.

If we run the build script we get the following output:

We notice the allCompile tasks doesn‘t have any dependencies, because those task dependencies didn‘t exist when we used the dependsOn() method. The allCopy task has all task dependencies even though we created them later in the build script.

Bonus: we can use the findAll method to look for task dependencies, but we have to let Gradle evaluate this condition in a closure. So we can change our build script and use a closure with the dependsOn() method. Gradle will invoke the closure at execution time and not a configuration time. The dependencies tasks are then available and assigned as dependencies to the allCompile task:

If we invoke both tasks with Gradle we get the following output:

This blog post is based on Gradle version 1.0-rc-3

Gradle Goodness: Working with Live Task Collection