A Kobalt plug-in in ten minutes

In tnis example, we'll write a Kobalt plug-in that simply counts the number of source files and lines in your project. Starting from scratch, we'll have this plug-in published to JCenter and ready to use in ten minutes.

Let's start by creating our project:

$ mkdir linecount
$ mkdir -p src/main/kotlin/com/beust/plugin/linecount
$ touch src/main/kotlin/com/beust/plugin/linecount/Main.kt
$ $KOBALT_HOME/kobaltw --init

I create an empty Main.kt in the example above so that calling ./kobaltw --init will detect the project as a Kotlin one. This way, the Build.kt file generated is already configured for Kotlin. Since we will be publishing this project to a Maven repository, we need to make sure that its group, artifactId and version are correct. The only thing that the generator can't guess is the group, so let's go ahead and fix it:

val project = kotlinProject {
    name = "kobalt-line-count"
    group = "com.beust.kobalt"
    artifactId = name
    version = "0.1"
    ...

Next, we want the manifest of our jar file to point to our main Kobalt plug-in class:

val packProject = packaging(project) {
    jar {
        manifest {
            attributes("Kobalt-Plugin-Class", "com.beust.kobalt.plugin.linecount.Main")
        }
    }
}

Now we're ready to code.

Let's start by writing the simplest plug-in we can:

package com.beust.kobalt.plugin.linecount

import com.beust.kobalt.api.*

public class Main : BasePlugin() {
    override val name = "kobalt-line-count"

    override fun apply(project: Project, context: KobaltContext) {
        println("*** Applying plugin ${name} with project ${project}")
    }
}

Before we can upload it, we need to create the package in bintray, as explained here. Once this is done, we are ready to do our first upload:

$ ./kobaltw uploadJcenter
...
========== kobalt-line-count:uploadJcenter
  kobalt-line-count: Found 2 artifacts to upload
  All artifacts successfully uploaded
  ############# Time to Build: 3590 ms

If you go to the maven section of your bintray account, you will now see that the new package has two unpublished files. Your new plug-in won't be visible by clients until you publish those files, so let's update our build file to automatically publish files from now on:

val jc = jcenter(project) {
    publish = true
}

And now, let's implement our logic, which is pretty simple:

// Main.kt
@Task(name = "lineCount", description = "Count the lines", runBefore = arrayOf("compile"))
fun lineCount(project: Project): TaskResult {
    var fileCount = 0
    var lineCount : Long = 0
    val matcher = FileSystems.getDefault().getPathMatcher("glob:**.kt")
    project.sourceDirectories.forEach {
        Files.walkFileTree(Paths.get(it), object: SimpleFileVisitor() {
            override public fun visitFile(path: Path, attrs: BasicFileAttributes): FileVisitResult {
                if (matcher.matches(path)) {
                    fileCount++
                    lineCount += Files.lines(path).count()
                }
                return FileVisitResult.CONTINUE
            }
        })
    }
    log(1, "Found ${lineCount} lines in ${fileCount} files")
    return TaskResult()
}

We create a task called "lineCount" in which we look for all files ending in ".kt" in all the source directories of the project. Finally, we display a count of files and lines at the end by using KobaltLogger.log(), which is obtained by extending the trait KobaltLogger:

public class Main : BasePlugin(), KobaltLogger {

Let's bump our version to 0.2 (since version 0.1 is already uploaded and JCenter won't allow us to overwrite it) and upload our new plug-in:

$ ./kobaltw uploadJcenter
...
kobalt-line-count: Compilation succeeded
========== kobalt-line-count:assemble
Created /Users/beust/kotlin/kobalt-line-count/kobaltBuild/libs/kobalt-line-count-0.2.jar
========== kobalt-line-count:generatePom
 Wrote /Users/beust/kotlin/kobalt-line-count/kobaltBuild/libs/kobalt-line-count-0.2.pom
========== kobalt-line-count:uploadJcenter
kobalt-line-count: Found 2 artifacts to upload
All artifacts successfully uploaded

Time to Build: 5907 ms

Finally, let's use our plug-in from another project. Since we didn't link this project to JCenter, it's uploaded in the user's maven repository, so we will have to add this maven repository to the build file where we want to use the plug-in. Adjust this line to point to your own maven repo:

val repos = repos("https://dl.bintray.com/cbeust/maven/")
val plugins = plugins("com.beust.kobalt:kobalt-line-count:0.2")

Now let's launch a build:

$ ./kobaltw assemble
...
========== kobalt:lineCount
Found 4972 lines in 65 files
========== kobalt:compile
...

Note that our plugin ran before the compile task, as we requested in the @Task annotation. We can also verify that it's activated and we can invoke the task directly instead of having it run as part of the build:

$ ./kobaltw --tasks
  ===== kobalt-line-count =====
    lineCount		Count the lines
$ ./kobaltw lineCount
Found 4972 lines in 65 files

And that's it! You can now iterate on your plug-in and upload it with additional ./kobaltw uploadJcenter. This plug-in is available on github.