Why is PropertyState Necessary - gradle

Question

Suppose I'm developing a Gradle plugin and the inputs that some of the tasks the plugin configures depend on how it is configured via an extension. For example:
class MyTask extends DefaultTask {
#InputFile
File toTrack
#TaskAction
def run() {
println("The file now contains ${toTrack.text}")
}
}
class MyConfig {
File toTrack = new File('bad-default.txt')
}
class MyPlugin implements Plugin<Project> {
#Override
def apply(Project project) {
project.with {
extensions.create('config', MyConfig)
task('printChanges', type: MyTask) {
toTrack = config.toTrack
}
}
}
}
Unfortunately, this doesn't work correctly. The problem is that if I have a build.gradle like:
apply plugin: my-plugin
config {
toTrack = file('file-to-track.txt')
}
where I've specified a file to track, Gradle will evaluate the #InputFile on my task before the config block is run so it'll decide if the task is up to date or not by looking at bad-default.txt rather than file-to-track.txt.
The recommended fix to this seems to be to use PropertyState like so:
class MyTask extends DefaultTask {
PropertyState<File> toTrack = project.property(File)
#InputFile
File getToTrack { return toTrack.get() }
#TaskAction
def run() {
println("The file now contains ${toTrack.get().text}")
}
}
class MyConfig {
private PropertyState<File> toTrack
MyConfig(Project project) {
toTrack = = project.property(File)
toTrack.set('bad-default.txt')
}
void setToTrack(File fileToTrack) { toTrack.set(fileToTrack) }
}
class MyPlugin implements Plugin<Project> {
#Override
def apply(Project project) {
project.with {
extensions.create('config', MyConfig)
task('printChanges', type: MyTask) {
toTrack = config.toTrack
}
}
}
}
That works but it seems very verbose and the PropertyState stuff seems entirely unnecessary. It seems like the real fix was simply to change the #InputFile annotation to be on a getter instead of having it be on the property. In other words, I believe the following has the same effect and is less code and easier to understand:
class MyTask extends DefaultTask {
File toTrack
// This is the only change: put the annotation on a getter
#InputFile
File getToTrack() { return toTrack }
#TaskAction
def run() {
println("The file now contains ${toTrack.text}")
}
}
class MyConfig {
File toTrack = new File('bad-default.txt')
}
class MyPlugin implements Plugin<Project> {
#Override
def apply(Project project) {
project.with {
extensions.create('config', MyConfig)
task('printChanges', type: MyTask) {
toTrack = config.toTrack
}
}
}
}
With some experiments this does seem to have the desired effect. What am I missing? Is there ever a time when PropertyState is necessary?

Answer 1

The problem is not the time #InputFile is evaluated. #InputFile is evaluated just before the task is executed, so after the configuration phase has finished and during gradles execution phase. The problem PropertyState is solving is the wiring between an extension and a task.
let's look again on your apply method:
def apply(Project project) {
project.with {
extensions.create('config', MyConfig)
task('printChanges', type: MyTask) {
toTrack = config.toTrack
}
}
}
here you:
1) Create your custom extension with the default value provided in the MyConfig class.
2) Link the value currently set in MyConfig to the MyTask toTrack property.
Now looking at the plugin usage:
apply plugin: my-plugin
config {
toTrack = file('file-to-track.txt')
}
here you:
1) Apply the plugin (and basically executing the apply method of your plugin).
2) Reconfigure the MyConfig#toTrack extension property.
But what isn't happening here, is updating the value in the printChanges task. That's what PropertyState is solving. It has nothing todo with Task inputs and outputs evaluation.