JavaFX Model View ViewModel where should I implement tasks? - mvvm

Question

There are several tutorials and examples on this topic out there but they are all a sort of generic build only in one class to show how it works generally.
So my question is when I would like to follow the MVVM pattern where I have to implement all my tasks?
Given the following:
Model:
class Model {
/* When I place the Task here how can I deal with arguments and results from ViewController? */
public BufferedImage bigTask (String this, String and, Image that){
// Some code to build a BufferedImage
}
}
ViewModel:
class ViewController {
private BufferedImage myBufferedImage;
#FXML
private Button aButton;
/*Should I implement my Task here? But how I get information about progress? */
final Task<Integer> myTask = new Task<Integer>(){
#Override
protected Integer call() throws Exception{
updateProgress( // How to get here? Is it the right place? )
return null;
}
};
#FXML
void setOnAction(ActionEvent actionEvent){
myBufferedImage = Model.bigTask("this", "that", new Image("path"));
}
}
Hope I could explain the problem.
Thanks in advance!

Answer 1

In general your tasks should be implemented in the ViewModel.
The actual implementation of business logic should be done in the Model for example in a service class. The ViewModel can then use this service and handle all the ui specific actions like creating a Task for async execution and updating a progress value. However the ViewModel may not directly update a ProgressIndicator but instead the viewModel could have a DoubleProperty "progress" that is updated in the ViewModel. In the ViewController/CodeBehind you bind the actual ProgressIndicator to this progress property of the ViewModel. This way the ViewModel is independent from actual UI controls and the View doesn't contain any business logic.
Your example is a little bit special I think. Normally I would say that "BufferedImage" is a ui specific class that only belongs to the View and not the ViewModel nor the Model. However your example looks like BufferedImage is the result of a business action. In this case I would create a ObjectProperty<BufferedImage> in your ViewModel and put the task to load the image in the ViewModel too. In your ViewController I would add a listener to this property and put the image into the ui when it changes.
This way your View class is independet of how the image is loaded.