Change current implementation of basic MVVM to adhere to SOLID pattern

Question

I have been writing all my MVVM application with basic design pattern generally mentioned in MVVM examples available online. The pattern that I am following is described below:<

Answer 1

First of all an answer to your question 3

How can I start changing my current implementation to accommodate all those changes with best possible approach?

This is the very first step you need to take. It is not a case of some smart refactoring of your current code. You will need to take a step back and design the application. I once read some nice blog about (re)design.

Before starting to write any code, define how many different basic types of views you will want to show to the user? E.g.:

1. Just show (any type of) data
2. Edit data
3. Alert user
4. Ask user for input
5. ...

When you defined your different requirements, you can translate this to specific interfaces that are tailor made for the job they serve. For example a view that lets the user edit data will typically have an interface that will look something like:

public interface IEditViewModel
{
    public EditResult EditEntity(TEntity entityToEdit)();
}

Once you every detail of this design in place, you must decide how you will show your views to the user. I used another interface for this to handle this task for me. But you could also decide to let a navigation service handle this kind of task.

With this framework in place, you can start coding your implementations.

Currently View Datacontext is binded to ViewModel hence all the controls are controlled by a property in VM. How would I change this to your above mentioned pattern with Processor/Service or DialogHandler?

This will not change in this design. You will still bind your view to your viewmodel and set the datacontext to the viewmodel. With a lot of views the use of an MVVM framework like Caliburn Micro will come in handy. This will do a lot of the MVVM stuff for you, based on Convention over Configuration. To start with this model, would make the learning curve even higher, so my advice to start of by hand. You will learn this way what happens under the covers of such an MVVM tool.

I am using Delegatecommands which are binded to command property of UI element. Execution of these command certain action happens like animation, usercontrol is displayed. How to do it in command pattern?

I'm not sure if the command pattern you mention here is the command pattern I advised you in the previous answer. If so, I think you need to reread this blog, because this is totally unrelated to the commands I think you mean in this question.

Animation and that sort of stuff is the responsibility of the view, not the viewmodel. So the view should handle all this stuff. XAML has a lot of ways to handle this. More than I can explain here. Some ideas: Triggers, Dependency Properties

Another option: Code behind! If the logic is purely view related IMO it is not a mortal sin to place this code in the code behind of your view. Just don't be temped to do some gray area stuff!

For commands that just perform a method call in your viewmodel, ICommand is still possible and MVVM tools like Caliburn will do this automagically...

And still: Loose the base class....