How to handle state transitions and yet replace "if" statements with polymorphic types?

Question

Recently I was listening to a tech talk on clean coding. The speaker was a test engineer, who emphasized on avoiding the "if" statements in the code and use polymorphism as much as possible. Also he advocated against global states.

I quite agree with him, yet i need a clarification on replacing the global state and "if" statement using polymorphism for the below scenario,

I have 3 states in my document. I want to change the state of the UI components based on the document state. Right now, i use "if" blocks and an enumeration type holding the current state of document to transition the states of UI components.

eg:

enum DOC_STATE
{
DOC_STATE_A = 0,
DOC_STATE_B,
DOC_STATE_C
};

void QMainWindow::handleUi(_docState)
{
switch(_docState)
{
case (DOC_STATE_A):
{
menu.disable();
....
}
case (DOC_STATE_B):
{
menu.enable();
...
}
case (DOC_STATE_C):
{
...
}
}

I think i can have separate child classes for each state and have the handleUI() method in each class. Calling handleUi() method calls the right method call. But say i maintain these objects in my doc, how do i switch from one object to other each time there is a transition in state?

In other words, how to handle UI transition by tracking the change in state of document without using a global state and "if" or Switch statements?

I use Qt. Thanks.

Answer 1

I don't think I understand the problem because the answer is too trivial: you replace the pointer to your state instance with a new state instance and discard the old one.

Answer 2

If you are using Qt, take a look at The Qt State Machine Framework and the State Machine Examples. No need to re-invent the wheel when your framework already provides a sports car :)