State Manager to manage screens - unit.pas files for separate scenes in each file

[anthonyedwardstark]

Hello my friend!

Seems that this framework is missing a class to manage the screens/scenes unit.pas files for separate scenes in each file .pas

Here is my sample using a class provided by Andreas Lago from PhoenixLib, I'm using ZenGL on this sample.

To jump on the screens you can use "F" from your keyboard.

Thanks

[anthonyedwardstark]

Here is a sample showing how to use multiple screens using just a Form of Delphi and Canvas.

[anthonyedwardstark]

### The Problems with the Singleton

**Invisible coupling** — any unit can access and modify the state, and you don’t know who is changing what.

**Hard to test** — you cannot create an isolated manager for tests; it is always the same global instance.

**Creation order** — if two units try to use the Singleton before it is created, you get a crash or undefined behavior.

**Multiple instances are impossible** — what if you want two managers? One for the game and another for the HUD? With a Singleton, you can’t.

**Silent death** — who is responsible for destroying it? No one really knows.

---

### This Class — The Real Advantages

**Explicit dependency** — whoever needs the manager receives it in the constructor. It becomes clear in the code who depends on whom:

```delphi
TScreenMenu.Create(AManager, ...)
// obvious: TScreenMenu needs a TScreenManager
```

**Controlled lifecycle** — the Form creates it, the Form destroys it. No mystery:

```delphi
procedure TFormMain.FormCreate;
begin
FScreenManager := TScreenManager.Create(Canvas, 800, 600);
end;

procedure TFormMain.FormDestroy;
begin
FScreenManager.Free; // you know exactly where it dies
end;
```

**Multiple instances** — if you need two separate contexts, it’s trivial:

```delphi
FMenuManager := TScreenManager.Create(Canvas1, 800, 600);
FHUDManager := TScreenManager.Create(Canvas2, 200, 100);
```

**Testable** — you can pass a fake canvas and test the logic in isolation.

**Naturally thread-safe** — each instance has its own state, without contention for a global resource.

---

| Aspect | Singleton | This Class |
| ------------------- | ---------- | ---------- |
| Coupling | High | Low |
| Testability | Difficult | Easy |
| Lifecycle | Obscure | Clear |
| Multiple instances | Impossible | Trivial |
| Who depends on whom | Invisible | Explicit |


---

**The Singleton solves the problem of “I need to access this from anywhere” — but that problem is usually a symptom of poor design.**

**This class solves the problem at its root.**

[anthonyedwardstark]

Here is a sample showing how to use multiple screens using just a Form of Delphi and Canvas.

please Lifepower and DraculaLin, test this sample with pure Delphi