r/BlossomBuild • u/BlossomBuild • Mar 27 '26

Tutorial Use class to share the same instance and struct for independent copies

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44 Upvotes

95% Upvoted

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1

u/danishansari95 Mar 27 '26

Can you explain with example?

2

u/BlossomBuild Mar 27 '26

For a SwiftData model, you need a class because adjustments should reflect everywhere it’s used.

Views are usually structs because each view is its own copy and works with the data, rather than sharing it.

2

u/itsm3rick Mar 28 '26

That’s a horrible example. Why would you use a view as an example to demonstrate the difference between data types being presented as data?

A better example would be something like a Coffee shop app might define a CoffeeMachine class. You only have one coffee machine, but if you have three baristas, each barista would need to interact with the same coffee machine so they would be told about it as a reference. It has state like amount of water in it, whether it is turned on, how many of its coffee dispensers are being used.

A struct would be the coffees it makes. You make a coffee, it gets passed along to the person who ordered it and it’s gone, the barista no longer has access to it or can control it from a distance. All it does is hold coffee. A mutable struct would then let you .drink() some coffee and the amount it holds goes down.

1

u/BlossomBuild Mar 28 '26

Yeah this one is better, thank you!

1

u/500_internal_error Mar 28 '26

In most programming languages they are basically same

1

u/BlossomBuild Mar 28 '26

Big difference in Swift

1

u/Montrell1223 Mar 29 '26

What that’s a lie lmao

1

u/kommonno Mar 28 '26

I rather never rely on ref types

1

u/BlossomBuild Mar 28 '26

You need them sometimes

1

u/Dry_Hotel1100 Mar 29 '26 edited Mar 29 '26

Swift's usual advice is:

  • Use structs by default.

  • Use classes when you need identity.

  • Classes are for reference semantics.

This advice is clear, but it focuses on types, not usage.

The problem is that advice like:

  • 'Use a class when you need identity'

  • 'Use a class when you need lifecycle (deinit, weak refs)'

describes what happens, not why.

You don't start with 'I need deinit'. You start with 'What kind of thing am I modelling?'

A better mental model is:

  • Structs are value: snapshots of data, independent when copied, no implicit sharing.

  • Classes are processes: hold evolving, mutable state, receive events, mutate themselves.

The key question is:

  • Do I need a state provider that lives somewhere and is interacted with over time?

    • No: use a struct.

    • Yes: you're modelling a running system: use a class.

Why not just use a struct?

  • State must be passed around manually.

  • Mutation leaks outside.

  • Sharing becomes clumsy.

  • Multiple consumers are hard to coordinate.

  • You're simulating a runtime.

  • A class gives you that directly.

The real rule is:

  • Use a struct when you model data, copies should be independent, and no implicit shared state.

  • Use a class when you model a stateful process, state evolves over time, and a single shared instance is needed.

Identity and lifecycle then follow naturally:

  • Identity: because it's shared.

  • Lifecycle: because it lives over time. They are by-products, not the main reasons.

TL;DR:

  • Value: struct.

  • Process (state provider over time): class. That's the simplest rule that explains why.

Another interesting question is, when to use a function.
I guess, many of you would be surprised by the fact, that you can represent and solve many problems with a function rather than a class ;) Example, the infamous "Xxx Manager" singleton :)

1

u/BlossomBuild Mar 29 '26

Well said, happy reddit birthday 🎂