tree 23b1ee6c363ff7f109bed19ecbd58045bfc204e8
parent f929cf4dd2e8e56c8ecc921228d57279b933f19c
author Leland Richardson <lelandr@google.com> 1589414339 -0700
committer Leland Richardson <lelandr@google.com> 1589481153 +0000

Deprecate @Model

Relnote: “
@Model annotation is now deprecated. Use state and mutableStateOf as alternatives. This deprecation decision was reached after much careful discussion.

Justification
=============

Rationale includes but is not limited to:
- Reduces API surface area and concepts we need to teach
- More closely aligns with other comparable toolkits (Swift UI, React, Flutter)
- Reversible decision. We can always bring @Model back later.
- Removes corner-case usage and difficult to answer questions about configuring @Model as things we need to handle
  - @Model data classes, equals, hashcode, etc.
  - How do I have some properties “observed” and others not?
  - How do I specify structural vs. referential equality to be used in observation?
- Reduces “magic” in the system. Would reduce the likelihood of someone assuming system was smarter than it is (ie, it knowing how to diff a list)
- Makes the granularity of observation more intuitive.
- Improves refactorability from variable -> property on class
- Potentially opens up possibilities to do hand-crafted State-specific optimizations
- More closely aligns with the rest of the ecosystem and reduces ambiguity towards immutable or us “embracing mutable state” 

Migration Notes
===============

Almost all existing usages of @Model are fairly trivially transformed in one of two ways. The example below has a @Model class with two properties just for the sake of example, and has it being used in a composable.

```
@Model class Position(
 var x: Int,
 var y: Int
)

@Composable fun Example() {
 var p = remember { Position(0, 0) }
 PositionChanger(
   position=p,
   onXChange={ p.x = it }
   onYChange={ p.y = it }
 )
}
```

Alternative 1: Use State<OriginalClass> and create copies.
----------------------------------------------------------

This approach is made easier with Kotlin’s data classes. Essentially, make all previously `var` properties into `val` properties of a data class, and then use `state` instead of `remember`, and assign the state value to cloned copies of the original using the data class `copy(...)` convenience method.

It’s important to note that this approach only works when the only mutations to that class were done in the same scope that the `State` instance is created. If the class is internally mutating itself outside of the scope of usage, and you are relying on the observation of that, then the next approach is the one you will want to use.

```
data class Position(
 val x: Int,
 val y: Int
)

@Composable fun Example() {
 var p by state { Position(0, 0) }
 PositionChanger(
   position=p,
   onXChange={ p = p.copy(x=it) }
   onYChange={ p = p.copy(y=it) }
 )
}
```

Alternative 2: Use mutableStateOf and property delegates
--------------------------------------------------------

This approach is made easier with Kotlin’s property delegates and the `mutableStateOf` API which allows you to create MutableState instances outside of composition. Essentially, replace all `var` properties of the original class with `var` properties with `mutableStateOf` as their property delegate. This has the advantage that the usage of the class will not change at all, only the internal implementation of it. The behavior is not completely identical to the original example though, as each property is now observed/subscribed to individually, so the recompositions you see after this refactor could be more narrow (a good thing).

```
class Position(x: Int, y: Int) {
 var x by mutableStateOf(x)
 var y by mutableStateOf(y)
}

// source of Example is identical to original
@Composable fun Example() {
 var p = remember { Position(0, 0) }
 PositionChanger(
   position=p,
   onXChange={ p.x = it }
   onYChange={ p.y = it }
 )
}
```
“

Bug: 156546430
Bug: 152993135
Bug: 152050010
Bug: 148866188
Bug: 148422703
Bug: 148394427
Bug: 146362815
Bug: 146342522
Bug: 143413369
Bug: 135715219
Bug: 126418732
Bug: 147088098
Bug: 143263925
Bug: 139653744

Change-Id: I409e8c158841eae1dd548b33f1ec80bb609cba31