Rules
State¶
Hoist all the things¶
Compose is built upon the idea of a unidirectional data flow, which can be summarised as: data/state flows down, and events fire up. To implement that, Compose advocates for the pattern of hoisting state upwards, enabling the majority of your composable functions to be stateless. This has many benefits, including far easier testing.
In practice, there are a few common things to look out for:
- Do not pass ViewModels (or objects from DI) down.
- Do not pass
MutableState<Bar>
instances down. - Do not pass inherently mutable types, that can't be observed types, down.
Instead pass down the relevant data to the function, and optional lambdas for callbacks.
More information: State and Jetpack Compose
Related rule: compose:vm-forwarding-check
State should be remembered in composables¶
Be careful when using mutableStateOf
(or any of the other state builders) to make sure that you remember
the instance. If you don't remember
the state instance, a new state instance will be created when the function is recomposed.
Related rule: compose:remember-missing-check
Use Immutable annotation whenever possible¶
The Compose Compiler tries to infer immutability and stability on value classes, but sometimes it gets it wrong, which then means that your UI will be doing more work than it needs. To force the compiler to see a class as 'immutable' you can apply the @Immutable
annotation to the class.
More info: Immutable docs and Composable metrics blog post
Related rule: TBD
Avoid using unstable collections¶
Collections are defined as interfaces (e.g. List<T>
, Map<T>
, Set<T>
) in Kotlin, which can't guarantee that they are actually immutable. For example, you could write:
// ❌ The compiler won't be able to infer that the list is immutable
val list: List<String> = mutableListOf<String>()
The variable is constant, its declared type is not mutable but its implementation is still mutable. The Compose compiler cannot be sure of the immutability of this class as it just sees the declared type and as such declares it as unstable.
To force the compiler to see a collection as truly 'immutable' you have a couple of options.
You can use Kotlinx Immutable Collections:
// ✅ The compiler knows that this list is immutable
val list: ImmutableList<String> = persistentListOf<String>()
Alternatively, you can wrap your collection in an annotated stable class to mark it as immutable for the Compose compiler.
// ✅ The compiler knows that this class is immutable
@Immutable
data class StringList(val items: List<String>)
// ...
val list: StringList = StringList(yourList)
Note: It is preferred to use Kotlinx Immutable Collections for this. As you can see, the wrapped case only includes the immutability promise with the annotation, but the underlying List is still mutable.
More info: Jetpack Compose Stability Explained, Kotlinx Immutable Collections
Related rule: compose:unstable-collections
Use mutableStateOf type-specific variants when possible¶
mutableIntStateOf
, mutableLongStateOf
, mutableDoubleStateOf
, mutableFloatStateOf
are essentially counterparts to mutableStateOf
, but with the added advantage of circumventing autoboxing on JVM platforms. This distinction renders them more memory efficient, making them the preferable choice when dealing with primitive types such as double, float, int, and long.
Functionally are the same, but they are preferred when dealing with these specific types.
Related rule: compose:mutable-state-autoboxing
Composables¶
Do not use inherently mutable types as parameters¶
This practice follows on from the 'Hoist all the things' item above, where we said that state flows down. It might be tempting to pass mutable state down to a function to mutate the value.
This is an anti-pattern though as it breaks the pattern of state flowing down, and events firing up. The mutation of the value is an event which should be modelled within the function API (a lambda callback).
There are a few reasons for this, but the main one is that it is very easy to use a mutable object which does not trigger recomposition. Without triggering recomposition, your composables will not automatically update to reflect the updated value.
Passing ArrayList<T>
or ViewModel
are common examples of this (but not limited to those types).
Related rule: compose:mutable-params-check
Do not use MutableState as a parameter¶
This practice also follows on from the 'Hoist all the things' item above. When using MutableState<T>
in a @Composable function signature as a parameter, this is promoting joint ownership over a state between a component and its user.
Instead, if possible, consider making the component stateless and concede the state change to the caller. If mutation of the parent’s owned property is required in the component, consider creating a ComponentState class with the domain specific meaningful field that is backed by mutableStateOf(...)
.
When a component accepts MutableState as a parameter, it gains the ability to change it. This results in the split ownership of the state, and the usage side that owns the state now has no control over how and when it will be changed from within the component’s implementation.
More info: Compose API guidelines
Related rule: compose:mutable-state-param-check
Be mindful of the arguments you use inside of a restarting effect¶
In Compose, effects like LaunchedEffect
, produceState
, or DisposableEffect
can take multiple keys as arguments to control when the effect restarts. The typical form for these APIs is:
EffectName(key1, key2, key3, ...) { block }
- Bugs if the effect restarts less often than needed.
- Inefficiency if the effect restarts more often than necessary.
To ensure proper behavior:
- Include mutable and immutable variables from the effect block as parameters.
- Additional parameters can be added for explicit restart control.
- Use
rememberUpdatedState
to prevent unnecessary restarts.- This is usually useful whenever it wouldn't be a good idea to restart the effect, e.g. it's invoked inside of a flow collector method.
- If a variable never changes due to remember with no keys, no need to pass it as a key to the effect.
Let's see some sample cases.
// ❌ onClick changes, but the effect won't be pointing to the right one!
@Composable
fun MyComposable(onClick: () -> Unit) {
LaunchedEffect(Unit) {
onClick()
}
// ...
}
// ✅ onClick changes and the LaunchedEffect won't be rebuilt -- but will point at the correct onClick!
@Composable
fun MyComposable(onClick: () -> Unit) {
val latestOnClick by rememberUpdatedState(onClick)
LaunchedEffect(Unit) {
latestOnClick()
}
// ...
}
// ✅ _If we don't care about rebuilding the effect_, we can also use the parameter as key
@Composable
fun MyComposable(onClick: () -> Unit) {
// This effect will be rebuilt every time onClick changes, so it will always point to the latest one.
LaunchedEffect(onClick) {
onClick()
}
}
More info: Restarting effects and rememberUpdatedState
Related rule: compose:lambda-param-in-effect
Do not emit content and return a result¶
Composable functions should either emit layout content, or return a value, but not both.
If a composable should offer additional control surfaces to its caller, those control surfaces or callbacks should be provided as parameters to the composable function by the caller.
More info: Compose API guidelines
Related rule: compose:content-emitter-returning-values-check
Note: To add your custom composables so they are used in this rule (things like your design system composables), you can add
composeEmitters
to this rule config in Detekt, orcompose_emitters
to your .editorconfig in ktlint.
Do not emit multiple pieces of content¶
A composable function should emit either 0 or 1 pieces of layout, but no more. A composable function should be cohesive, and not rely on what function it is called from.
You can see an example of what not to do below. InnerContent()
emits a number of layout nodes and assumes that it will be called from a Column
:
// This will render:
// <text>
// <image>
// <button>
Column {
InnerContent()
}
// ❌ Unclear UI, as we emit multiple pieces of content at the same time
@Composable
private fun InnerContent() {
Text(...)
Image(...)
Button(...)
}
However InnerContent could just as easily be called from a Row
or a Box
which would break all assumptions. Some other examples of interaction with InnerContent
could be:
// ❌ This will render: <text><image><button>
Row {
InnerContent()
}
// ❌ This will render all elements on top of each other.
Box {
InnerContent()
}
Instead, InnerContent should be cohesive and emit a single layout node itself:
// ✅
@Composable
private fun InnerContent() {
Column {
Text(...)
Image(...)
Button(...)
}
}
There is a slight exception to this rule, which is when the function is defined as an extension function of an appropriate scope, like so:
// ✅
@Composable
private fun ColumnScope.InnerContent() {
Text(...)
Image(...)
Button(...)
}
Related rule: compose:multiple-emitters-check
Note: To add your custom composables so they are used in this rule (things like your design system composables), you can add
composeEmitters
to this rule config in Detekt, orcompose_emitters
to your .editorconfig in ktlint.
Naming CompositionLocals properly¶
CompositionLocal
s should be named by using the adjective Local
as prefix, followed by a descriptive noun that describes the value they hold. This makes it easier to know when a value comes from a CompositionLocal
. Given that these are implicit dependencies, we should make them obvious.
More information: Naming CompositionLocals
Related rule: compose:compositionlocal-naming
Naming multipreview annotations properly¶
Multipreview annotations should be named by using Previews
as a prefix. These annotations have to be explicitly named to make sure that they are clearly identifiable as a @Preview
alternative on its usages.
More information: Multipreview annotations and Google's own predefined annotations
Related rule: compose:preview-annotation-naming
Naming @Composable functions properly¶
Composable functions that return Unit
should start with an uppercase letter. They are considered declarative entities that can be either present or absent in a composition and therefore follow the naming rules for classes.
However, Composable functions that return a value should start with a lowercase letter instead. They should follow the standard Kotlin Coding Conventions for the naming of functions for any function annotated @Composable
that returns a value other than Unit
More information: Naming Unit @Composable functions as entities and Naming @Composable functions that return values
Related rule: compose:naming-check
Naming Composable annotations properly¶
Custom Composable annotations (tagged with @ComposableTargetMarker
) should have the Composable
suffix (for example, @GoogleMapComposable
or @MosaicComposable
).
Related rule: compose:composable-annotation-naming-check
Ordering @Composable parameters properly¶
When writing Kotlin, it's a good practice to write the parameters for your methods by putting the mandatory parameters first, followed by the optional ones (aka the ones with default values). By doing so, we minimize the number times we will need to write the name for arguments explicitly.
Modifiers occupy the first optional parameter slot to set a consistent expectation for developers that they can always provide a modifier as the final positional parameter to an element call for any given element's common case.
More information: Kotlin default arguments, Modifier docs and Elements accept and respect a Modifier parameter.
Related rule: compose:param-order-check
Movable content should be remembered¶
The methods used to create movable composable content (movableContentOf
and movableContentWithReceiverOf
) need to be used inside a remember
function.
To work as intended, they need to persist through compositions - as if they get detached from the composition, they will be immediately recycled.
Related rule: compose:remember-content-missing-check
Make dependencies explicit¶
ViewModels¶
When designing our composables, we should always try to be explicit about the dependencies they take in. If you acquire a ViewModel or an instance from DI in the body of the composable, you are making this dependency implicit, which has the downsides of making it hard to test and harder to reuse.
To solve this problem, you should inject these dependencies as default values in the composable function.
Let's see it with an example.
// ❌ The VM dependency is implicit here.
@Composable
private fun MyComposable() {
val viewModel = viewModel<MyViewModel>()
// ...
}
But, if you change it to pass these instances via the composable function parameters, you could provide the instance you want directly in your tests without any extra effort. It would also have the upside of the function being explicit about its external dependencies in its signature.
// ✅ The VM dependency is explicit
@Composable
private fun MyComposable(
viewModel: MyViewModel = viewModel(),
) {
// ...
}
Related rule: compose:vm-injection-check
CompositionLocal
s¶
CompositionLocal
makes a composable's behavior harder to reason about. As they create implicit dependencies, callers of composables that use them need to make sure that a value for every CompositionLocal is satisfied.
Although uncommon, there are legit usecases for them, so this rule provides an allowlist so that you can add your CompositionLocal
names to it so that they are not flagged by the rule.
Related rule: compose:compositionlocal-allowlist
Note: To add your custom
CompositionLocal
to your allowlist, you can addallowedCompositionLocals
to this rule config in Detekt, orallowed_composition_locals
to your .editorconfig in ktlint.
Preview composables should not be public¶
When a composable function exists solely because it's a @Preview
, it doesn't need to have public visibility because it won't be used in actual UI. To prevent folks from using it unknowingly, we should restrict its visibility to private
.
Related rule: compose:preview-public-check
Note: If you are using Detekt, this may conflict with Detekt's UnusedPrivateMember rule. Be sure to set Detekt's ignoreAnnotated configuration to ['Preview'] for compatibility with this rule.
Modifiers¶
When should I expose modifier parameters?¶
Modifiers are the beating heart of Compose UI. They encapsulate the idea of composition over inheritance, by allowing developers to attach logic and behavior to layouts.
They are especially important for your public components, as they allow callers to customize the component to their wishes.
More info: Always provide a Modifier parameter
Related rule: compose:modifier-missing-check
Modifier order matters¶
The order of modifier functions is very important. Each function makes changes to the Modifier returned by the previous function, the sequence affects the final result. Let's see an example of this:
// ❌ The UI will be off, as the pressed state ripple will extend beyond the intended shape
@Composable
fun MyCard(modifier: Modifier = Modifier) {
Column(
modifier
// Tapping on it does a ripple, the ripple is bound incorrectly to the composable
.clickable { /* TODO */ }
// Create rounded corners
.clip(shape = RoundedCornerShape(8.dp))
// Background with rounded corners
.background(color = backgroundColor, shape = RoundedCornerShape(8.dp))
) {
// rest of the implementation
}
}
We can address this by simply reordering the modifiers.
// ✅ The UI will be now correct, as the pressed state ripple will have the same shape as the element
@Composable
fun MyCard(modifier: Modifier = Modifier) {
Column(
modifier
// Create rounded corners
.clip(shape = RoundedCornerShape(8.dp))
// Background with rounded corners
.background(color = backgroundColor, shape = RoundedCornerShape(8.dp))
// Tapping on it does a ripple, the ripple is bound correctly now to the composable
.clickable { /* TODO */ }
) {
// rest of the implementation
}
}
More info: Modifier documentation
Related rule: compose:modifier-clickable-order
Modifiers should be used at the top-most layout of the component¶
Modifiers should be applied once as a first modifier in the chain to the root-most layout in the component implementation. Since modifiers aim to modify the external behaviors and appearance of the component, they must be applied to the top-most layout and be the first modifiers in the hierarchy. It is allowed to chain other modifiers to the modifier passed as a param if needed.
More info: Compose Component API Guidelines
Related rule: compose:modifier-not-used-at-root
Don't re-use modifiers¶
Modifiers which are passed in are designed so that they should be used by a single layout node in the composable function. If the provided modifier is used by multiple composables at different levels, unwanted behaviour can happen.
In the following example we've exposed a public modifier parameter, and then passed it to the root Column, but we've also passed it to each of the descendant calls, with some extra modifiers on top:
// ❌ When changing `modifier` at the call site, it will the whole layout in unintended ways
@Composable
private fun InnerContent(modifier: Modifier = Modifier) {
Column(modifier) {
Text(modifier.clickable(), ...)
Image(modifier.size(), ...)
Button(modifier, ...)
}
}
// ✅ When changing `modifier` at the call site, it will only affect the external container of the UI
@Composable
private fun InnerContent(modifier: Modifier = Modifier) {
Column(modifier) {
Text(Modifier.clickable(), ...)
Image(Modifier.size(), ...)
Button(Modifier, ...)
}
}
Related rule: compose:modifier-reused-check
Modifiers should have default parameters¶
Composables that accept a Modifier as a parameter to be applied to the whole component represented by the composable function should name the parameter modifier and assign the parameter a default value of Modifier
. It should appear as the first optional parameter in the parameter list; after all required parameters (except for trailing lambda parameters) but before any other parameters with default values. Any default modifiers desired by a composable function should come after the modifier parameter's value in the composable function's implementation, keeping Modifier as the default parameter value.
More info: Modifier documentation
Related rule: compose:modifier-without-default-check
Naming modifiers properly¶
Composables that accept a Modifier as a parameter to be applied to the whole component represented by the composable function should name the parameter modifier
.
In cases where Composables accept modifiers to be applied to a specific subcomponent should name the parameter xModifier
(e.g. fooModifier
for a Foo
subcomponent) and follow the same guidelines above for default values and behavior.
More info: Modifier documentation
Related rule: compose:modifier-naming
Avoid Modifier extension factory functions¶
Using @Composable
builder functions for modifiers is not recommended, as they cause unnecessary recompositions. To avoid this, you should use Modifier.Node instead. It will allow you to accomplish the same things while being very performant.
There is another API for creating custom modifiers, composed {}
. This API is no longer recommended due to the performance issues it created, and like with the extension factory functions case, Modifier.Node is recommended instead.
More info: Modifier.Node, Compose Modifier.Node and where to find it, by Merab Tato Kutalia, Compose modifiers deep dive, with Leland Richardson and Composed modifier docs.
Related rules: compose:modifier-composable-check and compose:modifier-composed-check
ComponentDefaults¶
ComponentDefaults object should match the composable visibility¶
If your composable has an associated Defaults
object to contain its default values, this object should have the same visibility as the composable itself. This will allow consumers to be able to interact or build upon the original intended defaults, as opposed to having to maintain their own set of defaults by copy-pasting.
More info: Compose Component API Guidelines
Related rule: compose:defaults-visibility
Opt-in rules¶
Note: These rules are disabled by default, you'll need to explicitly enable them individually in detekt/ktlint.
Don't use Material 2¶
Material Design 3 is the next evolution of Material Design. It includes updated theming, components, and Material You personalization features like dynamic color. It supersedes Material 2, and using Material 3 usage is recommended instead of Material 2.
More info: Migration to Material 3
Related rule: compose:material-two