Android Studio brings a lot of new features specifically for Jetpack Compose. It embraces a code-first approach while improving the developer productivity without having to choose between using design interface or code editor.
A fundamental difference between View-based UI and Jetpack Compose is that
Compose doesn’t rely on View to render its composables. As a consequence of
this architecture approach, Android Studio offers extended features for Jetpack
Compose without having to open an emulator or connect to a device. Compared to
Android Views, this allows a faster, iterative process for developers to
implement their UI designs.
To enable Android Studio-specific features for Jetpack Compose, you need to add
these dependencies in your application build.gradle file:
debugImplementation "androidx.compose.ui:ui-tooling:1.3.2"
implementation "androidx.compose.ui:ui-tooling-preview:1.3.2"
Composable Preview
A composable is defined by a function, and annotated with @Composable:
@Composable
fun SimpleComposable() {
Text("Hello World")
}
To enable a preview of this composable, create another composable, annotated
with @Composable and @Preview. This new, annotated, composable now
contains the composable you created initially, SimpleComposable:
@Preview
@Composable
fun SimpleComposablePreview() {
SimpleComposable()
}
The @Preview annotation tells Android Studio that this composable should be shown in the design view of this file. Starting with Android Studio Electric Eel, you can see live updates to your composable preview as you make your edits.
You can add parameters manually in your code to customize the way Android Studio
renders @Preview. You can even add the @Preview annotation to the same
function multiple times to preview a composable with different properties.
@Preview features
Android Studio offers some features to extend composable previews. You can change their container design, interact with them, or deploy them directly to an emulator or device.
LocalInspectionMode
You can read from the LocalInspectionMode
CompositionLocal to see if the composable is rendered in a preview (inside an
inspectable component). If the composition is rendered in a preview,
LocalInspectionMode.currentevaluates to true. This information allows you to
do customize your preview; for example, you can show a placeholder image in the
preview window instead of showing real data.
@Composable
fun GreetingScreen(name: String) {
if (LocalInspectionMode.current) {
// Show this text in a preview window:
Text("Hello preview user!")
} else {
// Show this text in the app:
Text("Hello $name!")
}
}
Interactive mode
The interactive mode allows you to interact with a preview similarly to how you would on a device running your program, like a phone or tablet. The interactive mode is isolated in a sandbox environment (meaning, isolated from other previews), where you can click elements and enter user input in the preview. It’s a quick way to test different states, gestures, and even animations of your composable.
Preview interactive mode runs directly inside Android Studio without an emulator running, which results in some limitations:
- No network access
- No file access
- Some
ContextAPIs may not be fully available
Deploy Preview
You can deploy a specific @Preview to an emulator or physical device. The
preview is deployed within the same project app as a new activity, so it shares
the same context and permissions. It does not require you to write
boilerplate code asking for a permission if it has already been granted!
Click the Deploy to Device icon
next to the @Preview annotation or at the top of the preview, and Android
Studio deploys that @Preview to your connected device or emulator.
Use with different devices
In Android Studio Electric Eel, you can edit the device parameter of the
Preview annotation to define configurations for your Composables in different
devices.
When the device parameter has an empty string (@Preview(device = “”)), you can
invoke autocomplete by pressing Ctrl + Space. Then, you can set the
values of each parameter.
From autocomplete, you can select any device option from the list–for example,
@Preview(device = “id:pixel_4”). Alternatively, you can enter a custom device
by choosing spec:width=px,height=px,dpi=int… to set the individual values of
each parameter.
To apply, press Enter, or cancel with Esc.
If you set an invalid value, the declaration is underlined in red and a fix may
be available (Alt + Enter (⌥ + ⏎ for macOS) > Replace with ….
The Inspection attempts to provide a fix that is closest to resembling your
input.
Multipreview Annotations
With multipreview, you can define an annotation class that itself has multiple
@Preview annotations with different configurations. Adding this annotation to
a composable function automatically renders all of the different previews at
once. For example, you can use this annotation to preview multiple devices, font
sizes, or themes at the same time without repeating those definitions for every
single composable.
Start by creating your own custom annotation class:
@Preview(
name = "small font",
group = "font scales",
fontScale = 0.5f
)
@Preview(
name = "large font",
group = "font scales",
fontScale = 1.5f
)
annotation class FontScalePreviews
You can use this custom annotation for your preview composables:
@FontScalePreviews
@Composable
fun HelloWorldPreview() {
Text("Hello World")
}
You can combine multiple multipreview annotations and normal preview annotations, to create a more complete set of previews. Combining multipreview annotations doesn't mean all the different combinations are shown. Instead, each multipreview annotation acts independently and renders only its own variants.
@Preview(
name = "dark theme",
group = "themes",
uiMode = UI_MODE_NIGHT_YES
)
@FontScalePreviews
@DevicePreviews
annotation class CombinedPreviews
@CombinedPreviews
@Composable
fun HelloWorldPreview2() {
MaterialTheme { Surface { Text("Hello world") } }
}
The mix-and-match nature of multipreview-- and normal preview!-- lets you more comprehensively test many properties of larger scale projects.
Code navigation and composable outlines
You can hover over a preview to see the outlines of the composables contained within. Clicking on a composable outline triggers your editor view to navigate to its definition.
Copy @Preview render
Every rendered preview can be copied as an image by right clicking on it.
Set background color
By default, your composable is displayed with a transparent background.
To add a background, add the showBackground and backgroundColor parameters.
Keep in mind that backgroundColor is an ARGB Long, not a Color value:
@Preview(showBackground = true, backgroundColor = 0xFF00FF00)
@Composable
fun WithGreenBackground() {
Text("Hello World")
}
Dimensions
By default, @Preview dimensions are chosen automatically to wrap its content.
If you want to set the dimensions manually, you can add heightDp and widthDp
parameters. Keep in mind those values are already interpreted as dp-- you
don't need to add .dp at the end of the value:
@Preview(widthDp = 50, heightDp = 50)
@Composable
fun SquareComposablePreview() {
Box(Modifier.background(Color.Yellow)) {
Text("Hello World")
}
}
Locale
To test different user locales, you need to add the locale parameter:
@Preview(locale = "fr-rFR")
@Composable
fun DifferentLocaleComposablePreview() {
Text(text = stringResource(R.string.greeting))
}
System UI
If you need to display the status and action bars inside a preview, add the
showSystemUi parameter:
@Preview(showSystemUi = true)
@Composable
fun DecoratedComposablePreview() {
Text("Hello World")
}
UI mode
The parameter uiMode can take any of the Configuration.UI_* constants and allows you to change the behavior of the preview accordingly. For example, you can set the preview to Night Mode to see how the theme reacts.
@Preview tips
Benefits and best practices
One of the primary benefits of using @Preview composables is to avoid reliance
on the emulator in Android Studio. You can save the memory-heavy startup of the
emulator for more final look-and-feel changes, and @Preview's ability to make
and test small code changes with ease.
To leverage @Preview annotation most effectively, make sure to define your
screen(s) in terms of the state it receives as input and the events that it
outputs. In addition to improved testability, your screen(s) render easily
in your preview!
@Preview and large data sets
Very often, a need arises where you must pass a large data set to your
composable preview. To do this, simply pass sample data to a Composable Preview
function by adding a parameter with the @PreviewParameter
annotation.
@Preview
@Composable
fun UserProfilePreview(
@PreviewParameter(UserPreviewParameterProvider::class) user: User
) {
UserProfile(user)
}
To provide the sample data, create a class that implements
PreviewParameterProvider
and returns the sample data as a sequence.
class UserPreviewParameterProvider : PreviewParameterProvider<User> {
override val values = sequenceOf(
User("Elise"),
User("Frank"),
User("Julia")
)
}
This renders one preview per data element in the sequence:
You can use the same provider class for multiple previews. If necessary, limit the number of previews by setting the limit parameter.
@Preview
@Composable
fun UserProfilePreview2(
@PreviewParameter(UserPreviewParameterProvider::class, limit = 2) user: User
) {
UserProfile(user)
}
annotation class Preview
You can always 'command + Click' on the @Preview annotation in Android Studio
for a full list of parameters that can be adjusted when customizing your
preview.
annotation class Preview(
val name: String = "",
val group: String = "",
@IntRange(from = 1) val apiLevel: Int = -1,
val widthDp: Int = -1,
val heightDp: Int = -1,
val locale: String = "",
@FloatRange(from = 0.01) val fontScale: Float = 1f,
val showSystemUi: Boolean = false,
val showBackground: Boolean = false,
val backgroundColor: Long = 0,
@UiMode val uiMode: Int = 0,
@Device val device: String = Devices.DEFAULT
)
Additional Resources
To read more about how Android Studio promotes @Preview ease of use, and more
Tooling tips, check out the blog Compose Tooling.
Editor actions
Android Studio has also features inside the editor area to improve your productivity with Jetpack Compose.
Live Templates
Android Studio has added these Compose-related live templates, which allow you to enter code snippets for fast insertion by typing the corresponding template abbreviation:
compto set up a@Composablefunctionprevto create a@Previewcomposable functionpaddpto add apaddingModifier in dpweightto add aweightModifierW,WR,WCto surround the current composable with aBox,Row, orColumncontainer
Gutter icons
Gutter icons are contextual actions visible on the sidebar, next to the line numbers. Android Studio introduces several gutter icons specific to Jetpack Compose to ease your developer experience.
Deploy preview
You can deploy a @Preview to the emulator or physical device directly from the gutter icon:
Color picker
Whenever a color is defined inside or outside a composable, its preview is shown on the gutter. You can change the color via the color picker by clicking on it like this:
Image resource picker
Whenever a drawable, vector, or image is defined inside or outside a composable, its preview is shown on the gutter. You can change it via the image resource picker by clicking on it like this:
Iterative code development
As a mobile developer, you’re often developing your app’s UI step by step rather than developing everything at once. Android Studio embraces this approach with Jetpack Compose by providing tools that don’t require a full build to inspect, modify values and verify the final result.
Live edit of literals
Android Studio can update in real time some constant literals used in composables within previews, emulator, and physical device. Here are some supported types:
IntStringColorDpBoolean
You can view constant literals that trigger real time updates without the compilation step by enabling literal decorations through the Live Edit of literals UI indicator:
Live Edit
You can accelerate your Compose development experience by using Live Edit in the canary releases of Android Studio Giraffe. Live Edit is a more powerful version of Live edit of literals. The functionality allows you to see the effect of updates to composables in real time by automatically deploying code changes to an emulator or device.
Apply Changes
Apply Changes allows you to update code and resources without having to redeploy your app to an emulator or physical device (with some limitations).
Whenever you add, modify, or delete composables, you can update your app without having to redeploy it by clicking on this button:
Layout Inspector
Layout inspector allows you to inspect a Compose layout inside a running app in an emulator or physical device.
Get recomposition counts
You can use the layout inspector to check how often a composable is recomposed or skipped. If your UI has poor performance, this is often because of a coding error that forces your UI to be recomposed excessively. On the other hand, some coding errors can prevent your UI from being recomposed when it needs to be, which means UI changes aren't showing up on the screen. Tracking recompositions can help find both of these kinds of problems.
To track recomposition, turn on Show Recomposition Counts in the view options:
.
Once you've enabled it, the layout inspector shows recomposition counts on the left, and skipped recompositions on the right:
If you double-click a composable in the layout inspector, you're taken to the corresponding code for analysis.
Animation Preview
Android Studio allows you to inspect animations from Animation Preview. If an animation is described in a composable preview, you can inspect the exact value of each animated value at a given time, pause the animation, loop it, fast-forward it, or slow it, to help you debug the animation throughout its transitions:
You can also use Animation Preview to graph visualize animation curves, which is useful for making sure that the animation values are choreographed properly:
Animation Preview automatically detects inspectable animations, which are
indicated by the Start Animation Preview icon
.
If you have multiple animations, you can use Animation Preview to inspect and coordinate them all at once. You can also freeze a specific animation.
Animation Preview currently supports the updateTransition and
AnimatedVisibility
APIs. To access the latest features, use Animation Preview with
Android Studio Electric Eel and Compose 1.2.0-alpha01 and higher.
Enable experimental features
Some features are only available after enabling them manually in the experimental section within Android Studio preferences: File > Settings > Experimental (Android Studio > Preferences > Experimental on a Mac.)
