Modifiers allow you to decorate or augment a composable. Modifiers let you do these sorts of things:
- Change the composable's size, layout, behavior, and appearance
- Add information, like accessibility labels
- Process user input
- Add high-level interactions, like making an element clickable, scrollable, draggable, or zoomable
Modifiers are standard Kotlin objects. Create a modifier by calling one of the
Modifier
class functions:
import androidx.compose.ui.Modifier
@Composable
private fun Greeting(name: String) {
Column(modifier = Modifier.padding(24.dp)) {
Text(text = "Hello,")
Text(text = name)
}
}
You can chain these functions together to compose them:
@Composable
private fun Greeting(name: String) {
Column(modifier = Modifier
.padding(24.dp)
.fillMaxWidth()
) {
Text(text = "Hello,")
Text(text = name)
}
}
In the code above, notice different modifier functions used together.
padding
puts space around an element.fillMaxWidth
makes the composable fill the maximum width given to it from its parent.
It's a best practice to have all of your Composables accept a modifier
parameter, and pass that modifier to its first child that emits UI.
Doing so makes your
code more reusable and makes its behavior more predictable and intuitive. For
more information, see the Compose API guidelines, Elements accept and respect a
Modifier parameter.
Order of modifiers matters
The order of modifier functions is significant. Since 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:
@Composable
fun ArtistCard(/*...*/) {
val padding = 16.dp
Column(
Modifier
.clickable(onClick = onClick)
.padding(padding)
.fillMaxWidth()
) {
// rest of the implementation
}
}
In the code above the whole area is clickable, including the surrounding
padding, because the padding
modifier has been applied after the clickable
modifier. If the modifiers order is reversed, the space added by padding
does
not react to user input:
@Composable
fun ArtistCard(/*...*/) {
val padding = 16.dp
Column(
Modifier
.padding(padding)
.clickable(onClick = onClick)
.fillMaxWidth()
) {
// rest of the implementation
}
}
Built-in modifiers
Jetpack Compose provides a list of built-in modifiers to help you decorate or augment a composable. Here are some common modifiers you'll use to adjust your layouts.
padding and size
By default, layouts provided in Compose wrap their children. However,
you can set a size by using the size
modifier:
@Composable
fun ArtistCard(/*...*/) {
Row(
modifier = Modifier.size(width = 400.dp, height = 100.dp)
) {
Image(/*...*/)
Column { /*...*/ }
}
}
Note that the size you specified might not be respected if it does not satisfy
the constraints coming from the layout's parent. If you require the composable
size to be fixed regardless of the incoming constraints, use the requiredSize
modifier:
@Composable
fun ArtistCard(/*...*/) {
Row(
modifier = Modifier.size(width = 400.dp, height = 100.dp)
) {
Image(
/*...*/
modifier = Modifier.requiredSize(150.dp)
)
Column { /*...*/ }
}
}
In this example, even with the parent height
set to 100.dp
, the height of
the Image
will be 150.dp
, as the requiredSize
modifier takes
precedence.
If you want a child layout to fill all the available height allowed by the
parent, add the fillMaxHeight
modifier (Compose also provides
fillMaxSize
and fillMaxWidth
):
@Composable
fun ArtistCard(/*...*/) {
Row(
modifier = Modifier.size(width = 400.dp, height = 100.dp)
) {
Image(
/*...*/
modifier = Modifier.fillMaxHeight()
)
Column { /*...*/ }
}
}
To add padding all around an element, set a padding
modifier.
If you want to add padding above a text baseline such that you achieve a
specific distance from the top of the layout to the baseline, use the
paddingFromBaseline
modifier:
@Composable
fun ArtistCard(artist: Artist) {
Row(/*...*/) {
Column {
Text(
text = artist.name,
modifier = Modifier.paddingFromBaseline(top = 50.dp)
)
Text(artist.lastSeenOnline)
}
}
}
Offset
To position a layout relative to its original position, add the offset
modifier and set the offset in the x and y axis. Offsets can be positive
as well as non-positive. The difference between padding
and offset
is that
adding an offset
to a composable does not change its measurements:
@Composable
fun ArtistCard(artist: Artist) {
Row(/*...*/) {
Column {
Text(artist.name)
Text(
text = artist.lastSeenOnline,
modifier = Modifier.offset(x = 4.dp)
)
}
}
}
The offset
modifier is applied horizontally according to the layout direction.
In a left-to-right context, a positive offset
shifts the element to the
right, while in a right-to-left context, it shifts the element to the left.
If you need to set an offset without considering layout direction, see the
absoluteOffset
modifier, in which a positive offset value always shifts the element to the
right.
Type safety in Compose
In Compose, there are modifiers that only work when applied to children of
certain composables. For example, if you want to make a child as big as the
parent Box
without affecting the Box
size, use the
matchParentSize
modifier.
Compose enforces this type safety by means of custom scopes.
For example, matchParentSize
is only available in
BoxScope
.
Therefore, it can only be used when the child is used within a Box
.
Scoped modifiers notify the parent about some information the parent should know about the child. These are also commonly referred to as parent data modifiers. Their internals are different from the general purpose modifiers, but from a usage perspective, these differences don't matter.
matchParentSize in Box
As mentioned above, if you want a child layout to be the same size as a parent
Box
without affecting the Box
size, use the matchParentSize
modifier.
Note that matchParentSize
is only available within a Box
scope, meaning that
it only applies to direct children of Box
composables.
In the example below, the child Spacer
takes its size from its parent Box
,
which in turn takes its size from the biggest children,
ArtistCard
in this case.
@Composable
fun MatchParentSizeComposable() {
Box {
Spacer(Modifier.matchParentSize().background(Color.LightGray))
ArtistCard()
}
}
If fillMaxSize
were used instead of matchParentSize
, the Spacer
would take
all the available space allowed to the parent, in turn causing the parent to
expand and fill all the available space.
weight in Row and Column
As you have seen in the previous section on Padding and
size, by default, a composable size is defined by the
content it is wrapping. You can set a composable size to be flexible within its
parent using the weight
Modifier that is only available in RowScope
, and
ColumnScope
.
Let’s take a Row
that contains two Box
composables.
The first box is given twice the weight
of the second, so it's given twice the
width. Since the Row
is 210.dp
wide, the first Box
is 140.dp
wide, and
the second is 70.dp
:
@Composable
fun ArtistCard(/*...*/) {
Row(
modifier = Modifier.fillMaxWidth()
) {
Image(
/*...*/
modifier = Modifier.weight(2f)
)
Column(
modifier = Modifier.weight(1f)
) {
/*...*/
}
}
}
Learn more
We provide a full list of modifiers, with their parameters and scopes.