Compose et permet definir la interfície gràfica mitjançant funcions descriptives.

Introducció

Compose està construit al voltant de funcions composables.

Aquestes funcions et permeten definir la UI de la teva aplicació mitjançant codi, descrivint quin aspecte tindria que tenir i proporcionant les dependències respecte les dades, enlloc de tenir que centrar-te en la construcció de la UI (inicialitzant un element, afegint aquest a un element superior, etc.).

Per crear una funció composable només has d'afegir l'anotació @Composable al nom de la funció.

Projecte de suport: https://gitlab.com/xtec/kotlin/compose

Projecte

Instal.la Fleet

Crea un projecte amb Kotlin Multiplatform wizard:

  1. En el tab New project, cnavia el nom del projecte a compose i la ID del projecte a dev.xtec.

  2. Selecciona les opcions Android i Desktop.

  3. Fes clic al boto Download

  4. Descomprimeix el zip.

  5. Obre el projecte amb Fleet

TODO https://www.jetbrains.com/help/kotlin-multiplatform-dev/compose-multiplatform-create-first-app.html#examine-the-project-structure

Modifica el fitxer composeApp/build.gradle.kts:

        commonMain.dependencies {
            // ...
            implementation(compose.material3)
        }

Funcions composables

Afegeix un element Text

Modifica el fitxer composeApp/src/commonMain/kotlin/dev/xtec/App.kt:

package dev.xtec

// ...

@Composable
fun App() {
    MaterialTheme {
       Text("Hello World!")
    }
}

First, display a “Hello world!” text by adding a text element inside the onCreate method. You do this by defining a content block, and calling the Text composable function. The setContent block defines the activity's layout where composable functions are called. Composable functions can only be called from other composable functions.

Compose uses a Kotlin compiler plugin to transform these composable functions into the app's UI elements. For example, the Text composable function that is defined by the Compose UI library displays a text label on the screen.

Run your application on desktop:

Una altra opció és obrir un terminal:

> .\gradlew composeApp:run

Define a composable function

To make a function composable, add the @Composable annotation.

To try this out, define a MessageCard function which is passed a name, and uses it to configure the text element.

// ...
import androidx.compose.runtime.Composable

@Composable
fun MessageCard(name: String) {
    Text(text = "Hello $name")
}

Preview your function

The @Preview annotation lets you preview your composable functions within Android Studio without having to build and install the app to an Android device or emulator. The annotation must be used on a composable function that does not take in parameters. For this reason, you can't preview the MessageCard function directly.

Instead, make a second function named PreviewMessageCard, which calls MessageCard with an appropriate parameter. Add the @Preview annotation before @Composable.

Layouts

UI elements are hierarchical, with elements contained in other elements. In Compose, you build a UI hierarchy by calling composable functions from other composable functions.

Add multiple texts

So far you’ve built your first composable function and preview! To discover more Compose capabilities, you’re going to build a simple messaging screen containing a list of messages that can be expanded with some animations.

Start by making the message composable richer by displaying the name of its author and a message content. You need to first change the composable parameter to accept a Message object instead of a String, and add another Text composable inside the MessageCard composable. Make sure to update the preview as well.

// ...

@Composable
fun App() {
    MaterialTheme {
        MessageCard(Message("Eva", "En el pot petit hi ha la bona confitura"))
    }
}

data class Message(val author: String, val body: String)

@Composable
fun MessageCard(msg: Message) {
    Text(text = msg.author)
    Text(text = msg.body)
}

This code creates two text elements inside the content view. However, since you haven't provided any information about how to arrange them, the text elements are drawn on top of each other, making the text unreadable.

Using a Column

The Column function lets you arrange elements vertically. Add Column to the MessageCard function. You can use Row to arrange items horizontally and Box to stack elements.

// ...

@Composable
fun MessageCard(msg: Message) {
    Column {
        Text(text = msg.author)
        Text(text = msg.body)
    }
}

Afegeix una imatge

Compose - Recursos

Enrich your message card by adding a profile picture of the sender.

Has d'afegir les imatges al directori composeApp/src/commonMain/composeResources/drawable.

Has d'executar el projecte per generar la classe Res.

Add a Row composable to have a well structured design and an Image composable inside it.

// ...
import org.jetbrains.compose.resources.painterResource
import compose.composeapp.generated.resources.Res
import compose.composeapp.generated.resources.eva

@Composable
fun MessageCard(msg: Message) {
    Row {
        Image(
            painter = painterResource(Res.drawable.eva),
            contentDescription = "Contact profile picture"

        )
        Column {
            Text(text = msg.author)
            Text(text = msg.body)
        }
    }
}

Configure your layout

Your message layout has the right structure but its elements aren't well spaced and the image is too big!

To decorate or configure a composable, Compose uses modifiers. They allow you to change the composable's size, layout, appearance or add high-level interactions, such as making an element clickable. You can chain them to create richer composables. You'll use some of them to improve the layout.

// ...
import androidx.compose.foundation.layout.Spacer
import androidx.compose.foundation.layout.height
import androidx.compose.foundation.layout.padding
import androidx.compose.foundation.layout.size
import androidx.compose.foundation.layout.width
import androidx.compose.foundation.shape.CircleShape
import androidx.compose.ui.Modifier
import androidx.compose.ui.draw.clip
import androidx.compose.ui.unit.dp

@Composable
fun MessageCard(msg: Message) {
    // Add padding around our message
    Row(modifier = Modifier.padding(all = 16.dp)) {
        Image(
            painter = painterResource(R.drawable.profile_picture),
            contentDescription = "Contact profile picture",
            modifier = Modifier
                // Set image size to 80 dp
                .size(80.dp)
                // Clip image to be shaped as a circle
                .clip(CircleShape)
        )

        // Add a horizontal space between the image and the column
        Spacer(modifier = Modifier.width(16.dp))

        Column {
            Text(text = msg.author)
            // Add a vertical space between the author and message texts
            Spacer(modifier = Modifier.height(8.dp))
            Text(text = msg.body)
        }
    }
}

Material Design

Compose is built to support Material Design principles. Many of its UI elements implement Material Design out of the box. In this lesson, you'll style your app with Material Design widgets.

Has d'afegir a composeApp/build.gradle.kts

  commonMain.dependencies {
            // ...
            implementation(compose.material3)
        }

Use Material Design

Your message design now has a layout, but it doesn't look great yet.

Compose provides an implementation of Material Design 3 and its UI elements out of the box. You'll improve the appearance of our MessageCard composable using Material Design styling.

To start, wrap the MessageCard function with the Material theme created in your project, ComposeTheme, as well as a Surface. Do it both in the @Preview and in the setContent function. Doing so will allow your composables to inherit styles as defined in your app's theme ensuring consistency across your app.

// ...

@Composable
fun App() {
    MaterialTheme {
        Surface(modifier = Modifier.fillMaxSize()) {
            MessageCard(Message("Eva", "En el pot petit hi ha la bona confitura"))
        }
    }
}

Material Design is built around three pillars: Color, Typography, and Shape. You will add them one by one.

Note. the Empty Compose Activity template generates a default theme for your project that allows you to customize MaterialTheme.

Color

Use MaterialTheme.colors to style with colors from the wrapped theme. You can use these values from the theme anywhere a color is needed. This example uses dynamic theming colors (defined by device preferences).

You can set dynamicColor to false in the MaterialTheme.kt file to change this. TODO on??

Style the title and add a border to the image.

// ...
import androidx.compose.foundation.border
import androidx.compose.material3.MaterialTheme

@Composable
fun MessageCard(msg: Message) {
    // Add padding around our message
    Row(modifier = Modifier.padding(all = 16.dp)) {
        Image(
            painter = painterResource(Res.drawable.eva),
            contentDescription = "Contact profile picture",
            modifier = Modifier
                // Set image size to 80 dp
                .size(80.dp)
                // Clip image to be shaped as a circle
                .clip(CircleShape).border(3.dp, color = MaterialTheme.colors.primary, CircleShape)
        )

        // Add a horizontal space between the image and the column
        Spacer(modifier = Modifier.width(16.dp))

        Column {
            Text(
                text = msg.author,
                color = MaterialTheme.colors.secondary  
            )
            // Add a vertical space between the author and message texts
            Spacer(modifier = Modifier.height(8.dp))
            Text(text = msg.body)
        }
    }
}

Typography

Material Typography styles are available in the MaterialTheme, just add them to the Text composables.

// ...

@Composable
fun MessageCard(msg: Message) {
   Row(modifier = Modifier.padding(all = 16.dp)) {
       Image(
           painter = painterResource(R.drawable.profile_picture),
           contentDescription = null,
           modifier = Modifier
               .size(80.dp)
               .clip(CircleShape)
               .border(3.dp, MaterialTheme.colors.primary, CircleShape)
       )

       Spacer(modifier = Modifier.width(16.dp))

       Column {
           Text(
               text = msg.author,
               color = MaterialTheme.colors.secondary,
               style = MaterialTheme.typography.titleMedium
           )

           Spacer(modifier = Modifier.height(8.dp))
           Text(
               text = msg.body,
               style = MaterialTheme.typography.bodyLarge
           )
       }
   }
}

Shape

With Shape you can add the final touches. First, wrap the message body text around a Surface composable. Doing so allows customizing the message body's shape and elevation. Padding is also added to the message for a better layout.

// ...
import androidx.compose.material3.Surface

@Composable
fun MessageCard(msg: Message) {
    Row(modifier = Modifier.padding(all = 16.dp)) {
        Image(
            painter = painterResource(R.drawable.coyote),
            contentDescription = "Contact profile picture",
            modifier = Modifier
                .size(80.dp)
                .clip(CircleShape)
                .border(
                    3.dp, MaterialTheme.colors.primary,
                    CircleShape
                )
        )
        Spacer(modifier = Modifier.width(16.dp))
        Column {
            Text(
                text = msg.author, color = MaterialTheme.colors.secondary,
                style = MaterialTheme.typography.titleMedium
            )
            Spacer(modifier = Modifier.height(8.dp))
            Surface(shape = MaterialTheme.shapes.large, shadowElevation = 2.dp) {
                Text(
                    text = msg.body,
                    modifier = Modifier.padding(all = 4.dp),
                    style = MaterialTheme.typography.bodyLarge
                )
            }
        }
    }
}

Lists and animations

Lists and animations are everywhere in apps. In this lesson, you will learn how Compose makes it easy to create lists and fun to add animations.

Create a list of messages

A chat with one message feels a bit lonely, so we are going to change the conversation to have more than one message.

val messages = listOf(
    Message(
        "Josep Pla",
        "Antigament, el viatjar era un privilegi de gran senyor. Generalment, era la coronació normal dels estudis d'un home. Ara el viatjar s'ha generalitzat i democratitzat considerablement. Viatja molta gent"
    ),
    Message(
        "Josep Pla",
        "Però, potser, les persones que viatgen per arrodonir i afermar la seva visió del món i dels homes són més rares avui que fa cent anys. "
    ),
    Message(
        "Josep Pla",
        "En el nostre país hi ha tres pretextos essencials per a passar la frontera: la peregrinació a Lourdes, la lluna de mel i els negocis. Hom no pot tenir idea de la quantitat de gent del nostre país que ha estat a Lourdes. És incomptable. "
    ),
    Message(
        "Josep Pla",
        "Fa trenta anys, les persones riques de Catalunya feien el viatge de noces a Madrid. Avui van a París o a Niça i de vegades a Itàlia. La lluna de mel, però, és un mal temps per veure res i per formar-se. No es poden pas fer dues coses importants a la vegada. El pitjor temps, potser, per a viatjar, de la vida, és la temporada de la lluna de mel."
    ),
)

You'll need to create a Conversation function that will show multiple messages. For this use case, use Compose's LazyColumn and LazyRow. These composables render only the elements that are visible on screen, so they are designed to be very efficient for long lists.

In this code snippet, you can see that LazyColumn has an items child. It takes a List as a parameter and its lambda receives a parameter we've named message (we could have named it whatever we want) which is an instance of Message. In short, this lambda is called for each item of the provided List.

// ...
import androidx.compose.foundation.lazy.LazyColumn
import androidx.compose.foundation.lazy.items

@Composable
fun App() {
    MaterialTheme {
        Surface(modifier = Modifier.fillMaxSize()) {
            Conversation(messages)
        }
    }
}

@Composable
fun Conversation(messages: List<Message>) {
    LazyColumn {
        items(messages) { message -> MessageCard(message) }
    }
}

Animate messages while expanding

The conversation is getting more interesting. It's time to play with animations! You will add the ability to expand a message to show a longer one, animating both the content size and the background color. To store this local UI state, you need to keep track of whether a message has been expanded or not. To keep track of this state change, you have to use the functions remember and mutableStateOf.

Composable functions can store local state in memory by using remember, and track changes to the value passed to mutableStateOf. Composables (and their children) using this state will get redrawn automatically when the value is updated. This is called recomposition.

By using Compose's state APIs like remember and mutableStateOf, any changes to state automatically update the UI.

// ...
import androidx.compose.foundation.clickable
import androidx.compose.runtime.getValue
import androidx.compose.runtime.mutableStateOf
import androidx.compose.runtime.remember
import androidx.compose.runtime.setValue

@Composable
fun MessageCard(msg: Message) {

        // ...    

        // We keep track if the message is expanded or not in this
        // variable
        var isExpanded by remember { mutableStateOf(false) }

        Column(modifier = Modifier.clickable{isExpanded = !isExpanded}) {
            
            Text(
                text = msg.author,
                color = MaterialTheme.colors.secondary,
                style = MaterialTheme.typography.subtitle2
            )
            // Add a vertical space between the author and message texts
            Spacer(modifier = Modifier.height(8.dp))
            Surface(shape = MaterialTheme.shapes.large, elevation = 2.dp) {
                Text(
                    text = msg.body,
                    modifier = Modifier.padding(all = 4.dp),

                    // If the message is expanded, we display all its content
                    // otherwise we only display the first line
                    maxLines = if (isExpanded) Int.MAX_VALUE else 1,
                    
                    style = MaterialTheme.typography.body1
                )
            }
        }
    }
}

Now you can change the background of the message content based on isExpanded when we click on a message. You will use the clickable modifier to handle click events on the composable.

Instead of just toggling the background color of the Surface, you will animate the background color by gradually modifying its value from MaterialTheme.colors.surface to MaterialTheme.colors.primary and vice versa.

To do so, you will use the animateColorAsState function.

Lastly, you will use the animateContentSize modifier to animate the message container size smoothly:

// ...
import androidx.compose.animation.animateColorAsState
import androidx.compose.animation.animateContentSize

@Composable
fun MessageCard(msg: Message) {
    // Add padding around our message
    Row(modifier = Modifier.padding(all = 16.dp)) {
        Image(
            painter = painterResource(Res.drawable.josep),
            contentDescription = "Contact profile picture",
            modifier = Modifier
                // Set image size to 80 dp
                .size(80.dp)
                // Clip image to be shaped as a circle
                .clip(CircleShape).border(3.dp, color = MaterialTheme.colors.primary, CircleShape)
        )

        // Add a horizontal space between the image and the column
        Spacer(modifier = Modifier.width(16.dp))

        // We keep track if the message is expanded or not in this
        // variable
        var isExpanded by remember { mutableStateOf(false) }

        // surfaceColor will be updated gradually from one color to the other
        val surfaceColor by animateColorAsState(
            if (isExpanded) MaterialTheme.colors.primary else MaterialTheme.colors.surface
        )

        Column(modifier = Modifier.clickable { isExpanded = !isExpanded }) {
            Text(
                text = msg.author,
                color = MaterialTheme.colors.secondary,
                style = MaterialTheme.typography.subtitle2
            )
            // Add a vertical space between the author and message texts
            Spacer(modifier = Modifier.height(8.dp))
            Surface(
                shape = MaterialTheme.shapes.large,
                elevation = 2.dp,
                // surfaceColor color will be changing gradually from primary to surface
                color = surfaceColor,
                // animateContentSize will change the Surface size gradually
                modifier = Modifier.animateContentSize().padding(2.dp)
            ) {
                Text(
                    text = msg.body,
                    modifier = Modifier.padding(all = 4.dp),
                    // If the message is expanded, we display all its content
                    // otherwise we only display the first line
                    maxLines = if (isExpanded) Int.MAX_VALUE else 1,
                    style = MaterialTheme.typography.body1
                )
            }
        }
    }
}

Continue learning

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