Custom Drawing in Compose Multiplatform: A Jetpack Compose Guide

Jetpack Compose, known for its declarative UI paradigm, simplifies UI development across various platforms with Compose Multiplatform. A compelling feature is its capability for custom drawing, enabling developers to craft unique visual elements beyond the standard components. This article delves into the intricacies of custom drawing in Jetpack Compose Multiplatform, showcasing how to create platform-agnostic, visually rich applications.

Understanding Custom Drawing in Jetpack Compose

Custom drawing involves using the Canvas composable to draw directly on the screen, bypassing predefined UI components. It provides low-level control over rendering, allowing for the creation of custom shapes, animations, and visualizations.

Why Use Custom Drawing?

• Unique UI Elements: Create bespoke visual elements tailored to your application’s needs.
• Complex Visualizations: Render graphs, charts, and other complex data representations.
• Animations and Effects: Implement custom animations and visual effects not possible with standard components.
• Cross-Platform Consistency: Ensure visual consistency across different platforms with Compose Multiplatform.

Setting Up a Compose Multiplatform Project

Before diving into custom drawing, ensure you have a Compose Multiplatform project set up. This involves configuring your build.gradle.kts file to include the necessary dependencies and plugins.

Step 1: Project Setup

Create a new Kotlin Multiplatform project with Compose enabled or configure an existing one.

Step 2: Add Dependencies

Add the required dependencies to your build.gradle.kts file. Ensure that you include the Compose dependencies for each target platform.

dependencies {
    implementation(compose.components.runtime)
    implementation(compose.components.ui)
    implementation(compose.components.material)

    // Specific platform dependencies
    desktopMain(compose.components.desktop.currentOs)
    androidMain(compose.components.android.material)
    iosMain(compose.components.ios.uikit)
    // webMain(compose.components.web.core) // For web support
}

Implementing Custom Drawing

To implement custom drawing, use the Canvas composable along with various drawing operations.

Step 1: Create a Custom Composable

Create a composable function that uses the Canvas composable to draw on the screen.

import androidx.compose.foundation.Canvas
import androidx.compose.foundation.layout.size
import androidx.compose.runtime.Composable
import androidx.compose.ui.Modifier
import androidx.compose.ui.geometry.Offset
import androidx.compose.ui.graphics.Color
import androidx.compose.ui.graphics.StrokeCap
import androidx.compose.ui.tooling.preview.Preview
import androidx.compose.ui.unit.dp

@Composable
fun CustomDrawing() {
    Canvas(modifier = Modifier.size(200.dp)) {
        // Draw a circle
        drawCircle(
            color = Color.Blue,
            radius = 50.dp.toPx(),
            center = Offset(size.width / 2, size.height / 2)
        )

        // Draw a line
        drawLine(
            color = Color.Red,
            start = Offset(0f, 0f),
            end = Offset(size.width, size.height),
            strokeWidth = 5.dp.toPx(),
            cap = StrokeCap.Round
        )
    }
}

@Preview
@Composable
fun CustomDrawingPreview() {
    CustomDrawing()
}

In this example:

• Canvas creates a drawing surface.
• drawCircle draws a blue circle at the center of the canvas.
• drawLine draws a red line from the top-left to the bottom-right corner.

Step 2: Drawing Operations

Jetpack Compose provides various drawing operations such as:

• drawRect: Draws a rectangle.
• drawCircle: Draws a circle.
• drawLine: Draws a line.
• drawArc: Draws an arc.
• drawPath: Draws a complex path.
• drawImage: Draws an image.
• drawText: Draws text.

These operations can be customized with colors, stroke widths, stroke caps, and other properties to achieve the desired visual effect.

Step 3: Handling Platform Differences

When implementing custom drawing in a Compose Multiplatform project, you might encounter platform-specific differences. Use expect and actual declarations to handle these differences.

// Common code
expect fun platformSpecificDrawing(): @Composable () -> Unit

// Android implementation
import androidx.compose.runtime.Composable

actual fun platformSpecificDrawing(): @Composable () -> Unit = {
    // Android-specific drawing code
    Text("Drawing on Android")
}

// Desktop implementation
import androidx.compose.desktop.ui.tooling.preview.Preview
import androidx.compose.runtime.Composable

actual fun platformSpecificDrawing(): @Composable () -> Unit = {
    // Desktop-specific drawing code
    Text("Drawing on Desktop")
}

This allows you to define a common interface in the common code and provide platform-specific implementations in the respective platform modules.

Example: Drawing a Simple Graph

Here’s an example of drawing a simple graph using custom drawing in Jetpack Compose.

import androidx.compose.foundation.Canvas
import androidx.compose.foundation.layout.size
import androidx.compose.runtime.Composable
import androidx.compose.ui.Modifier
import androidx.compose.ui.geometry.Offset
import androidx.compose.ui.graphics.Color
import androidx.compose.ui.graphics.Path
import androidx.compose.ui.graphics.StrokeCap
import androidx.compose.ui.tooling.preview.Preview
import androidx.compose.ui.unit.dp

@Composable
fun GraphDrawing() {
    val dataPoints = listOf(
        Pair(0.0, 0.2),
        Pair(0.2, 0.5),
        Pair(0.4, 0.3),
        Pair(0.6, 0.7),
        Pair(0.8, 0.4),
        Pair(1.0, 0.6)
    )

    Canvas(modifier = Modifier.size(200.dp)) {
        val width = size.width
        val height = size.height

        // Draw axes
        drawLine(
            color = Color.Gray,
            start = Offset(0f, height),
            end = Offset(width, height),
            strokeWidth = 2.dp.toPx()
        )
        drawLine(
            color = Color.Gray,
            start = Offset(0f, 0f),
            end = Offset(0f, height),
            strokeWidth = 2.dp.toPx()
        )

        // Draw graph line
        val path = Path()
        if (dataPoints.isNotEmpty()) {
            val firstPoint = dataPoints.first()
            path.moveTo(firstPoint.first * width, height - firstPoint.second * height)

            dataPoints.forEach { point ->
                path.lineTo(point.first * width, height - point.second * height)
            }

            drawPath(
                path = path,
                color = Color.Green,
                style = Stroke(width = 3.dp.toPx(), cap = StrokeCap.Round)
            )
        }
    }
}

@Preview
@Composable
fun GraphDrawingPreview() {
    GraphDrawing()
}

Performance Considerations

Custom drawing can be resource-intensive, especially when dealing with complex shapes and animations. Optimize your drawing operations to ensure smooth performance:

• Cache frequently used shapes: Avoid recreating the same shapes on every frame.
• Use hardware acceleration: Ensure hardware acceleration is enabled for the Canvas.
• Minimize overdraw: Reduce the number of overlapping drawing operations.

Conclusion

Custom drawing in Jetpack Compose Multiplatform empowers developers to create visually stunning and platform-agnostic applications. By leveraging the Canvas composable and various drawing operations, you can implement unique UI elements, complex visualizations, and custom animations that enhance the user experience across different platforms. Understanding platform differences and optimizing performance are key to achieving the best results. With custom drawing, the possibilities for creative expression in Jetpack Compose are virtually limitless.