Exploring CustomScopes in Jetpack Compose: A Practical Guide

Jetpack Compose has revolutionized UI development for Android applications by introducing a declarative and concise way to build user interfaces. It allows developers to create UI components in a more intuitive and efficient manner. One of the powerful features in Jetpack Compose is the ability to define custom scopes, providing a structured and reusable approach to building UI elements.

Understanding CustomScopes

In Jetpack Compose, a CustomScope is a user-defined interface that encapsulates a set of functions for constructing specific UI elements. It serves as a contract between the user and the framework, allowing developers to express their UI logic in a clean and modular way.

Let’s take a closer look at the provided CustomScope interface:

interface CustomScope {
    fun item(textId: Int)
}

In this example, the CustomScope interface has a single function item, which takes an integer parameter representing a text resource ID. This function defines the building block for a custom UI element.

Implementing CustomScope

To implement the CustomScope interface, we have the CustomScopeImpl class:

private class CustomScopeImpl : CustomScope {

    val items = mutableListOf<@Composable () -> Unit>()

    override fun item(textId: Int) {
        items += {
            OutlinedButtonDS(textId = textId)
        }
    }
}

CustomScopeImpl maintains a list of @Composable functions (items) that will be used to construct UI elements. In this example, the item function appends a new @Composable function to the list, representing an OutlinedButtonDS with the specified text resource ID.

Leveraging CustomScopes with ColumnListCustomScope

Now, let’s explore the ColumnListCustomScope composable function:

@Composable
fun ColumnListCustomScope(
    modifier: Modifier = Modifier,
    verticalArrangement: Arrangement.HorizontalOrVertical = Arrangement.Center,
    horizontalAlignment: Alignment.Horizontal = Alignment.CenterHorizontally,
    content: CustomScope.() -> Unit
) {
    val scope = remember(content) { CustomScopeImpl().apply(content) }

    LazyColumn(
        modifier = modifier,
        verticalArrangement = verticalArrangement,
        horizontalAlignment = horizontalAlignment,
    ) {
        items(
            items = scope.items,
            key = { it.hashCode() }
        ) { item ->
            item()
        }
    }
}

The ColumnListCustomScope composable function takes parameters such as modifier, verticalArrangement, and horizontalAlignment to customize the layout of the LazyColumn. The crucial part is the content parameter, which is a lambda expression of type CustomScope.() -> Unit. This lambda is invoked on a CustomScope instance, allowing the user to construct UI elements using the functions defined in the CustomScope interface.

The remember function is used to create and remember the CustomScopeImpl instance based on the provided content lambda. The LazyColumn then iterates through the list of @Composable functions stored in the items property of the CustomScopeImpl instance.

Example Usage

Here’s an example of how you might use the ColumnListCustomScope:

ColumnListCustomScope {
    item(R.string.button_text_one)
    item(R.string.button_text_two)
    // Add more items as needed
}

In this example, the item function is called within the ColumnListCustomScope lambda, creating a list of OutlinedButtonDS instances with different text resource IDs.

Providing on GitHub

The source code related to this implementation can be found on the GitHub repository. Feel free to explore, clone, and contribute to the project!

Conclusion

Custom scopes in Jetpack Compose provide a structured and reusable approach to building UI elements. By defining a clear interface and implementing it in a dedicated class, developers can create composable functions that encapsulate specific UI logic. The ColumnListCustomScope composable function demonstrates how to leverage custom scopes to build a dynamic list of UI elements in a clean and concise way. This approach promotes code modularity and readability, making it easier to manage and maintain UI code in large-scale applications.