Props are the mechanism by which components communicate with each other. Components pass data down to their children and "send" events back up the component hierarchy through props.
Props are passed to the component
function as a strongly typed record, as defined by the
component. Event callbacks are are also passed via props in the form of a function. The component
can then call the event callback when an event occurs, notifying the parent. In stateless functional
components, props are not required since data is simply passed to the component as arguments.
Let's take a look at how props and events work in a functioning example. In this example, the
counter
component is passing a prop called on_click
to the two button
components. The counter
component is also passing a prop called count
to the display
component. When the button
components are clicked, they call the on_click
event handler that was passed to them as a prop.
The counter
component then increments its count
state and re-renders. The display
component is
also re-rendered because its count
prop has changed.
import gleam/int import gleam/option.{None, Option, Some}
import sprocket/context.{Context, dep}
import sprocket/hooks.{reducer}
import sprocket/component.{component, render}
import sprocket/html/elements.{button_text, div, span, text}
import sprocket/html/attributes.{class, classes}
type Model =
Int
type Msg {
UpdateCounter(Int) ResetCounter
}
fn update(_model: Model, msg: Msg) -> Model {
case msg {
UpdateCounter(count) -> {
count
} ResetCounter -> 0
}
}
pub type CounterProps {
CounterProps
}
pub fn counter(ctx: Context, _props: CounterProps) {
// Define a reducer to handle events and update the state
use ctx, count, dispatch <- reducer(ctx, 0, update)
render(
ctx, div(
[class("flex flex-row m-4")], [
component(
button, StyledButtonProps(
class: "rounded-l", label: "-", on_click: fn() { dispatch(UpdateCounter(count - 1)) },
),
), component(
display, DisplayProps(count: count),
), component(
button, StyledButtonProps(
class: "rounded-r", label: "+", on_click: fn() { dispatch(UpdateCounter(count + 1)) },
),
),
],
),
)
}
pub type ButtonProps {
ButtonProps(label: String, on_click: fn() -> Nil)
StyledButtonProps(class: String, label: String, on_click: fn() -> Nil)
}
pub fn button(ctx: Context, props: ButtonProps) {
// here we unpack the different types of ButtonProps that can be passed to the button component
let #(class, label, on_click) = case props {
ButtonProps(label, on_click) -> #(None, label, on_click) StyledButtonProps(class, label,
on_click) -> #(Some(class), label, on_click)
}
let handle_click = fn(_) { on_click() }
render(
ctx,
button_text(
[
attributes.on_click(handle_click), classes([
class, Some(
"p-1 px-2 border dark:border-gray-500 bg-gray-100 dark:bg-gray-800 hover:bg-gray-200 dark:hover:bg-gray-700 active:bg-gray-300 dark:active:bg-gray-600",
),
]),
], label,
),
)
}
pub type DisplayProps {
DisplayProps(count: Int)
}
pub fn display(ctx: Context, props: DisplayProps) {
let DisplayProps(count: count) = props
render(
ctx, span(
[
class(
"p-1 px-2 w-10 bg-white dark:bg-gray-900 border-t border-b dark:border-gray-500 align-center text-center",
),
], [text(int.to_string(count))],
),
)
}
So data flows down the component tree while events bubble up. In this sense, Sprocket uses a unidirectional data-flow architecture. This is a powerful pattern that makes it easy to reason about your application and to understand how data is flowing through it.
We'll cover state management more in-depth in the next section, but it's useful to start thinking about how this data-flow will inform where state should live in your component hierarchy. And since we have the safety provided by the Gleam type system, we aren't afraid of refactoring our state to a different part of the hierarchy when our requirements or designs inevitably change!