State: A Component’s Memory

Components often need to change what’s on the screen as a result of an interaction. Typing into a form should update the input field, clicking “Next” on an image carousel should change which image is displayed, and clicking “Buy” should put a product in the shopping cart. Components need to “remember” things: the current input value, the current image, the shopping cart. In deephaven.ui, this kind of component-specific memory is called state.

When a regular variable is not enough

Here’s a component that renders a word from a list. Clicking the “Next” button should show the next word by changing the index to 1, then 2, and so on. However, this does not work.

from deephaven import ui

word_list = ["apple", "banana", "cherry", "orange", "kiwi", "strawberry"]


@ui.component
def word_display():
    index = 0

    def handle_press():
        nonlocal index
        index = index + 1

    word = word_list[index]

    return [
        ui.button("Next", on_press=handle_press),
        ui.text(f"({index+1} of {len(word_list)})"),
        ui.heading(word),
    ]


word_display_example = word_display()

The handle_press event handler is updating a local variable, index. But two things prevent that change from being visible:

  1. Local variables do not persist between renders. When deephaven.ui renders this component a second time, it renders it from scratch. It does not consider any changes to the local variables.
  2. Changes to local variables do not trigger renders. deephaven.ui does not realize it needs to render the component again with the new data.

To update a component with new data, two things need to happen:

  1. Retain the data between renders.
  2. Trigger deephaven.ui to render the component with new data (re-rendering).

The use_state hook provides those two things:

  1. A state variable to retain the data between renders.
  2. A state setter function to update the variable and trigger deephaven.ui to render the component again.

Add a state variable

To add a state variable, replace this line:

index = 0

with

index, set_index = ui.use_state(0)

index is a state variable and set_index is the setter function.

This is how they work together in handle_press:

set_index(index + 1)

Now clicking the “Next” button switches the current word:

from deephaven import ui

word_list = ["apple", "banana", "cherry", "orange", "kiwi", "strawberry"]


@ui.component
def word_display():
    index, set_index = ui.use_state(0)

    def handle_press():
        set_index(index + 1)

    word = word_list[index]

    return [
        ui.button("Next", on_press=handle_press),
        ui.text(f"({index+1} of {len(word_list)})"),
        ui.heading(word),
    ]


word_display_example = word_display()

Meet your first hook

In deephaven.ui, use_state, as well as any other function starting with “use”, is called a hook.

Hooks are special functions that are only available while deephaven.ui is rendering. They let you “hook into” different deephaven.ui features.

State is just one of those features, but you will meet the other hooks later.

Hooks can only be called at the top level of your components or your own hooks. You cannot call hooks inside conditions, loops, or other nested functions. Hooks are functions, but it is helpful to think of them as unconditional declarations about your component’s needs. You “use” deephaven.ui features at the top of your component similar to how you “import” at the top of your file.

Anatomy of use_state

When you call use_state, you are telling deephaven.ui that you want this component to remember something:

index, set_index = ui.use_state(0)

In this case, you want deephaven.ui to remember index.

The convention is to name this pair consistently, like something and set_something. You can name it anything you like, but conventions make things easier to understand across projects.

The only argument to use_state is the initial value of your state variable. In this example, the index’s initial value is set to 0 with ui.use_state(0).

Every time your component renders, use_state gives you an array containing two values:

  1. The state variable index with the value you stored.
  2. The state setter function set_index, which can update the state variable and trigger deephaven.ui to render the component again.

Here’s how that happens in action:

index, set_index = ui.use_state(0)
  1. Your component renders the first time. Because you passed 0 to use_state as the initial value for index, it will return 0, set_index. deephaven.ui remembers 0 is the latest state value.
  2. You update the state. When a user clicks the button, it calls set_index(index + 1). Since index is 0, it becomes set_index(1). This instructs deephaven.ui to remember that index is now 1 and triggers another render.
  3. During your component’s second render, deephaven.ui still sees use_state(0), but because deephaven.ui remembers that you set index to 1, it returns 1, set_index instead.
  4. And so on with each render.

Give a component multiple state variables

You can have as many state variables of as many types as you like in one component. This component has two state variables, a number index and a boolean show_more that is toggled when you click “Show details”:

from deephaven import ui

word_list = ["apple", "banana", "cherry", "orange", "kiwi", "strawberry"]
detail_list = [
    "An apple is a round, edible fruit produced by an apple tree.",
    "A banana is an elongated, edible fruit.",
    "A cherry is the fruit of many plants of the genus Prunus.",
    "The oranges are the fruit of a tree in the family Rutaceae.",
    "Kiwi has a thin, fuzzy, fibrous, tart but edible, light brown skin and light green or golden flesh with rows of tiny, black, edible seeds.",
    "The genus Fragaria, strawberries, is in the rose family, Rosaceae.",
]


@ui.component
def word_display():
    index, set_index = ui.use_state(0)
    show_more, set_show_more = ui.use_state(False)

    def handle_press():
        set_index(index + 1)

    def handle_more_press():
        set_show_more(not show_more)

    word = word_list[index]
    detail = detail_list[index]

    return [
        ui.button("Next", on_press=handle_press),
        ui.text(f"({index+1} of {len(word_list)})"),
        ui.heading(word),
        ui.button(
            f"{'Hide' if show_more else 'Show'} Details", on_press=handle_more_press
        ),
        show_more and ui.text(detail),
    ]


word_display_example = word_display()

It is a good idea to have multiple state variables if their state is unrelated, like index and show_more in this example. But if you find that you often change two state variables together, it might be easier to combine them into one. For example, if you have a form with many fields, it’s more convenient to have a single state variable that holds a dictionary than a state variable per field.

State is isolated and private

State is local to a component instance on the screen. In other words, if you render the same component twice, each copy will have completely isolated state! Changing one of them will not affect the other.

In this example, the word_display component from earlier is rendered twice with no changes to its logic. Try clicking the buttons inside each of the component. Notice that their state is independent:

from deephaven import ui

word_list = ["apple", "banana", "cherry", "orange", "kiwi", "strawberry"]
detail_list = [
    "An apple is a round, edible fruit produced by an apple tree.",
    "A banana is an elongated, edible fruit.",
    "A cherry is the fruit of many plants of the genus Prunus.",
    "The oranges the fruit of a tree in the family Rutaceae.",
    "Kiwi has a thin, fuzzy, fibrous, tart but edible, light brown skin and light green or golden flesh with rows of tiny, black, edible seeds.",
    "The genus Fragaria, the strawberries, is in the rose family, Rosaceae.",
]


@ui.component
def word_display():
    index, set_index = ui.use_state(0)
    show_more, set_show_more = ui.use_state(False)

    def handle_press():
        set_index(index + 1)

    def handle_more_press():
        set_show_more(not show_more)

    word = word_list[index]
    detail = detail_list[index]

    return ui.flex(
        ui.button("Next", on_press=handle_press),
        ui.text(f"({index+1} of {len(word_list)})"),
        ui.heading(word),
        ui.button("Show Details", on_press=handle_more_press),
        show_more and ui.text(detail),
        direction="column",
    )


@ui.component
def page():
    return ui.flex(word_display(), word_display())


page_example = page()

This is what makes state different from regular variables that you might declare at the top of your script. State is not tied to a particular function call or a place in the code, but it’s “local” to the specific place on the screen. You rendered two word_display components, so their state is stored separately.

Also notice how the page component does not “know” anything about the word_display state or even whether it has any. Unlike props, state is fully private to the component declaring it. The parent component can’t change it. This lets you add state to or remove it from any component without impacting the rest of the components.