Reactive calculations

Agenda

Reactive calculations
Your turn
Overriding reactivity
Side effects

Reactivity scales

Every Shiny app uses this pattern
Works for dynamic UIs
Shiny is lazy

Saving and reusing calculated values

So far we’ve been working with shallow reactive graphs
Each input is passed to a rendering function which produces an output
Input -> Recipe -> Output can produce repetitive, inefficient applications

Important

@reactive.calc creates calculations whose results can be used by one or more outputs

This adds intermediate nodes to the reactive graph

Reactive Calc example

View the app on this page:

200-simple-reactive-calc

...

Identify repetition

    @output
    @render.table
    def df():
        rand = np.random.rand(input.n_rows(), 1)
        df = pd.DataFrame(rand, columns=["col_1"])
        return df

    @output
    @render.plot
    def hist():
        rand = np.random.rand(input.n_rows(), 1)
        df = pd.DataFrame(rand, columns=["col_1"])
        plot = (
            ggplot(df, aes(x="col_1"))
            + geom_histogram(binwidth=0.1, fill="blue", color="black")
            + labs(x="Random Values", y="Frequency", title="Histogram of Random Data")
        )
        return plot

Problems with repetition

Warning

Code is in multiple places
The app is taking the sample twice
The table and graph are not using the same sample!

Reactive calculation to the rescue

    @reactive.calc
    def sampled_df():
        rand = np.random.rand(input.n_rows(), 1)
        df = pd.DataFrame(rand, columns=["col_1"])

    @render.table
    def df():
        return sampled_df()

    @render.plot
    def hist():
        return (
            ggplot(sampled_df(), aes(x="col_1"))
            + geom_histogram(binwidth=0.1, fill="blue", color="black")
            + labs(x="Random Values", y="Frequency", title="Histogram of Random Data")
        )

Reactive calculations

Defined with the @reactive.calc decorator
Called like other inputs
Can read inputs, reactive values, or other reactive calculations
Caches its value, so it’s cheap to call repeatedly
Adds a node to the reactive graph
- Discards cached value when upstream nodes invalidate
- Notifies downstream nodes when it invalidates

Initial state

Calculate table

Calculate sample

Calculate plot

Deep reactive graph

Slider changes

Invalidated

Recalculate table

Recalculate sample

Recalculate plot

Updated

Your turn

Agenda

Reactive calculations
Your turn
Overriding reactivity
Side effects

Your turn

Complete the exercise on this page:

209-reactive-effect

...

Exercise 2.3 answer

Overriding reactivity

Agenda

Reactive calculations
Your turn
Overriding reactivity
Side effects

Overriding reactivity

By default, when an input changes, all outputs that use it will recalculate

Tip

This isn’t always the user interaction you want:

Database queries
Expensive modeling
Grouping multiple filters together
Side effects

Important

Use @reactive.event to explicitly specify the trigger for an output or calc

Example of reactive event

View the app on this page:

202-reactive-event

...

@reactive.event

from shiny.express import ui, render, input
from shiny import reactive

ui.input_text("input_txt", "Enter text")
ui.input_action_button("send", "Enter")

@render.text
@reactive.event(input.send)
def output_txt():
    return input.input_txt()

Important

@reactive.event overrides the usual implicit dependency detection with an explicit trigger
It can be applied to rendering functions or to @reactive.calc
It tells Shiny to invalidate the object whenever or more inputs change
@reactive.event is often used with action buttons or action links

Your turn

Complete the exercise on this page:

203-reactive-event

...

Side effects

Agenda

Reactive calculations
Your turn
Overriding reactivity
Side effects

What we’ve learned

How Shiny re-renders elements
How Shiny detects dependencies between elements (inputs and outputs)
How to create reusable calculations with @reactive.calc
How to explicitly control reactivity with @reactive.event

Is that enough?

Shiny has a function ui.show_modal which triggers a modal window.
Using what you know so far, how would you call this function?

View the app on this page:

201-modal

...

Return values vs. side effects

Inputs, outputs, and calculations all produce values
When an input value changes, downstream calculations and outputs might need to have their values updated as well
But sometimes we want to execute some code that doesn’t produce a value
- Doesn’t fit in the categories of “calculation” or “output”

Reactive Effects

The @reactive.effect decorator allows you to react to an input without returning a value
Usually paired with @reactive.event

@reactive.effect
@reactive.event(input.show)
def toggle_modal():
    m = ui.modal(
        "This is a somewhat important message.",
        title="Click outside the modal to close",
        easy_close=True,
        footer=None,
    )
    ui.modal_show(m)

The rule

Code that produces:

Desired effect	Code
an output	`@render_*`
an intermediate value	`@reactive.calc`
only side effects	`@reactive.effect`

Caution

Code that produces a value and a side effect: Don’t do this!
The “command-query separation” principle

What’s a side effect?

Somewhat context dependent
Multiple strategies can work
If your solution feels complicated and painful, ask whether you should use the other strategy

Your turn

Complete the exercise on this page:

209-reactive-calc

...