vide/docs/tut/crash-course/11-control-flow.md

Control Flow

Eventually you will need a way to dynamically create and destroy UI elements resulting from source updates. Vide provides functions to help you do this, known as control flow functions.

These functions return new sources, which hold the instances to be displayed. These sources can be assigned as children, meaning the displayed children will update when the input source updates.

Control flow functions are special, because they run their components in a new reactive scope, which can be destroyed independently of the reactive scope that called the control flow function itself. This means that parts of your app can be independently created then destroyed.

show()

The most basic control flow function is show(), which is used to conditionally show a component.

local source = vide.source
local show = vide.show

local function JoinMenu()
    local joined = source(false)

    local function JoinButton()
        return Button {
            Activated = function() joined(true) end
        }
    end

    return create "Frame" {
        show(function() return not joined() end, JoinButton)
    }
end

This will make a button to join if you have not joined already.

You can also pass a third argument, a fallback to show if the condition is falsey.

local function JoinMenu()
    local joined = source(false)

    local function JoinButton()
        return Button {
            Activated = function() joined(true) end
        }
    end

    local function LeaveButton()
        return Button {
            Activated = function() joined(false) end
        }
    end

    return create "Frame" {
        show(joined, LeaveButton, JoinButton)
    }
end

The reactive graph for the above example:

%%{init: {
    "theme": "base",
    "themeVariables": {
        "primaryColor": "#1B1B1F",
        "primaryTextColor": "#fff",
        "primaryBorderColor": "#1B1B1F",
        "lineColor": "#79B8FF",
        "tertiaryColor": "#161618",
        "tertiaryBorderColor": "#1C1C1F"
    }
}}%%

graph

subgraph root["mount() scope"]
    direction LR
    joined --> show -.- subroot

    subgraph subroot["show() scope"]
        direction LR
        Button
    end
end

show() will implicitly create an effect depending on joined, which can be seen as show on the graph. This effect manages, and can create or destroy a separate reactive scope seen as show() scope on the graph. The dotted line indicates that it isn't actually connected, only indirectly managed through code.

switch()

Similar to show(), switch(), also condtionally displays one instance at a time. It is more flexible since it can show one of many components, based on a table used to map a source value to a component.

local source = vide.source
local switch = vide.switch

local function JoinMenu()
    local joined = source(false)

    local function JoinButton()
        return Button {
            Activated = function() joined(true) end
        }
    end

    local function LeaveButton()
        return Button {
            Activated = function() joined(false) end
        }
    end

    return create "Frame" {
        switch(joined) {
            [true] = LeaveButton,
            [false] = JoinButton
        }
    }
end

This example is equivalent to the previous one.

The switch can map any value to any component.

type ActiveMenu = "none" | "inventory" | "shop" | "settings"

local menu = source "inventory"

switch(menu) {
    inventory = InventoryMenu,
    shop = ShopMenu,
    settings = SettingsMenu
}

The reactive graph for the above example:

%%{init: {
    "theme": "base",
    "themeVariables": {
        "primaryColor": "#1B1B1F",
        "primaryTextColor": "#fff",
        "primaryBorderColor": "#1B1B1F",
        "lineColor": "#79B8FF",
        "tertiaryColor": "#161618",
        "tertiaryBorderColor": "#1C1C1F"
    }
}}%%

graph

subgraph root["mount() scope"]
    direction LR
    menu --> switch -.- subroot

    subgraph subroot["switch() scope"]
        direction LR
        Menu
    end
end

indexes()

Often, you will have a table of values with each value displayed in a similar manner. Rather than manually looping over each value to generate a corresponding UI element, indexes() allows you to create elements for each table index, to display the value at that index.

local todoList = source {
    "finish the crash course",
    "star vide's GitHub"
}

local function TodoList(props: { list: () -> Array<string> })
    return create "Frame" {
        create "UIListLayout" {},

        indexes(todoList, function(todo, i)
            return create "TextLabel" {
                Text = function()
                    return i .. ": " .. todo()
                end,

                LayoutOrder = i
            }
        end)
    }
end

TodoList { list = todoList }

For each index in the given source table, the given function will be called with:

1. a source containing the value of the index
2. the index itself

When the value at an index is changed, the function is not reran. Instead, the given source for that index is updated.

Any time the input source table is updated, the given function will be ran for any newly added indexes, while any removed indexes (indexes now with a nil value), will have its corresponding reactive scope destroyed to clean up that element.

indexes() is said to map each table index to a new UI element that can update to display the current value at that index.

The reactive graph for the above example:

%%{init: {
    "theme": "base",
    "themeVariables": {
        "primaryColor": "#1B1B1F",
        "primaryTextColor": "#fff",
        "primaryBorderColor": "#1B1B1F",
        "lineColor": "#79B8FF",
        "tertiaryColor": "#161618",
        "tertiaryBorderColor": "#1C1C1F"
    }
}}%%

graph

subgraph root ["mount() scope"]
    direction LR
    todoList --> indexes -.- subroot1 & subroot2

    subgraph subroot1 ["indexes() scope 1"]
        direction LR
        value1[todo] --> prop1["prop binding"]
    end

    subgraph subroot2 ["indexes() scope 2"]
        direction LR
        value2[todo] --> prop2[prop binding]
    end
end

One thing to note regarding table sources, is that when you edit a table in a source, you must set that table again to actually update the source.

local src = source { 1, 2 }
local data = src()
table.insert(data, 3) -- no effects will run
src(data) -- effects will run

Together, these control flow functions cover the majority of cases where you need to dynamically create and destroy parts of your UI.

If you need to do something that these control flow functions cannot, you can always use mount() within an effect to dynamically create and destroy components on your own terms. Just remember to use cleanup() to unmount when the effect reruns.