Add source to show() callback

CHANGELOG.md

@@ -16,6 +16,8 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
   effects to set children.
 - `spring()` returns a second value, a setter to set position, velocity and
   impulse.
+- `show()` now receives a source to its callback returning the current value
+  of the condition.
 
 ### Changed
 

src/show.luau

@@ -1,16 +1,28 @@
-local switch = require "./switch"
+local derive = require "./derive"
+local untrack = require "./untrack"
 
-local function show<T>(source: () -> any, component: () -> T, fallback: (() -> T)?): () -> T?
-    local function truthy()
+local function show<T, U>(source: () -> T?, component: (() -> T) -> U, fallback: (() -> U)?): () -> U?
+    local truthy = derive(function()
         return not not source()
+    end)
+
+    -- seemingly redundant derivation to extend the reactive graph so that
+    -- the propogation of the source's update is delayed, giving time for
+    -- the show() scope to be destroyed before potential effects registered
+    -- by the component can run when they should not run
+    -- todo: are there cases this method does not cover?
+    local derived = derive(function()
+        return source()
+    end)
+
+    return derive(function()
+        return
+            if truthy() then untrack(function() return component(derived :: () -> T) end)
+            elseif fallback then untrack(fallback)
+            else nil
+    end)
 end
 
-    return switch(truthy) {
-        [true] = component,
-        [false] = fallback,
-    }
-end
-
-return show ::
-    (<T>(source: () -> any, component: () -> T) -> () -> T?) &
-    (<T, U>(source: () -> any, component: () -> T, fallback: () -> U) -> () -> (T | U)?)
+return show :: (
+    ( <T, U>(source: () -> T?, component: (() -> T) -> U) -> () -> U? )
+)

test/tests.luau

@@ -1073,20 +1073,73 @@ TEST("show()", wrap_root(function()
     local show = vide.show
     local root = vide.root
 
-    do CASE "main"
-        -- uses switch() internally, more extensive testing of scoping not needed
-        local value = source("truey" :: unknown)
+    do CASE "show component"
+        local input = source(true)
+        local function one() return 1 end
+
+        local output = show(input, one)
+
+        CHECK(output() == 1)
+        input(false)
+        CHECK(output() == nil)
+    end
+
+    do CASE "fallback component"
+        local input = source(true)
         local function one() return 1 end
         local function two() return 2 end
 
-        local output = show(value, one, two)
+        local output = show(input, one, two)
 
         CHECK(output() == 1)
-        value(nil)
+        input(false)
         CHECK(output() == 2)
     end
 
-    do CASE "alt"
+    do CASE "updating truth to truthy does not rerun"
+        local input = source(1)
+        local count = 0
+
+        local function component()
+            count += 1
+            return 1
+        end
+
+        local output = show(input, component)
+
+        CHECK(count == 1)
+        CHECK(output() == 1)
+        input(2)
+        CHECK(count == 1)
+    end
+
+    do CASE "updating source passed to component"
+        local input = source(1 :: number?)
+        local count = 0
+
+        show(input :: () -> number?, function(value: () -> number)
+            vide.cleanup(function() print "destroyed" end)
+            effect(function()
+                local v = value()
+    
+                count += 1
+
+                CHECK(v == count)
+                if v ~= count then error(count) end
+            end)
+
+            return true
+        end)
+
+        input(2)
+        CHECK(count == 2)
+        input(3)
+        CHECK(count == 3)
+        input(nil)
+        CHECK(count == 3)
+    end
+
+    do CASE "alt" -- todo: move test
         local visible = vide.source(true)
         local count = vide.source(0)