bborgesr · March 5, 2017 19:20 · Mar 5, 2017
diff --git a/reactives-in-loops.R b/reactives-in-loops.R
@@ -0,0 +1,188 @@
+# -------------------------------------------------------------------
+# ------------------ REACTIVES INSIDE FOR LOOPS ---------------------
+# -------------------------------------------------------------------
+
+# -------------------------------------------------------------------
+# --- EXAMPLE 1: this works fine, because there are no reactives in -
+# --- the for lopp --------------------------------------------------
+# -------------------------------------------------------------------
+library(shiny)
+
+ui <- fluidPage(
+  verbatimTextOutput("txt")
+)
+
+server <- function(input, output, session) {
+  rv <- reactiveValues(one = 1, two = 2, three = 3)
+
+  output$txt <- renderPrint({
+    vals <- list()
+    for (val in reactiveValuesToList(rv)) {
+      vals[[val]] <- val
+    }
+    for (i in seq_len(length(vals))) {
+      print(vals[[i]])
+    }
+  })
+}
+
+shinyApp(ui, server)
+#> [1] 1
+#> [1] 2
+#> [1] 3
+
+
+# -------------------------------------------------------------------
+# --- EXAMPLE 2: this works fine, because even though there is a ----
+# --- reactive in the for lopp, it is invoked immediately (inside ---
+# --- the for loop) -------------------------------------------------
+# -------------------------------------------------------------------
+library(shiny)
+
+ui <- fluidPage(
+  verbatimTextOutput("txt")
+)
+
+server <- function(input, output, session) {
+  rv <- reactiveValues(one = 1, two = 2, three = 3)
+
+  output$txt <- renderPrint({
+    vals <- list()
+    for (val in reactiveValuesToList(rv)) {
+      vals[[val]] <- reactive({ val })()
+    }
+    for (i in seq_len(length(vals))) {
+      print(vals[[i]])
+    }
+  })
+}
+
+shinyApp(ui, server)
+#> [1] 1
+#> [1] 2
+#> [1] 3
+
+
+# -------------------------------------------------------------------
+# --- EXAMPLE 3: this does not work, because there is a reactive in -
+# --- the for lopp that is only invoked outside the for loop --------
+# -------------------------------------------------------------------
+library(shiny)
+
+ui <- fluidPage(
+  verbatimTextOutput("txt")
+)
+
+server <- function(input, output, session) {
+  rv <- reactiveValues(one = 1, two = 2, three = 3)
+
+  output$txt <- renderPrint({
+    vals <- list()
+    for (val in reactiveValuesToList(rv)) {
+      vals[[val]] <- reactive({ val })
+    }
+    for (i in seq_len(length(vals))) {
+      print(vals[[i]]())
+    }
+  })
+}
+
+shinyApp(ui, server)
+#> [1] 3
+#> [1] 3
+#> [1] 3
+
+# --- Joe on this behavior:
+# --- > It's because all the iterations of the for loop share the same
+# --- > reference to el. So when any of the created reactive expressions 
+# --- > execute, they're using whatever the final value of el was.
+
+
+# -------------------------------------------------------------------
+# --- EXAMPLE 4: this works, because even though there is a reactive 
+# --- in the for lopp that is only invoked outside of it, it is: ----
+# ---                                                            ----  
+# ---    * wrapped in a call to `local({ })`;                    ----
+# ---                                                            ----
+# ---    * the iter variable (`val`) is assigned to `local({ })` ----
+# ---      specific variable (`myVal`);                          ----
+# ---                                                            ----
+# ---    * the super-assign operator (`<-`) is used              ----
+# -------------------------------------------------------------------
+library(shiny)
+
+ui <- fluidPage(
+  verbatimTextOutput("txt")
+)
+
+server <- function(input, output, session) {
+  rv <- reactiveValues(one = 1, two = 2, three = 3)
+
+  output$txt <- renderPrint({
+    vals <- list()
+    for (val in reactiveValuesToList(rv)) {
+      local({
+        myVal <- val
+        vals[[myVal]] <<- reactive({ myVal })
+      })
+    }
+    for (i in seq_len(length(vals))) {
+      print(vals[[i]]())
+    }
+  })
+}
+
+shinyApp(ui, server)
+#> [1] 1
+#> [1] 2
+#> [1] 3
+
+# --- Joe on this behavior:
+# --- > You can fix this by using a for loop but introducing a 
+# --- > local({...}) inside of there, and creating a local variable 
+# --- > in there whose value is assigned to el outside of the reactive.
+
+
+# -------------------------------------------------------------------
+# --- EXAMPLE 5: this works, because we use lapply instead of a for -
+# --- loop ----------------------------------------------------------
+# -------------------------------------------------------------------
+library(shiny)
+
+ui <- fluidPage(
+  verbatimTextOutput("txt")
+)
+
+server <- function(input, output, session) {
+  rv <- reactiveValues(one = 1, two = 2, three = 3)
+
+  output$txt <- renderPrint({
+    vals <- lapply(
+      reactiveValuesToList(rv),
+      function(x) { reactive(x) }
+    )
+
+    # --- Note that this alternative would not work because if doesn't
+    # --- get its own reference to the iter variable, so we always ---
+    # --- need to create a wrapper for the reactive ------------------
+    # vals <- lapply(
+    #   reactiveValuesToList(rv), 
+    #   reactive
+    # )
+
+    for (i in seq_len(length(vals))) {
+      print(vals[[i]]())
+    }
+  })
+}
+
+shinyApp(ui, server)
+#> [1] 1
+#> [1] 2
+#> [1] 3
+
+# --- Joe on this behavior:
+# --- > You can fix this by using lapply instead of a for loop; since 
+# --- > each iteration executes as its own function call, it gets its 
+# --- > own reference to el.
+