Reference ropensci/drake#345 (comment)

12-caution.Rmd

@@ -148,36 +148,70 @@ You can call `make(..., timeout = 10)` to time out all each target after 10 seco
 
 ## Dependencies
 
-### Dependency objects containing functions may change unexpectedly
+### Objects that contain functions may trigger unwanted builds
 
 For example, an `R6` class changes whenever a new `R6` object of that class is created.
 
 ```{r r6change11}
 library(digest)
 library(R6)
-example_class <- R6Class(
-  "example_class",
-  private = list(data = list()),
+circle_class <- R6Class(
+  "circle_class",
+  private = list(radius = NULL),
   public = list(
-    initialize = function(data = list()) {
-      private$data <- data
+    initialize = function(radius){
+      private$radius <- radius
+    },
+    area = function(){
+      pi * private$radius ^ 2
     }
   )
 )
-digest(example_class)
-example_object <- example_class$new(data = 1234)
-digest(example_class) # example_class changed
+digest(circle_class)
+circle <- circle_class$new(radius = 5)
+digest(circle_class) # example_class changed
+rm(circle)
 ```
 
-`Drake` detects this type strange change and unavoidably rebuilds targets.
+Ordinarily, `drake` overreacts to this change and builds targets repeatedly.
 
-```{r r6rebuild11, eval = FALSE}
-plan <- drake_plan(example_target = example_class$new(1234))
-make(plan) # `example_class` changes because it is referenced.
-make(plan) # Builds `example_target` again because `example_class` changed.
+```{r r6rebuild11}
+clean()
+plan <- drake_plan(
+  circle = circle_class$new(radius = 10),
+  area = circle$area()
+)
+make(plan) # `circle_class` changes because it is referenced.
+make(plan) # Builds `circle` again because `circle_class` changed.
 ```
 
-The same behavior affects objects containing functions more broadly, not just `R6` classes. For more on this edge case, see [issue 345](https://github.com/ropensci/drake/issues/345).
+The solution is to define your `R6` class inside a function. `drake` does the right thing when it comes to tracking changes to functions.
+
+
+```{r r6build2}
+clean()
+new_circle <- function(radius){
+  circle_class <- R6Class(
+    "circle_class",
+    private = list(radius = NULL),
+    public = list(
+      initialize = function(radius){
+        private$radius <- radius
+      },
+      area = function(){
+        pi * private$radius ^ 2
+      }
+    )
+  )
+  circle_class$new(radius = radius)
+}
+plan <- drake_plan(
+  circle = new_circle(radius = 10),
+  area = circle$area()
+)
+make(plan)
+make(plan)
+```
 
 
 ### The `magrittr` dot (`.`) is always ignored.