get_layer_data(), get_layer_grob() can use layer names

[lgaborini]

A tiny PR that builds upon #5967.

Now the names can be used by get_layer_data() and get_layer_grob() in place of the layer index.
Only scalars are accepted, just like the current behavior.

If a name is supplied, it is used, but the layer must exist.
If both the name and the index are supplied (either explicitly or as a default argument), the name takes precedence.

The names must be unique (this is currently enforced by #5967 — should revisit whether this is no longer the case).

devtools::load_all(".")
p <- ggplot(head(mpg), aes(displ, hwy)) +
  geom_point(name = "foo") +
  geom_line(name = "bar")

names(p$layers)
#> [1] "foo" "bar"

get_layer_data(p, i = 2L)
#>     x  y PANEL group flipped_aes colour linewidth linetype alpha
#> 1 1.8 29     1    -1       FALSE  black       0.5        1    NA
#> 2 1.8 29     1    -1       FALSE  black       0.5        1    NA
#> 3 2.0 31     1    -1       FALSE  black       0.5        1    NA
#> 4 2.0 30     1    -1       FALSE  black       0.5        1    NA
#> 5 2.8 26     1    -1       FALSE  black       0.5        1    NA
#> 6 2.8 26     1    -1       FALSE  black       0.5        1    NA
get_layer_data(p, name = "bar")
#>     x  y PANEL group flipped_aes colour linewidth linetype alpha
#> 1 1.8 29     1    -1       FALSE  black       0.5        1    NA
#> 2 1.8 29     1    -1       FALSE  black       0.5        1    NA
#> 3 2.0 31     1    -1       FALSE  black       0.5        1    NA
#> 4 2.0 30     1    -1       FALSE  black       0.5        1    NA
#> 5 2.8 26     1    -1       FALSE  black       0.5        1    NA
#> 6 2.8 26     1    -1       FALSE  black       0.5        1    NA

# silently use the name
get_layer_data(p, i = 1L, name = "bar")
#>     x  y PANEL group flipped_aes colour linewidth linetype alpha
#> 1 1.8 29     1    -1       FALSE  black       0.5        1    NA
#> 2 1.8 29     1    -1       FALSE  black       0.5        1    NA
#> 3 2.0 31     1    -1       FALSE  black       0.5        1    NA
#> 4 2.0 30     1    -1       FALSE  black       0.5        1    NA
#> 5 2.8 26     1    -1       FALSE  black       0.5        1    NA
#> 6 2.8 26     1    -1       FALSE  black       0.5        1    NA

# no match by name: error

get_layer_data(p, name = "none")
#> Error in `get_layer_data()`:
#> ! `name` must be one of "foo" or "bar", not "none".

# no match by subscript: error (uglier)

get_layer_data(p, i = 4L)
#> Error in ggplot_build(plot)@data[[idx]]: subscript out of bounds

# Similarly for get_layer_grob()

get_layer_grob(p, i = 2L)
#> [[1]]
#> polyline[GRID.polyline.1]
get_layer_grob(p, name = "bar")
#> [[1]]
#> polyline[GRID.polyline.2]

Perhaps it's better to make i and name mutually exclusive, like id and number for recipes::tidy.recipe()?

[teunbrand]

Thank you for the PR! I agree that being able to use the name to get layer data/grob would be useful. I think it'd make more sense to merge to name approach in this PR with i. Using vctrs::vec_as_location2() to construct the index would handle both the integer and named cases.

[lgaborini]

Thanks, I didn't know that helper existed!

Now the parameter i is a bit inconsistent with get_panel_scales() since it indexes into panels instead of layers.
Probably not an issue until the panels get names too, or layers become panel-dependent.

[teunbrand]

Thanks for these updates, I like the direction of the PR!

Now the parameter i is a bit inconsistent with get_panel_scales() since it indexes into panels instead of layers.

I agree that it is unlikely to be an issue.

[teunbrand]

It is good that these errors are thrown, but I don't think ggplot2 needs to test for them. They are outside ggplot2's control as they are thrown by the vctrs package. If the message changes, that doesn't imply that ggplot2 needs to change.

[teunbrand]

Likewise, I think we don't need these tests.

[teunbrand]

I think the intent of this test is good. However, I think the test would also pass if the index was mismatched because the layer data of both layers would be the same.