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S3 generic to calculate a `pkg_ref` field

Source: R/pkg_ref_cache.R
riskmetric_metadata_caching.Rd

Reactively retrieve and cache `pkg_ref` metadata

Value

a pkg_ref field

Caching Details

pkg_ref class fields

The pkg_ref class structures an environment with special handling for indexing into the pkg_ref class using the $ or [[ operators. For all intents and purposes, the pkg_ref class is works conceptually similar to a lazy, immutable list, and uses the pkg_ref_cache function internally to lazily retrieve package reference fields.

Lazy metadata caching

Laziness in a pkg_ref object refers to the delayed evaluation of the contents of its fields. Since some metadata is time or computationally intensive to retrieve, and unnessary for some assessments, we want to avoid that retrieval until it is needed.

The first time that a field is accessed within a pkg_ref object x, a corresponding pkg_ref_cache S3 generic is called. For example, when x$description is first accessed, the pkg_ref object uses the function pkg_ref_cache.description to attempt to retrieve the contents of the corresponding DESCRIPTION file.

Often, the way that this data is collected might be different depending on the subclass of the pkg_ref. In the case of the description metadata, a reference to a local install might be able to read in a local file directly, whereas a reference to a remote source of metadata might require first downloading the file. For this reason, many pkg_ref_cache.* functions are themselves S3 generics that dispatch on the class of the pkg_ref object, allowing for divergent behaviors for different source of package metadata.

pkg_ref field immutability

Once a field has been calculated, its value is immutable. This behavior was chosen because of the long time frame over which package metadata changes, rendering it unnecessary to continually reevaluate fields each time they are accesssed.

This means that within an assessment, a given field for a package will only ever be calculated once and preserved for downstream use.

Examples

if (FALSE) { # \dontrun{
# implementing a new field called "first_letter" that is consistently derived
# across all pkg_ref objects:

  pkg_ref_cache.first_letter <- function(x, name, ...) {
    substring(x$name, 1, 1)
  }

  x <- pkg_ref("riskmetric")
  x$first_letter



# implementing a new field called "subclass_enum" that dispatches on
# the subclass of the pkg_ref object:

  pkg_ref_cache.subclass_enum <- function(x, name, ...) {
    UseMethod("pkg_ref_cache.subclass_enum")
  }

  pkg_ref_cache.subclass_enum.pkg_ref <- function(x, name, ...) {
    0
  }

  pkg_ref_cache.subclass_enum.pkg_install <- function(x, name, ...) {
    1
  }

  x$subclass_enum
} # }