Allow Hierarchy to delay computation of children

This adds an option to allow callers to specify that computing
the children of the hierarchy should be delayed to when the
user calls for them, by utilizing the tree-widget :expander
property.
* lisp/emacs-lisp/hierarchy.el (hierarchy-add-tree)
(hierarchy-add-trees): Add parameter 'delay-children-p'.
* lisp/emacs-lisp/hierarchy.el
(hierarchy--create-delayed-tree-widget): Add function.
* lisp/emacs-lisp/hierarchy.el (hierarchy-convert-to-tree-widget):
Utilize ':expander' if delaying children.  (Bug#55900)

* test/lisp/emacs-lisp/hierarchy-tests.el: Add tests for
delayed-children functionality.

1 files changed, 67 insertions, 18 deletions

diff --git a/lisp/emacs-lisp/hierarchy.el b/lisp/emacs-lisp/hierarchy.el
index 6c95d86b47e..4cb5ba64a8d 100644
--- a/lisp/emacs-lisp/hierarchy.el
+++ b/lisp/emacs-lisp/hierarchy.el
@@ -71,7 +71,8 @@
                (:conc-name hierarchy--))
   (roots (list)) ; list of the hierarchy roots (no parent)
   (parents (make-hash-table :test 'equal)) ; map an item to its parent
-  (children (make-hash-table :test 'equal)) ; map an item to its childre
+  (children (make-hash-table :test 'equal)) ; map an item to its children
+  (delaying-parents (make-hash-table :test 'equal)) ; map an item to its childrenfn
   ;; cache containing the set of all items in the hierarchy
   (seen-items (make-hash-table :test 'equal)))  ; map an item to t
 
@@ -133,7 +134,8 @@ keys are :key and :test."
   "Create a hierarchy and return it."
   (hierarchy--make))
 
-(defun hierarchy-add-tree (hierarchy item parentfn &optional childrenfn acceptfn)
+(defun hierarchy-add-tree (hierarchy item parentfn
+                                     &optional childrenfn acceptfn delay-children-p)
   "In HIERARCHY, add ITEM.
 
 PARENTFN is either nil or a function defining the child-to-parent
@@ -151,27 +153,39 @@ CHILDRENFN are expected to be coherent with each other.
 
 ACCEPTFN is a function returning non-nil if its parameter (any object)
 should be an item of the hierarchy.  By default, ACCEPTFN returns non-nil
-if its parameter is non-nil."
+if its parameter is non-nil.
+
+DELAY-CHILDREN-P is a predicate determining whether the children that would
+normally be processed by CHILDRENFN should, instead, have their processing be
+delayed and stored to be processed by CHILDRENFN when the child is selected
+during use of the hierarchy."
   (unless (hierarchy-has-item hierarchy item)
     (let ((acceptfn (or acceptfn #'identity)))
       (hierarchy--seen-items-add hierarchy item)
       (let ((parent (and parentfn (funcall parentfn item))))
         (when (funcall acceptfn parent)
           (hierarchy--add-relation hierarchy item parent acceptfn)
-          (hierarchy-add-tree hierarchy parent parentfn childrenfn)))
-      (let ((children (and childrenfn (funcall childrenfn item))))
-        (mapc (lambda (child)
-                (when (funcall acceptfn child)
-                  (hierarchy--add-relation hierarchy child item acceptfn)
-                  (hierarchy-add-tree hierarchy child parentfn childrenfn)))
-              children)))))
-
-(defun hierarchy-add-trees (hierarchy items parentfn &optional childrenfn acceptfn)
+          (hierarchy-add-tree hierarchy parent
+                              parentfn (if delay-children-p nil childrenfn))))
+      (if (and childrenfn delay-children-p)
+          (map-put! (hierarchy--delaying-parents hierarchy) item childrenfn)
+        (let ((children (and childrenfn (funcall childrenfn item))))
+          (map-put! (hierarchy--delaying-parents hierarchy) item nil)
+          (mapc (lambda (child)
+                  (when (funcall acceptfn child)
+                    (hierarchy--add-relation hierarchy child item acceptfn)
+                    (hierarchy-add-tree hierarchy child parentfn childrenfn)))
+                children))))))
+
+(defun hierarchy-add-trees (hierarchy items parentfn
+                                      &optional childrenfn acceptfn delay-children-p)
   "Call `hierarchy-add-tree' on HIERARCHY and each element of ITEMS.
 
-PARENTFN, CHILDRENFN and ACCEPTFN have the same meaning as in `hierarchy-add'."
+PARENTFN, CHILDRENFN, ACCEPTFN, and DELAY-CHILDREN-P have the same meaning as in
+`hierarchy-add'."
   (seq-map (lambda (item)
-             (hierarchy-add-tree hierarchy item parentfn childrenfn acceptfn))
+             (hierarchy-add-tree hierarchy item parentfn
+                                 childrenfn acceptfn delay-children-p))
            items))
 
 (defun hierarchy-add-list (hierarchy list &optional wrap childrenfn)
@@ -541,6 +555,30 @@ nil.  The buffer is returned."
     buffer))
 
 (declare-function widget-convert "wid-edit")
+(defun hierarchy--create-delayed-tree-widget (elem labelfn indent childrenfn)
+  "Return a list of tree-widgets for the children generated.
+
+ELEM is the element of the hierarchy passed from
+`hierarchy-convert-to-tree-widget'; it and the CHILDRENFN are used to generate
+the children of the element dynamically.
+
+LABELFN is the same function passed to `hierarchy-convert-to-tree-widget'.
+
+INDENT is the same function passed to `hierarchy-convert-to-tree-widget'.
+
+CHILDRENFN is the function used to discover the children of ELEM."
+  (lambda (widget)
+    (mapcar
+     (lambda (item)
+       (widget-convert
+        'tree-widget
+        :tag (hierarchy-labelfn-to-string labelfn item indent)
+        :expander (hierarchy--create-delayed-tree-widget
+                   item
+                   labelfn
+                   (1+ indent)
+                   childrenfn)))
+     (funcall childrenfn elem))))
 (defun hierarchy-convert-to-tree-widget (hierarchy labelfn)
   "Return a tree-widget for HIERARCHY.
 
@@ -550,10 +588,21 @@ node label."
   (require 'wid-edit)
   (require 'tree-widget)
   (hierarchy-map-tree (lambda (item indent children)
-                        (widget-convert
-                         'tree-widget
-                         :tag (hierarchy-labelfn-to-string labelfn item indent)
-                         :args children))
+                        (let ((childrenfn (map-elt
+                                           (hierarchy--delaying-parents hierarchy)
+                                           item)))
+                          (apply
+                           #'widget-convert
+                           (list 'tree-widget
+                                 :tag (hierarchy-labelfn-to-string labelfn item indent)
+                                 (if childrenfn :expander :args)
+                                 (if childrenfn
+                                     (hierarchy--create-delayed-tree-widget
+                                      item
+                                      labelfn
+                                      (1+ indent)
+                                      childrenfn)
+                                   children)))))
                       hierarchy))
 
 (defun hierarchy-tree-display (hierarchy labelfn &optional buffer)