From 5f1acf3c66d9e128883a7a54409a08a42ab9ea05 Mon Sep 17 00:00:00 2001
From: Stephen Blott
Date: Fri, 18 Mar 2016 06:12:55 +0000
Subject: Add Utils.makeIdempotent.

The intention is to use this to clean up some of the initialisation
sequences in the front end.
---
 lib/utils.coffee                   |  4 ++++
 tests/unit_tests/utils_test.coffee | 23 +++++++++++++++++++++++
 2 files changed, 27 insertions(+)

diff --git a/lib/utils.coffee b/lib/utils.coffee
index 9969811a..31f4bec6 100644
--- a/lib/utils.coffee
+++ b/lib/utils.coffee
@@ -236,6 +236,10 @@ Utils =
   # Like Nodejs's nextTick.
   nextTick: (func) -> @setTimeout 0, func
 
+  # Make an idempotent function.
+  makeIdempotent: (func) ->
+    (args...) -> ([previousFunc, func] = [func, null])[0]? args...
+
 # Utility for parsing and using the custom search-engine configuration.  We re-use the previous parse if the
 # search-engine configuration is unchanged.
 SearchEngines =
diff --git a/tests/unit_tests/utils_test.coffee b/tests/unit_tests/utils_test.coffee
index 67c3b333..9b71ca72 100644
--- a/tests/unit_tests/utils_test.coffee
+++ b/tests/unit_tests/utils_test.coffee
@@ -102,3 +102,26 @@ context "compare versions",
     assert.equal -1, Utils.compareVersions("1.40.1", "1.40.2")
     assert.equal -1, Utils.compareVersions("1.40.1", "1.41")
     assert.equal 1, Utils.compareVersions("1.41", "1.40")
+
+context "makeIdempotent",
+  setup ->
+    @count = 0
+    @func = Utils.makeIdempotent (n = 1) => @count += n
+
+  should "call a function once", ->
+    @func()
+    assert.equal 1, @count
+
+  should "call a function once with an argument", ->
+    @func 2
+    assert.equal 2, @count
+
+  should "not call a function a second time", ->
+    @func()
+    assert.equal 1, @count
+
+  should "not call a function a second time", ->
+    @func()
+    assert.equal 1, @count
+    @func()
+    assert.equal 1, @count
-- 
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