Cocoa Fundamentals Guide<\/a> goes to a lot more trouble than it needs to in order to *ensure* that a class is a singleton.<\/p>\nThis is almost **never** what you want.<\/p>\n
First off, you probably want your class to be testable in a variety of configurations. In your unit tests, instead of getting your shared singleton instance in your `-setUp` method and “resetting” its state in `-tearDown`, you’d be better off just instantiating a *new* instance in `-setUp` and *releasing* it in `-tearDown`.<\/p>\n
Also, the example in the Cocoa Fundamentals Guide does a lot of work that it simply doesn’t need to. This is all you *really* need to do to create a singleton in Cocoa:<\/p>\n
@interface SomeManager : NSObject
\n + (id)sharedManager;
\n @end<\/p>\n
@implementation SomeManager<\/p>\n
+ (id)sharedManager {
\n static id sharedManager = nil;<\/p>\n
if (sharedManager == nil) {
\n sharedManager = [[self alloc] init];
\n }<\/p>\n
return sharedManager;
\n }<\/p>\n
@end<\/p>\n
That’s it! The astute reader will notice, of course, that this isn’t thread-safe. I got rid of the `@synchronized (self)` because it won’t do the right thing; depending on what actual class is sent `+sharedManager`, the value of `self` will be different!<\/p>\n
For the sake of argument, though, let’s say that you do want a singleton with which you can interact from multiple threads at once. One way to do this would be to create your singleton instance in `+initialize` since it will always be run, on a single thread, before any other methods in your class:<\/p>\n
@implementation SomeManager<\/p>\n
static id sharedManager = nil;<\/p>\n
+ (void)initialize {
\n if (self == [SomeManager class]) {
\n sharedManager = [[self alloc] init];
\n }
\n }<\/p>\n
+ (id)sharedManager {
\n return sharedManager;
\n }<\/p>\n
@end<\/p>\n
By doing this, you avoid the performance bottleneck of `@synchronized` taking a recursive lock every time `+sharedManager` is invoked.<\/p>\n
If you want to get fancier, and it’s OK to temporarily have more than one instance of your singleton created, you could even use `objc_atomicCompareAndSwapGlobalBarrier` to assign the value to return from `+sharedManager`, though this is probably also more work than it’s worth; after all, `+initialize` will only be invoked once for your class. (Though it can be re-invoked as a side-effect of initializing subclasses, hence the `if (self == [SomeManager class]) { }` idiom.)<\/p>\n
In all of the above cases, you’ve done a whole lot less work than the example in the Cocoa Fundamentals Guide, and your code is a lot more likely to be correct as a result.<\/p>\n","protected":false},"excerpt":{"rendered":"
I’ll preface this post with the standard advice: *Don’t create singletons if you don’t absolutely have to.* In general, if you’re creating a global “manager” object of some sort, you’re doing something wrong. That said, there’s still occasionally a reason to have such a global singleton, such as a “default something.” The sample code in…<\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"jetpack_post_was_ever_published":false,"_jetpack_newsletter_access":"","_jetpack_dont_email_post_to_subs":false,"_jetpack_newsletter_tier_id":0,"_jetpack_memberships_contains_paywalled_content":false,"_jetpack_memberships_contains_paid_content":false,"footnotes":"","jetpack_publicize_message":"","jetpack_publicize_feature_enabled":true,"jetpack_social_post_already_shared":false,"jetpack_social_options":{"image_generator_settings":{"template":"highway","default_image_id":0,"font":"","enabled":false},"version":2}},"categories":[3],"tags":[9,127,18,7],"class_list":["post-178","post","type-post","status-publish","format-standard","hentry","category-technology","tag-cocoa","tag-design-patterns","tag-objective-c","tag-unit-testing"],"jetpack_publicize_connections":[],"jetpack_featured_media_url":"","jetpack_sharing_enabled":true,"jetpack_shortlink":"https:\/\/wp.me\/p74loH-2S","_links":{"self":[{"href":"https:\/\/eschatologist.net\/blog\/index.php?rest_route=\/wp\/v2\/posts\/178","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/eschatologist.net\/blog\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/eschatologist.net\/blog\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/eschatologist.net\/blog\/index.php?rest_route=\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/eschatologist.net\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=178"}],"version-history":[{"count":10,"href":"https:\/\/eschatologist.net\/blog\/index.php?rest_route=\/wp\/v2\/posts\/178\/revisions"}],"predecessor-version":[{"id":188,"href":"https:\/\/eschatologist.net\/blog\/index.php?rest_route=\/wp\/v2\/posts\/178\/revisions\/188"}],"wp:attachment":[{"href":"https:\/\/eschatologist.net\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=178"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/eschatologist.net\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=178"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/eschatologist.net\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=178"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}