{"id":13651,"date":"2016-02-22T11:39:58","date_gmt":"2016-02-22T16:39:58","guid":{"rendered":"http:\/\/mjtsai.com\/blog\/?p=13651"},"modified":"2025-12-16T15:49:44","modified_gmt":"2025-12-16T20:49:44","slug":"use-and-misuse-of-nsuserdefaults","status":"publish","type":"post","link":"https:\/\/mjtsai.com\/blog\/2016\/02\/22\/use-and-misuse-of-nsuserdefaults\/","title":{"rendered":"Use and Misuse of NSUserDefaults"},"content":{"rendered":"

David Smith<\/a> (tweet<\/a>):<\/p>\n

If you find yourself needing to do anything else to read a preference, you should take a step back and reconsider: caching values from NSUserDefaults is usually unnecessary, since it’s extremely fast to read from. Calling -synchronize<\/code> before reading a value is always unnecessary. Responding when the value changes is almost always unnecessary, since the nature of “settings” is that they control what a program does when it does it, rather than actually causing it to do something. Having an alternate code path for “no value set” is also generally unnecessary, as you can provide a default value instead (see Providing Default Values below).<\/p>

[…]<\/p>

You can call -registerDefaults:<\/code> as many times as you like, and it will combine the dictionaries that you pass it, which means you can keep registration of settings near the code that cares about them.<\/p><\/blockquote>

I used to use -registerDefaults:<\/code> more, but now I mostly use a category method like:<\/p>\n

- (id)mjtObjectForKey:(NSString *)key defaultValue:(id)defaultValue;<\/pre>\n
that returns defaultValue<\/code> if NSUserDefaults<\/code> returns nil<\/code>. This is more convenient (one line of code) and makes it impossible to look up a key that has not been registered.<\/p>\n
In the sandboxed world of modern OSX and all iOS versions, NSUserDefaults is initially limited to operating in your app’s sandbox; if you use -initWithSuiteName:<\/code> you’ll just get a new store of user defaults that’s still not shared.<\/p>
[…]<\/p>
NSUserDefaults does not have any form of transaction system, so there’s no way to guarantee that multiple changes will only be seen all at once. Another program could see the first change before the second finishes.<\/p>
[…]<\/p>
Neither NSUserDefaultsDidChangeNotification<\/code> nor KVO notify of changes made by other programs<\/p>
-registerDefaults:<\/code> operates on every NSUserDefaults<\/code> instance, not just the one you call it on<\/p><\/blockquote>\n
Update (2016-02-22): One of the issues I’ve found is that, starting with Mac OS X 10.10, NSUserDefaults<\/code> ignores any attempt to set an object that is a dictionary or array bridged from Python.<\/p>","protected":false},"excerpt":{"rendered":"
David Smith (tweet): If you find yourself needing to do anything else to read a preference, you should take a step back and reconsider: caching values from NSUserDefaults is usually unnecessary, since it’s extremely fast to read from. Calling -synchronize before reading a value is always unnecessary. Responding when the value changes is almost always […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"apple_news_api_created_at":"2025-12-16T20:49:49Z","apple_news_api_id":"d879c354-729d-4390-ad95-b736a31e0773","apple_news_api_modified_at":"2025-12-16T20:49:49Z","apple_news_api_revision":"AAAAAAAAAAD\/\/\/\/\/\/\/\/\/\/w==","apple_news_api_share_url":"https:\/\/apple.news\/A2HnDVHKdQ5Ctlbc2ox4Hcw","apple_news_coverimage":0,"apple_news_coverimage_caption":"","apple_news_is_hidden":false,"apple_news_is_paid":false,"apple_news_is_preview":false,"apple_news_is_sponsored":false,"apple_news_maturity_rating":"","apple_news_metadata":"\"\"","apple_news_pullquote":"","apple_news_pullquote_position":"","apple_news_slug":"","apple_news_sections":"\"\"","apple_news_suppress_video_url":false,"apple_news_use_image_component":false,"footnotes":""},"categories":[4],"tags":[69,800,31,1137,275,30,1199,2868,54,138,71,53],"class_list":["post-13651","post","type-post","status-publish","format-standard","hentry","category-programming-category","tag-cocoa","tag-concurrency","tag-ios","tag-ios-9","tag-keyvalueobserving","tag-mac","tag-mac-os-x-10-11","tag-nsuserdefaults","tag-objective-c","tag-optimization","tag-programming","tag-sandboxing"],"apple_news_notices":[],"_links":{"self":[{"href":"https:\/\/mjtsai.com\/blog\/wp-json\/wp\/v2\/posts\/13651","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/mjtsai.com\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/mjtsai.com\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/mjtsai.com\/blog\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/mjtsai.com\/blog\/wp-json\/wp\/v2\/comments?post=13651"}],"version-history":[{"count":2,"href":"https:\/\/mjtsai.com\/blog\/wp-json\/wp\/v2\/posts\/13651\/revisions"}],"predecessor-version":[{"id":13663,"href":"https:\/\/mjtsai.com\/blog\/wp-json\/wp\/v2\/posts\/13651\/revisions\/13663"}],"wp:attachment":[{"href":"https:\/\/mjtsai.com\/blog\/wp-json\/wp\/v2\/media?parent=13651"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/mjtsai.com\/blog\/wp-json\/wp\/v2\/categories?post=13651"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/mjtsai.com\/blog\/wp-json\/wp\/v2\/tags?post=13651"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}