While doing research for my next Swift article, I implemented a very simple future class, which might be of interest:<\/p>\n
import Foundation\r\n\r\nfunc PerformOnMain(work: () -> Void) {\r\n\tCFRunLoopPerformBlock(NSRunLoop.mainRunLoop().getCFRunLoop(), kCFRunLoopCommonModes, work)\r\n}\r\n\r\nfunc PerformAsync(work: () -> Void) {\r\n\tdispatch_async(dispatch_get_global_queue(0, 0), work)\r\n}\r\n\r\nclass Future <T> {\r\n\tlet lock = NSCondition()\r\n\tvar result: [T] = []\r\n\t\r\n\tfunc _run(work: () -> T) {\r\n\t\tPerformAsync {\r\n\t\t\tlet value = work()\r\n\t\t\tself.lock.lock()\r\n\t\t\tself.result = [ value ]\r\n\t\t\tself.lock.broadcast()\r\n\t\t\tself.lock.unlock()\r\n\t\t}\r\n\t}\r\n\t\r\n\tinit(work: () -> T) {\r\n\t\t_run(work)\r\n\t}\r\n\t\r\n\tinit(_ work: @auto_closure ()-> T) {\r\n\t\t_run(work)\r\n\t}\r\n\t\r\n\tvar value: T {\r\n\t\tlock.lock()\r\n\t\twhile result.isEmpty {\r\n\t\t\tlock.wait()\r\n\t\t}\r\n\t\tlet r = result[0]\r\n\t\tlock.unlock()\r\n\t\treturn r\r\n\t}\r\n}<\/code><\/pre>\nIt’s used as follows:<\/p>\n
let futureString: Future<String?> = Future {\r\n\t\/\/ ... protracted action that may return a string\r\n\treturn didItWork ? aString : nil\r\n}\r\n\r\n\/\/ alternatively, if you just have a function call or expression:\r\nlet anotherString: Future<String> = Future(SomeFunctionThatTakesALongTime())\r\n\/\/ ... do other things, then when you need the future's value:\r\nlet theString = futureString.value<\/code><\/pre>\nIn other words, the closure will be executed asynchronously, then when you need the result,\u00a0getting value<\/code> will block until it’s ready. The result of the closure can be of any type and even (as in the first example) be optional (that is, may be\u00a0nil<\/code>). Once the result is ready, getting value<\/code> will not block.<\/p>\nMy interest in futures began with Mike Ash’s excellent article<\/a> about an Objective-C implementation<\/a>\u00a0of the concept. Mike’s implementation (MAFuture<\/code>) is of an implicit future; that is, it returns a proxy object which transparently stands in for the future value and doesn’t block when operations like retain\/release<\/code> are performed; therefore, you can add the future to an NSArray<\/code> or NSDictionary<\/code> while it’s still being computed. The downside is that, because of the intricacies of message forwarding in Objective-C, the future can’t return a nil value.<\/p>\nMAFuture<\/code>\u00a0has grown to be rather complex, now catering to iOS memory triggers, archiving and whatnot, so I made a very bare-bones variation for my own projects, where it has been very useful.<\/p>\nSwift has no proxy objects (yet?), so implicit futures can’t be done; I tried to code the proxy in Objective-C and subclass in Swift, but ran into too many compiler\/runtime bugs \u2014 it’s a beta, after all. However, the explicit implementation above has the advantage that the future can return any Swift type, including optional.<\/p>\n
An additional Future<\/code>\u00a0property that might be useful in some places:<\/p>\nvar resolved: Bool {\r\n\tlock.lock()\r\n\tlet r = !result.isEmpty\r\n\tlock.unlock()\r\n\treturn r\r\n}<\/code><\/pre>\nyou can use this to test if the result is available without blocking.<\/p>\n
There are more complex futures available already. I looked at Swiftz<\/a> (by Maxwell Swadling) and BrightFutures<\/a> (by Thomas Visser), which also implement other constructs. If you have a functional programming background you should examine them, too.<\/p>\nUpdate:<\/strong> Mike Ash suggested a small change in my code above (have to broadcast<\/code> before unlock<\/code>), and confirmed that everything should work properly. Thanks Mike!<\/p>\nUpdate#2:<\/strong>\u00a0some more small changes, added the autoclosure form and a PerformOnMain<\/code> function that fills in for the Objective-C performSelectorOnMainThread:<\/code> method – useful for updating UI at the conclusion of a future’s closure.<\/p>\nUpdate#3:<\/strong> final form, in line with the published sample app on github<\/a>. There are extensive comments<\/a> on details.<\/p>\nUpdate#4:\u00a0<\/strong>slight change for the new Array declaration syntax in Xcode 6.0b3.<\/p>\n","protected":false},"excerpt":{"rendered":"While doing research for my next Swift article, I implemented a very simple future class, which might be of interest: import Foundation func PerformOnMain(work: () -> Void) { CFRunLoopPerformBlock(NSRunLoop.mainRunLoop().getCFRunLoop(), kCFRunLoopCommonModes, work) } func PerformAsync(work: () -> Void) { dispatch_async(dispatch_get_global_queue(0, 0), work) } class Future <T> { let lock = NSCondition() var result: [T] = [] […]<\/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":""},"categories":[3,4,19],"tags":[47],"class_list":["post-2868","post","type-post","status-publish","format-standard","hentry","category-apple","category-dev","category-software","tag-swift"],"featured_image_src":null,"author_info":{"display_name":"Rainer Brockerhoff","author_link":"https:\/\/brockerhoff.net\/blog\/author\/rbrockerhoff\/"},"jetpack_featured_media_url":"","jetpack_shortlink":"https:\/\/wp.me\/p1q3Zc-Kg","jetpack_sharing_enabled":true,"_links":{"self":[{"href":"https:\/\/brockerhoff.net\/blog\/wp-json\/wp\/v2\/posts\/2868","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/brockerhoff.net\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/brockerhoff.net\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/brockerhoff.net\/blog\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/brockerhoff.net\/blog\/wp-json\/wp\/v2\/comments?post=2868"}],"version-history":[{"count":0,"href":"https:\/\/brockerhoff.net\/blog\/wp-json\/wp\/v2\/posts\/2868\/revisions"}],"wp:attachment":[{"href":"https:\/\/brockerhoff.net\/blog\/wp-json\/wp\/v2\/media?parent=2868"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/brockerhoff.net\/blog\/wp-json\/wp\/v2\/categories?post=2868"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/brockerhoff.net\/blog\/wp-json\/wp\/v2\/tags?post=2868"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}