问题
I have already asked this on In Swift, how to stop all the process until datas retrieved from parse.com in UICOLLECTIONVIEW. I couldn't retrieve the datas from parse.com before next function get executed. I don't know how to access asynchronous thread. I have declared main queue to be "first_fun()", so this should be run first. Likewise, it is running first, but ending at last. Before that, next function ("second_fun()") gets executed. How to QUEUE this function block? How to finish asynchronous thread first? Kindly check my code.
MY CODE IS BELOW:
override func viewDidLoad() {
println("START")
let queue : dispatch_queue_t = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0)
dispatch_async(queue, {
dispatch_async(dispatch_get_main_queue(), { () -> Void in
self.first_fun()
})
})
second_fun()
println("END")
}
//FIRST_FUN
func first_fun() {
println("FIRST CLASS TOP")
self.par_query.findObjectsInBackgroundWithBlock({(NSArray objects, NSError error) in
if (error != nil) {
NSLog("error " + error.localizedDescription)
}
else {
println("WELCOME to PARSE")
}//ELSE ENDING
})//PARSE ENDING
println("FIRST CLASS BOTTOM")
}
//SECOND_FUN
func second_fun() {
println("WELCOME to SECOND")
}
回答1:
What you could do is add a callback to the first_fun and call second_fun inside that callback like so:
func first_fun(callback: () -> ()) {
//do something async e.g your call to parse
self.par_query.findObjectsInBackgroundWithBlock({(NSArray objects, NSError error) in
if (error != nil) {
NSLog("error " + error.localizedDescription)
}
else {
println("WELCOME to PARSE")
}//ELSE ENDING
callback()
})//PARSE ENDING
}
Your viewDidLoad would look like this:
override func viewDidLoad() {
let queue : dispatch_queue_t = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0)
dispatch_async(queue, {
dispatch_async(dispatch_get_main_queue(), { () -> Void in
self.first_fun() {
self.second_fun()
}
})
})
}
You should of course also parameterize that callback to have access to the data or error from parse
For reference: Further info for completion blocks in swift
回答2:
The essence of your question is "how do I turn an asynchronous design into a synchronous one," which doesn't make a lot of sense. People hit this wall when they are trained in traditional procedural-style programming and then try to solve problems in a functional/event based system.
The answer to your question is "don't do that." You have to learn a new style of system design where everything that happens in second_fun() is not dependent on the results of first_fun(). If the first and second are truly dependent then you should be calling second_fun() as the last operation in first_fun().
For example, if you have a view that depends on data you pull down from the internet (which can be a long running operation), you would typically setup your view to show a spinning wait-indicator, and then you would make your call to findObjectsInBackgroundWithBlock(). In the callback you would process the found results, initialize other UI elements, and then replace the waiting-indicator with your desired view contents.
You have to stop thinking procedurally and start thinking functionally.
来源：https://stackoverflow.com/questions/28249105/in-swift-why-gcd-is-not-working-with-parse