xcode - dispatch_after in swift explanation -


i working on project , part of need unhighlight button after set period of time. decided use dispatch_after.

i have managed working, can please explain me how line of code works? have been unable understand how dispatch_after works.

dispatch_after(dispatch_time(dispatch_time_now, int64(1000 * double(nsec_per_msec))), dispatch_get_main_queue()) {                  self.redbutton.highlighted = false              }

let's break down multiple statements:

let when = dispatch_time(dispatch_time_now, int64(1000 * double(nsec_per_msec))) let queue = dispatch_get_main_queue()  dispatch_after(when, queue) {     self.redbutton.highlighted = false }

dispatch_after() enqueues block execution @ time on queue. in case, queue "main queue" "the serial dispatch queue associated application’s main thread". ui elements must modified on main thread only.

the when: parameter of dispatch_after() dispatch_time_t documented "a abstract representation of time". dispatch_time() utility function compute time value. takes initial time, in case dispatch_time_now "indicates time occurs immediately", , adds offset specified in nanoseconds:

let when = dispatch_time(dispatch_time_now, int64(1000 * double(nsec_per_msec)))

nsec_per_msec = 1000000 number of nanoseconds per millisecond, 

int64(1000 * double(nsec_per_msec))

is offset of 1000*1000000 nanoseconds = 1000 milliseconds = 1 second.

the explicit type conversions necessary because swift not implicitly convert between types. using double ensures works in cases like

let when = dispatch_time(dispatch_time_now, int64(0.3 * double(nsec_per_sec)))

to specify offset of 0.3 seconds.

summary: code enqueues block executed on main thread in 1000 ms now.


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