ios - Why is NSOperation::completionBlock called too soon? -

i have simple asynchronous operation

@interface idbwebpvalidationoperation()  @property bool executing; @property bool finished;  @end  @implementation idbwebpvalidationoperation  @synthesize executing; @synthesize finished;  - (instancetype)init { self = [super init]; if (self) {     self.completionblock = ^{         nsdlog(@"webp validation has finished");     }; } return self; }  - (void)main { idbassert0(self.bestcapture.webpcandidate);  self.finished = no; if(self.postprocessingvalidator) {     self.executing = yes;     // starts async operation, see callback below     self.postprocessingvalidator(self.bestcapture.webpcandidate); }else{     idbassert0(0);     //      self.bestcapture.jpegnsdata = self.bestcapture.webpcandidate;     idbassert0(self.bestcapture.jpegnsdata);     self.executing = no;     self.finished = yes; } }  - (void)scanningviewcontroller:    (uiviewcontroller<ppscanningviewcontroller>*)scanningviewcontroller           didoutputresults:(nsarray*)results { if([results count]>0) {     self.bestcapture.jpegnsdata = self.bestcapture.webpcandidate;     idbassert0(self.bestcapture.jpegnsdata); }else{     idbassert0(self.microblinkfailurehandler);     self.microblinkfailurehandler(); } idbassert0(!self.finished); self.executing = no; self.finished = yes; }  -(bool)isasynchronous { return yes;    //default no overriding return yes; } 

here have synthesized 2 atomic properties executing & finished avoid lots of (in opinion dumb & extraneous) code swift force kvo , synthesided atomic properties me out of box (i think).

the issue completionblock called before scanningviewcontroller: callback invoked. why?????

this on ios 9.3.x in case matters