IOS Dev Intro - Blocks Programming Series 03

https://developer.apple.com/library/ios/documentation/Cocoa/Conceptual/Blocks/Articles/bxDeclaringCreating.html#//apple_ref/doc/uid/TP40007502-CH4-SW1

DeclaringandCreatingBlocks

DeclaringaBlockReference

Blockvariablesholdreferencestoblocks.Youdeclarethemusingsyntaxsimilartothatyouusetodeclareapointertoafunction,exceptthatyouuse 

^

 instead
of 

*

.TheblocktypefullyinteroperateswiththerestoftheCtypesystem.Thefollowingareallvalidblockvariabledeclarations:

void(^blockReturningVoidWithVoidArgument)(void);

int(^blockReturningIntWithIntAndCharArguments)(int,char);

void(^arrayOfTenBlocksReturningVoidWithIntArgument[10])(int);

Blocksalsosupportvariadic(

...

)arguments.Ablockthattakesnoargumentsmustspecify 

void

 in
theargumentlist.
Blocksaredesignedtobefullytypesafebygivingthecompilerafullsetofmetadatatousetovalidateuseofblocks,parameterspassedtoblocks,andassignmentofthereturnvalue.Youcancastablock
referencetoapointerofarbitrarytypeandviceversa.Youcannot,however,dereferenceablockreferenceviathepointerdereferenceoperator(

*

)—thus
ablock'ssizecannotbecomputedatcompiletime.
Youcanalsocreatetypesforblocks—doingsoisgenerallyconsideredtobebestpracticewhenyouuseablockwithagivensignatureinmultipleplaces:

typedeffloat(^MyBlockType)(float,float);

MyBlockTypemyFirstBlock=//...;

MyBlockTypemySecondBlock=//...;

CreatingaBlock

Youusethe 

^

 operatortoindicatethebeginningofablockliteralexpression.Itmaybefollowedbyan
argumentlistcontainedwithin 

()

.Thebodyoftheblockiscontainedwithin 

{}

.
Thefollowingexampledefinesasimpleblockandassignsittoapreviouslydeclaredvariable(

oneFrom

)—heretheblockisfollowedbythenormal 

;

 that
endsaCstatement.

float(^oneFrom)(float);

oneFrom=^(floataFloat){

floatresult=aFloat-1.0;

returnresult;

};

Ifyoudon’texplicitlydeclarethereturnvalueofablockexpression,itcanbeautomaticallyinferredfromthecontentsoftheblock.Ifthereturntypeisinferredandtheparameterlistis 

void

,
thenyoucanomitthe 

(void)

 parameterlistaswell.Iforwhenmultiplereturnstatementsarepresent,theymustexactlymatch(usingcastingifnecessary).

GlobalBlocks

Atafilelevel,youcanuseablockasagloballiteral:

#import<stdio.h>

intGlobalInt=0;

int(^getGlobalInt)(void)=^{returnGlobalInt;};