IOS Dev Intro - Blocks Programming Series 05

https://developer.apple.com/library/ios/documentation/Cocoa/Conceptual/Blocks/Articles/bxUsing.html#//apple_ref/doc/uid/TP40007502-CH5-SW1

UsingBlocks

InvokingaBlock

Ifyoudeclareablockasavariable,youcanuseitasyouwouldafunction,asshowninthesetwoexamples:

int(^oneFrom)(int)=^(intanInt){

returnanInt-1;

};

printf("1from10is%d",oneFrom(10));

//Prints"1from10is9"

float(^distanceTraveled)(float,float,float)=

^(floatstartingSpeed,floatacceleration,floattime){

floatdistance=(startingSpeed*time)+(0.5*acceleration*time*time);

returndistance;

};

floathowFar=distanceTraveled(0.0,9.8,1.0);

//howFar=4.9

Frequently,however,youpassablockastheargumenttoafunctionoramethod.Inthesecases,youusuallycreateablock“inline”.

UsingaBlockasaFunctionArgument

Youcanpassablockasafunctionargumentjustasyouwouldanyotherargument.Inmanycases,however,youdon’tneedtodeclareblocks;insteadyousimplyimplementtheminlinewherethey’rerequiredasan
argument.Thefollowingexampleusesthe 

qsort_b

 function. 

qsort_b

 is
similartothestandard 

qsort_r

 function,buttakesablockasitsfinalargument.

char*myCharacters[3]={"TomJohn","George","CharlesCondomine"};

qsort_b(myCharacters,3,sizeof(char*),^(constvoid*l,constvoid*r){

char*left=*(char**)l;

char*right=*(char**)r;

returnstrncmp(left,right,1);

});

//Blockimplementationendsat"}"

//myCharactersisnow{"CharlesCondomine","George","TomJohn"}

Noticethattheblockiscontainedwithinthefunction’sargumentlist.
Thenextexampleshowshowtouseablockwiththe 

dispatch_apply

 function. 

dispatch_apply

 is
declaredasfollows:

voiddispatch_apply(size_titerations,dispatch_queue_tqueue,void(^block)(size_t));

Thefunctionsubmitsablocktoadispatchqueueformultipleinvocations.Ittakesthreearguments;thefirstspecifiesthenumberofiterationstoperform;thesecondspecifiesaqueuetowhichtheblockis
submitted;andthethirdistheblockitself,whichinturntakesasingleargument—thecurrentindexoftheiteration.
Youcanuse 

dispatch_apply

 triviallyjusttoprintouttheiterationindex,asshown:

#include<dispatch/dispatch.h>

size_tcount=10;

dispatch_queue_tqueue=dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT,0);

dispatch_apply(count,queue,^(size_ti){

printf("%u\n",i);

});

UsingaBlockasaMethodArgument

Cocoa provides
anumberofmethodsthatuseblocks.Youpassablockasamethodargumentjustasyouwouldanyotherargument.
Thefollowingexampledeterminestheindexesofanyofthefirstfiveelementsinanarraythatappearinagivenfilterset.

NSArray*array=@[@"A",@"B",@"C",@"A",@"B",@"Z",@"G",@"are",@"Q"];

NSSet*filterSet=[NSSetsetWithObjects:@"A",@"Z",@"Q",nil];

BOOL(^test)(idobj,NSUIntegeridx,BOOL*stop);

test=^(idobj,NSUIntegeridx,BOOL*stop){

if(idx<5){

if([filterSetcontainsObject:obj]){

returnYES;

}

}

returnNO;

};

NSIndexSet*indexes=[arrayindexesOfObjectsPassingTest:test];

NSLog(@"indexes:%@",indexes);

/*

Output:

indexes:<NSIndexSet:0x10236f0>[numberofindexes:2(in2ranges),indexes:(03)]

*/

Thefollowingexampledetermineswhetheran 

NSSet

 objectcontainsawordspecifiedbyalocalvariableand
setsthevalueofanotherlocalvariable(

found

)to 

YES

 (and
stopsthesearch)ifitdoes.Noticethat 

found

 isalsodeclaredasa 

__block

 variable,
andthattheblockisdefinedinline:

__blockBOOLfound=NO;

NSSet*aSet=[NSSetsetWithObjects:@"Alpha",@"Beta",@"Gamma",@"X",nil];

NSString*string=@"gamma";

[aSetenumerateObjectsUsingBlock:^(idobj,BOOL*stop){

if([objlocalizedCaseInsensitiveCompare:string]==NSOrderedSame){

*stop=YES;

found=YES;

}

}];

//Atthispoint,found==YES

CopyingBlocks

Typically,youshouldn’tneedtocopy(orretain)ablock.Youonlyneedtomakeacopywhenyouexpecttheblocktobeusedafterdestructionofthescopewithinwhichitwasdeclared.Copyingmovesablock
totheheap.
YoucancopyandreleaseblocksusingCfunctions:

Block_copy();

Block_release();

Toavoidamemoryleak,youmustalwaysbalancea 

Block_copy()

 with 

Block_release()

.

PatternstoAvoid

Ablockliteral(thatis, 

^{...}

)istheaddressofa stack-local datastructurethatrepresents
theblock. Thescopeofthestack-localdatastructureisthereforetheenclosingcompoundstatement,soyoushould avoid thepatternsshowninthefollowingexamples:

voiddontDoThis(){

void(^blockArray[3])(void); //anarrayof3blockreferences

for(inti=0;i<3;++i){

blockArray[i]=^{printf("hello,%d\n",i);};

//WRONG:Theblockliteralscopeisthe"for"loop.

}

}

voiddontDoThisEither(){

void(^block)(void);

inti=random():

if(i>1000){

block=^{printf("gotiat:%d\n",i);};

//WRONG:Theblockliteralscopeisthe"then"clause.

}

//...

}

Debugging

Youcansetbreakpointsandsinglestepintoblocks.YoucaninvokeablockfromwithinaGDBsessionusing 

invoke-block

,
asillustratedinthisexample:

$invoke-blockmyBlock1020

IfyouwanttopassinaCstring,youmustquoteit.Forexample,topass 

thisstring

 intothe 

doSomethingWithString

 block,
youwouldwritethefollowing:

$invoke-blockdoSomethingWithString"\"thisstring\""