Objective C Blocks Sample Code

Objective C Blocks Sample Code

Written By: Debasis Das (19-Apr-2015)

In this post we will create sample codes for using blocks in an Cocoa Objective C Application

Block with 1 input Parameter and 1 return Type

int (^testBlock)(int) = ^int (int num)
    int multiplier = 7;
    return num * multiplier;
//    NSLog(@"number %d",testBlock(10));      //This will print 70

Block with 2 input parameters and 1 return type as int

int (^addBlock)(int, int) = ^int(int va1, int val2)
    return va1 + val2;
//    NSLog(@"addBlock %d",addBlock(10,12));  //Prints 22

Two input parameters and block definition

    float (^areaBlock)(float length, float breadth); //Declare the block variable
    //Create and assign the block
    areaBlock = ^float(float len, float brd) //Defines the return type to be float
    //areaBlock = ^(float len, float brd)
    //Does not define the return type. Defining the return type is not mandatory
        return len * brd;
    float newArea = areaBlock(10.87,87.65);
    NSLog(@"newArea is %2.2f",newArea);    //Prints 952.76

Block with No input Parameters

u_int32_t (^randomNumber)(void) = ^{
    return arc4random();
//    NSLog(@"random number is %d",(int)randomNumber);


    //Blocks can access to non local variables defined in the blocks enclosing lexical scope but outside the block itself
    NSString *firstName = @"John";
    NSString * (^getFullName)(NSString *lastName) = ^(NSString *lName){
        // firstName = @"Jane"; //Variable cannot be assigned here untill and unless the variable is declared as a block type
        return [firstName stringByAppendingFormat:@" %@",lName];
    NSLog(@"Full Name is %@",getFullName(@"Doe"));    //Full Name is John Doe
    //Non Local Variables are copied and stored with the blocks are constants (read only)
    //The non local variable cannot be modified
    firstName = @"Jane";
    NSLog(@"Full Name after first name is changed is still %@",getFullName(@"Doe"));
    //Full Name after first name is changed is still John Doe

Changing a non local variable within a block

-(void) blocksSampleFive{
    __block NSString *title = @"Mr";
    //The block modifier serves as a memory between multiple calls to a block
    NSString *(^getCompleteName)(NSString *name) = ^(NSString *name){
        title = @"Ms"; //If we did not use the block modifier this will continue to hold Mr
        return [title stringByAppendingFormat:@" %@",name];
    NSLog(@"Complete Name is %@",getCompleteName(@"Jane Doe"));
    //Complete Name is Ms Jane Doe

Blocks as Method Parameters

    NSArray *stringsArray = @[ @"string 1",
                               @"String 21",
                               @"string 12",
                               @"String 11",
                               @"String 02" ];
    static NSStringCompareOptions comparisonOptions = NSCaseInsensitiveSearch | NSNumericSearch | NSWidthInsensitiveSearch | NSForcedOrderingSearch;

    NSLocale *currentLocale = [NSLocale currentLocale];
    NSComparator finderSortBlock = ^(id string1, id string2) {
        NSRange string1Range = NSMakeRange(0, [string1 length]);
        return [string1 compare:string2 options:comparisonOptions range:string1Range locale:currentLocale];
    //In the below line of code the finderSortBlock is sent as a method parameter
    NSArray *finderSortArray = [stringsArray sortedArrayUsingComparator:finderSortBlock];
    NSLog(@"finderSortArray: %@", finderSortArray);
     finderSortArray: (
     "string 1",
     "String 02",
     "String 11",
     "string 12",
     "String 21"

Enumerate Objects Using Block

    NSArray *numberArray = @[@12,@13,@10,@9,@100,@109,@432];
    [numberArray enumerateObjectsUsingBlock:^(id obj, NSUInteger idx, BOOL *stop) {
        NSLog(@"index = %d  value = %d %@",idx,[obj intValue],stop?@"Yes":@"No");
    //index = 0  value = 12 Yes
    //index = 1  value = 13 Yes
    //index = 2  value = 10 Yes
    //index = 3  value = 9 Yes
    //index = 4  value = 100 Yes
    //index = 5  value = 109 Yes
    //index = 6  value = 432 Yes

A simple block that simply executes without taking any input parameter or returning any value

    void (^callSimpleBlock)(void) = ^{
        NSLog(@"This is a block, Takes nothing, gives nothing");

Test Block to find the index of objects in an array present in a NSSet

    NSArray *array = @[@"A", @"B", @"C", @"A", @"B", @"Z", @"G", @"are", @"Q"];
    NSSet *filterSet = [NSSet setWithObjects: @"A", @"Z", @"Q", nil];

    BOOL (^test)(id obj, NSUInteger idx, BOOL *stop);
    test = ^(id obj, NSUInteger idx, BOOL *stop) {
        if (idx < 5) {
            if ([filterSet containsObject: obj]) {
                return YES;
        return NO;
    NSIndexSet *indexes = [array indexesOfObjectsPassingTest:test];
    NSLog(@"indexes: %@", indexes);
    //indexes: [number of indexes: 2 (in 2 ranges), indexes: (0 3)]

 Block Completion- Sample 1

    [self doSomething:^{
        NSLog(@"This is the completion block. This will get called after the job at doSomething is completed");

typedef void (^EndBlock)();

// A method that returns a completion block
    NSLog(@"Do Something first");
    //This can be a lengthy calculation here.
    //Once this is done then the completion block is called
    NSLog(@"Then may be something else");

 Block Completion- Sample 2

    [self countToHundredAndReturnCompletionBLock:^(BOOL completed){
            NSLog(@"Hundred Counts Finished");
            NSLog(@"Actually Completed but showing not completed. Returning NO here");


// A method that returns a completion block
-(void)countToHundredAndReturnCompletionBLock:(void (^)(BOOL))completed
    int x = 1;
    while (x < 100) {

Display Progress Indicator and Hide Progress Indicator in Completion Block

 Using Blocks to Show/hide progress indicator while a task is being executed
 A simple block implementation to show a progress indicator while a task is in progress.
 Once the task is completed the progress indicator needs to hide.

    [self getDataFromDatabase:self];

/* The order of execution is as follows
-[AppDelegate getDataFromDatabase:]
-[AppDelegate showProgressIndicator]
-[AppDelegate doActualDatabringingActivity:]
-[AppDelegate hideProgressIndicator]
    [self showProgressIndicator];
    [self doActualDatabringingActivity:^{
        [self hideProgressIndicator];
         Avoid Strong reference cycles when capturing self inside blocks
         Capturing a strong reference to self creates a strong reference cycle.
         //To avoid this, its better to capture a wek reference to self
         InterfaceName *__weak weakSelf = self;
         [weakSelf doSomething];

-(void)doActualDatabringingActivity:(void (^) (void))callBackBlock



Get Data from WebService / Database using Blocks

    [[SampleService sharedInstance] fetchCompanyDataWithCompletionBlock:^(NSDictionary *companyRecord, NSError *error) {
        if (!error) {
            NSLog(@"companyRecord %@",companyRecord);
            [[NSOperationQueue mainQueue] addOperationWithBlock:^{
                [NSApp presentError:error];
#import <Foundation/Foundation.h>
@interface SampleService : NSObject

+(SampleService *)sharedInstance;
-(void) fetchCompanyDataWithCompletionBlock:(void(^)(NSDictionary *companyRecord, NSError *error)) completionBlock;

//  SampleService.m
//  ObjcBlocks
//  Created by Debasis Das on 4/19/15.

#import "SampleService.h"
@implementation SampleService

+(SampleService *) sharedInstance
    static SampleService *_sharedInstance = nil;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        _sharedInstance = [[SampleService alloc] init];
    return _sharedInstance;

-(void) fetchCompanyDataWithCompletionBlock:(void(^)(NSDictionary *companyRecord, NSError *error)) completionBlock
    //Create a service request
    //Post the service request
    //Actual Data receiveing goes here
    NSDictionary *companyRecord = [NSDictionary dictionaryWithObjectsAndKeys:@"1000",@"companyId", nil];
   // NSError *error = nil;
    NSError *error = [NSError errorWithDomain:@"Database Connection probably failed" code:7654310 userInfo:[NSDictionary dictionaryWithObjectsAndKeys:@"Some Error Desc",@"errorDesc", nil]];
    completionBlock (companyRecord, error);

Download the Source code from ObjcBlocks

Posted in Cocoa, Objective C Tagged with: , ,
5 comments on “Objective C Blocks Sample Code
  1. Imran Khan says:

    Hi … Great Tutorial for beginners..

    • Imran Khan says:

      Hi If u could elaborate why we are going to use shared instance. how it is useful. That would be good to understand end to end. Thanks

      • Debasis Das says:

        Hi Imran,

        The sample service class is a singleton and thus the sharedInstance.
        The only objective of the SampleService class is to get data from the db through a webservice. We can create a SampleService as subclass of NSObject and do not make it a singleton and still things will work, however everytime we need to call the fetchCompanyDataWithCompletionBlock we have to create an object of SampleService.

        I am writing another article to fetch data from webservice using two different approaches. (Call Back vs NSURLSession Delegate). Once you go through this new article, I think things will be much more clearer.


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