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Difference between blocks and completion handler in iOS

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Blocks are a language-level feature added to C, Objective-C and C++, which allow you to create distinct segments of code that can be passed around to methods or functions as if they were values. Blocks are Objective-C objects, which means they can be added to collections like NSArray or NSDictionary.

They can be executed in a later time, and not when the code of the scope they have been implemented is being executed.
Their usage leads eventually to a much cleaner and tidier code writing, as they can be used instead of delegate methods, written just in one place and not spread to many files.
Syntax: ReturnType (^blockName)(Parameters)

see example:

int anInteger = 42;

void (^testBlock)(void) = ^{

    NSLog(@"Integer is: %i", anInteger);   // anInteger outside variables


// calling blocks like
Block with argument:

double (^multiplyTwoValues)(double, double) =

                          ^(double firstValue, double secondValue) {

                              return firstValue * secondValue;

// calling with parameter
double result = multiplyTwoValues(2,4);

NSLog(@"The result is %f", result);

Completion handler:
Whereas completion handler is a way (technique) for implementing callback functionality using blocks.
A completion handler is nothing more than a simple block declaration passed as a parameter to a method that needs to make a callback at a later time.
Note: completion handler should always be the last parameter in a method. A method can have as many arguments as you want, but always have the completion handler as the last argument in the parameters list.
- (void)beginTaskWithName:(NSString *)name completion:(void(^)(void))callback;
// calling
[self beginTaskWithName:@"MyTask" completion:^{
    NSLog(@"Task completed ..");
More example with UIKit classes methods.

[self presentViewController:viewController animated:YES completion:^{
        NSLog(@"xyz View Controller presented ..");
        // Other code related to view controller presentation...
[UIView animateWithDuration:0.5
                         // Animation-related code here...
                         [self.view setAlpha:0.5];
                     completion:^(BOOL finished) {
                         // Any completion handler related code here...
                         NSLog(@"Animation over..");

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