Ans :
Blocks:
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
testBlock();
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.
Example:
- (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
animations:^{
// Animation-related code here...
[self.view setAlpha:0.5];
}
completion:^(BOOL finished) {
// Any completion handler related code here...
NSLog(@"Animation over..");
}];
