# Block

# Custom completion block for Custom Methods

1- Define Your own custom Block

typedef void(^myCustomCompletion)(BOOL);

2- Create custom method which takes your custom completion block as a parameter.

-(void) customMethodName:(myCustomCompletion) compblock{
    //do stuff
    // check if completion block exist; if we do not check it will throw an exception

3- How to use block in your Method

[self customMethodName:^(BOOL finished) {

# UIView Animations

[UIView animateWithDuration:1.0
        someView.alpha = 0;
        otherView.alpha = 1;
    completion:^(BOOL finished) {
    [someView removeFromSuperview];

The carat “^” character defines a block. For example, ^{ … } is a block. More specifically, it is a block that returns “void” and accepts no arguments. It is equivalent to a method such like: “- (void)something;” but there is no inherent name associated with the code block.

Define a block that can accept arguments work very similarly. To supply an argument to a block, you define the block like so: ^(BOOL someArg, NSString someStr) { … }*. When you use API calls that support blocks, you’ll be writing blocks that look similar to this, especially for animation blocks or NSURLConnection blocks as shown in the above example.

# Modify captured variable

Block will capture variables that appeared in the same lexical scope. Normally these variables are captured as "const" value:

int val = 10;
void (^blk)(void) = ^{
    val = 20; // Error! val is a constant value and cannot be modified!

In order to modify the variable, you need to use the __block storage type modifier.

__block int val = 10;
void (^blk)(void) = ^{
    val = 20; // Correct! val now can be modified as an ordinary variable.

# Syntax

  • As a variable: returnType (^blockName)(parameterTypes) = ^returnType(parameters) {...};
  • As a property: @property (nonatomic, copy) returnType (^blockName)(parameterTypes);
  • As a method parameter: - (void)methodWithBlock:(returnType (^)(parameterTypes))blockName;
  • As a typedef: typedef returnType (^TypeName)(parameterTypes); TypeName blockName = ^returnType(parameters) {...};