didBeginContact:(SKPhysicsContact *)contact not in

I have created SKScene inherited class. Problem is that on contact of physics body method

- (void)didBeginContact:(SKPhysicsContact *)contact

is not invoked solution may be simple but as beginner with sprite kit i am stuck with this.

Below is the code

#import "MyScene.h"
@interface MyScene ()
@property BOOL contentCreated;
@end
@implementation MyScene
- (id)initWithSize:(CGSize)size {
    self = [super initWithSize:size];
    if (self) {
        self.physicsWorld.contactDelegate = self;
        self.physicsBody = [SKPhysicsBody bodyWithEdgeLoopFromRect:self.frame];
    }
    return self;
}
- (void)didMoveToView:(SKView *)view
{
    if (!self.contentCreated) {
        [self buildWorld];
        self.physicsWorld.contactDelegate = self;
    }
}

#pragma mark - World Building
- (void)buildWorld {
    NSLog(@"Building the world");
    SKSpriteNode * sprite1 = [[SKSpriteNode alloc] initWithColor:[SKColor grayColor] size:CGSizeMake(100,100)];
    sprite1.physicsBody = [SKPhysicsBody bodyWithRectangleOfSize:CGSizeMake(100,100)];
    sprite1.position = CGPointMake(CGRectGetMidX(self.frame), CGRectGetMidY(self.frame) +100);

    SKSpriteNode * sprite2 = [[SKSpriteNode alloc] initWithColor:[SKColor grayColor] size:CGSizeMake(100,100)];
    sprite2.physicsBody = [SKPhysicsBody bodyWithRectangleOfSize:CGSizeMake(100,100)];
    sprite2.position = CGPointMake(CGRectGetMidX(self.frame), CGRectGetMidY(self.frame) - 100);


    [self addChild:sprite1];
    [self addChild:sprite2];
}
- (void)didBeginContact:(SKPhysicsContact *)contact
{
    NSLog(@"contact");
}

@end

Thanks in advance.

From the SKPhysicsWorld documentation:

A contact is created when two physics bodies overlap and one of the physics bodies has a contactTestBitMask property that overlaps with the other body’s categoryBitMask property.

You have to assign the physics bodies a categoryBitMask and a contactTestBitMask. You want to first create your categories:

static const uint32_t sprite1Category = 0x1 << 0;
static const uint32_t sprite2Category = 0x1 << 1;

Next, assign the category and contact test bit masks:

sprite1.physicsBody.categoryBitMask = sprite1Category;
sprite1.physicsBody.contactTestBitMask = sprite2Category;

sprite2.physicsBody.categoryBitMask = sprite2Category;
sprite2.physicsBody.contactTestBitMask = sprite1Category;

Note from the SKPhysicsBody documentation: