- (void) didFinishAutoLayout {

    // Do some stuff here.

    NSLog(@"didFinishAutoLayout");
}

- (void) viewDidLayoutSubviews {
    [super viewDidLayoutSubviews];


    [NSObject cancelPreviousPerformRequestsWithTarget:self
                                 selector:@selector(didFinishAutoLayout)
                                           object:nil];
    [self performSelector:@selector(didFinishAutoLayout) withObject:nil
               afterDelay:0];
}

self.cnsTableviewHeight.constant = 50;
    [self layoutIfNeeded];

- (void) layoutSubviews { //This method when auto layout engine finishes
    }

someConstraint.constant = 100; // the change

// Animate just to make sure the constraint change is fully applied
[UIView animateWithDuration:0.1f animations:^{
    [self.view setNeedsLayout];
} completion:^(BOOL finished) {
    // Here do whatever you need to do after constraint change
}];