// WGDigitField<WGDigitView<UIView *> *> means you have to initialize this WGDigitField with an instance of WGDigitView<UIView *> as digitView
WGDigitField<WGDigitView<UIView *> *> *field = [[WGDigitField<WGDigitView<UIView *> *> alloc] initWithDigitViewInitBlock:^WGDigitView<UIView *> * (NSInteger index){
    // initializing a background view (UIView or any subclass of UIView if you want)
    UIView *background = [[UIView alloc] initWithFrame:CGRectMake(0, 0, 40, 45)];
    background.backgroundColor = [UIColor whiteColor];
    background.layer.borderColor = [UIColor grayColor].CGColor;
    background.layer.borderWidth = 1.f;
    background.layer.cornerRadius = 3.f;
    // `WGDigitView<UIView *>` means you have to initialize this WGDigitView with an instance of UIView as backgroundView
    return [[WGDigitView<UIView *> alloc] initWithBackgroundView:background digitFont:[UIFont systemFontOfSize:25.f] digitColor:[UIColor blackColor]];

} numberOfDigits:6 leadSpacing:25 tailSpacing:25 weakenBlock:^(WGDigitView<UIView *> * _Nonnull digitView) {
    // getting it normal
    digitView.backgroundView.layer.borderColor = [UIColor grayColor].CGColor;
    digitView.backgroundView.layer.borderWidth = 1.f;

} highlightedBlock:^(WGDigitView<UIView *> * _Nonnull digitView) {
    // making some UI changes
    digitView.backgroundView.layer.borderColor = [UIColor redColor].CGColor;
    digitView.backgroundView.layer.borderWidth = 2.f;
    // or implementing animations
    CAKeyframeAnimation *animation = ...
} fillCompleteBlock:^(WGDigitField * _Nonnull digitField, NSArray<WGDigitView<UIView *> *> * _Nonnull digitViewArray, NSString * _Nonnull text) {
    // dismissing keyboard and starting requests
    [digitField resignFirstResponder];
}];