Validation feature

[janheinrichmerker]

Would be nice to have the ability to block users from getting past a given page based on some sort of policy:

public interface PageContinuePolicy{
    boolean canGoToPage(int oldPosition, int newPosition);
}

That way we could also check for permissions and such. You could also disallow users to go back.

[AkshayChordiya]

@heinrichreimer Sure, there's a need of such policy. Hoping to see it soon. Thank you.

[Jucedik]

Hi.
Please apply my pull request.
I added possibility which allow to lock and unlock next/previous (and swipe left/right) navigation.

Instructions:
1. Create FragmentSlide with build option "allowNext(boolean value)" or "allowPrevious(boolean value)". Warning!!! - Your fragment should be extended from SlideFragment. E.g.:

slides.add(new FragmentSlide.Builder()
                    .background(R.color.color_custom_fragment_1)
                    .backgroundDark(R.color.color_dark_custom_fragment_1)
                    .fragment(LoginFragment.newInstance())
                    **.allowNext(false)**
                    .build());

2. Set NavigationInterface in MaterialIntroActivity by calling
setNavigationInterface(NavigationInterface navigationInterface). E.g.:

setNavigationInterface(new NavigationInterface() {
            @Override
            public boolean onNextClick(int position) {
                return slides.get(position).isAllowSlideNext();
            }

            @Override
            public boolean onPreviousClick(int position) {
                return slides.get(position).isAllowSlidePrevious();
            }

            @Override
            public void onImpossibleToNavigate(int position, int direction) {
                Toast.makeText(getApplicationContext(), R.string.toast_impossible_to_navigate, Toast.LENGTH_LONG).show();
            }
        });

3. For lock or unlock slide page you need to call allowNextSlide() or allowPreviousSlide() from your fragment (e.g.: you may call this methods after successfull user autorization). Also you may call showNextSlide() and showPreviousSlide() for imidiately go to next or previous slide.

You can see the sample code in demo files.

[janheinrichmerker]

Fixed by #24