ParallaxPagerTransformer.java

/*
 * Copyright 2015 Alexandre Piveteau
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.alexandrepiveteau.library.tutorial;

import android.support.v4.view.ViewPager;
import android.view.View;

import java.util.ArrayList;
import java.util.List;

/**
 * Created by Alexandre on 14.07.2015.
 */
public class ParallaxPagerTransformer implements ViewPager.PageTransformer {

    private List<ParallaxTransformInformation> mViewsToParallax = new ArrayList<ParallaxTransformInformation>();

    public ParallaxPagerTransformer() {
    }

    public ParallaxPagerTransformer(List<ParallaxTransformInformation> viewsToParallax) {
        mViewsToParallax = viewsToParallax;
    }

    public ParallaxPagerTransformer addViewToParallax(ParallaxTransformInformation viewInfo) {
        if (mViewsToParallax != null) {
            mViewsToParallax.add(viewInfo);
        }
        return this;
    }

    public void transformPage(View view, float position) {

        int pageWidth = view.getWidth();

        if (position < -1) {
            // This page is way off-screen to the left.
            view.setAlpha(1);

        } else if (position <= 1 && mViewsToParallax != null) { // [-1,1]
            for (ParallaxTransformInformation parallaxTransformInformation : mViewsToParallax) {
                applyParallaxEffect(view, position, pageWidth, parallaxTransformInformation,
                        position > 0);
            }
        } else {
            // This page is way off-screen to the right.
            view.setAlpha(1);
        }
    }

    private void applyParallaxEffect(View view, float position, int pageWidth,
                                     ParallaxTransformInformation information, boolean isEnter) {
        if (information.isValid() && view.findViewById(information.resource) != null) {
            if (isEnter && !information.isEnterDefault()) {
                view.findViewById(information.resource)
                        .setTranslationX(-position * (pageWidth / information.parallaxEnterEffect));
            } else if (!isEnter && !information.isExitDefault()) {
                view.findViewById(information.resource)
                        .setTranslationX(-position * (pageWidth / information.parallaxExitEffect));
            }
        }
    }


    /**
     * Information to make the parallax effect in a concrete view.
     *
     * parallaxEffect positive values reduces the speed of the view in the translation
     * ParallaxEffect negative values increase the speed of the view in the translation
     * Try values to see the different effects. I recommend 2, 0.75 and 0.5
     */
    public static class ParallaxTransformInformation {

        public static final float PARALLAX_EFFECT_DEFAULT = -101.1986f;

        int resource = -1;
        float parallaxEnterEffect = 1f;
        float parallaxExitEffect = 1f;

        public ParallaxTransformInformation(int resource, float parallaxEnterEffect,
                                            float parallaxExitEffect) {
            this.resource = resource;
            this.parallaxEnterEffect = parallaxEnterEffect;
            this.parallaxExitEffect = parallaxExitEffect;
        }

        public boolean isValid() {
            return parallaxEnterEffect != 0 && parallaxExitEffect != 0 && resource != -1;
        }

        public boolean isEnterDefault() {
            return parallaxEnterEffect == PARALLAX_EFFECT_DEFAULT;
        }

        public boolean isExitDefault() {
            return parallaxExitEffect == PARALLAX_EFFECT_DEFAULT;
        }
    }
}