com.matthewtamlin.sliding_intro_screen_library.ParallaxTransformer.java Source code

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Here is the source code for com.matthewtamlin.sliding_intro_screen_library.ParallaxTransformer.java

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/*
 * Copyright 2016 Matthew Tamlin
 *
 * 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.matthewtamlin.sliding_intro_screen_library;

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

import java.util.HashMap;

/**
 * Transforms a {@code ParallaxPage} by translating its views left and right when scrolling. Front
 * images are translated faster than back images, which creates a parallax scrolling effect. This
 * class is designed to function with {@code ParallaxPage} elements and may not function with other
 * {@code Page} subclasses.
 */
public final class ParallaxTransformer implements ViewPager.PageTransformer {
    /**
     * Stores references to the views to animate. This is more efficient than frequent calls to
     * {@code findViewById(int)}.
     */
    private final HashMap<View, ImageView> cachedViews = new HashMap<>();

    @Override
    public void transformPage(final View pageRootView, final float position) {
        ImageView frontImageHolder = getFrontImageHolder(pageRootView);

        boolean pageIsSelected = (position == 0f);
        boolean pageIsScrolling = (position > -1f && position < 1f);

        if (pageIsSelected) {
            pageRootView.invalidate();
        } else if (pageIsScrolling) {
            //TODO clarify how the scale is quantified
            float n = 0.5f;

            // Transform front image holder
            frontImageHolder.setTranslationX(pageRootView.getWidth() * position * n / 2);
        }
    }

    /**
     * Returns the front image for {@code Page}. Using this method is more efficient than calling
     * {@code findViewById} each time the {@code View} is needed.
     *
     * @param pageRootView
     *       the root view of the {@code ParallaxPage} to transform
     * @return the front image holder of the {@code ParallaxPage} to transform
     */
    private ImageView getFrontImageHolder(final View pageRootView) {
        ImageView frontImage = cachedViews.get(pageRootView);

        if (frontImage == null) {
            frontImage = (ImageView) pageRootView.findViewById(R.id.page_fragment_imageHolderFront);
            cachedViews.put(pageRootView, frontImage);
        }

        return frontImage;
    }
}