ParallaxViewPager：ViewPager的视差背景效果

本文主要是介绍ParallaxViewPager：ViewPager的视差背景效果，希望对大家解决编程问题提供一定的参考价值，需要的开发者们随着小编来一起学习吧！

源码：

Java代码

import android.annotation.SuppressLint;
import android.content.Context;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.graphics.Canvas;
import android.graphics.Rect;
import android.graphics.drawable.BitmapDrawable;
import android.graphics.drawable.Drawable;
import android.support.v4.view.ViewPager;
import android.util.AttributeSet;
import android.util.Log;
@SuppressLint("NewApi")
public class ParallaxViewPager extends ViewPager {
public static final int FIT_WIDTH = 0;
public static final int FIT_HEIGHT = 1;
public static final float OVERLAP_FULL = 1f;
public static final float OVERLAP_HALF = 0.5f;
public static final float OVERLAP_QUARTER = 0.25f;
private static final float CORRECTION_PERCENTAGE = 0.01f;
public Bitmap bitmap;
private Rect source, destination;
private int scaleType;
private int chunkWidth;
private int projectedWidth;
private float overlap;
private OnPageChangeListener secondOnPageChangeListener;
public ParallaxViewPager(Context context) {
super(context);
init();
}
public ParallaxViewPager(Context context, AttributeSet attrs) {
super(context, attrs);
init();
}
private void init() {
source = new Rect();
destination = new Rect();
scaleType = FIT_HEIGHT;
overlap = OVERLAP_HALF;
setOnPageChangeListener(new OnPageChangeListener() {
@Override public void onPageScrolled(int position, float positionOffset, int positionOffsetPixels) {
if (bitmap != null) {
source.left = (int) Math.floor((position + positionOffset - CORRECTION_PERCENTAGE) * chunkWidth);
source.right = (int) Math.ceil((position + positionOffset + CORRECTION_PERCENTAGE) * chunkWidth + projectedWidth);
destination.left = (int) Math.floor((position + positionOffset - CORRECTION_PERCENTAGE) * getWidth());
destination.right = (int) Math.ceil((position + positionOffset + 1 + CORRECTION_PERCENTAGE) * getWidth());
invalidate();
}
if (secondOnPageChangeListener != null) {
secondOnPageChangeListener.onPageScrolled(position, positionOffset, positionOffsetPixels);
}
}
@Override public void onPageSelected(int position) {
if (secondOnPageChangeListener != null) {
secondOnPageChangeListener.onPageSelected(position);
}
}
@Override public void onPageScrollStateChanged(int state) {
if (secondOnPageChangeListener != null) {
secondOnPageChangeListener.onPageScrollStateChanged(state);
}
}
});
}
@Override protected void onSizeChanged(int w, int h, int oldw, int oldh) {
super.onSizeChanged(w, h, oldw, oldh);
destination.top = 0;
destination.bottom = h;
if (getAdapter() != null && bitmap != null)
calculateParallaxParameters();
}
private void calculateParallaxParameters() {
if (bitmap.getWidth() < getWidth() && bitmap.getWidth() < bitmap.getHeight() && scaleType == FIT_HEIGHT) {
Log.w(ParallaxViewPager.class.getName(), "Invalid bitmap bounds for the current device, parallax effect will not work.");
}
final float ratio = (float) getHeight() / bitmap.getHeight();
if (ratio != 1) {
switch (scaleType) {
case FIT_WIDTH:
source.top = (int) ((bitmap.getHeight() - bitmap.getHeight() / ratio) / 2);
source.bottom = bitmap.getHeight() - source.top;
chunkWidth = (int) Math.ceil((float) bitmap.getWidth() / (float) getAdapter().getCount());
projectedWidth = chunkWidth;
break;
case FIT_HEIGHT:
default:
source.top = 0;
source.bottom = bitmap.getHeight();
projectedWidth = (int) Math.ceil(getWidth() / ratio);
chunkWidth = (int) Math.ceil((bitmap.getWidth() - projectedWidth) / (float) getAdapter().getCount() * overlap);
break;
}
}
}
/**
* Sets the background from a resource file.
*
* @param resid
*/
@Override public void setBackgroundResource(int resid) {
bitmap = BitmapFactory.decodeResource(getResources(), resid);
}
/**
* Sets the background from a Drawable.
*
* @param background
*/
@Override public void setBackground(Drawable background) {
bitmap = ((BitmapDrawable) background).getBitmap();
}
/**
* Deprecated.
* Sets the background from a Drawable.
*
* @param background
*/
@Override public void setBackgroundDrawable(Drawable background) {
bitmap = ((BitmapDrawable) background).getBitmap();
}
/**
* Sets the background from a bitmap.
*
* @param bitmap
* @return The ParallaxViewPager object itself.
*/
public ParallaxViewPager setBackground(Bitmap bitmap) {
this.bitmap = bitmap;
return this;
}
/**
* Sets how the view should scale the background. The available choices are:
* <ul>
* <li>FIT_HEIGHT - the height of the image is resized to matched the height of the View, also stretching the width to keep the aspect ratio. The non-visible part of the bitmap is divided into equal parts, each of them sliding in at the proper position.</li>
* <li>FIT_WIDTH - the width of the background image is divided into equal chunks, each taking up the whole width of the screen.</li>
* </ul>
*
* @param scaleType
* @return
*/
public ParallaxViewPager setScaleType(final int scaleType) {
if (scaleType != FIT_WIDTH && scaleType != FIT_HEIGHT)
throw new IllegalArgumentException("Illegal argument: scaleType must be FIT_WIDTH or FIT_HEIGHT");
this.scaleType = scaleType;
return this;
}
/**
* Sets the amount of overlapping with the setOverlapPercentage(final float percentage) method. This is a number between 0 and 1, the smaller it is, the slower is the background scrolling.
*
* @param percentage
* @return The ParallaxViewPager object itself.
*/
public ParallaxViewPager setOverlapPercentage(final float percentage) {
if (percentage <= 0 || percentage >= 1)
throw new IllegalArgumentException("Illegal argument: percentage must be between 0 and 1");
overlap = percentage;
return this;
}
/**
* Recalculates the parameters of the parallax effect, useful after changes in runtime.
*
* @return The ParallaxViewPager object itself.
*/
public ParallaxViewPager invalidateParallaxParameters() {
calculateParallaxParameters();
return this;
}
@Override protected void onDraw(Canvas canvas) {
if (bitmap != null)
canvas.drawBitmap(bitmap, source, destination, null);
}
public void addOnPageChangeListener(OnPageChangeListener listener) {
secondOnPageChangeListener = listener;
}
}

使用：
在layout xml或者程序中创建了ParallaxViewPager之后，可以使用下面的方法来设置背景，或者也可以xml设置：

Java代码

setBackgroundResource(int resid)
setBackground(Drawable background) or setBackgroundDrawable(Drawable background)
setBackground(Bitmap bitmap)

这就好了，你现在可以使用ParallaxViewPager的全部功能了。你可以修改背景的滚动效果来优化用户体验。你也可以使用setScaleType(final int scaleType)方法来配置视图的图像缩放方式。这个方法只能和FIT_HEIGHT搭配使用，从下面的参数中进行选择：

FIT_HEIGHT

表示缩放图像的高度以便适配视图的高度，同时缩放图像的宽度以便保持宽高比。bitmap的不可见部分被划分成相同的区域，每个区域插入到合适的位置。FIT_HEIGHT是默认值。

FIT_WIDTH

表示背景图像的宽度被划分成相同的块，每一块占满整个屏幕的宽度。这个模式不适用于视差效果，因为背景和视图的滚动速度一样。

你也可以使用setOverlapPercentage(final float percentage) 方法来设置重叠的程度。重叠程度值介于0到1之间，这个值越小背景就滚动地越慢，默认值是50%。

Java代码

setContentView(R.layout.activity_main);
final ParallaxViewPager parallaxViewPager = ((ParallaxViewPager) findViewById(R.id.parallaxviewpager));
parallaxViewPager
.setOverlapPercentage(0.25f)
.setAdapter(new PagerAdapter()
//...

Xml代码