Android 自定义轮播图（Banner）

本文主要是介绍Android 自定义轮播图（Banner），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！

先上图看效果：

知识要点：

1. 轮播图是怎么做到无限循环的呢
   让ViewPagerAdapter里面的getCount返回MAX_VALUE，instantiateItem方法中，获取图片的时候，让position % drawableList.size(),来得到真实值

    @Overridepublic int getCount() {return Integer.MAX_VALUE;}@NonNull@Overridepublic Object instantiateItem(@NonNull ViewGroup container, int position) {if (drawableList != null && drawableList.size() > 0) {View imageView = null;if (mCache.isEmpty()) {imageView = new ImageView(mContext);}else {imageView = mCache.remove(0);}((ImageView) imageView).setImageDrawable(drawableList.get(position % drawableList.size()));container.addView(imageView);return imageView;}return null;}
   

2. 自动轮播是怎么实现的呢
   通过Handler每隔一段时间发送一次消息，来进行循环的，每循环一次，就切换到下一个ViewPager的位置

    @Overridepublic void handleMessage(Message msg) {super.handleMessage(msg);if (msg.what == MSG_CHANGE_SELECTION) {if (mBannerRef == null || mBannerRef.get() == null) {return;}AutoScrollViewPager banner = mBannerRef.get();int current = banner.getCurrentItem();banner.setCurrentItem(current + 1);sendEmptyMessageDelayed(MSG_CHANGE_SELECTION,AUTO_SCROLL_TIME);}}
   

3. 指示器是怎么实现的
   首先自定义一个BannerItemView，代表每个小圆点，再自定义个BannerIndicator，把总共的小圆点添加进去，通过伸缩动画和透明度动画来改变小圆点的变化

    //指示器增大同时设置透明度变化private void setLarge(int position) {if (getChildAt(position) instanceof BannerItemView) {AnimatorSet set = new AnimatorSet();ValueAnimator animator = getEnlarge((BannerItemView) getChildAt(position));ValueAnimator alpha = ObjectAnimator.ofFloat(getChildAt(position), "alpha", 0.4f, 1f);set.play(animator).with(alpha);set.setDuration(618);set.start();}}//放大动画private ValueAnimator getEnlarge(BannerItemView roundRectView) {return ObjectAnimator.ofFloat(roundRectView,"rectWidth",0, getOffset(roundRectView));}//根据大小变化方向获取指示器大小偏移量private int getOffset(BannerItemView bannerItemView) {int offsest = 0;switch (bannerItemView.getLoaction()) {case BannerItemView.CENTER:offsest = (slider_width - slider_height)/2;break;case BannerItemView.LEFT:offsest = slider_width - slider_height;break;case BannerItemView.RIGHT:offsest = slider_width - slider_height;break;}return offsest;}//缩小动画private ValueAnimator getShrink(BannerItemView roundRectView) {return ObjectAnimator.ofFloat(roundRectView,"rectWidth",getOffset(roundRectView), 0);}//缩小动画同事伴随透明度变化public void setSmall(int small) {if (getChildAt(small) instanceof BannerItemView) {AnimatorSet set = new AnimatorSet();ValueAnimator alpha = ObjectAnimator.ofFloat(getChildAt(position), "alpha", 1, 0.4f);ValueAnimator animator = getShrink((BannerItemView) getChildAt(small));set.play(animator).with(alpha);set.setDuration(618);set.start();}}
   

具体实现细节请移步github
https://github.com/xinhuashi/CustomBanner.git
如果感觉有所帮助，请帮忙star一下

参考了这两位大神的文章 https://www.jianshu.com/p/489b77424b63
https://www.jianshu.com/p/ef6ff245ed3e
在这里表示感谢

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