Android 自定义轮播图（Banner）

先上图看效果：
在这里插入图片描述
知识要点：

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

 @Override
 public int getCount() {
     return Integer.MAX_VALUE;
 }

 @NonNull
 @Override
 public 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;

 }

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

 @Override
 public 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);
     }
 }

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

Android 自定义轮播图（Banner）

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