android viewpager实现轮播「建议收藏」

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本文是基于ViewPager实现的无限自动轮播banner：

分为四步去实现：

第一步是有限手动轮播；

第二步是无限轮播；

第三步是自动轮播；

第四步是指示器适配

第一步：有限手动轮播实现

布局：

<androidx.viewpager.widget.ViewPager
    android:id="@+id/banner"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:layout_marginStart="12dp"
    android:layout_marginEnd="12dp" />

adapter实现：

public class BannerAdapter extends PagerAdapter {
    
    private List<String> bannerList;

    public BannerAdapter(List<String> bannerList) {
        this.bannerList = bannerList;
    }

    @Override
    public int getCount() {
        return bannerList.size();
    }

    @Override
    public boolean isViewFromObject(@NonNull View view, @NonNull Object object) {
        return view == object;
    }

    @NonNull
    @Override
    public Object instantiateItem(@NonNull ViewGroup container, int position) {
        ImageView bannerImageView = new ImageView(container.getContext());
        ViewGroup.LayoutParams lp = new ViewGroup.LayoutParams(ViewGroup.LayoutParams.MATCH_PARENT, ViewGroup.LayoutParams.MATCH_PARENT);
        bannerImageView.setLayoutParams(lp);
        bannerImageView.setScaleType(ImageView.ScaleType.FIT_XY);
        Glide.with(container.getContext()).load(bannerList.get(position)).into(bannerImageView);

        container.addView(bannerImageView);
        return bannerImageView;
    }

    @Override
    public void destroyItem(@NonNull ViewGroup container, int position, @NonNull Object object) {
        container.removeView((View) object);
    }
}

Activity中：

// scrollview中viewpager一定要设置高度，此处根据图片的宽高比来设定高度

int bannerWidth = (Utils.getScreenWidth(getContext()) - Utils.dip2pixel(getContext(), 24));
LinearLayout.LayoutParams lp = (LinearLayout.LayoutParams) bannerView.getLayoutParams();
lp.width = LinearLayout.LayoutParams.MATCH_PARENT;
lp.height = (int) (bannerWidth * 90f / 345);
bannerView.setLayoutParams(lp);

bannerView.setAdapter(new BannerAdapter(getUrlList()));

注意：ScrollView包裹ViewPager时，ViewPager的高度一定要有确定值，否则内容无法加载出来，可以在xml中指定，也可以代码设定，但一定要有确定值。

第二步：无限轮播

无限轮播只需要在有限轮播的基础上，做以下两个改动点，修改getCount返回值且在加载数据时获取正确的数据源即可

public class BannerAdapter extends PagerAdapter {

    private List<String> bannerList;

    public BannerAdapter(List<String> bannerList) {
        this.bannerList = bannerList;
    }

    @Override
    public int getCount() {
//        return bannerList.size();  // before
        return Integer.MAX_VALUE;   // now
    }

    @Override
    public boolean isViewFromObject(@NonNull View view, @NonNull Object object) {
        return view == object;
    }

    @NonNull
    @Override
    public Object instantiateItem(@NonNull ViewGroup container, int position) {
        ImageView bannerImageView = new ImageView(container.getContext());
        int realPosition = position % bannerList.size(); // 获取要加载数据的真实位置
        ViewGroup.LayoutParams lp = new ViewGroup.LayoutParams(ViewGroup.LayoutParams.MATCH_PARENT, ViewGroup.LayoutParams.MATCH_PARENT);
        bannerImageView.setLayoutParams(lp);
        bannerImageView.setScaleType(ImageView.ScaleType.FIT_XY);
//        Glide.with(container.getContext()).load(bannerList.get(position)).into(bannerImageView); // before
        Glide.with(container.getContext()).load(bannerList.get(realPosition)).into(bannerImageView); // now

        container.addView(bannerImageView);
        return bannerImageView;
    }

    @Override
    public void destroyItem(@NonNull ViewGroup container, int position, @NonNull Object object) {
        container.removeView((View) object);
    }
}

修改完发现banner只能向右无限轮播，第一次左滑滑不动，这个时候我们强制设置viewpager位置在中间就可以解决这个问题了

bannerView.setAdapter(new BannerAdapter(getUrlList()));
bannerView.setCurrentItem(getUrlList().size() * 5);

第三步：自动轮播

handler每隔轮播间隔发送消息，设置viewpager为下一个位置

private Runnable bannerRunnable = new Runnable() {
        @Override
        public void run() {
            bannerView.setCurrentItem(bannerView.getCurrentItem() + 1);
            mHandler.postDelayed(bannerRunnable, 3000);
        }
    };

bannerView.addOnPageChangeListener(new ViewPager.OnPageChangeListener() {
            @Override
            public void onPageScrolled(int position, float positionOffset, int positionOffsetPixels) {

            }

            @Override
            public void onPageSelected(int position) {
                // 手滑动到某一位置，重新开始计时
                start();
            }

            @Override
            public void onPageScrollStateChanged(int state) {

            }
        });

private void start() {
        mHandler.removeCallbacksAndMessages(null);
        mHandler.postDelayed(bannerRunnable, 3000);
    }

第四步：添加指示器

指示器样式及表现可以自己去根据需求实现，以相对简单和常见的小圆圈指示器为例，添加和banner数量相同的小圆圈，小圆圈设置selector，在选中时为黑色选中样式，在非选中时为灰色默认样式，根据当前选中的banner的实际position，设置指示器的selected属性，从而展示不同的样式

private void initIndicator() {
        for (int i = 0; i < getUrlList().size(); i++) {
            View view = new View(getActivity());
            LinearLayout.LayoutParams lp = new LinearLayout.LayoutParams(Utils.dip2pixel(getActivity(), 6), Utils.dip2pixel(getActivity(), 6));
            lp.rightMargin = Utils.dip2pixel(getActivity(), 8);
            view.setLayoutParams(lp);
            view.setBackgroundResource(R.drawable.selector_indicator_view);
            view.setSelected(i == 0);
            llIndicatorView.addView(view);
        }
    }


private void initBannerView() {
        bannerView.addOnPageChangeListener(new ViewPager.OnPageChangeListener() {
            @Override
            public void onPageScrolled(int position, float positionOffset, int positionOffsetPixels) {

            }

            @Override
            public void onPageSelected(int position) {
                // 手滑动到某一位置，重新开始计时
                realPosition = 0;
                realPosition = position % getUrlList().size();
                // 根据当前滑动到的banner设置指示器的状态
                for (int i = 0; i < llIndicatorView.getChildCount(); i++) {
                    llIndicatorView.getChildAt(i).setSelected(i == realPosition);
                }
                start();
            }

            @Override
            public void onPageScrollStateChanged(int state) {

            }
        });
    }

附：Utils文件

public class Utils {
    public static void setFullScreen(Activity activity) {
        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.M) {
            ViewGroup decorView = (ViewGroup) activity.getWindow().getDecorView();
            decorView.setSystemUiVisibility(View.SYSTEM_UI_FLAG_LAYOUT_FULLSCREEN | View.SYSTEM_UI_FLAG_LIGHT_STATUS_BAR);
            activity.getWindow().setStatusBarColor(Color.TRANSPARENT);
        }
    }

    public static int dip2pixel(Context context, float n) {
        int value = (int) TypedValue.applyDimension(TypedValue.COMPLEX_UNIT_DIP, n, context.getResources().getDisplayMetrics());
        return value;
    }

    /**
     * 获取屏幕宽度
     * @param context
     * @return 屏幕宽度
     */
    public static int getScreenWidth(Context context) {
        WindowManager wm = (WindowManager) context
                .getSystemService(Context.WINDOW_SERVICE);
        DisplayMetrics outMetrics = new DisplayMetrics();
        wm.getDefaultDisplay().getMetrics(outMetrics);
        return outMetrics.widthPixels;
    }
}

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