写一个Android的轮播图

好的，以下是一个简单的Android轮播图的实现方法：

1. 在布局文件中添加ViewPager和指示器Indicator：

<RelativeLayout
    android:layout_width="match_parent"
    android:layout_height="match_parent">
    
    <android.support.v4.view.ViewPager
        android:id="@+id/view_pager"
        android:layout_width="match_parent"
        android:layout_height="match_parent" />
        
    <LinearLayout
        android:id="@+id/indicator"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentBottom="true"
        android:layout_centerHorizontal="true"
        android:orientation="horizontal" />
        
</RelativeLayout>

2. 创建一个PagerAdapter来管理ViewPager的内容：

public class MyPagerAdapter extends PagerAdapter {
    private Context mContext;
    private List<Integer> mImageList;

    public MyPagerAdapter(Context context, List<Integer> imageList) {
        mContext = context;
        mImageList = imageList;
    }

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

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

    @Override
    public Object instantiateItem(ViewGroup container, int position) {
        ImageView imageView = new ImageView(mContext);
        imageView.setScaleType(ImageView.ScaleType.CENTER_CROP);
        imageView.setImageResource(mImageList.get(position));
        container.addView(imageView);
        return imageView;
    }

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

3. 在Activity中初始化ViewPager和指示器，并设置PagerAdapter：

public class MainActivity extends AppCompatActivity {
    private ViewPager mViewPager;
    private LinearLayout mIndicator;
    private int mCurrentPosition;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        mViewPager = findViewById(R.id.view_pager);
        mIndicator = findViewById(R.id.indicator);

        List<Integer> imageList = new ArrayList<>();
        imageList.add(R.drawable.image1);
        imageList.add(R.drawable.image2);
        imageList.add(R.drawable.image3);

        MyPagerAdapter adapter = new MyPagerAdapter(this, imageList);
        mViewPager.setAdapter(adapter);

        addIndicator(imageList.size());
        setIndicator(mCurrentPosition);

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

            }

            @Override
            public void onPageSelected(int position) {
                mCurrentPosition = position;
                setIndicator(position);
            }

            @Override
            public void onPageScrollStateChanged(int state) {

            }
        });
    }

    private void addIndicator(int count) {
        for (int i = 0; i < count; i++) {
            ImageView imageView = new ImageView(this);
            imageView.setImageResource(R.drawable.indicator_selector);
            LinearLayout.LayoutParams params = new LinearLayout.LayoutParams(
                    LinearLayout.LayoutParams.WRAP_CONTENT,
                    LinearLayout.LayoutParams.WRAP_CONTENT
            );
            params.setMargins(10, 0, 10, 0);
            imageView.setLayoutParams(params);
            mIndicator.addView(imageView);
        }
    }

    private void setIndicator(int position) {
        for (int i = 0; i < mIndicator.getChildCount(); i++) {
            ImageView imageView = (ImageView) mIndicator.getChildAt(i);
            if (position == i) {
                imageView.setSelected(true);
            } else {
                imageView.setSelected(false);
            }
        }
    }
}

4. 在drawable目录中添加indicator_selector.xml文件，设置指示器的选中和未选中状态：

<selector xmlns:android="http://schemas.android.com/apk/res/android">
    <item android:drawable="@drawable/indicator_selected" android:state_selected="true" />
    <item android:drawable="@drawable/indicator_unselected" />
</selector>

5. 在drawable目录中添加indicator_selected.png和indicator_unselected.png两个图片，分别是选中和未选中状态的指示器图片。

以上就是一个简单的Android轮播图的实现方法。