安卓开发——基于ViewPager的图片轮播

概述：
要用ViewPager实现图片轮播，主要是两步：
1：用PagerAdapter使图片可以滑动切换
2：用Handler来实现图片自动轮播

页面布局部分的代码：

<?xml version="1.0" encoding="utf-8"?>
<android.support.v4.widget.DrawerLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/drawerlayout"
    android:layout_width="match_parent" android:layout_height="match_parent">
   <LinearLayout
       android:layout_width="match_parent"
       android:layout_height="match_parent">
       <FrameLayout
           android:layout_width="wrap_content"
           android:layout_height="150dp"
           android:orientation="vertical">

           <android.support.v4.view.ViewPager
               android:id="@+id/viewPager"
               android:layout_width="fill_parent"
               android:layout_height="match_parent" />

           <AbsoluteLayout
               android:layout_width="match_parent"
               android:layout_height="match_parent"
               android:orientation="vertical">

               <LinearLayout
                   android:id="@+id/viewgroup"
                   android:layout_width="match_parent"
                   android:layout_height="30dp"
                   android:layout_alignParentRight="true"
                   android:orientation="horizontal"
                   android:gravity="right|bottom"
                   android:layout_y="120dip">
               </LinearLayout>
           </AbsoluteLayout>
       </FrameLayout>


   </LinearLayout>
</android.support.v4.widget.DrawerLayout>

控制逻辑部分的java代码：

public class MainActivity extends AppCompatActivity  implements ViewPager.OnPageChangeListener {
    private MyHandler handler = new MyHandler(new WeakReference<>(this));
    private ViewPager viewPaper;
    private ImageView[] tips = null;
    private ImageView[] imgs = null;
    private ViewGroup group;
    private int[] ids = new int[]{R.drawable.sc_1,R.drawable.sc_2,R.drawable.sc_3};//载入图片

    @Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        group = (ViewGroup)findViewById(R.id.viewgroup);
        viewPaper = (ViewPager)findViewById(R.id.viewPager);


        //将标识点装载入ViewGroup中
        tips = new ImageView[ids.length];
        for(int i=0;i<ids.length;i++){
            ImageView imageview = new ImageView(this);
            imageview.setLayoutParams(new LinearLayout.LayoutParams(7,7));//设置标识点的大小
            tips[i] = imageview;
            if(i==0){//加载图片
                tips[i].setBackgroundResource(R.drawable.page_indicator_focused);
            }else{
                tips[i].setBackgroundResource(R.drawable.page_indicator_focused);
            }
            //设置标识点的位置参数
            LinearLayout.LayoutParams layoutparams = new LinearLayout.LayoutParams(new ViewGroup.LayoutParams(LinearLayout.LayoutParams.WRAP_CONTENT, LinearLayout.LayoutParams.WRAP_CONTENT));
            layoutparams.leftMargin = 3;
            layoutparams.rightMargin = 3;
            group.addView(imageview,layoutparams);
        }
        group.bringToFront();
        //将轮播图片加载到数组中
        imgs = new ImageView[ids.length];
        for(int i=0;i<ids.length;i++){
            ImageView imageview = new ImageView(this);
            imgs[i] = imageview;
            imageview.setBackgroundResource(ids[i]);
        }

        //设置Adapter
        viewPaper.setAdapter(new MyAdapter());
        //设置监听。主要是设置标识点背景
        viewPaper.setOnPageChangeListener(this);
        viewPaper.setCurrentItem((imgs.length)*100);
        //启动轮播
        handler.sendEmptyMessageDelayed(MyHandler.MSG_UPDATE_IMG, MyHandler.MSG_DELAY);
    }

    public class MyAdapter extends PagerAdapter{

        @Override
        public int getCount(){
            return Integer.MAX_VALUE;
        }
        @Override
        public boolean isViewFromObject(View arg0, Object arg1){
            return arg0 == arg1;
        }
        @Override
        public void destroyItem(View container,int  position,Object object){
//            ((ViewPager)container).removeView(imgs[position % imgs.length]);
        }

        //载入图片进去
        @Override
        public Object instantiateItem(View container, int position){
            position %= imgs.length;
            ImageView  view = imgs[position % imgs.length];
            ViewParent vp = view.getParent();
            if(vp != null){
                ViewGroup parent = (ViewGroup)vp;
                parent.removeView(view);
            }
            try {
                ((ViewPager) container).addView(imgs[position],0);
            }catch(Exception e){

            }
            return imgs[position % imgs.length];
        }
    }

    @Override
    public void onPageScrollStateChanged(int arg0) {
        switch(arg0){
            case ViewPager.SCROLL_STATE_DRAGGING://正在滑动
                handler.sendEmptyMessage(MyHandler.MSG_STOP_IMG);
                break;
            case ViewPager.SCROLL_STATE_IDLE://已经完成切换
                handler.sendEmptyMessageDelayed(MyHandler.MSG_UPDATE_IMG,MyHandler.MSG_DELAY);
                break;
            default:
                break;
        }
    }

    @Override
    public void onPageScrolled(int arg0, float arg1, int arg2) {

    }

    @Override
    public void onPageSelected(int arg0) {
        setImageBackground(arg0 % imgs.length);
        handler.sendMessage(Message.obtain(handler,MyHandler.MSG_PAGE_OLD,arg0,0));//记录切换到的页码
    }

    /**
     * 设置选中的tip的背景
     * @param selectItems
     */
    private void setImageBackground(int selectItems){
        for(int i=0; i<tips.length; i++){
            if(i == selectItems){
                tips[i].setBackgroundResource(R.drawable.page_indicator_focused);
            }else{
                tips[i].setBackgroundResource(R.drawable.page_indicator_unfocused);
            }
        }
    }

    public class MyHandler extends Handler{
        /*
        请求轮播
         */
        protected static final int MSG_UPDATE_IMG = 1;
        /*
        请求暂停轮播
         */
        protected static final int MSG_STOP_IMG = 2;
        /*
        请求开始轮播
         */
        protected static final int MSG_START_IMG = 3;
        /*
        记录要开始轮播的页号
         */
        protected static final int MSG_PAGE_OLD = 4;
        /*
        轮播时间间隔
         */
        protected static final long MSG_DELAY = 1000;

        //使用弱引用
        private WeakReference<MainActivity> weakReference;
        private int currentItem = 0;

        protected MyHandler(WeakReference<MainActivity> wk){
            this.weakReference = wk;
        }
        public void handleMessage(Message msg) {
            super.handleMessage(msg);
            MainActivity activity = weakReference.get();
            if(activity == null){
                return;
            }
            //检查并移除队列中未发送的消息
            if (activity.handler.hasMessages(MSG_UPDATE_IMG) && currentItem > 1){
                activity.handler.removeMessages(MSG_UPDATE_IMG);
            }

            switch(msg.what){
                case MSG_UPDATE_IMG:
                    currentItem ++;
                    //设置一个显示图片的下标，实现切换效果
                    activity.viewPaper.setCurrentItem(currentItem);
                    //准备下次切换
                    activity.handler.sendEmptyMessageDelayed(MSG_UPDATE_IMG,MSG_DELAY);
                    break;
                case MSG_STOP_IMG:
                    //不发送消息就暂停了
                    break;
                case MSG_START_IMG:
                    activity.handler.sendEmptyMessageDelayed(MSG_UPDATE_IMG,MSG_DELAY);
                    break;
                case MSG_PAGE_OLD:
                    //记录页号
                    currentItem = msg.arg1;
                    break;
                default:
                    break;
            }
        }
    }
}