android - 我的 View Pager Adapter 中的 instantiateItem() 如何工作？

Question

我想达到什么目的？

我有 5 张图片的 url 链接。我希望它们以幻灯片的形式显示在我的主屏幕上。

我知道该怎么做吗？

据我所知，是的。我能够在主屏幕内以自动幻灯片的形式显示 5 张图像

那问题是什么？

正在加载的第一个图像实际上来自第二个索引位置的 url，即第三个 url。

我在文本中所做的事情的概述

我已经将图像的 url 存储在一个字符串数组中。我将我的主屏幕上下文和 url 数组传递给我的 View Pager Adapter。我在 Adapter 的 instantiateItem() 方法中使用 Glide 来显示图像

代码

HomeScreenTopPagerAdapter.java

public class HomeScreenTopPagerAdapter extends PagerAdapter {

    private String[] urls;
    private LayoutInflater layoutInflater;
    private Context context;
    private int custom_position = 0;

    public  HomeScreenTopPagerAdapter(Context context, String[] urls){
        this.context = context;
        this.urls = urls;
        layoutInflater = (LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
    }

    @Override
    public int getCount() {
        return Integer.MAX_VALUE;           //to make automatic image slider come back to first image smoothly, by hypothetically making the image views infinite
    }

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

    @NonNull
    @Override
    public Object instantiateItem(@NonNull final ViewGroup container, int position) {

        if(custom_position>urls.length-1)        //done for a smooth transition when there is a slideshow between last image and the first one
            custom_position=0;

        View view = layoutInflater.inflate(R.layout.home_screen_topimageslider,container,false);
        ImageView imageView = view.findViewById(R.id.topImageSliderImageView);
        final ProgressBar progressBar = ((HomeScreen)context).findViewById(R.id.progresBarHomeScreenTop);
        progressBar.setVisibility(View.VISIBLE);

        GlideApp.with(context)
                .load(urls[custom_position])
                .listener(new RequestListener<Drawable>() {
                    @Override
                    public boolean onLoadFailed(@Nullable GlideException e, Object model, Target<Drawable> target, boolean isFirstResource) {
                        progressBar.setVisibility(View.GONE);
                        Toast.makeText(context,"Please Check Your Internet Connection",Toast.LENGTH_LONG).show();
                        Log.i("In Glide Loading Failed",Integer.toString(custom_position));
                        return false;
                    }

                    @Override
                    public boolean onResourceReady(Drawable resource, Object model, Target<Drawable> target, DataSource dataSource, boolean isFirstResource) {
                        progressBar.setVisibility(View.GONE);
                        Log.i("In Glide Resource Ready",Integer.toString(custom_position));
                        //((HomeScreen)context).prepareDots(custom_position);
                        return false;
                    }
                })
                .diskCacheStrategy(DiskCacheStrategy.NONE)
                .into(imageView);

        container.addView(view);
        Log.i("Outside Glide ",Integer.toString(custom_position));
        custom_position++;
        return view;
    }

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

Logcat

2019-06-19 09:50:46.728 29185-29185/? I/Outside Glide: 0
2019-06-19 09:50:46.731 29185-29185/? I/Outside Glide: 1
2019-06-19 09:50:46.741 29185-29210/? I/Adreno: QUALCOMM build                   : 4c638fb, I557c585805
    Build Date                       : 10/06/18
    OpenGL ES Shader Compiler Version: EV031.25.03.01
    Local Branch                     : 
    Remote Branch                    : 
    Remote Branch                    : 
    Reconstruct Branch               : 
2019-06-19 09:50:46.742 29185-29210/? I/Adreno: Build Config                     : S L 6.0.7 AArch64
2019-06-19 09:50:46.745 29185-29210/? I/Adreno: PFP: 0x005ff112, ME: 0x005ff066
2019-06-19 09:50:46.748 29185-29210/? I/ConfigStore: android::hardware::configstore::V1_0::ISurfaceFlingerConfigs::hasWideColorDisplay retrieved: 0
2019-06-19 09:50:46.748 29185-29210/? I/ConfigStore: android::hardware::configstore::V1_0::ISurfaceFlingerConfigs::hasHDRDisplay retrieved: 0
2019-06-19 09:50:46.749 29185-29210/? I/OpenGLRenderer: Initialized EGL, version 1.4
2019-06-19 09:50:46.770 29185-29216/? I/DpmTcmClient: RegisterTcmMonitor from: com.android.okhttp.TcmIdleTimerMonitor
2019-06-19 09:50:48.034 29185-29185/com.example.gofresh I/In Glide Resource Ready: 2
2019-06-19 09:50:48.204 29185-29185/com.example.gofresh I/In Glide Resource Ready: 2
2019-06-19 09:50:52.324 29185-29185/? I/Outside Glide: 2
2019-06-19 09:50:52.953 29185-29185/? I/In Glide Resource Ready: 3
2019-06-19 09:50:55.066 29185-29185/? I/Outside Glide: 3
2019-06-19 09:50:55.722 29185-29185/? I/In Glide Resource Ready: 4
2019-06-19 09:50:57.273 29185-29185/? I/Outside Glide: 4
2019-06-19 09:50:58.611 29185-29185/? I/In Glide Resource Ready: 5
2019-06-19 09:50:59.327 29185-29185/? I/Outside Glide: 0
2019-06-19 09:50:59.342 29185-29185/? I/In Glide Resource Ready: 1
2019-06-19 09:51:03.633 29185-29185/? I/Outside Glide: 1
2019-06-19 09:51:03.655 29185-29185/? I/In Glide Resource Ready: 2

正如您从 Logcat 中看到的那样，仅当发生 Outside Glide: 0 和 Outside Glide: 1 时，它才会进入 Glide Method。我无法理解这个 instantiateItem() 方法实际上是如何工作的。

此外，我假设每当 In Glide Resource Ready 将显示在 Log 中时，紧接着将显示 Outside Glide(即对于特定索引位置的图像，两者将同时显示)，但是 Outside Glide 是ONLY AFTER 幻灯片显示到下一张图片。简而言之，每当幻灯片放映到一张图片时，就会显示带有前一张图片索引位置的 Outside Glide，然后显示带有当前图片索引的 In Glide Resource Ready。

Homescreen.java

    private ViewPager viewPager;
    private HomeScreenTopPagerAdapter adapter;
    private String[] urls = {"https://picsum.photos/420/200/?temp=1",               ///?temp=randomNo/String doesn't change the URL. Added so that each image is random
                     "https://picsum.photos/420/200/?temp=2",
                     "https://picsum.photos/420/200/?temp=3",
                     "https://picsum.photos/420/200/?temp=4",
                     "https://picsum.photos/420/200/?temp=5"};
    private Timer timer;
    private int current_position = 0;
    private LinearLayout dotsLayout;
    private int custom_position = 0;


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

        instantiateViews();
        viewPager.setAdapter(adapter);
        createSlideShow();
    }

public void instantiateViews(){

        viewPager = findViewById(R.id.topImageSliderViewPager);
        adapter = new HomeScreenTopPagerAdapter(HomeScreen.this,urls);
        dotsLayout = findViewById(R.id.hs_dotsLayout);

    }

public void createSlideShow(){
        final Handler handler = new Handler();
        final Runnable runnable = new Runnable() {
            @Override
            public void run() {
                if(current_position==Integer.MAX_VALUE)
                    current_position=0;
                viewPager.setCurrentItem(current_position++,true);
            }
        };

        timer = new Timer();
        timer.schedule(new TimerTask() {
            @Override
            public void run() {
                handler.post(runnable);
            }
        },1000,3000);
    }
}

Answer 1

花了很多时间后，我找到了如何加载第一个图像而不是第三个图像的答案。

public Object instantiateItem(@NonNull final ViewGroup container, final int position) {
        .
        .
        .

        GlideApp.with(context)
                .load(urls[position%urls.length])   //THE CHANGE MADE HERE, completely removed the usage of custom_position variable
                .
                .
}

注意:这只回答了如何获得所需的结果，它仍然没有回答 instantiateItem() 是如何工作的，显示出意想不到的结果。我是初学者，只有有经验的人才能用上面的方法解释错误。