如果页面已经加载，如何使 viewpager 不再加载页面

Question

我有带有 7 个标签的 Viewpager，就像在 Play 商店中一样，如果我们点击主屏幕上的标签它会加载，如果我们再次点击该标签它不会加载，我希望如果我点击标签 1 它应该加载然后我让点击在标签 7 上，然后我再次点击标签 1，它不应该加载，只有在满足某些条件时才加载，我该怎么做？现在它只是带有适配器的简单视图寻呼机，例如：

  public class PagerNewProductAdapter extends FragmentStatePagerAdapter { 
      RealmList<SubCategory> categoryData;
      String categoryName;
      String categoryId;
      Env env;

      public PagerNewProductAdapter(Fragment fragment, RealmList<SubCategory> category, String categoryName, String categoryId, Env env) {
        super(fragment.getChildFragmentManager());
        categoryData = category;
        this.categoryId = categoryId;
        this.env = env;
        this.categoryName = categoryName;
      }

      @Override
      public int getCount() {
        return categoryData.size() + 2;
      }

      @Override
      public Fragment getItem(int position) {
        Fragment fragment;
        if (position == 0) {
          fragment= ProductFragment.newInstance("2", categoryId, categoryName);
        } else if (position == 1) {
          fragment= ProductFragment.newInstance("3", categoryId, categoryName);
        } else {
          fragment= ProductFragment.newInstance("1", categoryData.get(position - 2).getCategoryId(), categoryName);
        }
        return fragment;
      }

      @Override
      public CharSequence getPageTitle(int position) {
        if (position == 0) {
          // return env.appSubcategoryViewAll;
          return env.appSubcategoryViewAll;
        } else if (position == 1) {
          return env.appDashboardCBrands;
        } else {
          return categoryData.get(position - 2).getName();
        }
     }
  }

谢谢

Answer 1

只需像这样添加 setOffscreenPageLimit：

    viewPager = findViewById(R.id.viewpager);
    viewPager.setOffscreenPageLimit(3);

3 = viewpager 内的片段数。

Answer 2

我认为在这种情况下你可以使用SparseArray

public class PagerNewProductAdapter extends FragmentStatePagerAdapter {
  private final SparseArray<Fragment> registeredFragments = new SparseArray<>();
  RealmList<SubCategory> categoryData;
  String categoryName;
  String categoryId;
  Env env;

  public PagerNewProductAdapter(Fragment fragment, RealmList<SubCategory> category, String categoryName, String categoryId, Env env) {
    super(fragment.getChildFragmentManager());
    categoryData = category;
    this.categoryId = categoryId;
    this.env = env;
    this.categoryName = categoryName;
  }

  @NonNull
  @Override
  public Object instantiateItem(ViewGroup container, int position) {
    if (registeredFragments.get(position) == null) {
      Object fragment = super.instantiateItem(container, position);
      registeredFragments.put(position, (BaseFragment) fragment);
      return fragment;
    } else {
      return registeredFragments.get(position);
    }
  }

  @Override
  public void destroyItem(ViewGroup container, int position, Object object) {
    super.destroyItem(container, position, object);
    registeredFragments.remove(position);
  }

  @Override
  public Fragment getItem(int position) {
    Fragment fragment;
    if (position == 0) {
        fragment= ProductFragment.newInstance("2", categoryId, categoryName);
    } else if (position == 1) {
        fragment= ProductFragment.newInstance("3", categoryId, categoryName);
    } else {
        fragment= ProductFragment.newInstance("1", categoryData.get(position - 2).getCategoryId(), categoryName);
    }
    return fragment;
  }

  @Override
  public int getCount() {
    return categoryData.size() + 2;
  }



  @Override
  public CharSequence getPageTitle(int position) {
    if (position == 0) {
       // return env.appSubcategoryViewAll;
        return env.appSubcategoryViewAll;
    } else if (position == 1) {
        return env.appDashboardCBrands;
    } else {
        return categoryData.get(position - 2).getName();
    }
  }
}

Answer 3

我认为您需要使用 Fragmentation。它正在按照您的要求工作。如果您有任何疑问，请告诉我。

第 1 步： 实现分片https://github.com/YoKeyword/Fragmentation

第 2 步： 从 SupportActivity 而不是 AppCompatActivity 扩展您的活动 从 SupportFragment 而不是 Fragment 扩展您的片段

第 3 步： 从片段中删除 onresume 和 onpause 方法

第 4 步： 调用 onPause();在 onViewCreated 方法中

@Override
    public void onSupportVisible() {
        super.onSupportVisible();
       onResume();
        if(!isLoaded){
            isLoaded = true;
//Load your data here 
}
}
@Override
    public void onSupportInvisible() {
        super.onSupportInvisible();
       onPause();
    }

第 5 步： 设置限制 viewPager.setOffscreenPageLimit(numberi);

快乐编码。

Answer 4

您可以将之前创建的 Fragment 存储在一个数组中。这确保我们将使用先前创建的实例（如果它存在于我们的 Array 中）。

public class PagerNewProductAdapter extends FragmentStatePagerAdapter { 
      Fragment[] cache;
      RealmList<SubCategory> categoryData;
      ...

      public PagerNewProductAdapter(Fragment fragment, RealmList<SubCategory> category, String categoryName, String categoryId, Env env) {
        ...
        this.categoryName = categoryName;
        this.cache = new Fragment[getCount()];
      }

      @Override
      public int getCount() {
        return categoryData.size() + 2;
      }

      @Override
      public Fragment getItem(int position) {

        Fragment cachedFragment = cache[position]
        if(cachedFragment == null){
            if (position == 0) {
                cache[position] = ProductFragment.newInstance("2", categoryId, categoryName);
                cachedFragment = cache[position];
            } else if (position == 1) {
                cache[position] = ProductFragment.newInstance("3", categoryId, categoryName);
                cachedFragment = cache[position];
            } else {
                cache[position]= ProductFragment.newInstance("1", categoryData.get(position - 2).getCategoryId(), categoryName);
                cachedFragment = cache[position];
            }
        }
        return cachedFragment;
      }
      ...
  }