使用静态newInstance方法实例化片段覆盖以前的参数

Question

我正在尝试使用多个片段来实现ViewPager，并使用静态newInstance来实例化activity循环中的片段

for(String name : list) {          
  viewPagerAdapter.addFragment(ProductListFragment.newInstance(name), name);
        } 


 public static ProductListFragment newInstance(String category) {
    ProductListFragment fragment = new ProductListFragment();
    ProductListFragment.category = category;
    Bundle args = new Bundle();
    args.putString(CATEGORY_PARAM, category);
    fragment.setArguments(args);
    return fragment;
}

因为片段的生命周期只有在循环结束后才会被调用，所以我不能保留我发送的参数值。为了更好地理解，假设有三个选项卡，在每个选项卡中我将显示与"name“值相关的产品，并且我希望使用片段中productList中的"name”值来过滤这些产品。一旦第一个片段生命周期获取调用，我将拥有最后一个值"name“，因为我已经提到过，片段生命周期只有在循环结束后才开始，这意味着我将只过滤与最后一个值"name”相关的产品，相同的产品将显示在所有三个标签中。如何解决这个问题？

Answer 1

不要试图使用ViewPagerAdapter.addFragment()，应该在getItem()方法中创建片段。

如果您需要动态FragmentPagerAdapter，则可以执行以下操作：

public class DynamicFragmentPagerAdapter extends PagerAdapter {
    private static final String TAG = "DynamicFragmentPagerAdapter";
  
    private final FragmentManager fragmentManager;
  
    public static abstract class FragmentIdentifier implements Parcelable {
        private final String fragmentTag;
        private final Bundle args;
      
        public FragmentIdentifier(@NonNull String fragmentTag, @Nullable Bundle args) {
            this.fragmentTag = fragmentTag;
            this.args = args;
        }
      
        protected FragmentIdentifier(Parcel in) {
            fragmentTag = in.readString();
            args = in.readBundle(getClass().getClassLoader());
        }

        @Override
        public void writeToParcel(Parcel dest, int flags) {
            dest.writeString(fragmentTag);
            dest.writeBundle(args);
        }
      
        protected final Fragment newFragment() {
            Fragment fragment = createFragment();
            Bundle oldArgs = fragment.getArguments();
            Bundle newArgs = new Bundle();
            if(oldArgs != null) {
                newArgs.putAll(oldArgs);
            }
            if(args != null) {
                newArgs.putAll(args);
            }
            fragment.setArguments(newArgs);
            return fragment;
        }

        protected abstract Fragment createFragment();
    }
  
    private ArrayList<FragmentIdentifier> fragmentIdentifiers = new ArrayList<>();

    private FragmentTransaction currentTransaction = null;

    private Fragment currentPrimaryItem = null;

    public DynamicFragmentPagerAdapter(FragmentManager fragmentManager) {
        this.fragmentManager = fragmentManager;
    }

    private int findIndexIfAdded(FragmentIdentifier fragmentIdentifier) {
        for (int i = 0, size = fragmentIdentifiers.size(); i < size; i++) {
            FragmentIdentifier identifier = fragmentIdentifiers.get(i);
            if (identifier.fragmentTag.equals(fragmentIdentifier.fragmentTag)) {
                return i;
            }
        }
        return -1;
    }

    public void addFragment(FragmentIdentifier fragmentIdentifier) {
        if (findIndexIfAdded(fragmentIdentifier) < 0) {
            fragmentIdentifiers.add(fragmentIdentifier);
            notifyDataSetChanged();
        }
    }

    public void removeFragment(FragmentIdentifier fragmentIdentifier) {
        int index = findIndexIfAdded(fragmentIdentifier);
        if (index >= 0) {
            fragmentIdentifiers.remove(index);
            notifyDataSetChanged();
        }
    }

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

    @Override
    public void startUpdate(@NonNull ViewGroup container) {
        if (container.getId() == View.NO_ID) {
            throw new IllegalStateException("ViewPager with adapter " + this
                    + " requires a view id");
        }
    }

    @SuppressWarnings("ReferenceEquality")
    @NonNull
    @Override
    public Object instantiateItem(@NonNull ViewGroup container, int position) {
        if (currentTransaction == null) {
            currentTransaction = fragmentManager.beginTransaction();
        }
        final FragmentIdentifier fragmentIdentifier = fragmentIdentifiers.get(position);
        // Do we already have this fragment?
        final String name = fragmentIdentifier.fragmentTag;
        Fragment fragment = fragmentManager.findFragmentByTag(name);
        if (fragment != null) {
            currentTransaction.attach(fragment);
        } else {
            fragment = fragmentIdentifier.newFragment();
            currentTransaction.add(container.getId(), fragment, fragmentIdentifier.fragmentTag);
        }
        if (fragment != currentPrimaryItem) {
            fragment.setMenuVisibility(false);
            fragment.setUserVisibleHint(false);
        }
        return fragment;
    }

    @Override
    public void destroyItem(@NonNull ViewGroup container, int position, @NonNull Object object) {
        if (currentTransaction == null) {
            currentTransaction = fragmentManager.beginTransaction();
        }
        currentTransaction.detach((Fragment) object);
    }

    @SuppressWarnings("ReferenceEquality")
    @Override
    public void setPrimaryItem(@NonNull ViewGroup container, int position, @NonNull Object object) {
        Fragment fragment = (Fragment) object;
        if (fragment != currentPrimaryItem) {
            if (currentPrimaryItem != null) {
                currentPrimaryItem.setMenuVisibility(false);
                currentPrimaryItem.setUserVisibleHint(false);
            }
            fragment.setMenuVisibility(true);
            fragment.setUserVisibleHint(true);
            currentPrimaryItem = fragment;
        }
    }

    @Override
    public void finishUpdate(@NonNull ViewGroup container) {
        if (currentTransaction != null) {
            currentTransaction.commitNowAllowingStateLoss();
            currentTransaction = null;
        }
    }

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

    @Override
    public Parcelable saveState() {
        Bundle bundle = new Bundle();
        bundle.putParcelableArrayList("fragmentIdentifiers", fragmentIdentifiers);
        return bundle;
    }

    @Override
    public void restoreState(Parcelable state, ClassLoader loader) {
        Bundle bundle = ((Bundle)state);
        bundle.setClassLoader(loader);
        fragmentIdentifiers = bundle.getParcelableArrayList("fragmentIdentifiers");
    }
}