如何在视图分页器选项卡中切换列表片段和详细信息片段？

Question

我正在尝试在视图寻呼机的第一个选项卡中实现片段的切换。我尝试了在link 找到的方法，它在第一个选项卡中使用根片段，然后切换子片段。但是，当第二个子片段屏幕旋转发生时，应用程序会切换回第一个子片段，除此之外，这似乎是一种 hack。

我尝试搜索其他方法，但发现了几十个似乎都很混乱的答案。 here 的解释似乎是对的，但我不完全理解。这是该链接的解释和代码：

这种方法是通过覆盖getItem(...) 方法以及每次调用viewPager.getAdapter().notifyDataSetChanged() 时调用的getItemPosition(Object object) 来切换显示在FragmentPagerAdapter 内的片段。例如：

public static class MyPagerAdapter extends FragmentStatePagerAdapter {
// Return a different fragment for position based on additional state tracked in a member variable
@Override
public Fragment getItem(int position) {
    // For a given position, return two different potential fragments based on a condition
}

// Force a refresh of the page when a different fragment is displayed
@Override
public int getItemPosition(Object object) {
    // this method will be called for every fragment in the ViewPager
    if (object instanceof SomePermanantCachedFragment) {
        return POSITION_UNCHANGED; // don't force a reload
    } else {
        // POSITION_NONE means something like: this fragment is no longer valid
        // triggering the ViewPager to re-build the instance of this fragment.
        return POSITION_NONE;
    }
}
}

假设我有 ListFragment 和 DetailFragment 应该在第一个选项卡中切换。

1. 我会在哪里打电话给viewPager.getAdapter().notifyDataSetChanged()？
2. （...基于成员变量中跟踪的其他状态）我该如何实现？
3. SomePermanantCachedFragment 在我的情况下指的是ListFragment？

例如，我也尝试搜索 github，但没有成功...

Answer 1

如果您在 PagerAdapter 中的片段是动态的，那么您应该使用 Dynamic FragmentPagerAdapter。

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");
    }
}

完成后，您可以使用removeFragment 和addFragment 更改 ViewPager 中的片段。

从技术上讲，您可以添加一个 setFragment(int index) 方法来专门在索引 0 处替换它。

如果片段似乎没有更新，刷新 ViewPager 的技巧始终是setAdapter(null); setAdapter(adapter);。