在“FragmentStateAdapter"中覆盖“containsItem"和“getItemId"的目的是什么? [英] What is purpose of overriding `containsItem` and `getItemId` in `FragmentStateAdapter`?

问题描述

在FragmentStateAdapter 中覆盖containsItem 和getItemId 的目的是什么?

What is purpose of overriding containsItem and getItemId in FragmentStateAdapter?

如果将新列表传递给 FragmentStatePagerAdapter 实现并调用 notifyDataSetChanged，是否需要这些覆盖?

Are these overrides required if one is passing in new lists to the FragmentStatePagerAdapter implementation and calling notifyDataSetChanged?

从文档中可以看出，如果基础数据集正在发生变化，则两个覆盖都是必需的，并且您需要调用 notifyDataSetChanged?

From the doc, appears both overrides are require if the underlying data set is changing, and you need to call notifyDataSetChanged?

/**
 * Default implementation works for collections that don't add, move, remove items.
 * <p>
 * TODO(b/122670460): add lint rule
 * When overriding, also override {@link #getItemId(int)}
 */
public boolean containsItem(long itemId) {
    return itemId >= 0 && itemId < getItemCount();
}

/**
 * Default implementation works for collections that don't add, move, remove items.
 * <p>
 * TODO(b/122670460): add lint rule
 * When overriding, also override {@link #containsItem(long)}.
 * <p>
 * If the item is not a part of the collection, return {@link RecyclerView#NO_ID}.
 *
 * @param position Adapter position
 * @return stable item id {@link RecyclerView.Adapter#hasStableIds()}
 */
@Override
public long getItemId(int position) {
    return position;
}

例如，这是我的实现

class MyAdapter(host : Fragment) : FragmentStateAdapter(host) {

    data class FragItem(val newInstance : () -> Fragment, val fragId : Long)

    private var fragments = mapOf<Integer,FragItem>()

    override fun createFragment(position: Int): Fragment {
        return fragments[position]?.newInstance?.invoke() ?: DefaultFragment()
    }
    
    fun applyDataSet(newFragments : Map<Integer, FragItem>) {
        fragments = newFragments
        notifyDataSetChanged()
    }

    override fun getItemCount() = fragments.size

    override fun containsItem(itemId: Long): Boolean{
        return fragments.any { it.value.fragId == itemId }
    }

    override fun getItemId(position: Int) : Long{
        return fragments.values.elementAt(position).fragId
    }
}

现在我可以去掉 getItemId 和 containsItem 吗?还是离开他们?getItemCount 怎么样?

Now can I get rid of getItemId and containsItem? Or leave them? What about getItemCount?

推荐答案

• getItemCount 将指示回收者要渲染多少项.当前索引将作为 position 传递给 createFragment，所以很明显你需要它
• getItemId 和 containsItem 如果您想为回收商提供稳定的 ID，则实施.拥有稳定的 ID 允许进行某些优化，通常是当它涉及对列表中不相邻的元素进行操作时.
• getItemCount will instruct the recycler how many items are to be rendered. the current index will be passed as position to createFragment, so clearly you need it
• the getItemId and containsItem implemented if you want to provide stable ID's for your recycler. having stable ID's allows for a certain optimisations to be made, usually when it involves operations on elements that are not next to each-other in the list.

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