RecyclerView.ViewHolder - getLayoutPosition 与 getAdapterPosition [英] RecyclerView.ViewHolder - getLayoutPosition vs getAdapterPosition

问题描述

自新的支持库版本 (22.x) 起，RecyclerView.ViewHolder 类的 getPosition() 方法已被弃用，以代替话题.我并没有真正理解阅读文档的区别.有人可以用外行的术语解释其中的区别吗?

Since the new support library version (22.x) the getPosition() method of the RecyclerView.ViewHolder class has been deprecated in lieu of the methods mentioned in the topic. I don't really get the difference from reading the docs. Could somebody explain the difference in layman's terms?

我有以下用例 - 我给我的适配器一个 List，并且还希望能够为每个列表项关联额外的信息.我有一个位置到额外的映射，并且该映射可供持有者使用，这样他们就可以为他们的位置获取额外的信息并对其进行处理.在持有人中，我应该使用哪种方法?

I have the following use case - I give my adapter a List, and also want to be able to associate extra info for each list item. I have a position-to-extra mapping, and the mapping is available for the holders so that they can fetch the extra for their position and do stuff with it. In the holder, which method should I use?

当索引 0 和 1 的列表项交换位置时，​​持有者的位置会发生什么变化?方法返回什么?

What happens with the holder positions when list items at indices 0 and 1 are switched places? What do the methods return?

推荐答案

这是一个棘手的情况，抱歉文档不够.

This is a tricky situation, sorry that docs are not sufficient.

当适配器内容改变时(你调用 notify***())RecyclerView 请求一个新的布局.从那一刻起，直到布局系统决定计算新布局(<16 毫秒)，布局位置和适配器位置可能不匹配，因为布局尚未反映适配器更改.

When adapter contents change (and you call notify***()) RecyclerView requests a new layout. From that moment, until layout system decides to calculate a new layout (<16 ms), the layout position and adapter position may not match because layout has not reflected adapter changes yet.

在您的用例中，由于您的数据与您的适配器内容相关(并且我假设数据与适配器更改同时更改)，您应该使用 adapterPosition.

In your use case, since your data is related to your adapter contents (and I assume that data is changed at the same time with adapter changes), you should be using adapterPosition.

但是请注意，如果您正在调用 notifyDataSetChanged()，因为它会使所有内容无效，RecyclerView 在计算下一个布局之前不知道 ViewHolder 的适配器位置.在这种情况下，getAdapterPosition() 将返回 RecyclerView#NO_POSITION (-1).

Be careful though, if you are calling notifyDataSetChanged(), because it invalidates everything, RecyclerView does not know that ViewHolder's adapter position until next layout is calculated. In that case, getAdapterPosition() will return RecyclerView#NO_POSITION (-1).

但是假设您调用了 notifyItemInserted(0)，之前位于 0 位置的 ViewHolder 的 getAdapterPosition() 将立即开始返回 1.因此，只要您正在调度粒度通知事件，您就始终处于良好状态(即使尚未计算新布局，我们也知道适配器位置).

But lets say if you've called notifyItemInserted(0), the getAdapterPosition() of ViewHolder which was previously at position 0 will start returning 1 immediately. So as long as you are dispatching granular notify events, you are always in good state (we know adapter position even though new layout is not calculated yet).

另一个例子，如果你在用户点击时做某事，如果getAdapterPosition()返回NO_POSITION，最好忽略那个点击，因为你不知道什么用户点击(除非您有其他机制，例如使用稳定 ID 来查找项目).

Another example, if you are doing something on user click, if getAdapterPosition() returns NO_POSITION, it is best to ignore that click because you don't know what user clicked (unless you have some other mechanism, e.g. stable ids to lookup the item).

在布局位置良好时进行编辑

假设您正在使用 LinearLayoutManager 并希望访问当前单击的项目上方的 ViewHolder.在这种情况下，您应该使用布局位置来获取上面的项目.

Lets say you are using LinearLayoutManager and want to access the ViewHolder above the currently clicked item. In that case, you should use layout position to get the item above.

mRecyclerView.findViewHolderForLayoutPosition(myViewHolder.getLayoutPosition() - 1)

您必须使用布局位置，因为它匹配用户当前在屏幕上看到的内容.

You have to use layout position because it matches what user is currently seeing on the screen.

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