RecyclerView：getLayoutPosition 和 getAdapterPosition

一、getPosition

/**
 * @deprecated This method is deprecated because its meaning is ambiguous due to the async
 * handling of adapter updates. Please use {@link #getLayoutPosition()} or
 * {@link #getAdapterPosition()} depending on your use case.
 *
 * @see #getLayoutPosition()
 * @see #getAdapterPosition()
 */
@Deprecated
public final int getPosition() {
    return mPreLayoutPosition == NO_POSITION ? mPosition : mPreLayoutPosition;
}

二、getLayoutPosition

/**
 * Returns the position of the ViewHolder in terms of the latest layout pass.
 * 

 * This position is mostly used by RecyclerView components to be consistent while
 * RecyclerView lazily processes adapter updates.
 * 

 * For performance and animation reasons, RecyclerView batches all adapter updates until the
 * next layout pass. This may cause mismatches between the Adapter position of the item and
 * the position it had in the latest layout calculations.
 * 

 * LayoutManagers should always call this method while doing calculations based on item
 * positions. All methods in {@link RecyclerView.LayoutManager}, {@link RecyclerView.State},
 * {@link RecyclerView.Recycler} that receive a position expect it to be the layout position
 * of the item.
 * 

 * If LayoutManager needs to call an external method that requires the adapter position of
 * the item, it can use {@link #getAdapterPosition()} or
 * {@link RecyclerView.Recycler#convertPreLayoutPositionToPostLayout(int)}.
 *
 * @return Returns the adapter position of the ViewHolder in the latest layout pass.
 * @see #getAdapterPosition()
 */
public final int getLayoutPosition() {
    return mPreLayoutPosition == NO_POSITION ? mPosition : mPreLayoutPosition;
}

返回布局中最新的计算位置，和用户所见到的位置一致，当做用户输入（例如点击事件）的时候考虑使用

三、getAdapterPosition

/**
 * Returns the Adapter position of the item represented by this ViewHolder.
 * 

 * Note that this might be different than the {@link #getLayoutPosition()} if there are
 * pending adapter updates but a new layout pass has not happened yet.
 * 

 * RecyclerView does not handle any adapter updates until the next layout traversal. This
 * may create temporary inconsistencies between what user sees on the screen and what
 * adapter contents have. This inconsistency is not important since it will be less than
 * 16ms but it might be a problem if you want to use ViewHolder position to access the
 * adapter. Sometimes, you may need to get the exact adapter position to do
 * some actions in response to user events. In that case, you should use this method which
 * will calculate the Adapter position of the ViewHolder.
 * 

 * Note that if you've called {@link RecyclerView.Adapter#notifyDataSetChanged()}, until the
 * next layout pass, the return value of this method will be {@link #NO_POSITION}.
 *
 * @return The adapter position of the item if it still exists in the adapter.
 * {@link RecyclerView#NO_POSITION} if item has been removed from the adapter,
 * {@link RecyclerView.Adapter#notifyDataSetChanged()} has been called after the last
 * layout pass or the ViewHolder has already been recycled.
 */
public final int getAdapterPosition() {
    if (mOwnerRecyclerView == null) {
        return NO_POSITION;
    }
    return mOwnerRecyclerView.getAdapterPositionFor(this);
}

返回数据在 Adapter 中的位置（也许位置的变化还未来得及刷新到布局中），当使用 Adapter 的时候（例如调用 Adapter 的 notify 的刷新相关方法时）考虑使用。

总结：

getLayoutPosition 和 getAdapterPosition 通常情况下是一样的，只有当 Adapter 里面的内容改变了，而 Layout 还没来得及绘制的这段时间之内才有可能不一样，这个时间小于16ms

如果调用的是 notifyDataSetChanged()，因为要重新绘制所有 Item，所以在绘制完成之前 RecyclerView 是不知道 adapterPosition 的，这时会返回-1（NO_POSITION）

但如果用的是 notifyItemInserted(0)，那立即就能获取到正确的 adapterPosition，即使新的 Layout 还没绘制完成，比如之前是0的现在就会变成1，因为插入了0, 相当于 RecyclerView 提前帮你计算的，此时getLayoutPosition 还只能获取到旧的值。

总的来说，大多数情况下用 getAdapterPosition，只要不用 notifyDataSetChanged() 来刷新数据就总能立即获取到正确 position 值。

什么情况下用 getLayoutPosition 呢？

就是调用 findViewHolderForLayoutPosition 获取当前点击的 Item 的 ViewHolder 时，因为此时 layout position 和用户在屏幕上看到的一定是一样的。

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