RecyclerView 介绍 基本使用

文档位置：https://developer.android.google.cn/reference/android/support/v7/widget/RecyclerView.html

A flexible view for providing a limited window into a large data set.

RecylerView是support-v7包中的新组件，是一个强大的滑动组件，与经典的ListView相比，同样拥有item回收复用的功能，这一点从它的名字recylerview即回收view也可以看出。从它类名上看，RecyclerView代表的意义是，我只管Recycler View，也就是说RecyclerView只管回收与复用View，其他的你可以自己去设置。可以看出其高度的解耦，给予你充分的定制自由（所以你才可以轻松的通过这个控件实现ListView,GirdView，瀑布流等效果）。

看到这也许有人会问，不是已经有ListView了吗，为什么还要RecylerView呢？

根据官方的介绍，RecylerView是ListView的升级版，既然如此那RecylerView必然有它的优点：

添加依赖【compile ‘com.android.support:appcompat-v7:25.3.1‘】

TMD添加后竟然找不到RecyclerView，因为你还要再添加【compile ‘com.android.support:recyclerview-v7:25.3.1‘】

<android.support.v7.widget.RecyclerView
        android:id="@+id/rv"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"/>

android.support.v7.widget.RecyclerView
public class RecyclerView extends ViewGroup implements ScrollingView, NestedScrollingChild

1、官方文档

RecyclerView introduces引入 an additional额外 level of abstraction抽象 between the RecyclerView.Adapter and RecyclerView.LayoutManager to be able to detect发现 data set changes in batches during a layout calculation. This saves LayoutManager from tracking adapter changes to calculate animations. It also helps with performance because all view bindings happen at the same time and unnecessary bindings are avoided.

For this reason, there are two types of position related methods in RecyclerView:

These two positions are the same except the time between dispatching adapter.notify* events and calculating the updated最新的 layout.

Methods that return or receive *LayoutPosition* use position as of the latest layout calculation (e.g. getLayoutPosition(), findViewHolderForLayoutPosition(int)). These positions include all changes until the last layout calculation. You can rely on these positions to be consistent with what user is currently seeing on the screen. For example, if you have a list of items on the screen and user asks for the 5th element, you should use these methods as they‘ll match what user is seeing.

The other set of position related methods are in the form of *AdapterPosition*. (e.g. getAdapterPosition(), findViewHolderForAdapterPosition(int)) You should use these methods when you need to work with up-to-date最近的 adapter positions even if they may not have been reflected to layout yet. For example, if you want to access使用 the item in the adapter on a ViewHolder click, you should use getAdapterPosition(). Beware当心 that these methods may not be able to calculate adapter positions if notifyDataSetChanged() has been called and new layout has not yet been calculated. For this reasons, you should carefully handle NO_POSITION or null results from these methods.

When writing a RecyclerView.LayoutManager you almost always want to use layout positions whereas反之 when writing an RecyclerView.Adapter, you probably want to use adapter positions.

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

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.

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.

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).

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颗粒状的 notify events, you are always in good state (we know adapter position even though new layout is not calculated yet).

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).