RecyclerView发布已经挺久了，博客上也有好多优秀的使用教程，相比较ListView，RecyclerView的使用确实更加灵活，本文就针对RecyclerView的源码进行分析，分析一下它的工作原理，关于它的使用教程，大家可以去看下hongyang大神的博文，以下我就自己针对源码的查看写了自己的分析，如有不对的地方，欢迎提出指教。
我们首先看下RecyclerView这个类的第一条注释说明：A flexible view for providing a limited window into a large data set.可见该View的定义就是一种灵活的、能够装载大量数据的View。该类的源码有1w多行，我们就从使用它相关的代码进行查看：
recyclerView = (RecyclerView) findViewById(R.id.id_recyclerview);
recyclerView.setLayoutManager(new StaggeredGridLayoutManager(2, StaggeredGridLayoutManager.VERTICAL));
recyclerView.setAdapter(new MyAdapter(this,datas));
recyclerView.addItemDecoration(new DividerItemDecoration(this, DividerItemDecoration.VERTICAL_LIST));这是我们常用的一段基本的代码，可知，我们会用到以下几个方法：setLayoutManager，（关键点，灵活的设置布局，简单），setAdapter，addItemDecoration。另外，还有这个类我们也一起看下：Recycler。
关于Recycler：
A Recycler is responsible for managing scrapped or detached item views for reufinal
public final class Recycler {
final ArrayList<ViewHolder> mAttachedScrap = new ArrayList<>();
private ArrayList<ViewHolder> mChangedScrap = null;
final ArrayList<ViewHolder> mCachedViews = new ArrayList<ViewHolder>();
private final List<ViewHolder>
mUnmodifiableAttachedScrap = Collections.unmodifiableList(mAttachedScrap);
private int mViewCacheMax = DEFAULT_CACHE_SIZE;
private RecycledViewPool mRecyclerPool;
private ViewCacheExtension mViewCacheExtension;
private static final int DEFAULT_CACHE_SIZE = 2;它为RecyclerView中的item views提供了复用，用于管理已经废弃或与RecyclerView分离的（scrapped or detached）item view。
/**
* Set the {@link LayoutManager} that this RecyclerView will use.
*
* <p>In contrast to other adapter-backed views such as {@link android.widget.ListView}
* or {@link android.widget.GridView}, RecyclerView allows client code to provide custom
* layout arrangements for child views. These arrangements are controlled by the
* {@link LayoutManager}. A LayoutManager must be provided for RecyclerView to function.</p>
*
* <p>Several default strategies are provided for common uses such as lists and grids.</p>
*
* @param layout LayoutManager to use
*/
public void setLayoutManager(LayoutManager layout) {
if (layout == mLayout) {
return;
}
stopScroll();
// TODO We should do this switch a dispachLayout pass and animate children. There is a good
// chance that LayoutManagers will re-use views.
if (mLayout != null) {
if (mIsAttached) {
mLayout.dispatchDetachedFromWindow(this, mRecycler);
}
mLayout.setRecyclerView(null);
}
mRecycler.clear();
mChildHelper.removeAllViewsUnfiltered();
mLayout = layout;
if (layout != null) {
if (layout.mRecyclerView != null) {
throw new IllegalArgumentException("LayoutManager " + layout +
" is already attached to a RecyclerView: " + layout.mRecyclerView);
}
mLayout.setRecyclerView(this);
if (mIsAttached) {
mLayout.dispatchAttachedToWindow(this);
}
}
requestLayout();
} 我们看这句代码:mLayout.setRecyclerView(this); 继续追踪下去：
/** * These measure specs might be the measure specs that were passed into RecyclerView's * onMeasure method OR fake measure specs created by the RecyclerView. * For example, when a layout is run, RecyclerView always sets these specs to be * EXACTLY because a LayoutManager cannot resize RecyclerView during a layout pass. */
private int mWidthSpec, mHeightSpec;
void setRecyclerView(RecyclerView recyclerView) {
if (recyclerView == null) {
mRecyclerView = null;
mChildHelper = null;
mWidthSpec = MeasureSpec.makeMeasureSpec(0, MeasureSpec.EXACTLY);
mHeightSpec = MeasureSpec.makeMeasureSpec(0, MeasureSpec.EXACTLY);
} else {
mRecyclerView = recyclerView;
mChildHelper = recyclerView.mChildHelper;
mWidthSpec = MeasureSpec
.makeMeasureSpec(recyclerView.getWidth(), MeasureSpec.EXACTLY);
mHeightSpec = MeasureSpec
.makeMeasureSpec(recyclerView.getHeight(), MeasureSpec.EXACTLY);
}
} 也就是测量RecyclerView 中itemView高度和宽度，然后对其绘制，可以实现水平滚动、垂直滚动、方块表格等列表形式。具体可以看LayoutManager这个类。
接下来我们看下这句代码：
recyclerView.setAdapter(new MyAdapter(this,datas));可知，它是为recyclerView存放数据集的，其中MyAdapter是我继承RecyclerView.Adapter
/** * Base class for an Adapter * * <p>Adapters provide a binding from an app-specific data set to views that are displayed * within a {@link RecyclerView}.</p> */
public static abstract class Adapter<VH extends ViewHolder> {
private final AdapterDataObservable mObservable = new AdapterDataObservable();
private boolean mHasStableIds = false;
/** * Called when RecyclerView needs a new {@link ViewHolder} of the given type to represent * an item. * <p> * This new ViewHolder should be constructed with a new View that can represent the items * of the given type. You can either create a new View manually or inflate it from an XML * layout file. * <p> * The new ViewHolder will be used to display items of the adapter using * {@link #onBindViewHolder(ViewHolder, int, List)}. Since it will be re-used to display * different items in the data set, it is a good idea to cache references to sub views of * the View to avoid unnecessary {@link View#findViewById(int)} calls. * * @param parent The ViewGroup into which the new View will be added after it is bound to * an adapter position. * @param viewType The view type of the new View. * * @return A new ViewHolder that holds a View of the given view type. * @see #getItemViewType(int) * @see #onBindViewHolder(ViewHolder, int) */
public abstract VH onCreateViewHolder(ViewGroup parent, int viewType);
public abstract void onBindViewHolder(VH holder, int position);
public void onBindViewHolder(VH holder, int position, List<Object> payloads) {
onBindViewHolder(holder, position);
}
/** * This method calls {@link #onCreateViewHolder(ViewGroup, int)} to create a new * {@link ViewHolder} and initializes some private fields to be used by RecyclerView. * * @see #onCreateViewHolder(ViewGroup, int) */
public final VH createViewHolder(ViewGroup parent, int viewType) {
TraceCompat.beginSection(TRACE_CREATE_VIEW_TAG);
final VH holder = onCreateViewHolder(parent, viewType);
holder.mItemViewType = viewType;
TraceCompat.endSection();
return holder;
}
...... 我们可以重写该类的onBindViewHolder方法，进行自己的数据处理，而在onCreateViewHolder方法中我们可以读取item文件进行设置。相比ListView的优点就是RecyclerView直接将ViewHolder内置：
ViewHolder 部分代码：
/** * A ViewHolder describes an item view and metadata about its place within the RecyclerView. * * <p>{@link Adapter} implementations should subclass ViewHolder and add fields for caching * potentially expensive {@link View#findViewById(int)} results.</p> * * <p>While {@link LayoutParams} belong to the {@link LayoutManager}, * {@link ViewHolder ViewHolders} belong to the adapter. Adapters should feel free to use * their own custom ViewHolder implementations to store data that makes binding view contents * easier. Implementations should assume that individual item views will hold strong references * to <code>ViewHolder</code> objects and that <code>RecyclerView</code> instances may hold * strong references to extra off-screen item views for caching purposes</p> */
public static abstract class ViewHolder {
public final View itemView;
int mPosition = NO_POSITION;
int mOldPosition = NO_POSITION;
long mItemId = NO_ID;
int mItemViewType = INVALID_TYPE;
int mPreLayoutPosition = NO_POSITION;
// The item that this holder is shadowing during an item change event/animation
ViewHolder mShadowedHolder = null;
// The item that is shadowing this holder during an item change event/animation
ViewHolder mShadowingHolder = null;
.......提供了很多属性。
ItemDecoration：
/** * An ItemDecoration allows the application to add a special drawing and layout offset * to specific item views from the adapter's data set. This can be useful for drawing dividers * between items, highlights, visual grouping boundaries and more. * * <p>All ItemDecorations are drawn in the order they were added, before the item * views (in {@link ItemDecoration#onDraw(Canvas, RecyclerView, RecyclerView.State) onDraw()} * and after the items (in {@link ItemDecoration#onDrawOver(Canvas, RecyclerView, * RecyclerView.State)}.</p> */
public static abstract class ItemDecoration {
/** * Draw any appropriate decorations into the Canvas supplied to the RecyclerView. * Any content drawn by this method will be drawn before the item views are drawn, * and will thus appear underneath the views. * * @param c Canvas to draw into * @param parent RecyclerView this ItemDecoration is drawing into * @param state The current state of RecyclerView */
public void onDraw(Canvas c, RecyclerView parent, State state) {
onDraw(c, parent);
}
/** * @deprecated * Override {@link #onDraw(Canvas, RecyclerView, RecyclerView.State)} */
@Deprecated
public void onDraw(Canvas c, RecyclerView parent) {
}
/** * Draw any appropriate decorations into the Canvas supplied to the RecyclerView. * Any content drawn by this method will be drawn after the item views are drawn * and will thus appear over the views. * * @param c Canvas to draw into * @param parent RecyclerView this ItemDecoration is drawing into * @param state The current state of RecyclerView. */
public void onDrawOver(Canvas c, RecyclerView parent, State state) {
onDrawOver(c, parent);
}
/** * @deprecated * Override {@link #onDrawOver(Canvas, RecyclerView, RecyclerView.State)} */
@Deprecated
public void onDrawOver(Canvas c, RecyclerView parent) {
}
/** * @deprecated * Use {@link #getItemOffsets(Rect, View, RecyclerView, State)} */
@Deprecated
public void getItemOffsets(Rect outRect, int itemPosition, RecyclerView parent) {
outRect.set(0, 0, 0, 0);
}
/** * Retrieve any offsets for the given item. Each field of <code>outRect</code> specifies * the number of pixels that the item view should be inset by, similar to padding or margin. * The default implementation sets the bounds of outRect to 0 and returns. * * <p> * If this ItemDecoration does not affect the positioning of item views, it should set * all four fields of <code>outRect</code> (left, top, right, bottom) to zero * before returning. * * <p> * If you need to access Adapter for additional data, you can call * {@link RecyclerView#getChildAdapterPosition(View)} to get the adapter position of the * View. * * @param outRect Rect to receive the output. * @param view The child view to decorate * @param parent RecyclerView this ItemDecoration is decorating * @param state The current state of RecyclerView. */
public void getItemOffsets(Rect outRect, View view, RecyclerView parent, State state) {
getItemOffsets(outRect, ((LayoutParams) view.getLayoutParams()).getViewLayoutPosition(),
parent);
}
}像我们以前用ListView都是在ItemView中去写好布局，现在我们的ItemView和要装饰的UI是分离出来的，用ItemDecoration这个方法即可装饰ItemView，比如项之间的分割线、Margin、绘制颜色等。我们也可以自定义ItemDecoration引用自己绘制的UI进行使用，功能上更加强大。
