BRVAH分组功能原理分析-原创手记-慕课网

最近在diycode社区遇到一位同学提问，所以特写此文章来分析BRVAH分组功能的实现。如果还什么疑问都可以在这里进行提问因为开源项目和技术分享收到 Google 的面试邀请，大家有什么想要讨论的么？

问题分析的步骤：

如何使用
原理分析

如何该框架的分组功能

Adapter:

public class SectionAdapter extends BaseSectionQuickAdapter<MySection> { 

public SectionAdapter(int layoutResId, int sectionHeadResId, List data) { 
    super(layoutResId, sectionHeadResId, data); 
} 
@Override 
protected void convert(BaseViewHolder helper, MySection item) { 
    helper.setImageUrl(R.id.iv, (String) item.t); 
} 
@Override 
protected void convertHead(BaseViewHolder   helper,final MySection item) {
     helper.setText(R.id.header, item.header); 
 }

adapter的构造需要传入三个参数，分别是内容的布局和头部的布局和数据源，数据源需要继承SectionEntity如下：

Entity:

public class MySection extends SectionEntity<Video> {    
public MySection(boolean isHeader, String header, boolean isMroe) {        
    super(isHeader, header);        
}    
public MySection(Video t) {        
    super(t);    
}    
}

填充数据

public static List<MySection> getSampleData() {
    List<MySection> list = new ArrayList<>();
    list.add(new MySection(true, "Section 1"));
    list.add(new MySection(new Video(HTTPS_AVATARS1_GITHUBUSERCONTENT_COM_LINK, CYM_CHAD)));    
    list.add(new MySection(new Video(HTTPS_AVATARS1_GITHUBUSERCONTENT_COM_LINK, CYM_CHAD)));    
    list.add(new MySection(new Video(HTTPS_AVATARS1_GITHUBUSERCONTENT_COM_LINK, CYM_CHAD)));    
    list.add(new MySection(new Video(HTTPS_AVATARS1_GITHUBUSERCONTENT_COM_LINK, CYM_CHAD)));    
    list.add(new MySection(true, "Section 2"));    
    list.add(new MySection(new Video(HTTPS_AVATARS1_GITHUBUSERCONTENT_COM_LINK, CYM_CHAD)));    
    list.add(new MySection(new Video(HTTPS_AVATARS1_GITHUBUSERCONTENT_COM_LINK, CYM_CHAD)));    
    list.add(new MySection(new Video(HTTPS_AVATARS1_GITHUBUSERCONTENT_COM_LINK, CYM_CHAD)));    
    list.add(new MySection(true, "Section 3"));    
    list.add(new MySection(new Video(HTTPS_AVATARS1_GITHUBUSERCONTENT_COM_LINK, CYM_CHAD)));    
    list.add(new MySection(new Video(HTTPS_AVATARS1_GITHUBUSERCONTENT_COM_LINK, CYM_CHAD)));    
    list.add(new MySection(true, "Section 4"));    
    list.add(new MySection(new Video(HTTPS_AVATARS1_GITHUBUSERCONTENT_COM_LINK, CYM_CHAD)));    
    list.add(new MySection(new Video(HTTPS_AVATARS1_GITHUBUSERCONTENT_COM_LINK, CYM_CHAD)));    
    list.add(new MySection(true, "Section 5"));    
    list.add(new MySection(new Video(HTTPS_AVATARS1_GITHUBUSERCONTENT_COM_LINK, CYM_CHAD)));    
    list.add(new MySection(new Video(HTTPS_AVATARS1_GITHUBUSERCONTENT_COM_LINK, CYM_CHAD)));    
    return list;
}

原理分析

其实头部和内容部分就是通过不同的type来实现的，我们可以查看BaseSectionQuickAdapter源码

    @Override
    protected int getDefItemViewType(int position) {
        return ((SectionEntity) mData.get(position)).isHeader ? SECTION_HEADER_VIEW : 0;
    }

它是通过SectionEntity的isHeader属性来区别是否是头部的

public abstract class SectionEntity<T> {    
    public boolean isHeader;    
    public T t;    
    public String header;    
    public SectionEntity(boolean isHeader, String header) {        
        this.isHeader = isHeader;        
        this.header = header;        
        this.t = null;    
    }    
    public SectionEntity(T t) {        
        this.isHeader = false;        
        this.header = null;        
        this.t = t;    
    }
}

这就是为什么要求开发者的实体类必须继承SectionEntity的原因了，因为需要通过它的isHeader这个属性来改变type，onCreateViewHolder通过不同的type来加载不同的布局。

@Override
protected BaseViewHolder onCreateDefViewHolder(ViewGroup parent, int viewType) {    
    if (viewType == SECTION_HEADER_VIEW)        
        return new BaseViewHolder(getItemView(mSectionHeadResId, parent));    

    return super.onCreateDefViewHolder(parent, viewType);
}

然后在onBindViewHolder里面通过type来区分头部和内容部分调用不同的方法

protected void convert(BaseViewHolder holder, Object item) {    
    switch (holder.getItemViewType()) {        
        case SECTION_HEADER_VIEW:            
        setFullSpan(holder);            
        convertHead(holder, (T) item);            
        break;        
        default:            
        convert(holder, (T) item);            
        break;    
    }
}

protected abstract void convertHead(BaseViewHolder helper, T item);

protected abstract void convert(BaseViewHolder helper, T item);

在此有同学肯定会想这个setFullSpan方法是干嘛的呀？

因为要考虑到RecyclerView的setLayoutManager的三种不同类型的LayoutManager

LinearLayoutManager
GridLayoutManager
StaggeredGridLayoutManager

LinearLayoutManager：本身就可以占据一行，所以不需要做特殊的处理
GridLayoutManager：需要特殊处理,才能实现头部占据一行的效果，代码如下：

  @Override
  public void onAttachedToRecyclerView(RecyclerView recyclerView) {    
    super.onAttachedToRecyclerView(recyclerView);    
    RecyclerView.LayoutManager manager = recyclerView.getLayoutManager();    
    if (manager instanceof GridLayoutManager) {        
      final GridLayoutManager gridManager = ((GridLayoutManager) manager);        
      gridManager.setSpanSizeLookup(new GridLayoutManager.SpanSizeLookup() {            
        @Override            
        public int getSpanSize(int position) {                
          int type = getItemViewType(position);                
          return (type == SECTION_HEADER_VIEW) ? gridManager.getSpanCount() : 1;            
        });    
      }
  }

StaggeredGridLayoutManager：需要特殊处理,才能实现头部占据一行的效果，代码如下：

    protected void setFullSpan(RecyclerView.ViewHolder holder) {    
        if (holder.itemView.getLayoutParams() instanceof  StaggeredGridLayoutManager.LayoutParams) {        
            StaggeredGridLayoutManager.LayoutParams params = (StaggeredGridLayoutManager.LayoutParams) 
            holder.itemView.getLayoutParams();        
            params.setFullSpan(true);    
      }
    }

这就是为什么要在conver方法里判断是head类型的代码块中要调用setFullSpan方法的原因了。

如果还什么疑问都可以在这里进行提问因为开源项目和技术分享收到 Google 的面试邀请，大家有什么想要讨论的么？