实现RecyclerView粘性头部效果

RecyclerView粘性头部效果常用于分组列表，如微信账单的月份标题。实现这一效果需要结合自定义ItemDecoration和布局管理器。

核心原理

• 通过ItemDecoration判断当前组第一个可见项的位置
• 动态计算头部视图的偏移量
• 当新组头部到达旧组头部位置时进行替换

关键实现步骤

创建StickyHeaderItemDecoration类继承RecyclerView.ItemDecoration：

public class StickyHeaderItemDecoration extends RecyclerView.ItemDecoration {
    private StickyHeaderInterface listener;
    private View headerView;
    
    public StickyHeaderItemDecoration(StickyHeaderInterface listener) {
        this.listener = listener;
    }
    
    @Override
    public void onDrawOver(Canvas c, RecyclerView parent, RecyclerView.State state) {
        int topChildPos = ((LinearLayoutManager)parent.getLayoutManager())
            .findFirstVisibleItemPosition();
        if (topChildPos == RecyclerView.NO_POSITION) return;
            
        View currentHeader = getHeaderView(topChildPos, parent);
        fixLayoutSize(parent, currentHeader);
        
        int nextHeaderPos = getNextHeaderPos(topChildPos);
        if (nextHeaderPos != -1) {
            View nextHeader = getHeaderView(nextHeaderPos, parent);
            fixLayoutSize(parent, nextHeader);
            
            int overlap = nextHeader.getTop() - currentHeader.getHeight();
            if (overlap < 0) {
                c.save();
                c.translate(0, overlap);
                currentHeader.draw(c);
                c.restore();
                return;
            }
        }
        currentHeader.draw(c);
    }
}

定义粘性头部接口

创建接口用于获取头部视图和数据绑定：

public interface StickyHeaderInterface {
    int getHeaderPositionForItem(int itemPosition);
    View getHeaderView(RecyclerView recyclerView, int position);
    void bindHeaderData(View header, int position);
}

实现分组数据适配器

在RecyclerView.Adapter中实现分组逻辑：

public class BillAdapter extends RecyclerView.Adapter<RecyclerView.ViewHolder> 
    implements StickyHeaderInterface {
    
    private static final int TYPE_HEADER = 0;
    private static final int TYPE_ITEM = 1;
    private List<BillItem> data;
    
    @Override
    public int getItemViewType(int position) {
        return isPositionHeader(position) ? TYPE_HEADER : TYPE_ITEM;
    }
    
    private boolean isPositionHeader(int position) {
        // 判断是否为月份开头
    }
    
    @Override
    public View getHeaderView(RecyclerView recyclerView, int position) {
        View header = LayoutInflater.from(context)
            .inflate(R.layout.item_month_header, recyclerView, false);
        bindHeaderData(header, position);
        return header;
    }
}

处理头部平移动画

实现平滑的头部替换过渡效果：

private void handleHeaderTransition(Canvas c, View currentHeader, View nextHeader) {
    int overlap = nextHeader.getTop() - currentHeader.getHeight();
    if (overlap < 0) {
        c.save();
        c.translate(0, overlap);
        currentHeader.draw(c);
        c.restore();
    } else {
        currentHeader.draw(c);
    }
}

优化性能考虑

1. 使用ViewHolder模式缓存头部视图
2. 避免在滚动过程中频繁创建新视图
3. 对头部视图进行测量和布局的预计算
4. 使用RecyclerView的局部刷新方法

测量布局方法

private void fixLayoutSize(ViewGroup parent, View view) {
    int widthSpec = View.MeasureSpec.makeMeasureSpec(
        parent.getWidth(), View.MeasureSpec.EXACTLY);
    int heightSpec = View.MeasureSpec.makeMeasureSpec(
        parent.getHeight(), View.MeasureSpec.UNSPECIFIED);
    
    view.measure(widthSpec, heightSpec);
    view.layout(0, 0, view.getMeasuredWidth(), view.getMeasuredHeight());
}

完整集成流程

1. 准备分组数据源，确保数据已按月份排序
2. 实现StickyHeaderInterface接口方法
3. 创建RecyclerView并设置LayoutManager
4. 添加StickyHeaderItemDecoration
5. 设置适配器并绑定数据

示例初始化代码

RecyclerView recyclerView = findViewById(R.id.recycler_view);
BillAdapter adapter = new BillAdapter(data);
recyclerView.setLayoutManager(new LinearLayoutManager(this));
recyclerView.addItemDecoration(new StickyHeaderItemDecoration(adapter));
recyclerView.setAdapter(adapter);

处理边界情况

1. 空数据集处理
2. 单组数据显示逻辑
3. 快速滚动时的性能优化
4. 不同分辨率下的适配
5. 夜间模式等主题切换时的UI一致性

通过以上实现方案，可以创建出类似微信账单的平滑粘性头部效果，用户体验流畅自然。实际开发中可根据具体需求调整动画细节和交互逻辑。