Android中使用itemdecoration实现时间线效果

// 时间线装饰器
public class TimeLineDecoration extends RecyclerView.ItemDecoration {
  private Paint mPaint;

  public TimeLineDecoration() {
    mPaint = new Paint();
    mPaint.setStyle(Paint.Style.FILL);
    mPaint.setColor(Color.BLUE);
    mPaint.setStrokeWidth(5);
  }

  @Override
  public void onDraw(@NonNull Canvas c, @NonNull RecyclerView parent, @NonNull RecyclerView.State state) {
    super.onDraw(c, parent, state);
    RecyclerView.LayoutManager layoutManager = parent.getLayoutManager();
    // 这里的childcount为可见item的个数。 与item的个数不一定相同。
    int childCount = parent.getChildCount();
    for (int i = 0; i < childCount; i  ) {
      View child = parent.getChildAt(i);
      // 避免硬编码，这里通过代码获取在getItemOffsets方法中设置的宽度
      int leftDecoWidth = layoutManager.getLeftDecorationWidth(child);
      // 根据position获取当前的数据，然后根据数据状态绘制不同的形状
      int position = parent.getChildAdapterPosition(child);
      int cx = leftDecoWidth / 2;
      int cy = child.getTop()   child.getHeight() / 2;
      int radius = 20;
      if (position == 2) {
        c.drawRect(cx - radius, cy - radius, cx   radius, cy   radius, mPaint);
      } else if (position == 4) {
        // 绘制外圈为空心圆，内圈为实心圆
        mPaint.setStyle(Paint.Style.STROKE);
        c.drawCircle(cx, cy, radius, mPaint);
        mPaint.setStyle(Paint.Style.FILL);
        c.drawCircle(cx, cy, radius >> 1, mPaint);
      } else {
        c.drawCircle(cx, cy, radius, mPaint);
      }

      // 绘制item中间的连接线，第一个item与最后一个item的连接线需单独处理一下。
      if (position == 0) {
        c.drawLine(cx, cy   mPaint.getStrokeWidth()   radius, cx, child.getBottom(), mPaint);
      } else if (position == parent.getAdapter().getItemCount() - 1) {
        c.drawLine(cx, child.getTop(), cx, cy - mPaint.getStrokeWidth() - radius, mPaint);
      } else {
        c.drawLine(cx, cy   mPaint.getStrokeWidth()   radius, cx, child.getBottom(), mPaint);
        c.drawLine(cx, child.getTop(), cx, cy - mPaint.getStrokeWidth() - radius, mPaint);
      }
    }
  }

  @Override
  public void onDrawOver(@NonNull Canvas c, @NonNull RecyclerView parent, @NonNull RecyclerView.State state) {
    super.onDrawOver(c, parent, state);
    // 不受outRect设置的范围影响，可以绘制在item上。
  }

  @Override
  public void getItemOffsets(@NonNull Rect outRect, @NonNull View view, @NonNull RecyclerView parent, @NonNull RecyclerView.State state) {
    super.getItemOffsets(outRect, view, parent, state);
    // 在item左边留下100像素的空间。 item的布局会在减掉这100像素后处理。
    outRect.left = 100;
  }
}

RecyclerAdapter adapter = new RecyclerAdapter(this, data);
mRecyclerView.setLayoutManager(new LinearLayoutManager(this));
mRecyclerView.addItemDecoration(new TimeLineDecoration());
mRecyclerView.setAdapter(adapter);