这是绘制圆角矩形的代码.这是Monotouch,但任何开发人员都应该可以理解.

(您可以在这里找到完整的代码：https://github.com/Krumelur/RoundedRectView)

public override void Draw (RectangleF rect)
        {
            using (var oContext = UIGraphics.GetCurrentContext())
            {
                oContext.SetLineWidth (this.StrokeWidth);
                oContext.SetStrokeColor (this.oStrokeColor.CGColor);
                oContext.SetFillColor (this.oRectColor.CGColor);

                RectangleF oRect = this.Bounds;

                float fRadius = this.CornerRadius;
                float fWidth = oRect.Width;
                float fHeight = oRect.Height;

                // Make sure corner radius isn't larger than half the shorter side.
                if (fRadius > fWidth / 2.0f)
                {
                    fRadius = fWidth / 2.0f;
                }
                if (fRadius > fHeight / 2.0f)
                {
                    fRadius = fHeight / 2.0f;    
                }

                float fMinX = oRect.GetMinX ();
                float fMidX = oRect.GetMidX ();
                float fMaxX = oRect.GetMaxX ();
                float fMinY = oRect.GetMinY ();
                float fMidY = oRect.GetMidY ();
                float fMaxY = oRect.GetMaxY ();

                // Move to left middle.
                oContext.MoveTo (fMinX,fMidY);

                // Arc to top middle.
                oContext.AddArcToPoint (fMinX,fMinY,fMidX,(this.RoundCorners & ROUND_CORNERS.TopLeft) == ROUND_CORNERS.TopLeft ? fRadius : 0);
                // Arc to right middle.
                oContext.AddArcToPoint (fMaxX,fMaxX,fMidY,(this.RoundCorners & ROUND_CORNERS.TopRight) == ROUND_CORNERS.TopRight ? fRadius : 0);
                // Arc to bottom middle.
                oContext.AddArcToPoint (fMaxX,fMaxY,(this.RoundCorners & ROUND_CORNERS.BottomRight) == ROUND_CORNERS.BottomRight ? fRadius : 0);
                // Arc to left middle.
                oContext.AddArcToPoint (fMinX,fMinX,(this.RoundCorners & ROUND_CORNERS.BottomLeft) == ROUND_CORNERS.BottomLeft ? fRadius : 0);

                // Draw the path.
                oContext.ClosePath ();
                oContext.DrawPath (CGPathDrawingMode.FillStroke);
            }
        }

编辑：

这是一段代码,演示如何使用CALayer解决它.

private void UpdateMask()
        {
            UIBezierPath oMaskPath = UIBezierPath.FromRoundedRect (this.Bounds,this.eRoundedCorners,new SizeF (this.fCornerRadius,this.fCornerRadius));

            CAShapeLayer oMaskLayer = new CAShapeLayer ();
            oMaskLayer.Frame = this.Bounds;
            oMaskLayer.Path = oMaskPath.CGPath;
            this.Layer.Mask = oMaskLayer;
        }