Iphone 加速或优化持久drawrect

我使用下面的代码为手指绘画类型练习创建一个持久的DrawRect

当我在模拟器上运行时，它是好的，当我在iPad上运行时，它会以锯齿线延迟转弯。有人能帮助优化或建议更好的方法吗

提前谢谢

 - (void)drawRect:(CGRect)rect
 {
 UIGraphicsPushContext(drawingContext);
 CGImageRef cgImage = CGBitmapContextCreateImage(drawingContext); 
 image = [[UIImage alloc] initWithCGImage:cgImage];

 if (draw) {
 CGContextSetLineWidth(drawingContext, (2.0 * size) );
 CGContextSetStrokeColorWithColor(drawingContext, color);
 CGContextSetLineCap(drawingContext,kCGLineCapRound);
 CGContextMoveToPoint(drawingContext,    lastPt.x - (31.0 / self.transform.a),  lastPt.y - (31.0 / self.transform.a)  );
 CGContextAddLineToPoint(drawingContext, currPt.x - (31.0 / self.transform.a),  currPt.y - (31.0 / self.transform.a)  );
 CGContextStrokePath(drawingContext);
 } else {
 //                     CGContextSetFillColorWithColor(drawingContext, [[UIColor redColor] CGColor]);
 //                     CGContextFillRect(drawingContext,    CGRectMake( currPt.x - (31.0 / self.transform.a) - 16.0 , currPt.y - (31.0 / self.transform.a) - 16.0 , 40.0 , 40.0) );
 CGContextClearRect(drawingContext, CGRectMake( currPt.x - (31.0 / self.transform.a) - 16.0 , currPt.y - (31.0 / self.transform.a) - 16.0 , 40.0 , 40.0) );
 }

 UIGraphicsPopContext();
 CGImageRelease(cgImage);
 NSLog(@"    %@   "  ,  rect);
 [image drawInRect: rect];

 lastPt = currPt;
 }

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我发现把电话从

[self setNeedsDisplay]

到

根据仪器从1fps变为52fps

[self setNeedsDisplayInRect]