iOS CGPath提升阴影性能

     发现应用在itouch4上页面左右滑动或push进一个视图控制器时，非常的卡。最后发现时因为给导航控制器的边框添加了阴影的原因。

                之前的代码：           

                        - (void)addframeleftline {

                            uicolor *color = [uicolor blackcolor];

                            cgsize offset = cgsizemake(0, 3);

                            float radius = 2;

                            float opacity = 1;

                            self.navigationcontroller.view.layer.shadowcolor = color.cgcolor;

                            self.navigationcontroller.view.layer.shadowoffset = offset;

                            self.navigationcontroller.view.layer.shadowradius = radius;

                            self.navigationcontroller.view.layer.shadowopacity = opacity;

                        }

              修改后的代码：

                        - (void)addframeleftline {

                            cgpathref shadow = cgpathcreatewithrect(cgrectinset(self.navigationcontroller.view.bounds, -5, 0), null);

                            [self.navigationcontroller.view.layer setshadowpath:shadow];

                            [self.navigationcontroller.view.layer setshadowcolor:[[uicolor blackcolor] cgcolor]];

                            [self.navigationcontroller.view.layer setshadowopacity:0.3];

                            [self.navigationcontroller.view.layer setshadowradius:2];

                            cfrelease(shadow);

                        }