调整导航条上leftBarButtonItem和rightBarButtonItem与屏幕边界的间距

UIButton *btn = [UIButton buttonWithType:UIButtonTypeRoundedRect];
/**
* 设置frame只能控制按钮的大小
*/
btn.frame= CGRectMake(0, 0, 40, 44);
[btn addTarget:self action:@selector(buttonClicked) forControlEvents:UIControlEventTouchUpInside];
UIBarButtonItem *btn_right = [[UIBarButtonItem alloc] initWithCustomView:btn];
UIBarButtonItem *negativeSpacer = [[UIBarButtonItem alloc]
initWithBarButtonSystemItem:UIBarButtonSystemItemFixedSpace
target:nil action:nil];
/**
* width为负数时，相当于btn向右移动width数值个像素，由于按钮本身和边界间距为5pix，所以width设为-5时，间距正好调整
* 为0；width为正数时，正好相反，相当于往左移动width数值个像素
*/
negativeSpacer.width = -5;
self.navigationItem.rightBarButtonItems = [NSArray arrayWithObjects:negativeSpacer, btn_right, nil];
[btn_right release];