好久没写东西，转战简书，重装出发吧。
项目里用了两种自定义返回键，发现在Xcode9+iOS11上歇菜了，少不了折腾一番。
A. 自定义的navigationItem.backBarItem：
UIImage *backButtonImage = [UIImage imageNamed:@"icon_back"];
UIBarButtonItem *backItem = [[UIBarButtonItem alloc] init];
[backItem setBackButtonBackgroundImage:[backButtonImage resizableImageWithCapInsets:UIEdgeInsetsMake(0, backButtonImage.size.width, 0, 0)] forState:UIControlStateNormal barMetrics:UIBarMetricsDefault];
self.navigationItem.backBarButtonItem = backItem;
点进二级页就华丽的crash了，苹果就是任性。。
*** Terminating app due to uncaught exception 'NSInternalInconsistencyException',
reason: '_UIButtonBarButtonVisualProviderIOS cannot have a background without any content'
意思是必须设image or title，好吧，给它个@""。然后有个讨厌的蓝色箭头一直存在，而这货是根据tintColor来绘制的，所以你遍历bar层级把对应的view.hidden=YES没用，它还是会多次绘制展示。最终灵光一闪，把tintColor设成clearColor，so加上这两句就O了~
[backItem setTitle:@""];
backItem.tintColor = [UIColor clearColor];
当然，用leftItem+FDFullscreenPopGesture的同学可以跳过这段。
B. 自定义的navigationItem.leftBarItem
UIButton *leftButton = [UIButton buttonWithType:UIButtonTypeCustom];
[leftButton setFrame:CGRectMake(0, 0, 44, 44)];
UIBarButtonItem *negativeSpacer = [[UIBarButtonItem alloc]
initWithBarButtonSystemItem:UIBarButtonSystemItemFixedSpace
target:nil action:nil];
negativeSpacer.width = -15;
UIBarButtonItem *leftBarItem = [[UIBarButtonItem alloc] initWithCustomView:leftButton];
self.navigationItem.leftBarButtonItems = [NSArray arrayWithObjects:negativeSpacer, leftBarItem, nil];
现象是箭头距离屏幕会比较长的留白。一开始以为negativeSpacer要设成0，其实没用。只用@[leftBarItem]也一样。遍历view层级改结构也是坑。google上的 老外 说iOS11导航栏默认开启autolayout，所以想让setFrame生效，自然就是把它关了呗
leftButton.translatesAutoresizingMaskIntoConstraints = NO;
[leftButton setContentEdgeInsets:UIEdgeInsetsMake(0, -10, 0, 0)];
rightItem同理。
坑未完待续。
