方法一：设置BarButtonItem的文本样式为透明颜色，代码如下：
[[UIBarButtonItem appearance] setTitleTextAttributes:@{NSForegroundColorAttributeName: [UIColor clearColor]} forState:UIControlStateNormal];
[[UIBarButtonItem appearance] setTitleTextAttributes:@{NSForegroundColorAttributeName: [UIColor clearColor]} forState:UIControlStateHighlighted];
此外这种方法会导致title不能居中，被偏移很多，如下所示（虽然不被显示，也占了导航栏左边很大一部分位置）
方法二：给UIViewController添加类别，然后在load方法里面用Method Swzilling方法替换 交换ViewDidAppear，部分代码如下
+(void)load {
swizzleMethod([self class], @selector(viewDidAppear:), @selector(ac_viewDidAppear));
}
- (void)ac_viewDidAppear{
self.navigationItem.backBarButtonItem = [[UIBarButtonItem alloc]
initWithTitle:@""
style:UIBarButtonItemStylePlain
target:self
action:nil];
[self ac_viewDidAppear];
}
void swizzleMethod(Class class, SEL originalSelector, SEL swizzledSelector)
{
// the method might not exist in the class, but in its superclass
Method originalMethod = class_getInstanceMethod(class, originalSelector);
Method swizzledMethod = class_getInstanceMethod(class, swizzledSelector);
// class_addMethod will fail if original method already exists
BOOL didAddMethod = class_addMethod(class, originalSelector, method_getImplementation(swizzledMethod), method_getTypeEncoding(swizzledMethod));
// the method doesn't exist and we just added one
if (didAddMethod) {
class_replaceMethod(class, swizzledSelector, method_getImplementation(originalMethod), method_getTypeEncoding(originalMethod));
}
else {
method_exchangeImplementations(originalMethod, swizzledMethod);
}
}
注意事项：
要给整个backButtonItem赋值才可以，
