加载一个ViewController，我们通常使用init或者initWithNibName方法初始化，但是有的时候会出现一些需要初始化的值没有得到正确的设置，下面给出代码看下实际的效果。
第一种情况：正常初始化
FatherViewController： -(instancetype)initWithNibName:(NSString *)nibNameOrNil bundle:(NSBundle *)nibBundleOrNil{ self = [super initWithNibName:nibNameOrNil bundle:nibBundleOrNil]; if(self){ NSLog(@"F_init start "); self.FPContent = @"F_我是父类初始化的"; NSLog(@"F_init end"); } return self; } - (void)viewDidLoad { [super viewDidLoad]; NSLog(@"F_View"); NSLog(@"父类的属性值：%@",self.FPContent); } SubViewController: -(instancetype)initWithNibName:(NSString *)nibNameOrNil bundle:(NSBundle *)nibBundleOrNil{ self = [super initWithNibName:nibNameOrNil bundle:nibBundleOrNil]; if(self){ NSLog(@"S_init start"); self.FPContent = @"S_我是子类初始化的"; NSLog(@"S_init end"); } return self; } - (void)viewDidLoad { [super viewDidLoad]; NSLog(@"S_View"); NSLog(@"属性值：%@",self.FPContent); }
执行结果输出如下：
2018-02-06 16:13:07.236401+0800 YYDemo[70368:2809851] F_init start
2018-02-06 16:13:07.236593+0800 YYDemo[70368:2809851] F_init end
2018-02-06 16:13:07.236878+0800 YYDemo[70368:2809851] S_init start
2018-02-06 16:13:07.236967+0800 YYDemo[70368:2809851] S_init end
2018-02-06 16:13:07.411284+0800 YYDemo[70368:2809851] F_View
2018-02-06 16:13:07.411469+0800 YYDemo[70368:2809851] 父类的属性值：S_我是子类初始化的
2018-02-06 16:13:07.411604+0800 YYDemo[70368:2809851] S_View
2018-02-06 16:13:07.411700+0800 YYDemo[70368:2809851] 属性值：S_我是子类初始化的
第二种情况：在initWithNibName中调用了viewDidLoad
FatherViewController： -(instancetype)initWithNibName:(NSString *)nibNameOrNil bundle:(NSBundle *)nibBundleOrNil{ self = [super initWithNibName:nibNameOrNil bundle:nibBundleOrNil]; if(self){ NSLog(@"F_init start "); self.FPContent = @"F_我是父类初始化的"; NSLog(@"F_init end"); } return self; } - (void)viewDidLoad { [super viewDidLoad]; NSLog(@"F_View"); NSLog(@"父类的属性值：%@",self.FPContent); } SubViewController: -(instancetype)initWithNibName:(NSString *)nibNameOrNil bundle:(NSBundle *)nibBundleOrNil{ self = [super initWithNibName:nibNameOrNil bundle:nibBundleOrNil]; if(self){ NSLog(@"S_init start"); UILabel *la = [[UILabel alloc] initWithFrame:CGRectMake(88, 20, 100, 20)]; la.text = self.FPContent; [self.view addSubview:la]; self.FPContent = @"S_我是子类初始化的"; NSLog(@"S_init end"); } return self; } - (void)viewDidLoad { [super viewDidLoad]; NSLog(@"S_View"); NSLog(@"属性值：%@",self.FPContent); }
执行结果输出如下：
2018-02-06 16:19:37.594282+0800 YYDemo[71147:2825509] F_init start
2018-02-06 16:19:37.594492+0800 YYDemo[71147:2825509] F_init end
2018-02-06 16:19:37.594592+0800 YYDemo[71147:2825509] S_init start
2018-02-06 16:19:37.718493+0800 YYDemo[71147:2825509] F_View
2018-02-06 16:19:37.718672+0800 YYDemo[71147:2825509] 父类的属性值：F_我是父类初始化的
2018-02-06 16:19:37.718773+0800 YYDemo[71147:2825509] S_View
2018-02-06 16:19:37.718869+0800 YYDemo[71147:2825509] 属性值：F_我是父类初始化的
2018-02-06 16:19:37.719246+0800 YYDemo[71147:2825509] S_init end
由以上执行结果可知：
在初始化方法总调用View属性的时候就会调用viewDidLoad方法。
viewDidLoad方法一般在视图树构建完成以后调用一次，但是如果VC的View属性被引用，View为nil 的时候也会调用viewDidLoad方法，提前调动该方法可能就会导致某些值不能得到正确的初始化。
