viewDidLoad与initWithNibName的调用时机

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。

原文地址：https://www.cnblogs.com/yinyanlyy/p/8422971.html