IOS 子类对父类的属性修改必须在viewDidLoad函数中进行，否则该属性值将为nil

Question

我是弄了一个有公用组件的viewcontroller类作为基类，其他使用到该组件的类作为其子类。但是IOS 子类对父类的属性修改必须在viewDidLoad函数中进行，否则该属性值将为nil。
不知道是什么原因所致，求各位大大帮忙解决下
```object-c
//ViewController作为基类
ViewController.h

import <UIKit/UIKit.h>

define kWidth [UIScreen mainScreen].bounds.size.width

define kHeight [UIScreen mainScreen].bounds.size.height

@interface ViewController : UIViewController

@property (nonatomic,strong)UINavigationItem *navi;
@property (nonatomic,strong)UIButton *barButton;
@property (strong,nonatomic)UIButton *bButton;

-(void)goBack:(id)sender;
-(void)addChose:(id)sender;
@end
ViewController.m

import "ViewController.h"

import "SubViewController.h"

define kWidth [UIScreen mainScreen].bounds.size.width

define kHeight [UIScreen mainScreen].bounds.size.height

@interface ViewController ()

@end
@implementation ViewController

• (void)viewDidLoad {
  [super viewDidLoad];
  // Do any additional setup after loading the view, typically from a nib.
  [self initView];
  }

• (void)didReceiveMemoryWarning {
  [super didReceiveMemoryWarning];
  // Dispose of any resources that can be recreated.
  }
  -(void)initView{
  self.view.backgroundColor = [UIColor whiteColor];
  UIImageView *bgImage = [[UIImageView alloc] initWithFrame:[[UIScreen mainScreen] applicationFrame]];
  bgImage.image = [UIImage imageNamed:@"BG1.jpg"];
  bgImage.alpha = 1;
  [self.view addSubview:bgImage];

  UINavigationBar *naviBar = [[UINavigationBar alloc] initWithFrame:CGRectMake(0,20,kWidth,44)];
  //naviBar.barTintColor = [UIColor colorWithRed:1.0f green:1.0f blue:1.0f alpha:0.39f];
  _navi = [[UINavigationItem alloc] initWithTitle:@"personBlog"];

  _barButton = [UIButton buttonWithType:UIButtonTypeCustom];
  _barButton.frame = CGRectMake(0,0,38,38);
  [_barButton setImage:[UIImage imageNamed:@"btn_back_normal@2x.png"] forState:UIControlStateNormal];
  [_barButton addTarget:self action:@selector(goBack:) forControlEvents:UIControlEventTouchUpInside];
  UIBarButtonItem *leftButton = [[UIBarButtonItem alloc] initWithCustomView:_barButton];
  _navi.leftBarButtonItem = leftButton;

  _bButton = [UIButton buttonWithType:UIButtonTypeCustom];
  _bButton.frame = CGRectMake(0,0,38,38);
  [_bButton setImage:[UIImage imageNamed:@"btn_other_normal@2x.png"] forState:UIControlStateNormal];
  [_bButton addTarget:self action:@selector(addChose:) forControlEvents:UIControlEventTouchUpInside];
  UIBarButtonItem *rightButton = [[UIBarButtonItem alloc] initWithCustomView:_bButton];
  _navi.rightBarButtonItem = rightButton;

  [naviBar pushNavigationItem:_navi animated:NO];
  [self.view addSubview:naviBar];

}
-(void)goBack:(id)sender{
[self dismissViewControllerAnimated:YES completion:nil];
}
-(void)addChose:(id)sender{
SubViewController * sub = [[SubViewController alloc]init];
[self presentViewController:sub animated:YES completion:nil];
}
@end

//SubViewController.h作为子类
SubviewController.h

import "ViewController.h"

@interface SubViewController : ViewController
@end
SubviewController.m

import "SubViewController.h"

define kWidth [UIScreen mainScreen].bounds.size.width

define kHeight [UIScreen mainScreen].bounds.size.height

@interface SubViewController ()

@end

@implementation SubViewController

• (void)viewDidLoad {
  [super viewDidLoad];
  NSLog(@"%f,%f",kWidth,kHeight);
  [self.navi setTitle:@"Subview"];
  UIButton *helloButton = [[UIButton alloc]initWithFrame:CGRectMake(0,64,kWidth,kHeight)];
  [helloButton setTitle:@"subview" forState:UIControlStateNormal];
  [helloButton setTitleColor:[UIColor redColor]forState:UIControlStateNormal];
  [helloButton setBackgroundColor:[UIColor whiteColor]];

  UIButton * innerButton = [[UIButton alloc]initWithFrame:CGRectMake(0,0,kWidth,110)];
  [innerButton setTitle:@"hello,innerButton"forState:UIControlStateNormal];
  [innerButton setTitleColor:[UIColor redColor]forState:UIControlStateNormal];
  [innerButton setBackgroundColor:[UIColor blueColor]];

  [helloButton addSubview:innerButton];
  // UIButton * small = [[UIButton alloc]initWithFrame:CGRectMake(0,0,)]

  [self.view addSubview:helloButton];
  [self.bButton setImage:[UIImage imageNamed:@""]forState:UIControlStateNormal];
  }
  将属性修改放在viewDidLoad函数中进行，结果是正确的。如图所示：
  

如果将属性修改作为一个函数放在外面就会出现错误结果，代码如下，结果如图所示：
SubviewController.m

• (void)viewDidLoad {
  [super viewDidLoad];
  [self initView];
  NSLog(@"%f,%f",kWidth,kHeight);
  // [self.navi setTitle:@"Subview"];
  // UIButton *helloButton = [[UIButton alloc]initWithFrame:CGRectMake(0,64,kWidth,kHeight)];
  // [helloButton setTitle:@"subview" forState:UIControlStateNormal];
  // [helloButton setTitleColor:[UIColor redColor]forState:UIControlStateNormal];
  // [helloButton setBackgroundColor:[UIColor whiteColor]];
  //
  // UIButton * innerButton = [[UIButton alloc]initWithFrame:CGRectMake(0,0,kWidth,110)];
  // [innerButton setTitle:@"hello,innerButton"forState:UIControlStateNormal];
  // [innerButton setTitleColor:[UIColor redColor]forState:UIControlStateNormal];
  // [innerButton setBackgroundColor:[UIColor blueColor]];
  //
  // [helloButton addSubview:innerButton];
  // // UIButton * small = [[UIButton alloc]initWithFrame:CGRectMake(0,0,)]
  //
  // [self.view addSubview:helloButton];
  // [self.bButton setImage:[UIImage imageNamed:@""]forState:UIControlStateNormal];
  }
  -(void)initView{
  [self.navi setTitle:@"Subview"];
  UIButton *helloButton = [[UIButton alloc]initWithFrame:CGRectMake(0,64,kWidth,kHeight-110)];
  [helloButton setTitle:@"subview" forState:UIControlStateNormal];
  [helloButton setTitleColor:[UIColor redColor]forState:UIControlStateNormal];
  [helloButton setBackgroundColor:[UIColor whiteColor]];

  UIButton * innerButton = [[UIButton alloc]initWithFrame:CGRectMake(0,0,kWidth,110)];
  [innerButton setTitle:@"hello,innerButton"forState:UIControlStateNormal];
  [innerButton setTitleColor:[UIColor redColor]forState:UIControlStateNormal];
  [innerButton setBackgroundColor:[UIColor blueColor]];

  [helloButton addSubview:innerButton];
  // UIButton * small = [[UIButton alloc]initWithFrame:CGRectMake(0,0,)]

  [self.view addSubview:helloButton];
  [self.bButton setImage:[UIImage imageNamed:@""]forState:UIControlStateNormal];
  }

加断点调试发现：self.navi = nil，不知道原因，调试截图如下：

Answer 1

问题的原因是因为我的”属性“其实是局部变量，在函数的内部的变量，在函数外面就变为nil了。

Answer 2

对父类属性的修改肯定子类的任何方法里面都可以进行更改啊。你可以参照一下ViewController的生命周期，看看你获取属性时，你重载的方法是否已经执行了。要不然你不会是忘了调用super xxxmethod了吧

Answer 3

SubviewController中的-(void)initView 方法名不要和父类的-(void)initView 方法重名。

Answer 4

这是多态写法。子类调用[self initView] 不会执行父类的initView，你需要[super initView]