iOS：从头捋一遍VC的生命周期

一、介绍

UIViewController是iOS开发中的核心控件，没有它那基本上任何功能都无法实现，虽然系统已经做了所有控件的生命维护，但是，了解它的生命周期是如何管理还是非常有必要的。网上有很多教程，别人写的始终是别人的，自己动手实践一下，理解才能更深刻，本文就来捋一遍VC的生命周期。

二、思路

通过三个VC，第1个VC是storyBoard创建的，第2个VC是纯code创建的，第3个是xib创建的。分别称为BoardInitViewController、CodeInitViewController、XibInitViewController，这三个控制器采用导航模式进行交互，现在在VC中把所有跟初始化相关的方法都实现一下并做打印，如下：

BoardInitViewController：

//
//  BoardInitViewController.m
//  声明周期
//
//  Created by 夏远全 on 2019/11/3.
//  Copyright © 2019 Beijing Huayue Education Technology Co., Ltd. All rights reserved.
//

#import "BoardInitViewController.h"
#import "CodeInitViewController.h"

@interface BoardInitViewController ()

@end

@implementation BoardInitViewController

+(void)load {
    [super load];
    NSLog(@"boardVc---------%s",__func__);
}

+(void)initialize {
    [super initialize];
    NSLog(@"boardVc---------%s",__func__);
}

+(instancetype)alloc {
    NSLog(@"boardVc---------%s",__func__);
    return [super alloc];
}

- (nullable instancetype)initWithCoder:(NSCoder *)coder {
    NSLog(@"boardVc---------%s",__func__);
    return [super initWithCoder:coder];
}

- (instancetype)initWithNibName:(nullable NSString *)nibNameOrNil bundle:(nullable NSBundle *)nibBundleOrNil {
    NSLog(@"boardVc---------%s",__func__);
    return [super initWithNibName:nibNameOrNil bundle:nibBundleOrNil];
}

-(instancetype)init {
    NSLog(@"boardVc---------%s",__func__);
    return [super init];
}

- (void)loadView {
    [super loadView];
    NSLog(@"boardVc---------%s",__func__);
}

- (void)viewDidLoad {
    [super viewDidLoad];
    self.view.backgroundColor = [UIColor greenColor];
    self.title = @"boardVc";
    NSLog(@"boardVc---------%s",__func__);
}

- (void)viewWillAppear:(BOOL)animated  {
    [super viewWillAppear:animated];
    NSLog(@"boardVc---------%s",__func__);
}

- (void)viewDidAppear:(BOOL)animated {
    [super viewDidAppear:animated];
    NSLog(@"boardVc---------%s",__func__);
}

- (void)viewWillDisappear:(BOOL)animated {
    [super viewWillDisappear:animated];
    NSLog(@"boardVc---------%s",__func__);
}

- (void)viewDidDisappear:(BOOL)animated {
    [super viewDidDisappear:animated];
    NSLog(@"boardVc---------%s",__func__);
}

- (void)viewWillLayoutSubviews {
    [super viewWillLayoutSubviews];
    NSLog(@"boardVc---------%s",__func__);
}

- (void)viewDidLayoutSubviews {
    [super viewDidLayoutSubviews];
    NSLog(@"boardVc---------%s",__func__);
}

- (void)didReceiveMemoryWarning {
    [super didReceiveMemoryWarning];
    NSLog(@"boardVc---------%s",__func__);
}

-(void)dealloc {
    NSLog(@"boardVc---------%s",__func__);
}

-(void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event {

    CodeInitViewController *codeVc = [[CodeInitViewController alloc] init];
    [self.navigationController pushViewController:codeVc animated:YES];
}

@end

CodeInitViewController：

//
//  CodeInitViewController.m
//  声明周期
//
//  Created by 夏远全 on 2019/11/3.
//  Copyright © 2019 Beijing Huayue Education Technology Co., Ltd. All rights reserved.
//

#import "CodeInitViewController.h"
#import "XibInitViewController.h"

@interface CodeInitViewController ()

@end

@implementation CodeInitViewController

+(void)load {
    [super load];
    NSLog(@"codeVc---------%s",__func__);
}

+(void)initialize {
    [super initialize];
    NSLog(@"codeVc---------%s",__func__);
}

+(instancetype)alloc {
    NSLog(@"codeVc---------%s",__func__);
    return [super alloc];
}

- (nullable instancetype)initWithCoder:(NSCoder *)coder {
    NSLog(@"codeVc---------%s",__func__);
    return [super initWithCoder:coder];
}

- (instancetype)initWithNibName:(nullable NSString *)nibNameOrNil bundle:(nullable NSBundle *)nibBundleOrNil {
    NSLog(@"codeVc---------%s",__func__);
    return [super initWithNibName:nibNameOrNil bundle:nibBundleOrNil];
}

-(instancetype)init {
    NSLog(@"codeVc---------%s",__func__);
    return [super init];
}

- (void)loadView {
    [super loadView];
    NSLog(@"codeVc---------%s",__func__);
}

- (void)viewDidLoad {
    [super viewDidLoad];
    self.view.backgroundColor = [UIColor greenColor];
    self.title = @"codeVc";
    NSLog(@"codeVc---------%s",__func__);
}

- (void)viewWillAppear:(BOOL)animated  {
    [super viewWillAppear:animated];
    NSLog(@"codeVc---------%s",__func__);
}

- (void)viewDidAppear:(BOOL)animated {
    [super viewDidAppear:animated];
    NSLog(@"codeVc---------%s",__func__);
}

- (void)viewWillDisappear:(BOOL)animated {
    [super viewWillDisappear:animated];
    NSLog(@"codeVc---------%s",__func__);
}

- (void)viewDidDisappear:(BOOL)animated {
    [super viewDidDisappear:animated];
    NSLog(@"codeVc---------%s",__func__);
}

- (void)viewWillLayoutSubviews {
    [super viewWillLayoutSubviews];
    NSLog(@"codeVc---------%s",__func__);
}

- (void)viewDidLayoutSubviews {
    [super viewDidLayoutSubviews];
    NSLog(@"codeVc---------%s",__func__);
}

- (void)didReceiveMemoryWarning {
    [super didReceiveMemoryWarning];
    NSLog(@"codeVc---------%s",__func__);
}

-(void)dealloc {
    NSLog(@"codeVc---------%s",__func__);
}

-(void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event {

    //通过xib创建
    XibInitViewController *xibVc = [[XibInitViewController alloc] init];
    [self.navigationController pushViewController:xibVc animated:YES];
}

@end

XibInitViewController：

//
//  XibInitViewController.m
//  声明周期
//
//  Created by 夏远全 on 2019/11/3.
//  Copyright © 2019 Beijing Huayue Education Technology Co., Ltd. All rights reserved.
//

#import "XibInitViewController.h"

@interface XibInitViewController ()

@end

@implementation XibInitViewController

+(void)load {
    [super load];
    NSLog(@"xibVc---------%s",__func__);
}

+(void)initialize {
    [super initialize];
    NSLog(@"xibVc---------%s",__func__);
}

+(instancetype)alloc {
    NSLog(@"xibVc---------%s",__func__);
    return [super alloc];
}

- (nullable instancetype)initWithCoder:(NSCoder *)coder {
    NSLog(@"xibVc---------%s",__func__);
    return [super initWithCoder:coder];
}

- (instancetype)initWithNibName:(nullable NSString *)nibNameOrNil bundle:(nullable NSBundle *)nibBundleOrNil {
    NSLog(@"xibVc---------%s",__func__);
    return [super initWithNibName:nibNameOrNil bundle:nibBundleOrNil];
}

-(instancetype)init {
    NSLog(@"xibVc---------%s",__func__);
    return [super init];
}

- (void)loadView {
    [super loadView];
    NSLog(@"xibVc---------%s",__func__);
}

- (void)viewDidLoad {
    [super viewDidLoad];
    self.view.backgroundColor = [UIColor blueColor];
    self.title = @"xibVc";
    NSLog(@"xibVc---------%s",__func__);
}

- (void)viewWillAppear:(BOOL)animated  {
    [super viewWillAppear:animated];
    NSLog(@"xibVc---------%s",__func__);
}

- (void)viewDidAppear:(BOOL)animated {
    [super viewDidAppear:animated];
    NSLog(@"xibVc---------%s",__func__);
}

- (void)viewWillDisappear:(BOOL)animated {
    [super viewWillDisappear:animated];
    NSLog(@"xibVc---------%s",__func__);
}

- (void)viewDidDisappear:(BOOL)animated {
    [super viewDidDisappear:animated];
    NSLog(@"xibVc---------%s",__func__);
}

- (void)viewWillLayoutSubviews {
    [super viewWillLayoutSubviews];
    NSLog(@"xibVc---------%s",__func__);
}

- (void)viewDidLayoutSubviews {
    [super viewDidLayoutSubviews];
    NSLog(@"xibVc---------%s",__func__);
}

- (void)didReceiveMemoryWarning {
    [super didReceiveMemoryWarning];
    NSLog(@"xibVc---------%s",__func__);
}

-(void)dealloc {
    NSLog(@"xibVc---------%s",__func__);
}

@end

三、注解

 +load: 程序启动后，在系统的main函数调用之前，系统就会加载所有的load方法，提前进行一些资源包的配置或者hook，(可以打断点看看结果，本人亲测过)

 +initialize: 当前类或者其子类未被初始化过时会首次调用，若以后当前类或者子类再多次初始化时不会再调用，一般提前为初始化做一些工作

 +alloc: 系统为当前类分配内存时调用，在C语言中就是malloc这一步

 -initWithCoder: 通过storyBoard方式实例化的vc，需要经过反序列化，这个方法会被调用

 -initWithNibName:bundle: 通过xib文件或者init方法实例化的vc，这个方法都会被调用，其实init方法最终都会走该方法

 -init: 通过纯代码实例化Vc会调用，其最终会调用initWithNibName:bundle:方法

 -loadView: 实例化Vc后，可以加载一些系统常规的View

 -viewDidLoad: 一般加载自定义的view或者初始化属性，视图加载完毕后会调用

 -viewWillAppear: 视图即将出现会调用

 -viewWillDisappear: 视图即将消失会调用

 -viewWillLayoutSubviews: 视图加载完毕后即将要布局

 -viewDidLayoutSubviews: 视图加载完毕后布局也完成了

 -didReceiveMemoryWarning: 加载视图时，内存消耗太大，出现内存警告，会调用

 -dealloc: 实例化被销毁，进行内存的回收会调用

四、演示

[1] 程序启动，加载storyBoard实例化BoardInitViewController

-- ::45.724860+ 声明周期[:] codeVc---------+[CodeInitViewController load]
-- ::45.725520+ 声明周期[:] boardVc---------+[BoardInitViewController load]
-- ::45.725713+ 声明周期[:] xibVc---------+[XibInitViewController load]
-- ::45.875841+ 声明周期[:] boardVc---------+[BoardInitViewController initialize]
-- ::45.876016+ 声明周期[:] boardVc---------+[BoardInitViewController alloc]
-- ::45.876132+ 声明周期[:] boardVc----------[BoardInitViewController initWithCoder:]
-- ::45.932805+ 声明周期[:] boardVc----------[BoardInitViewController loadView]
-- ::45.935257+ 声明周期[:] boardVc----------[BoardInitViewController viewDidLoad]
-- ::45.935932+ 声明周期[:] boardVc----------[BoardInitViewController viewWillAppear:]
-- ::45.938655+ 声明周期[:] boardVc----------[BoardInitViewController viewWillLayoutSubviews]
-- ::45.938840+ 声明周期[:] boardVc----------[BoardInitViewController viewDidLayoutSubviews]
-- ::45.960117+ 声明周期[:] boardVc----------[BoardInitViewController viewDidAppear:]

[2]  从BoardInitViewController中push到纯代码实例化的CodeInitViewController中

-- ::21.027613+ 声明周期[:] codeVc---------+[CodeInitViewController initialize]
-- ::21.027858+ 声明周期[:] codeVc---------+[CodeInitViewController alloc]
-- ::21.027986+ 声明周期[:] codeVc----------[CodeInitViewController init]
-- ::21.028089+ 声明周期[:] codeVc----------[CodeInitViewController initWithNibName:bundle:]
-- ::21.032537+ 声明周期[:] codeVc----------[CodeInitViewController loadView]
-- ::21.032821+ 声明周期[:] codeVc----------[CodeInitViewController viewDidLoad]
-- ::21.032967+ 声明周期[:] boardVc----------[BoardInitViewController viewWillDisappear:]
-- ::21.033095+ 声明周期[:] codeVc----------[CodeInitViewController viewWillAppear:]
-- ::21.054149+ 声明周期[:] codeVc----------[CodeInitViewController viewWillLayoutSubviews]
-- ::21.054325+ 声明周期[:] codeVc----------[CodeInitViewController viewDidLayoutSubviews]
-- ::21.581912+ 声明周期[:] boardVc----------[BoardInitViewController viewDidDisappear:]
-- ::21.582262+ 声明周期[:] codeVc----------[CodeInitViewController viewDidAppear:]

[3]  从CodeInitViewController中push到Xib实例化的XibInitViewController中

-- ::12.506639+ 声明周期[:] codeVc---------+[CodeInitViewController alloc]
-- ::12.506831+ 声明周期[:] codeVc----------[CodeInitViewController init]
-- ::12.507020+ 声明周期[:] codeVc----------[CodeInitViewController initWithNibName:bundle:]
-- ::12.508456+ 声明周期[:] codeVc----------[CodeInitViewController loadView]
-- ::12.509038+ 声明周期[:] codeVc----------[CodeInitViewController viewDidLoad]
-- ::12.509353+ 声明周期[:] boardVc----------[BoardInitViewController viewWillDisappear:]
-- ::12.509780+ 声明周期[:] codeVc----------[CodeInitViewController viewWillAppear:]
-- ::12.520785+ 声明周期[:] codeVc----------[CodeInitViewController viewWillLayoutSubviews]
-- ::12.521002+ 声明周期[:] codeVc----------[CodeInitViewController viewDidLayoutSubviews]
-- ::13.023817+ 声明周期[:] boardVc----------[BoardInitViewController viewDidDisappear:]
-- ::13.024016+ 声明周期[:] codeVc----------[CodeInitViewController viewDidAppear:]

[4]  将XibInitViewController进行Pop后

-- ::55.287065+ 声明周期[:] xibVc----------[XibInitViewController viewWillDisappear:]
-- ::55.287238+ 声明周期[:] codeVc----------[CodeInitViewController viewWillAppear:]
-- ::55.803065+ 声明周期[:] xibVc----------[XibInitViewController viewDidDisappear:]
-- ::55.803504+ 声明周期[:] codeVc----------[CodeInitViewController viewDidAppear:]
-- ::55.803857+ 声明周期[:] xibVc----------[XibInitViewController dealloc]

[5] 将CodeInitViewController进行Pop后

-- ::07.038091+ 声明周期[:] codeVc----------[CodeInitViewController viewWillDisappear:]
-- ::07.038281+ 声明周期[:] boardVc----------[BoardInitViewController viewWillAppear:]
-- ::07.543910+ 声明周期[:] codeVc----------[CodeInitViewController viewDidDisappear:]
-- ::07.544089+ 声明周期[:] boardVc----------[BoardInitViewController viewDidAppear:]
-- ::07.544236+ 声明周期[:] codeVc----------[CodeInitViewController dealloc]

四、总结

• 程序一启动，系统在当前应用程序的所有方法被调用之前优先加载load方法；
• 控制器生命的正常流程是：初始化--->加载视图--->将要显示---->布局子视图---->完全显示---->将要显示---->完全消失----->对象销毁（有可能内存警告释放）
• 控制器交互的前后显隐流程是交叉的，在源VC即将消失和目的VC即将显示之后，目的VC会完成所有子视图的布局，然后源VC才会真正消失目的VC真正显示
• 控制器交互消失的流程也是交叉的，目的VC即将消失，源VC即将显示，目的VC完全消失，源VC完全显示，接着释放目的VC
• 通过stroyBoard创建的VC一定需要反序列化
• 通过纯代码和xib创建的VC，最终都会调用initWithNibName:bundle:方法