文章目录
- 一、生命周期
- 二、注意事项
- 案例讲解
一、生命周期
viewController有自己的生命周期，其生命周期如下图：
init- 初始化程序；loadView- 在UIViewController对象的view被访问且为空的时候调用；viewDidLoad- 视图加载完成后调用；viewWillAppear-UIViewController对象的视图即将加入窗口时调用；viewDidAppear-UIViewController对象的视图已经加入窗口时调用；viewWillDisappear-UIViewController对象的视图即将消失时调用；viewDidDisappear-UIViewController对象的视图已经消失时调用；didReceiveMemoryWarning- 出现内存警告；dealloc- 视图被销毁，此次需要在init和viewDidLoad中创建的对象进行释放；
初始化
init方法：初始化UIViewController，执行关键数据初始化操作。不要在此操作view，view在loadView: 方法中才初始化。
加载视图
loadView: 方法是在UIViewController对象的view被访问且为空的时候调用，这个方法会加载或者创建一个view并将其赋值给view属性。在ViewController的生命周期中没有特殊情况只会被调用一次。
你可以通过重写该方法来手动初始化views。在该方法的实现中，不能调用super。viewDidLoad方法：view加载完成后调用。通常，对于各种初始化数据的载入，初始设定、修改约束、移除视图等很多操作都可以这个方法中实现。通常在此方法中添加一些控件和视图。viewWillAppear方法：系统在载入所有的数据后，将会在屏幕上显示视图，这时会先调用这个方法，通常会在这个方法对即将显示的视图做进一步的设置。比如，设置设备不同方向时该如何显示；设置状态栏方向、设置视图显示样式等。
当APP有多个视图时,上下级视图切换是也会调用这个方法，如果在调入视图时，需要对数据做更新，就只能在这个方法内实现。viewDidAppear方法：UIViewController对象的视图已经加入窗口时调用。这时可以对正在显示的视图做设置。viewWillDisAppear方法：UIViewController对象的视图即将消失、被覆盖或是隐藏时调用。viewDidDisAppear方法：UIViewController对象的视图已经消失、被覆盖或是隐藏时调用。
内存警告
didReceiveMemoryWarning方法：在内存足够的情况下，app视图通常会一直保存在内存中，如果内存不够,会释放掉自己拥有的视图。系统只会释放内存，不会释放对象的所有权，通常需要在这里将不需要显示在内存中保留的对象释放它的所有权，置为nil。
视图销毁
dealloc 方法：销毁视图，释放资源。不要手动调用此方法，当引用计数值为0的时候，系统会自动调用此方法。
二、注意事项
loadView和viewDidLoad的区别就是，loadView时view还没有生成，viewDidLoad时，view已经生成了，loadView只会被调用一次，而viewDidLoad可能会被调用多次（View可能会被多次加载），当view被添加到其他view中之前，会调用viewWillAppear，之后会调用viewDidAppear。当view从其他view中移除之前，调用viewWillDisAppear，移除之后会调用viewDidDisappear。当view不再使用时，受到内存警告时，ViewController会将view释放并将其指向为nil。
案例讲解
分为A和B两个视图。
A视图：
AViewController.h 文件：
#import <UIKit/UIKit.h>
#import "BViewController.h"
NS_ASSUME_NONNULL_BEGIN
@interface AViewController : UIViewController
@end
NS_ASSUME_NONNULL_END
AViewController.m文件：
#import "AViewController.h"
@interface AViewController ()
@end
@implementation AViewController
- (void) loadView {
[super loadView];
NSLog(@"AViewController loadView");
}
- (void)viewDidLoad {
[super viewDidLoad];
// Do any additional setup after loading the view.
self.view.backgroundColor = [UIColor whiteColor];
UIButton* button = [UIButton buttonWithType:UIButtonTypeRoundedRect];
button.frame = CGRectMake(100, 100, 100, 50);
button.center = self.view.center;
[button setTitle:@"按键" forState:UIControlStateNormal];
[button addTarget:self action:@selector(press:) forControlEvents:UIControlEventTouchUpInside];
[self.view addSubview:button];
NSLog(@"AViewController viewDidLoad");
}
- (void) viewWillAppear:(BOOL)animated {
NSLog(@"AViewController viewWillAppear");
}
- (void) viewDidAppear:(BOOL)animated {
NSLog(@"AViewController viewDidAppear");
}
- (void) viewWillDisappear:(BOOL)animated {
NSLog(@"AViewController viewWillDisappear");
}
- (void) viewDidDisappear:(BOOL)animated {
NSLog(@"AViewController viewDidDisappear");
}
- (void) didReceiveMemoryWarning {
[super didReceiveMemoryWarning];
NSLog(@"AViewController didReceiveMemoryWarning");
}
- (void) press:(UIButton*)sender {
BViewController* bViewController = [[BViewController alloc] init];
bViewController.modalPresentationStyle = UIModalPresentationFullScreen;
NSLog(@"A->B");
[self presentViewController:bViewController animated:YES completion:nil];
}
@end
BViewController.h文件：
#import <UIKit/UIKit.h>
NS_ASSUME_NONNULL_BEGIN
@interface BViewController : UIViewController
@end
NS_ASSUME_NONNULL_END
BViewController.m文件：
#import "BViewController.h"
@interface BViewController ()
@end
@implementation BViewController
- (void) loadView {
[super loadView];
NSLog(@"BViewController loadView");
}
- (void)viewDidLoad {
[super viewDidLoad];
// Do any additional setup after loading the view.
self.view.backgroundColor = [UIColor yellowColor];
UIButton* button = [UIButton buttonWithType:UIButtonTypeRoundedRect];
button.frame = CGRectMake(100, 100, 100, 50);
button.center = self.view.center;
[button setTitle:@"按键" forState:UIControlStateNormal];
[button addTarget:self action:@selector(press:) forControlEvents:UIControlEventTouchUpInside];
[self.view addSubview:button];
NSLog(@"BViewController viewDidLoad");
}
- (void) viewWillAppear:(BOOL)animated {
NSLog(@"BViewController viewWillAppear");
}
- (void) viewDidAppear:(BOOL)animated {
NSLog(@"BViewController viewDidAppear");
}
- (void) viewWillDisappear:(BOOL)animated {
NSLog(@"BViewController viewWillDisappear");
}
- (void) viewDidDisappear:(BOOL)animated {
NSLog(@"BViewController viewDidDisappear");
}
- (void) press:(UIButton*)sender {
NSLog(@"B->A");
[self dismissViewControllerAnimated:YES completion:nil];
}
@end
运行结果：
2022-09-18 18:34:13.549033+0800 ViewController[25007:24031989] AViewController loadView
2022-09-18 18:34:13.554447+0800 ViewController[25007:24031989] AViewController viewDidLoad
2022-09-18 18:34:13.563963+0800 ViewController[25007:24031989] AViewController viewWillAppear
2022-09-18 18:34:13.697255+0800 ViewController[25007:24031989] AViewController viewDidAppear
2022-09-18 18:34:16.421472+0800 ViewController[25007:24031989] A->B
2022-09-18 18:34:16.422527+0800 ViewController[25007:24031989] BViewController loadView
2022-09-18 18:34:16.423160+0800 ViewController[25007:24031989] BViewController viewDidLoad
2022-09-18 18:34:16.428482+0800 ViewController[25007:24031989] AViewController viewWillDisappear
2022-09-18 18:34:16.428732+0800 ViewController[25007:24031989] BViewController viewWillAppear
2022-09-18 18:34:16.933891+0800 ViewController[25007:24031989] BViewController viewDidAppear
2022-09-18 18:34:16.934169+0800 ViewController[25007:24031989] AViewController viewDidDisappear
2022-09-18 18:34:31.586055+0800 ViewController[25007:24031989] B->A
2022-09-18 18:34:31.587979+0800 ViewController[25007:24031989] BViewController viewWillDisappear
2022-09-18 18:34:31.588890+0800 ViewController[25007:24031989] AViewController viewWillAppear
2022-09-18 18:34:32.093543+0800 ViewController[25007:24031989] AViewController viewDidAppear
2022-09-18 18:34:32.093834+0800 ViewController[25007:24031989] BViewController viewDidDisappear
