相关函数
init: 初始化ViewController。viewDidLoad:在我之前学习中最多使用的方法，在创建类后无论视图展示还是消失，该方法只调用一次在布局中，类成员对象和变量的初始化都会放在这个方法中。viewWilAppear:如同英语含义一样，在视图即将展示的时候调用该方法。viewDidAppear:在视图已经展示的时候调用该方法。viewWillLayoutSubviews，viewDidLayoutSubviews:这两个方法用于处理视图布局的变化，一般来说在这里处理视图布局的相关操作，will在重新布局前被调用，did在重新布局后被调用。viewWillDisappear:在视图即将消失前被调用。viewDidDisappear:在视图消失后被调用。traitCollectionDidChange:当视图控制器的特征集合发生变化的时候（例如横竖屏切换或者分屏时使用），这里笔者并没有使用过，后期对其了解透彻后再进行补充。didReceiveMemoryWarning:当系统可用内存不足时会调用这个函数，可以在这个函数中释放一些不必要的对象和可重建的资源，但是如果在这个方法中不写代码，系统不会自动释放内存，只会释放一些系统级别的内存资源（不甚理解）。
执行顺序
下面我通过两个视图控制器跳转来展示执行的顺序。
VCViewController:
objectivec
#import "ViewController.h"
#import "VCTwo.h"
@interface ViewController ()
@end
@implementation ViewController
- (void)viewDidLoad {
[super viewDidLoad];
self.view.backgroundColor = [UIColor redColor];
UIButton* btn = [[UIButton alloc] initWithFrame:CGRectMake(100, 200, 250, 100)];
[btn setTitle:@"跳转下一页" forState:UIControlStateNormal];
[btn addTarget:self action:@selector(press) forControlEvents:UIControlEventTouchUpInside];
[self.view addSubview: btn];
NSLog(@"viewDidLoad");
}
-(void) press
{
NSLog(@"A->B");
VCTwo* vc2 = [[VCTwo alloc] init];
vc2.modalPresentationStyle = UIModalPresentationFullScreen;
[self presentViewController:vc2 animated:YES completion:nil];
}
- (void)viewWillAppear:(BOOL)animated
{
NSLog(@"viewWillAppear:A");
}
- (void)viewDidAppear:(BOOL)animated
{
NSLog(@"viewDidAppear:A");
}
- (void)viewWillLayoutSubviews
{
NSLog(@"viewWillLaoutSubviews");
}
- (void)viewDidLayoutSubviews
{
NSLog(@"viewDidLayoutSubviews");
}
- (void)viewWillDisappear:(BOOL)animated
{
NSLog(@"viewWillDisappear:A");
}
- (void)viewDidDisappear:(BOOL)animated
{
NSLog(@"viewDidDisappear:A");
}
@endVCTwo:
objectivec
#import "VCTwo.h"
@interface VCTwo ()
@end
@implementation VCTwo
- (void)viewDidLoad {
[super viewDidLoad];
self.view.backgroundColor = [UIColor greenColor];
UIButton* btn = [[UIButton alloc] initWithFrame:CGRectMake(100, 200, 250, 100)];
[btn setTitle:@"跳转上一页" forState:UIControlStateNormal];
[btn addTarget:self action:@selector(press) forControlEvents:UIControlEventTouchUpInside];
[self.view addSubview: btn];
NSLog(@"viewDidLoad");
}
-(void) press
{
NSLog(@"B->A");
[self dismissViewControllerAnimated:YES completion:nil];
}
- (void)viewWillAppear:(BOOL)animated
{
NSLog(@"viewWillAppear:B");
}
- (void)viewDidAppear:(BOOL)animated
{
NSLog(@"viewDidAppear:B");
}
- (void)viewWillLayoutSubviews
{
NSLog(@"viewWillLaoutSubviews");
}
- (void)viewDidLayoutSubviews
{
NSLog(@"viewDidLayoutSubviews");
}
- (void)viewWillDisappear:(BOOL)animated
{
NSLog(@"viewWillDisappear:B");
}
- (void)viewDidDisappear:(BOOL)animated
{
NSLog(@"viewDidDisappear:B");
}
@end打印结果：
objectivec
viewDidLoad
viewWillAppear:A
viewWillLaoutSubviews
viewDidLayoutSubviews
viewDidAppear:A
A->B
viewDidLoad
viewWillDisappear:A
viewWillAppear:B
viewWillLaoutSubviews
viewDidLayoutSubviews
viewDidAppear:B
viewDidDisappear:A
B->A
viewWillDisappear:B
viewWillAppear:A
viewDidAppear:A
viewDidDisappear:B由打印的结果可以看出这些方法执行的顺序：
viewDidLoad->viewWillAppear->viewDidAppear->viewWillLaoutSubviews->viewDidLaoutSubviews->viewWillDisappear->viewDidDisappear在最后消失的时候，先准备加载新的视图，旧的视图才开始准备消失，在新的视图出现后，旧视图才消失。
GIF动图 ：
loadView
该方法在控制器的view为空的时候被调用，loadView是使用代码生成视图的时候，当视图第一次载入的时候调用的方法，用于使用代码来实现控件。当我们找不到view时，系统就会一直调用loadView方法来寻找view，就会陷入一个循环。