ARC模式下delloc()注意事项

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 一. 有一次面试问到：从VC1 推到VC2会不会调用dealloc呢？答案是不会

而从VC2 dismiss 会调VC2的dealloc 
不会调用VC1的dealloc方法

有代码为证：

#import "ViewController.h"
#import "ViewController2.h"
@interface ViewController ()

@end

@implementation ViewController

- (void)dealloc
{
    NSLog(@"ViewController dealloc");
}

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

- (IBAction)goVC2:(id)sender {
    ViewController2 *vc2 = [ViewController2 new];
    [self presentViewController:vc2 animated:YES completion:^{
        
    }];
    
}

@end

#import "ViewController2.h"

@interface ViewController2 ()

@end

@implementation ViewController2

- (void)dealloc
{
    NSLog(@"ViewController2 dealloc");
}

- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view from its nib.
}

二. ViewController中被加入了定时器，而没有及时的invalidate ，会怎样？

#import "ViewController.h"
#import "ViewController2.h"
@interface ViewController ()
{
    NSTimer *timer;
}
@end

@implementation ViewController

- (void)dealloc
{
    NSLog(@"ViewController dealloc %@",self);
}

- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.
    timer = [NSTimer scheduledTimerWithTimeInterval:3.0 target:self selector:@selector(updateTime:) userInfo:nil repeats:YES];
}

-(void)viewWillDisappear:(BOOL)animated
{
    [super viewWillDisappear:animated];
    [timer invalidate];
}

-(void)updateTime:(id)sender
{
    
    NSLog(@"ViewController updateTime");
}

- (IBAction)goVC2:(id)sender {
    ViewController2 *vc2 = [ViewController2 new];
    [self presentViewController:vc2 animated:YES completion:^{
        
    }];
    
}

@end

没调 [timer invalidate]; 打印

2020-02-29 17:23:04.608616+0800 YiJTApp[17656:482667] ViewController updateTime
2020-02-29 17:23:07.608558+0800 YiJTApp[17656:482667] ViewController updateTime
2020-02-29 17:23:10.607419+0800 YiJTApp[17656:482667] ViewController updateTime
2020-02-29 17:23:13.608551+0800 YiJTApp[17656:482667] ViewController updateTime
2020-02-29 17:23:16.608406+0800 YiJTApp[17656:482667] ViewController updateTime
2020-02-29 17:23:19.608554+0800 YiJTApp[17656:482667] ViewController updateTime
2020-02-29 17:23:22.608535+0800 YiJTApp[17656:482667] ViewController updateTime
2020-02-29 17:23:25.409616+0800 YiJTApp[17656:482667] ViewController2 dealloc
2020-02-29 17:23:25.607751+0800 YiJTApp[17656:482667] ViewController updateTime
2020-02-29 17:23:28.608624+0800 YiJTApp[17656:482667] ViewController updateTime
2020-02-29 17:23:31.607510+0800 YiJTApp[17656:482667] ViewController updateTime

调 [timer invalidate]; 打印

2020-02-29 17:26:12.271080+0800 YiJTApp[17763:485905] ViewController updateTime
2020-02-29 17:26:23.268043+0800 YiJTApp[17763:485905] ViewController2 dealloc

不管是有定时器，或者被代理，block强引用。VC1都不会调delloc方法。