1. NSTimer循环引用原因

如下代码，dealloc永远不会走，因为self引用timer，timer引用了target(self = BlockViewController2),造成了相互强引用

@interface BlockViewController2 ()

@property (nonatomic, strong) NSTimer *timer;

@end

@implementation BlockViewController2

- (void)viewDidLoad {
    [super viewDidLoad];
    self.timer = [NSTimer scheduledTimerWithTimeInterval:2.0 target:self selector:@selector(onTimer) userInfo:nil repeats:YES];
}

- (void)onTimer {
    NSLog(@"-- timer --");
}

- (void)dealloc {
    NSLog(@"dealloc 2 = %@", self);
    [self.timer invalidate];
}

@end

2. 解决方案

2.1 主动调用释放timer(viewDidDisappear)

- (void)viewDidDisappear:(BOOL)animated {
    [super viewDidDisappear:animated];
    
    [self.timer invalidate];
    self.timer = nil;
}

这个方法能解决循环引用问题，但可能导致新问题,比如在A vc 使用timer，在A push B的时候，释放timer，此时从b pop 回A，A timer已经不存在，需要重新创建

2.2 使用Timer的Block API

/// Creates and returns a new NSTimer object initialized with the specified block object. This timer needs to be scheduled on a run loop (via -[NSRunLoop addTimer:]) before it will fire.
/// - parameter: timeInterval The number of seconds between firings of the timer. If seconds is less than or equal to 0.0, this method chooses the nonnegative value of 0.1 milliseconds instead
/// - parameter: repeats If YES, the timer will repeatedly reschedule itself until invalidated. If NO, the timer will be invalidated after it fires.
/// - parameter: block The execution body of the timer; the timer itself is passed as the parameter to this block when executed to aid in avoiding cyclical references
+ (NSTimer *)timerWithTimeInterval:(NSTimeInterval)interval repeats:(BOOL)repeats block:(void (^)(NSTimer *timer))block API_AVAILABLE(macosx(10.12), ios(10.0), watchos(3.0), tvos(10.0));

/// Creates and returns a new NSTimer object initialized with the specified block object and schedules it on the current run loop in the default mode.
/// - parameter: ti The number of seconds between firings of the timer. If seconds is less than or equal to 0.0, this method chooses the nonnegative value of 0.1 milliseconds instead
/// - parameter: repeats If YES, the timer will repeatedly reschedule itself until invalidated. If NO, the timer will be invalidated after it fires.
/// - parameter: block The execution body of the timer; the timer itself is passed as the parameter to this block when executed to aid in avoiding cyclical references
+ (NSTimer *)scheduledTimerWithTimeInterval:(NSTimeInterval)interval repeats:(BOOL)repeats block:(void (^)(NSTimer *timer))block API_AVAILABLE(macosx(10.12), ios(10.0), watchos(3.0), tvos(10.0));

    self.timer = [NSTimer timerWithTimeInterval:1.0 repeats:YES block:^(NSTimer * _Nonnull timer) {
        NSLog(@"-- timer -- ");
    }];
    [[NSRunLoop currentRunLoop] addTimer:self.timer forMode:NSDefaultRunLoopMode];

    self.timer = [NSTimer scheduledTimerWithTimeInterval:1.0 repeats:YES block:^(NSTimer * _Nonnull timer) {
        NSLog(@"-- timer -- ");
    }];

使用这两个方法的时候需要注意避免Block的循环引用

2.3 将NSTimer做一层封装

因为NSTimer循环引用的核心问题是timer 的target引用了self，我们对target换成封装类的对象,这样可以解决timer引用self的问题

CCTimer.h

@interface CCTimer : NSObject

- (void)startTimer;

- (void)stopTimer;

@end

CCTimer.m

#import "CCTimer.h"

@implementation CCTimer
{
    NSTimer * _timer;
}

- (void)startTimer {
    _timer = [NSTimer scheduledTimerWithTimeInterval:1.0 target:self selector:@selector(onTimer) userInfo:nil repeats:YES];
}

- (void)stopTimer {
    if (_timer == nil) {
        return;
    }
    [_timer invalidate];
    _timer = nil;
}

- (void)onTimer {
    NSLog(@"-- timer --");
}

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

@end

调用：

- (void)viewDidLoad {
    [super viewDidLoad];
    self.ccTimer = [[CCTimer alloc] init];
    [self.ccTimer startTimer];
}

- (void)onTimer {
    NSLog(@"-- timer --");
}

- (void)dealloc {
    NSLog(@"dealloc 2 = %@", self);
    [self.ccTimer stopTimer];
    [self.timer invalidate];
}

这样的方式主要是让CCTimer强引用NSTimer， NSTimer强引用CCTimer，避免与ViewController强引用， 在ViewController dealloc方法中释放timer对NSTimer进行销毁，CCTimer也就会相应的销毁，避免了循环引用

2.4 使用NSProxy解决

@interface CCProxy : NSProxy

- (instancetype)initWithObject:(id)obj;

+ (instancetype)proxyWithObject:(id)obj;

@end

//
// CCProxy.m
// MemoryManageDemo
//
// Created by Ternence on 2021/5/17.
//

#import "CCProxy.h"

@interface CCProxy ()

@property (nonatomic, weak) id obj;

@end

@implementation CCProxy

- (instancetype)initWithObject:(id)obj {
    self.obj = obj;
    return self;
}

+ (instancetype)proxyWithObject:(id)obj {
    return [[self alloc] initWithObject:obj];
}

- (void)forwardInvocation:(NSInvocation *)invocation {
    if ([self.obj respondsToSelector:invocation.selector]) {
        [invocation invokeWithTarget: self.obj];
    }
}

- (NSMethodSignature *)methodSignatureForSelector:(SEL)sel {
    return [self.obj methodSignatureForSelector:sel];
}
@end

调用：

    CCProxy *proxy = [[CCProxy alloc] initWithObject:self];
    self.timer = [NSTimer scheduledTimerWithTimeInterval:1.0 target:proxy selector:@selector(onTimer) userInfo:nil repeats:YES];

通过CCProxy这个伪基类(相当于ViewContoller的复制类)，避免让timer引用ViewController