iOS - NSTimer 循環引用

1. NSTimer循環引用緣由

以下代碼，dealloc永遠不會走，由於self引用timer，timer引用了target(self = BlockViewController2),形成了相互強引用

@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
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2. 解決方案

2.1 主動調用釋放timer(viewDidDisappear)

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

這個方法能解決循環引用問題，但可能致使新問題,好比在A vc 使用timer，在A push B的時候，釋放timer，此時從b pop 回A，A timer已經不存在，須要從新建立

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));
複製代碼

self.timer = [NSTimer timerWithTimeInterval:1.0 repeats:YES block:^(NSTimer * _Nonnull timer) {
        NSLog(@"-- timer -- ");
    }];
    [[NSRunLoop currentRunLoop] addTimer:self.timer forMode:NSDefaultRunLoopMode];
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self.timer = [NSTimer scheduledTimerWithTimeInterval:1.0 repeats:YES block:^(NSTimer * _Nonnull timer) {
        NSLog(@"-- timer -- ");
    }];
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使用這兩個方法的時候須要注意避免Block的循環引用

2.3 將NSTimer作一層封裝

由於NSTimer循環引用的核心問題是timer 的target引用了self，咱們對target換成封裝類的對象,這樣能夠解決timer引用self的問題

CCTimer.h

@interface CCTimer : NSObject

- (void)startTimer;

- (void)stopTimer;

@end
複製代碼

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
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調用：

- (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];
}
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這樣的方式主要是讓CCTimer強引用NSTimer， NSTimer強引用CCTimer，避免與ViewController強引用， 在ViewController dealloc方法中釋放timer對NSTimer進行銷燬，CCTimer也就會相應的銷燬，避免了循環引用

2.4 使用NSProxy解決

@interface CCProxy : NSProxy

- (instancetype)initWithObject:(id)obj;

+ (instancetype)proxyWithObject:(id)obj;

@end
複製代碼

//
//  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
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調用：

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

經過CCProxy這個僞基類(至關於ViewContoller的複製類)，避免讓timer引用ViewController