NSTimer解决循环引用的几种方式

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NSTimer

NSTimer是日常常用控件，但在使用中需要注意避免循环引用的问题。NSTimer使用方式如下

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- (void)initTimer {
    self.timer = [NSTimer scheduledTimerWithTimeInterval:1 target:self selector:@selector(beginTimer) userInfo:nil repeats:YES]; 
}

- (void)dealloc {
    [self.timer invalidate];
    self.timer = nil;
}

由于timer与self相互强引用，导致走不到dealloc方法，造成内存泄露。
以下为解决NSTimer循环引用的几种方法。

选择合适时机手动释放

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-(void)viewDidDisappear:(BOOL)animated
{
   [super viewDidDisappear:animated];
    _textLabel.text = @"请按住说话";
    [_timer invalidate];
}

这种方案在有些情况下是不合理的，比如控制器push到下一个页面，返回时需要重新实例化timer。

在NSTimer分类中添加类方法

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@implementation NSTimer (BlockTimer)

+ (NSTimer *)bl_scheduledTimerWithTimeInterval:(NSTimeInterval)interval block:(void (^)(void))block repeats:(BOOL)repeats {
    
    return [self scheduledTimerWithTimeInterval:interval target:self selector:@selector(bl_blockSelector:) userInfo:[block copy] repeats:repeats];
}

+ (void)bl_blockSelector:(NSTimer *)timer {
    
    void(^block)(void) = timer.userInfo;
    if (block) {
        block();
    }
}
@end

设置timer的target为自身，解决了循环引用的问题。使用时要注意block引起的循环引用。

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__weak typeof(self) weakSelf = self;
self.timer = [NSTimer bl_scheduledTimerWithTimeInterval:1 block:^{
     [weakSelf doSomething];
} repeats:YES];

给target添加中间件NSProxy

在YYKit中，作者使用YYWeakProxy类作为NSTimer、CADisplayLink 的target，使得Timer得引用变为 VC->NSTimer->YYWeakProxy–>VC，YYWeakProxy弱引用VC，当触发timer的方法时，通过消息转发机制将消息转发至VC，避免了循环引用。
使用方式在头文件中可以看到：

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#import <Foundation/Foundation.h>
/**
 A proxy used to hold a weak object.
 It can be used to avoid retain cycles, such as the target in NSTimer or CADisplayLink.
 sample code:
 
     @implementation MyView {
        NSTimer *_timer;
     }
     
     - (void)initTimer {
        YYWeakProxy *proxy = [YYWeakProxy proxyWithTarget:self];
        _timer = [NSTimer timerWithTimeInterval:0.1 target:proxy selector:@selector(tick:) userInfo:nil repeats:YES];
     }
     
     - (void)tick:(NSTimer *)timer {...}
     @end
 */
@interface YYWeakProxy : NSProxy
/**
 The proxy target.
 */
@property (nullable, nonatomic, weak, readonly) id target;

/**
 Creates a new weak proxy for target.
 
 @param target Target object.
 
 @return A new proxy object.
 */
- (instancetype)initWithTarget:(id)target;

/**
 Creates a new weak proxy for target.
 
 @param target Target object.
 
 @return A new proxy object.
 */
+ (instancetype)proxyWithTarget:(id)target;

@end

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#import "YYWeakProxy.h"


@implementation YYWeakProxy

- (instancetype)initWithTarget:(id)target {
    _target = target;
    return self;
}

+ (instancetype)proxyWithTarget:(id)target {
    return [[YYWeakProxy alloc] initWithTarget:target];
}

- (id)forwardingTargetForSelector:(SEL)selector {
    return _target;
}

- (void)forwardInvocation:(NSInvocation *)invocation {
    void *null = NULL;
    [invocation setReturnValue:&null];
}

- (NSMethodSignature *)methodSignatureForSelector:(SEL)selector {
    return [NSObject instanceMethodSignatureForSelector:@selector(init)];
}

- (BOOL)respondsToSelector:(SEL)aSelector {
    return [_target respondsToSelector:aSelector];
}

- (BOOL)isEqual:(id)object {
    return [_target isEqual:object];
}

- (NSUInteger)hash {
    return [_target hash];
}

- (Class)superclass {
    return [_target superclass];
}

- (Class)class {
    return [_target class];
}

- (BOOL)isKindOfClass:(Class)aClass {
    return [_target isKindOfClass:aClass];
}

- (BOOL)isMemberOfClass:(Class)aClass {
    return [_target isMemberOfClass:aClass];
}

- (BOOL)conformsToProtocol:(Protocol *)aProtocol {
    return [_target conformsToProtocol:aProtocol];
}

- (BOOL)isProxy {
    return YES;
}

- (NSString *)description {
    return [_target description];
}

- (NSString *)debugDescription {
    return [_target debugDescription];
}

@end

使用GCD timer

以下为YYTimer,实现了线程安全的GCD timer,不会被runloop的mode影响。

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/**
 YYTimer is a thread-safe timer based on GCD. It has similar API with `NSTimer`.
 YYTimer object differ from NSTimer in a few ways:
 
 * It use GCD to produce timer tick, and won't be affected by runLoop.
 * It make a weak reference to the target, so it can avoid retain cycles.
 * It always fire on main thread.
 
 */
@interface YYTimer : NSObject

+ (YYTimer *)timerWithTimeInterval:(NSTimeInterval)interval
                            target:(id)target
                          selector:(SEL)selector
                           repeats:(BOOL)repeats;

- (instancetype)initWithFireTime:(NSTimeInterval)start
                        interval:(NSTimeInterval)interval
                          target:(id)target
                        selector:(SEL)selector
                         repeats:(BOOL)repeats NS_DESIGNATED_INITIALIZER;

@property (readonly) BOOL repeats;
@property (readonly) NSTimeInterval timeInterval;
@property (readonly, getter=isValid) BOOL valid;

- (void)invalidate;

- (void)fire;

@end
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#import "YYTimer.h"
#import <pthread.h>

#define LOCK(...) dispatch_semaphore_wait(_lock, DISPATCH_TIME_FOREVER); \
__VA_ARGS__; \
dispatch_semaphore_signal(_lock);


@implementation YYTimer {
    BOOL _valid;
    NSTimeInterval _timeInterval;
    BOOL _repeats;
    __weak id _target;
    SEL _selector;
    dispatch_source_t _source;
    dispatch_semaphore_t _lock;
}

+ (YYTimer *)timerWithTimeInterval:(NSTimeInterval)interval
                            target:(id)target
                          selector:(SEL)selector
                           repeats:(BOOL)repeats {
    return [[self alloc] initWithFireTime:interval interval:interval target:target selector:selector repeats:repeats];
}

- (instancetype)init {
    @throw [NSException exceptionWithName:@"YYTimer init error" reason:@"Use the designated initializer to init." userInfo:nil];
    return [self initWithFireTime:0 interval:0 target:self selector:@selector(invalidate) repeats:NO];
}

- (instancetype)initWithFireTime:(NSTimeInterval)start
                        interval:(NSTimeInterval)interval
                          target:(id)target
                        selector:(SEL)selector
                         repeats:(BOOL)repeats {
    self = [super init];
    _repeats = repeats;
    _timeInterval = interval;
    _valid = YES;
    _target = target;
    _selector = selector;
    
    __weak typeof(self) _self = self;
    _lock = dispatch_semaphore_create(1);
    _source = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0, dispatch_get_main_queue());
    dispatch_source_set_timer(_source, dispatch_time(DISPATCH_TIME_NOW, (start * NSEC_PER_SEC)), (interval * NSEC_PER_SEC), 0);
    dispatch_source_set_event_handler(_source, ^{[_self fire];});
    dispatch_resume(_source);
    return self;
}

- (void)invalidate {
    dispatch_semaphore_wait(_lock, DISPATCH_TIME_FOREVER);
    if (_valid) {
        dispatch_source_cancel(_source);
        _source = NULL;
        _target = nil;
        _valid = NO;
    }
    dispatch_semaphore_signal(_lock);
}

- (void)fire {
    if (!_valid) return;
#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Warc-performSelector-leaks"
    dispatch_semaphore_wait(_lock, DISPATCH_TIME_FOREVER);
    id target = _target;
    if (!target) {
        dispatch_semaphore_signal(_lock);
        [self invalidate];
    } else {
        dispatch_semaphore_signal(_lock);
        [target performSelector:_selector withObject:self];
        if (!_repeats) {
            [self invalidate];
        }
    }
#pragma clang diagnostic pop
}

- (BOOL)repeats {
    LOCK(BOOL repeat = _repeats); return repeat;
}

- (NSTimeInterval)timeInterval {
    LOCK(NSTimeInterval t = _timeInterval) return t;
}

- (BOOL)isValid {
    LOCK(BOOL valid = _valid) return valid;
}

- (void)dealloc {
    [self invalidate];
}

@end

更精确的Timer

纳秒级精度的Timer

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static const uint64_t NANOS_PER_USEC = 1000ULL;

 static const uint64_t NANOS_PER_MILLISEC = 1000ULL * NANOS_PER_USEC;
 static const uint64_t NANOS_PER_SEC = 1000ULL * NANOS_PER_MILLISEC;
 static mach_timebase_info_data_t timebase_info;

 static uint64_t nanos_to_abs(uint64_t nanos) {

 return nanos * timebase_info.denom / timebase_info.numer;

 }
 void waitSeconds(int seconds) {

 mach_timebase_info(&timebase_info);

 uint64_t time_to_wait = nanos_to_abs(seconds * NANOS_PER_SEC);

 uint64_t now = mach_absolute_time();

 mach_wait_until(now + time_to_wait);

 }

参考文档

NSTimer使用详解
NSProxy使用