文章以在TimerViewController中使用计时器为例，在VC中声明一个NSTimer属性。
创建NSTimer对象：
self.timer = [NSTimer scheduledTimerWithTimeInterval:1 target:self selector:@selector(startTimer) userInfo:nil repeats:YES];
timer作为VC的属性，被VC强引用，创建timer对象时VC作为target被timer强引用，即循环引用。
循环引用
解决循环引用的几种方式：
weak指针：
既然是强引用导致循环引用，那么用__weak修饰self就好了，想法是对的，但是做法是无效的。因为无论是weak还是strong修饰，在NSTimer中都会重新生成一个新的强引用指针指向self，导致循环引用的。
类方法：
创建一个继承NSObject的子类TempTarget，并创建开启计时器的方法。
TempTarget .h文件
@property (nonatomic, assign) SEL selector;
@property (nonatomic, weak) NSTimer *tempTimer;
@property (nonatomic, weak) id tempTarget;
+ (NSTimer *)scheduledTimerWithTimeInterval:(NSTimeInterval)interval target:(id)tempTarget selector:(SEL)selector userInfo:(id)userInfo repeats:(BOOL)repeats;
TempTarget .m文件
@implementation TempTarget
+ (NSTimer *)scheduledTimerWithTimeInterval:(NSTimeInterval)interval target:(id)tempTarget selector:(SEL)selector userInfo:(id)userInfo repeats:(BOOL)repeats {
TempTarget *target = [[TempTarget alloc] init];
target.tempTarget = tempTarget;
target.selector = selector;
target.tempTimer = [NSTimer scheduledTimerWithTimeInterval:interval target:target selector:@selector(timerSelector:) userInfo:userInfo repeats:repeats];
return target.tempTimer;
}
- (void)timerSelector:(NSTimer *)tempTimer {
if (self.tempTarget && [self.tempTarget respondsToSelector:self.selector]) {
[self.tempTarget performSelector:self.selector withObject:tempTimer.userInfo];
}else {
[self.tempTimer invalidate];
}
}
@end
VC调用
self.timer = [TempTarget scheduledTimerWithTimeInterval:1 target:self selector:@selector(timerStart:) userInfo:nil repeats:YES];
VC强引用timer，因为timer的target是TempTarget实例，所以timer强引用TempTarget实例，而TempTarget实例弱引用VC，解除循环引用。
强引用转移
WeakProxy：
创建一个继承NSProxy的子类WeakProxy，并实现消息转发的相关方法。
WeakProxy.h文件
@property (nonatomic, weak, readonly) id weakTarget;
+ (instancetype)proxyWithTarget:(id)target;
- (instancetype)initWithTarget:(id)target;
WeakProxy.m文件
@implementation WeakProxy
+ (instancetype)proxyWithTarget:(id)target {
return [[self alloc] initWithTarget:target];
}
- (instancetype)initWithTarget:(id)target {
_weakTarget = target;
return self;
}
- (void)forwardInvocation:(NSInvocation *)invocation {
SEL sel = [invocation selector];
if ([self.weakTarget respondsToSelector:sel]) {
[invocation invokeWithTarget:self.weakTarget];
}
}
//返回方法的签名。
- (NSMethodSignature *)methodSignatureForSelector:(SEL)sel {
return [self.weakTarget methodSignatureForSelector:sel];
}
- (BOOL)respondsToSelector:(SEL)aSelector {
return [self.weakTarget respondsToSelector:aSelector];
}
@end
VC调用
self.timer = [NSTimer scheduledTimerWithTimeInterval:1 target:[WeakProxy proxyWithTarget:self] selector:@selector(timerStart:) userInfo:nil repeats:YES];
- (void)dealloc {
[_timer invalidate];
NSLog(@"VC销毁了");
}
原理跟类方法相似，打破循环引用的环路。将timer的target设置为WeakProxy实例，利用消息转发机制实现执行VC中的计时方法，解决循环引用。
GCD:
VC的.m文件
@interface ViewController ()
@property (nonatomic, strong) dispatch_source_t timer;
@end
- (void)viewDidLoad {
[super viewDidLoad];
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
self.timer = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0, queue);
dispatch_source_set_timer(_timer, dispatch_time(DISPATCH_TIME_NOW, 0), 0.1*NSEC_PER_SEC, 0);
dispatch_source_set_event_handler(_timer, ^{
dispatch_async(dispatch_get_main_queue(), ^{
NSLog(@"执行了");
});
});
//开启计时器
dispatch_resume(_timer);
}
//结束计时
dispatch_source_cancel(_timer);
Block:
创建NSTimer的分类NSTimer+UnretainCycle。
+ (NSTimer *)lhScheduledTimerWithTimeInterval:(NSTimeInterval)inerval
repeats:(BOOL)repeats
block:(void(^)(NSTimer *timer))block {
return [NSTimer scheduledTimerWithTimeInterval:inerval target:self selector:@selector(blcokInvoke:) userInfo:[block copy] repeats:repeats];
}
+ (void)blcokInvoke:(NSTimer *)timer {
void (^block)(NSTimer *timer) = timer.userInfo;
if (block) {
block(timer);
}
}
VC调用
self.timer = [NSTimer mxScheduledTimerWithTimeInterval:0.5 repeats:YES block:^(NSTimer *timer) {
NSLog(@"执行了");
}];
