iOS 基于消息转发机制实现弱引用计时器
在iOS开发中,我们经常使用NSTimer.常使用下列几个方法:
+ (NSTimer *)timerWithTimeInterval:(NSTimeInterval)ti target:(id)aTarget selector:(SEL)aSelector userInfo:(nullable id)userInfo repeats:(BOOL)yesOrNo;
+ (NSTimer *)scheduledTimerWithTimeInterval:(NSTimeInterval)ti target:(id)aTarget selector:(SEL)aSelector userInfo:(nullable id)userInfo repeats:(BOOL)yesOrNo;
...
其中NSTimer示例都会对aTarget进行强持有
@interface NormalViewController ()
@property (nonatomic, strong) NSTimer *timer;
@end
@implementation NormalViewController
- (void)viewDidLoad {
[super viewDidLoad];
self.view.backgroundColor = [UIColor whiteColor];
_timer = [NSTimer scheduledTimerWithTimeInterval:1.0 target:self selector:@selector(action) userInfo:nil repeats:YES];
}
- (void)action{
NSLog(@"%s",__func__);
}
- (void)dealloc{
NSLog(@"%s",__func__);
}
上述逻辑如果不主动的指向timer的invalidate方法释放timer中的target, NormalViewController和 timer就构成了循环引用关系
ViewController中我们还好处理,不过有时我们需要在View或者其他对象中来开启计时器,这个时候没有完善的生命周期方法,我们可能完全找不到正确调用timer invalidate方法的时机.
下面使用OC的消息转发机制来实现了一个weak的timer,该方案肯定不是最好方案,不过只是提供一个解决循环引用关系的思路.
自定义WeakTimer基础于NSTimer
@interface WeakTimer : NSTimer
@end
+ (NSTimer *)scheduledTimerWithTimeInterval:(NSTimeInterval)ti target:(id)aTarget selector:(SEL)aSelector userInfo:(nullable id)userInfo repeats:(BOOL)yesOrNo{
// 将时机Timer的target设置成WeakTimerTarget对象
WeakTimerTarget *target = [[WeakTimerTarget alloc] init];
target.aTarget = aTarget;
target.aSelector = aSelector;
WeakTimer *timer = [[WeakTimer superclass] scheduledTimerWithTimeInterval:ti target:target selector:aSelector userInfo:userInfo repeats:yesOrNo];
target.timer = timer;
return timer;
}
- (void)dealloc{
NSLog(@"%s",__func__);
}
WeakTimerTarget:
@interface WeakTimerTarget : NSObject
@property (nonatomic, weak) id aTarget;
@property (nonatomic, assign) SEL aSelector;
@property (nonatomic, weak) NSTimer *timer;
@end
@implementation WeakTimerTarget
- (id)forwardingTargetForSelector:(SEL)aSelector{
if (aSelector == _aSelector){
if (!_aTarget){
if (_timer){
[_timer invalidate];
}
return [[WeakTimerTargetdonotCrash alloc] init];
}
return _aTarget;
}
return [super forwardingTargetForSelector:aSelector];
}
@end
- WeakTimerTarget对象中会有弱属性
aTarget方法aSelector并弱引用timer - timer实际会使用WeakTimerTarget对象调用aSelector方法
- WeakTimerTarget对象未实现aSelector方法
- WeakTimerTarget执行消息转发流程
在消息转发第二部中我们将方法调用转给实际的target - aTarget, 最后结果其实变成了
[aTarget aSelector]
aTarget对象强引用timer
timer强引用WeakTimerTarget对象
WeakTimerTarget对象弱引用aTarget
打破了循环引用关系
当aTarget被释放的时候,WeakTimerTarget对象中的aTarget == nil
这个时候执行
[_timer invalidate];
停止timer
这个时候我们在- (id)forwardingTargetForSelector:(SEL)aSelector方法中返回了一个WeakTimerTargetdonotCrash对象
该对象的作用是: 将该步的消息转发流程走完, 并避免此次方法调用导致crash
@interface WeakTimerTargetdonotCrash: NSObject
@end
@implementation WeakTimerTargetdonotCrash
void travel(id self, SEL _cmd){
NSLog(@"travel");
}
+ (BOOL)resolveInstanceMethod:(SEL)sel{
// 添加空方法
class_addMethod([self class], sel, (IMP)travel, "V@:");
return YES;
}
@end
该对象其实只做一个简单的事: 在消息转发流程第一步中, 补救WeakTimerTargetdonotCrash对象的未实现方法调用导致的crash
后续为完善demo,使用runtime直接修改NSTimer,将NSTimer改成弱引用类型的计时器.
