iOS如何巧妙解决NSTimer的循环引用详解

_timer = [NSTimer scheduledTimerWithTimeInterval:5.0
           target:self
selector:@selector(startTimer) userInfo:nil
repeats:YES];

#import <Foundation/Foundation.h>
#pragma mark -
#pragma mark - 内置weak对象（可用于分类定义weak属性）
@interface XWWeakObject : NSObject

@property (nullable, nonatomic, weak, readonly) id weakObject;

- (instancetype _Nullable )initWeakObject:(id _Nullable )obj;

+ (instancetype _Nullable )proxyWeakObject:(id _Nullable )obj;
@end

#import "XWWeakObject.h"
@implementation XWWeakObject
-(instancetype)initWeakObject:(id)obj{
  _weakObject = obj;
  return self;
}

+(instancetype)proxyWeakObject:(id)obj{
  
  return [[XWWeakObject alloc] initWeakObject:obj];
}


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

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

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

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

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

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

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

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

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

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

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

- (BOOL)isProxy {
  return YES;
}

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

- (NSString *)debugDescription {
  return [_weakObject debugDescription];
}
@end

- (void)viewDidLoad {
  [super viewDidLoad];
  XWWeakObject *target = [XWWeakObject proxyWeakObject:self];
  self.timer = [NSTimer scheduledTimerWithTimeInterval:1 target:target selector:@selector(timerCount) userInfo:nil repeats:YES];
}

-(void)timerCount{ 
}

-(void)dealloc{
  [_timer invalidate];
   _timer = nil;
}