NSTimer、CAdisplayLink会对target产生强引用,如果target同时对他们产生强引用,则会发生循环引用。
以NSTimer为例,解决循环引用的问题。
方法1:使用block
- (void)viewDidLoad { [super viewDidLoad]; // Do any additional setup after loading the view. __weak typeof(self) weakself = self; self.timer = [NSTimer scheduledTimerWithTimeInterval:1.0 repeats:YES block:^(NSTimer * _Nonnull timer) { [weakself func]; }]; } - (void)func { NSLog(@"%s",__func__); } - (void)dealloc { NSLog(@"%s",__func__); [self.timer invalidate]; }
方法2:使用NSObject作为中间对象
Proxy1.h @interface Proxy1 : NSObject + (instancetype)initWithTarget:(id)target; @end
Proxy1.m @interface Proxy1 () @property (nonatomic,weak) id target; @end @implementation Proxy1 + (instancetype)initWithTarget:(id)target { Proxy1 *proxy = [[Proxy1 alloc] init]; proxy.target = target; return proxy; } - (id)forwardingTargetForSelector:(SEL)aSelector { return self.target; } @end
- (void)viewDidLoad { [super viewDidLoad]; // Do any additional setup after loading the view. self.timer = [NSTimer scheduledTimerWithTimeInterval:1.0 target:[Proxy1 initWithTarget:self] selector:@selector(func) userInfo:nil repeats:YES]; } - (void)func { NSLog(@"%s",__func__); } - (void)dealloc { NSLog(@"%s",__func__); [self.timer invalidate]; }
Proxy2.h @interface Proxy2 : nsproxy + (instancetype)initWithTarget:(id)target; @end
Proxy2.m @interface Proxy2 () @property (nonatomic,weak) id target; @end @implementation Proxy2 + (instancetype)initWithTarget:(id)target { Proxy2 *proxy = [Proxy2 alloc]; proxy.target = target; return proxy; } - (NSMethodSignature *)methodSignatureForSelector:(SEL)sel { return [self.target methodSignatureForSelector:sel]; } - (void)forwardInvocation:(NSInvocation *)invocation { [invocation invokeWithTarget:self.target]; } @end
- (void)viewDidLoad { [super viewDidLoad]; // Do any additional setup after loading the view. self.timer = [NSTimer scheduledTimerWithTimeInterval:1.0 target:[Proxy2 initWithTarget:self] selector:@selector(func) userInfo:nil repeats:YES]; } - (void)func { NSLog(@"%s",__func__); } - (void)dealloc { NSLog(@"%s",__func__); [self.timer invalidate]; }
方法3的优点:
关于nsproxy补充:
通过调用isKindOfClass
Proxy1 *proxy1 = [Proxy1 initWithTarget:self]; Proxy2 *proxy2 = [Proxy2 initWithTarget:self]; NSLog(@"%d",[proxy1 isKindOfClass:[ViewController class]]); // 0 NSLog(@"%d",[proxy2 isKindOfClass:[ViewController class]]); // 1
proxy1为Proxy1类型,Proxy1继承自NSObject,可以正常处理isKindOfClass方法,所以判断结果为0.
proxy2为Proxy2类型,Proxy2继承自nsproxy,大部分方法会直接进入消息转发阶段,会改为使用target进行调用,所以判断结果为1.
通过观察nsproxy的源码发现,该方法直接进行了消息转发。
/** * Calls the -forwardInvocation: method to determine if the 'real' object * referred to by the proxy is an instance of the specified class. * Returns the result.<br /> * NB. The default operation of -forwardInvocation: is to raise an exception. */ - (BOOL) isKindOfClass: (Class)aClass { NSMethodSignature *sig; NSInvocation *inv; BOOL ret; sig = [self methodSignatureForSelector: _cmd]; inv = [NSInvocation invocationWithMethodSignature: sig]; [inv setSelector: _cmd]; [inv setArgument: &aClass atIndex: 2]; [self forwardInvocation: inv]; [inv getReturnValue: &ret]; return ret; }
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