[iOS]定时器NSTimer、CADisplayLink的内存管理

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];
}

方法3：使用nsproxy作为中间对象

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：

执行效率高，无需执行父类的方法搜索过程，直接进行消息转发。

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;
}