1. 使用block

    __weak typeof(self) weakSelf = self;
    self.timer = [NSTimer scheduledTimerWithTimeInterval:1.0 repeats:YES block:^(NSTimer * _Nonnull timer) {
        [weakSelf timerTest];
    }];

2. 利用Runtime的消息转发机制

//.h
@interface YZTimerProxy : NSProxy
+ (instancetype)proxyWithTarget:(id)target;
@property (weak, nonatomic) id target;
@end

//.m
@implementation YZTimerProxy

+ (instancetype)proxyWithTarget:(id)target {
    // NSProxy对象不需要调用init，因为它本来就没有init方法
    YZTimerProxy *proxy = [YZTimerProxy alloc];
    proxy.target = target;
    return proxy;
}

- (NSMethodSignature *)methodSignatureForSelector:(SEL)sel {
    return [self.target methodSignatureForSelector:sel];
}

- (void)forwardInvocation:(NSInvocation *)invocation {
    [invocation invokeWithTarget:self.target];
}
@end

self.timer = [NSTimer scheduledTimerWithTimeInterval:1.0 target:[YZTimerProxy proxyWithTarget:self] selector:@selector(timerTest) userInfo:nil repeats:YES];