在ios10以前，使用NSTimer的
+ (NSTimer *)scheduledTimerWithTimeInterval:(NSTimeInterval)ti target:(id)aTarget selector:(SEL)aSelector userInfo:(nullable id)userInfo repeats:(BOOL)yesOrNo;
会导致self被timer持有，如果timer不主动调用invalidate，或者invalidate放在dealloc里面，会产出保留环，导致内存泄露
在10以后，我们可以调用
+ (NSTimer *)scheduledTimerWithTimeInterval:(NSTimeInterval)interval repeats:(BOOL)repeats block:(void (^)(NSTimer *timer))block
放心的把invalidate写在dealloc里面（当然block中引用self需要weak - strong一下）
10之前应该怎么办呢？我们可以仿写系统的这个方法，用分类的方式加入：
.h
#import <Foundation/Foundation.h>
@interface NSTimer (Block)
+ (NSTimer *)Block_ScheduledTimerWithTimeInterval:(NSTimeInterval)interval repeats:(BOOL)repeats block:(void (^)(NSTimer *timer))block;
@end
.m
#import "NSTimer+Block.h"
@implementation NSTimer (Block)
+ (NSTimer *)Block_ScheduledTimerWithTimeInterval:(NSTimeInterval)interval repeats:(BOOL)repeats block:(void (^)(NSTimer *timer))block{
return [self scheduledTimerWithTimeInterval:interval target:self selector:@selector(Block_BlockInvoke:) userInfo:[block copy] repeats:repeats];
}
+ (void)Block_BlockInvoke:(NSTimer *)timer{
void(^block)(NSTimer *timer) = timer.userInfo;
!block?:block(timer);
}
@end
