NSTimer的问题

iOS开发中，涉及到定时的问题，我们通常使用NSTimer来解决，例如下面的代码。

SFClass.h

#import <Foundation/Foundation.h>

@interface SFClass : NSObject

- (void)startPolling;
- (void)stopPolling;

@end

SFClass.m

#import "SFClass.h"

@implementation SFClass {
    NSTimer *_pollTimer;
}

- (void)dealloc {
    [_pollTimer invalidate];
}

- (void)stopPolling {
    [_pollTimer invalidate];
    _pollTimer = nil;
}

- (void)doSomething {

}

- (void)startPolling {
    _pollTimer = [NSTimer scheduledTimerWithTimeInterval:1.0 target:self selector:@selector(doSomething) userInfo:nil repeats:YES];
}

@end

由于定时器会保留target对象，所以上面的代码会造成SFClass对象和NSTimer对象的相互引用，会导致内存泄漏问题。

我们可以通过下面的方式来解决。

NSTimer+SFBlocksSupport.h

#import <Foundation/Foundation.h>

@interface NSTimer (SFBlocksSupport)

+ (NSTimer *)sf_scheduleTimerWithTimeInterval:(NSTimeInterval)interval block:(void (^)(void))block repeats:(BOOL)repeats;

@end

NSTimer+SFBlocksSupport.m

#import "NSTimer+SFBlocksSupport.h"

@implementation NSTimer (SFBlocksSupport)

+ (NSTimer *)sf_scheduleTimerWithTimeInterval:(NSTimeInterval)interval block:(void (^)(void))block repeats:(BOOL)repeats {
    return [self scheduledTimerWithTimeInterval:interval target:self selector:@selector(sf_blockInvoke:) userInfo:[block copy] repeats:repeats];
}

+ (void)sf_blockInvoke:(NSTimer *)timer {
    void (^block)(void) = timer.userInfo;
    if (block) {
        block();
    }
}

@end

然后，我们使用定时器的代码就要修改为下面的样子

- (void)startPolling {
    _pollTimer = [NSTimer sf_scheduleTimerWithTimeInterval:1.0 block:^{
        [self doSomething];
    } repeats:YES];
}

仔细看看代码，发现依然有保留环，再修改一下:

- (void)startPolling {
    __weak typeof(self) weakSelf = self;
    _pollTimer = [NSTimer sf_scheduleTimerWithTimeInterval:1.0 block:^{
        SFClass *strongSelf = weakSelf;
        [strongSelf doSomething];
    } repeats:YES];
}

最终解决了相互引用的问题。