iOS之NSTimer循环引用的最佳解决方案

2021-09-09 本文已影响0人片片碎

最佳方案一：使用新API

如果你的系统只支持iOS10以上，强烈建议使用新API。

+ (NSTimer *)scheduledTimerWithTimeInterval:(NSTimeInterval)interval repeats:(BOOL)repeats block:(void (^)(NSTimer *timer))block API_AVAILABLE(macosx(10.12), ios(10.0), watchos(3.0), tvos(10.0));

+ (NSTimer *)timerWithTimeInterval:(NSTimeInterval)interval repeats:(BOOL)repeats block:(void (^)(NSTimer *timer))block API_AVAILABLE(macosx(10.12), ios(10.0), watchos(3.0), tvos(10.0));

- (instancetype)initWithFireDate:(NSDate *)date interval:(NSTimeInterval)interval repeats:(BOOL)repeats block:(void (^)(NSTimer *timer))block

这新的API，增加了block的参数。关于这个block参数，官方文档说明如下:

 block  The execution body of the timer; the timer itself is passed as the parameter to this block when executed to aid in avoiding cyclical references

翻译过来大概意思是：定时器在执行时，讲定时器自身作为参数传递给block，来帮助避免循环引用。使用很简单。只需要注意两点：

1、避免block的循环引用，使用__weak和__strong来避免
2、在持用NSTimer对象的类的方法中-(void)dealloc调用NSTimer 的- (void)invalidate方法；
代码示范

#import "SecondViewController.h"
#define WEAKSELF typeof(self) __weak weakSelf = self;
#define STRONGSELF typeof(weakSelf) __strong strongSelf = weakSelf;

@interface SecondViewController ()

@property (nonatomic, strong) NSTimer *timer;

@end

@implementation SecondViewController

- (void)viewDidLoad {
    [super viewDidLoad];

    WEAKSELF
    self.timer =  [NSTimer scheduledTimerWithTimeInterval:1.0 repeats:YES block:^(NSTimer * _Nonnull timer) {
        STRONGSELF //避免block循环引用
        [strongSelf timerAction];
    }];
}

-(void)viewWillAppear:(BOOL)animated {
    if (self.timer) {
        [self.timer setFireDate:[NSDate distantPast]];
    }
}

- (void)viewWillDisappear:(BOOL)animated {
    if (self.timer) {
        [self.timer setFireDate:[NSDate distantFuture]];
    }
}

- (void)dealloc {
    if (self.timer) {
        [self.timer invalidate]; //dealloc时候从runloop中删除timer
        self.timer = nil;
    }
    NSLog(@"Feng SecondViewController dealloc");
}

-(void)timerAction {
    NSLog(@"Feng timer test");
}
@end

最佳方案二：使用NSProxy

如果实在没有办法，需要兼容10以下版本，那就使用NSProxy吧。SDWebImage就使用了此方案。使用SDWeakProxy这个伪基类，避免timer和target造成循环。此处FengWeakProxy也就相当于SDWebImage中的SDWeakProxy。此方案的好处就是：不需要修改之前的代码，一句都不用修改

NSProxy的伪基类：FengWeakProxy

FengWeakProxy.h

#import <Foundation/Foundation.h>

NS_ASSUME_NONNULL_BEGIN

@interface FengWeakProxy : NSProxy

@property (nonatomic, weak, readonly, nullable) id target;

- (nonnull instancetype)initWithTarget:(nonnull id)target;
+ (nonnull instancetype)proxyWithTarget:(nonnull id)target;

@end

NS_ASSUME_NONNULL_END

FengWeakProxy.m

#import "FengWeakProxy.h"

@implementation FengWeakProxy
- (instancetype)initWithTarget:(id)target {
    _target = target;
    return self;
}

+ (instancetype)proxyWithTarget:(id)target {
    return [[FengWeakProxy alloc] initWithTarget:target];
}

#pragma mark --转发实现
//当不能识别方法时候,就会调用这个方法,在这个方法中,我们可以将不能识别的传递给其它对象处理
//由于这里对所有的不能处理的都传递给_target了,所以methodSignatureForSelector和forwardInvocation不可能被执行的,所以不用再重载了吧
//其实还是需要重载methodSignatureForSelector和forwardInvocation的,为什么呢?因为_target是弱引用的,所以当_target可能释放了,当它被释放了的情况下,那么forwardingTargetForSelector就是返回nil了.然后methodSignatureForSelector和forwardInvocation没实现的话,就直接crash了!!!
//这也是为什么这两个方法中随便写的!!!

// 转发目标选择器
- (id)forwardingTargetForSelector:(SEL)selector {
  return _target;
}

// 方法签名的选择器
- (NSMethodSignature *)methodSignatureForSelector:(SEL)sel {
    return [NSObject instanceMethodSignatureForSelector:@selector(init)];
}

// 函数执行器
- (void)forwardInvocation:(NSInvocation *)invocation {
    void *null = NULL;
    [invocation setReturnValue:&null];
}

#pragma mark --其他
- (BOOL)respondsToSelector:(SEL)aSelector {
    return [_target respondsToSelector:aSelector];
}

- (BOOL)isEqual:(id)object {
    return [_target isEqual:object];
}

- (NSUInteger)hash {
    return [_target hash];
}

- (Class)superclass {
    return [_target superclass];
}

- (Class)class {
    return [_target class];
}

- (BOOL)isKindOfClass:(Class)aClass {
    return [_target isKindOfClass:aClass];
}

- (BOOL)isMemberOfClass:(Class)aClass {
    return [_target isMemberOfClass:aClass];
}

- (BOOL)conformsToProtocol:(Protocol *)aProtocol {
    return [_target conformsToProtocol:aProtocol];
}

- (BOOL)isProxy {
    return YES;
}

- (NSString *)description {
    return [_target description];
}

- (NSString *)debugDescription {
    return [_target debugDescription];
}
@end

NSTimer方法替换：NSTimer+weak

NSTimer+weak.h

#import <Foundation/Foundation.h>
NS_ASSUME_NONNULL_BEGIN
@interface NSTimer (Weak)

@end
NS_ASSUME_NONNULL_END

NSTimer+weak.m

#import "NSTimer+weak.h"
#import <objc/runtime.h>
#import "NSObject+Swizzing.h"
#import "FengWeakProxy.h"
@implementation NSTimer (Weak)

+(void)load {
  static dispatch_once_t onceToken;
  dispatch_once(&onceToken, ^{
  @autoreleasepool {
      [self swizzleWithSysMethod:@selector(scheduledTimerWithTimeInterval:target:selector:userInfo:repeats:) swizzledMethod:@selector(scheduledSafeTimerWithTimeInterval:target:selector:userInfo:repeats:)];
      [self swizzleWithSysMethod:@selector(timerWithTimeInterval:target:selector:userInfo:repeats:) swizzledMethod:@selector(timerSafeWithTimeInterval:target:selector:userInfo:repeats:)];
        }
  });
}

+ (NSTimer *)scheduledSafeTimerWithTimeInterval:(NSTimeInterval)ti target:(id)aTarget selector:(SEL)aSelector userInfo:(id)userInfo repeats:(BOOL)yesOrNo {
  return [self scheduledSafeTimerWithTimeInterval:ti target:[FengWeakProxy proxyWithTarget:aTarget] selector:aSelector userInfo:userInfo repeats:yesOrNo];
}

+ (NSTimer *)timerSafeWithTimeInterval:(NSTimeInterval)ti target:(id)aTarget selector:(SEL)aSelector userInfo:(id)userInfo repeats:(BOOL)yesOrNo {
      return [self timerSafeWithTimeInterval:ti target:[FengWeakProxy proxyWithTarget:aTarget] selector:aSelector userInfo:userInfo repeats:yesOrNo];
 }

  @end

交换方法

NSObject+Swizzing.h

#import <Foundation/Foundation.h>

 NS_ASSUME_NONNULL_BEGIN

 @interface NSObject (Swizzing)
  + (void)swizzleWithSysMethod:(SEL)originalSelector swizzledMethod:(SEL)swizzledSelector;
  + (void)swizzleInstanceWithSysMethod:(SEL)originalSelector swizzledMethod:(SEL)swizzledSelector;

  @end

  NS_ASSUME_NONNULL_END

NSObject+Swizzing.m

#import "NSObject+Swizzing.h"
#import <objc/runtime.h>

@implementation NSObject (Swizzing)
 + (void)swizzleWithSysMethod:(SEL)originalSelector swizzledMethod:(SEL)swizzledSelector {
// Method中包含IMP函数指针，通过替换IMP，使SEL调用不同函数实现
   Method origMethod = class_getClassMethod(self, originalSelector);
   Method replaceMeathod = class_getClassMethod(self, swizzledSelector);
   Class metaKlass = objc_getMetaClass(NSStringFromClass(self).UTF8String);

 //    class_addMethod:如果发现方法已经存在，会失败返回，也可以用来做检查用,我们这里是为了避免源方法没有实现的情况;如果方法没有存在,我们则先尝试添加被替换的方法的实现
     BOOL didAddMethod = class_addMethod(metaKlass,
                                   originalSelector,
                                   method_getImplementation(replaceMeathod),
                                   method_getTypeEncoding(replaceMeathod));
     if (didAddMethod) {
     // 原方法未实现，则替换原方法防止crash
         class_replaceMethod(metaKlass,
                       swizzledSelector,
                       method_getImplementation(origMethod),
                       method_getTypeEncoding(origMethod));
     }else {
 //   添加失败：说明源方法已经有实现，直接将两个方法的实现交换即
         method_exchangeImplementations(origMethod, replaceMeathod);
       }
 }

   + (void)swizzleInstanceWithSysMethod:(SEL)originalSelector swizzledMethod:(SEL)swizzledSelector {
     Method origMethod = class_getInstanceMethod(self, originalSelector);
     Method replaceMeathod = class_getInstanceMethod(self, swizzledSelector);

       // class_addMethod:如果发现方法已经存在，会失败返回，也可以用来做检查用,我们这里是为了避免源方法没有实现的情况;如果方法没有存在,我们则先尝试添加被替换的方法的实现
     BOOL didAddMethod = class_addMethod(self,
                                   originalSelector,
                                   method_getImplementation(replaceMeathod),
                                   method_getTypeEncoding(replaceMeathod));
     if (didAddMethod) {
   // 原方法未实现，则替换原方法防止crash
         class_replaceMethod(self,
                       swizzledSelector,
                       method_getImplementation(origMethod),
                       method_getTypeEncoding(origMethod));
       }else {
         // 添加失败：说明源方法已经有实现，直接将两个方法的实现交换即
         method_exchangeImplementations(origMethod, replaceMeathod);
     }
   }

 @end

   ```

代码示范

#import "SecondViewController.h"
#define WEAKSELF typeof(self) __weak weakSelf = self;
#define STRONGSELF typeof(weakSelf) __strong strongSelf = weakSelf;

@interface SecondViewController ()

@property (nonatomic, strong) NSTimer *timer;

@end

@implementation SecondViewController

- (void)viewDidLoad {
    [super viewDidLoad];

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

-(void)viewWillAppear:(BOOL)animated {
    if (self.timer) {
        [self.timer setFireDate:[NSDate distantPast]];
    }
}

- (void)viewWillDisappear:(BOOL)animated {
    if (self.timer) {
        [self.timer setFireDate:[NSDate distantFuture]];
    }
}

- (void)dealloc {
    if (self.timer) {
        [self.timer invalidate]; //dealloc时候从runloop中删除timer
        self.timer = nil;
    }
    NSLog(@"Feng SecondViewController dealloc");
}

-(void)timerAction {
    NSLog(@"Feng timer test");
}
@end

参考：《iOS之NSTimer循环引用的解决方案》
Demo：点击下载

iOS之NSTimer循环引用的最佳解决方案

最佳方案一：使用新API

最佳方案二：使用NSProxy

猜你喜欢

热点阅读