有时我们需要在一个对象生命周期结束的时候触发一个操作,希望当该对象dealloc的时候调用一个外部指定的block,但又不希望直接hook dealloc方法,这样侵入性太强了.下面贴一段非常简单的实现方式,通过一个category给外部暴露一个block注入的接口,内部将该block封装到一个寄生对象中(Parasite),该寄生对象在dealoc的时候触发block调用,所有的寄生对象通过runtime的AssociatedObject机制与宿主共存亡,从而达到监控宿主生命周期的目的.
注意事项
1.block触发的线程与对象释放时的线程一致,请注意后续操作的线程安全.
2.不要在block中强引用对象,否则引用循环释放不了;
3.不要在block中通过weak引用对象,因为此时会返回nil;
(根据WWDC2011,Session322对对象释放时间的描述，associated objects清除在对象生命周期中很晚才执行，通过被NSObject -dealloc方法调用的object_dispose()函数完成);
NSObject+Guard.h
#import <Foundation/Foundation.h>
@interface NSObject (Guard)
/**
@brief 添加一个block,当该对象释放时被调用
**/
- (void)guard_addDeallocBlock:(void(^)(void))block;
@end
NSObject+Guard.m
#import "NSObject+Guard.h"
#import <objc/runtime.h>
@interface Parasite : NSObject
@property (nonatomic, copy) void(^deallocBlock)(void);
@end
@implementation Parasite
- (void)dealloc {
if (self.deallocBlock) {
self.deallocBlock();
}
}
@end
@implementation NSObject (Guard)
- (void)guard_addDeallocBlock:(void(^)(void))block {
@synchronized (self) {
static NSString *kAssociatedKey = nil;
NSMutableArray *parasiteList = objc_getAssociatedObject(self, &kAssociatedKey);
if (!parasiteList) {
parasiteList = [NSMutableArray new];
objc_setAssociatedObject(self, &kAssociatedKey, parasiteList, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}
Parasite *parasite = [Parasite new];
parasite.deallocBlock = block;
[parasiteList addObject: parasite];
}
}
@end
