1. 循環(huán)引用是如何產(chǎn)生的
循環(huán)引用顧名思義就是相互直接或間接強(qiáng)引用達(dá)成了一個(gè)環(huán)，造成了相互持有（eg：A--B B--A），釋放的時(shí)候由于這個(gè)持有關(guān)系，A要等待B釋放了對(duì)自己的持有才會(huì)釋放，同時(shí)B也要等待A釋放了對(duì)自己的持有才會(huì)釋放，最終導(dǎo)致都無(wú)法釋放，產(chǎn)生了內(nèi)存泄漏
常見的循環(huán)引用的產(chǎn)生場(chǎng)景
- 對(duì)象的Block成員內(nèi)部訪問了self或者self的屬性
- 代理的修飾符使用了強(qiáng)引用，例如：strong
- 對(duì)象A持有了對(duì)象B，對(duì)象B直接或間接的持有了對(duì)象A
2. weak的方式解決循環(huán)引用
weak是如何解決循環(huán)引用的細(xì)節(jié)可以參照這個(gè)文章weak如何解決循環(huán)引用
@property (copy, nonatomic) void(^testBreakRetainCycleBlockByWeak)(void);
- (void)testBreakRetainCycleByWeak {
__weak typeof(self) weakSelf = self;
self.testBreakRetainCycleBlockByWeak = ^(void) {
__strong typeof(self) strongSelf = weakSelf; // 涉及到多線程異步操作，最好加上一個(gè)強(qiáng)引用，這樣保證下面的任務(wù)可以被執(zhí)行
dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(2 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
NSLog(@"%@", strongSelf); // 執(zhí)行該段代碼之后，臨時(shí)變量strongSelf就釋放了，self就能正常的dealloc
});
};
self.testBlockTypeBlock();
}
使用weak，block內(nèi)部結(jié)構(gòu)捕獲變量使用的是弱引用關(guān)系，這樣就避免了相互強(qiáng)引用
3. 臨時(shí)變量持有self，設(shè)置為nil
@property (copy, nonatomic) void(^testBreakRetainCycleBlockBySetNil)(void);
- (void)testBreakRetainCycleBySetNil {
NSLog(@"testBreakRetainCycleBySetNil:self.retainCount before tmp refrence = %ld", CFGetRetainCount((__bridge CFTypeRef)(self))); // 1
__block BlockTest *strongSelf = self;
NSLog(@"testBreakRetainCycleBySetNil:self.retainCount after tmp refrence = %ld", CFGetRetainCount((__bridge CFTypeRef)(self))); // 2
self.testBreakRetainCycleBlockBySetNil = ^(void) {
NSLog(@"testBreakRetainCycle:%@", strongSelf);
strongSelf = nil;
};
self.testBreakRetainCycleBlockBySetNil();
NSLog(@"testBreakRetainCycleBySetNil:self.retainCount after set nil = %ld", CFGetRetainCount((__bridge CFTypeRef)(self))); // 1
}
Clang Rewrite 查看內(nèi)部實(shí)現(xiàn)
Clang rewrite可以參照weak如何解決循環(huán)引用
struct __Block_byref_strongSelf_3 {
void *__isa;
__Block_byref_strongSelf_3 *__forwarding;
int __flags;
int __size;
void (*__Block_byref_id_object_copy)(void*, void*);
void (*__Block_byref_id_object_dispose)(void*);
BlockTest *__strong strongSelf;
};
struct __BlockTest__testBreakRetainCycleBySetNil_block_impl_0 {
struct __block_impl impl;
struct __BlockTest__testBreakRetainCycleBySetNil_block_desc_0* Desc;
__Block_byref_strongSelf_3 *strongSelf; // by ref
__BlockTest__testBreakRetainCycleBySetNil_block_impl_0(void *fp, struct __BlockTest__testBreakRetainCycleBySetNil_block_desc_0 *desc, __Block_byref_strongSelf_3 *_strongSelf, int flags=0) : strongSelf(_strongSelf->__forwarding) {
impl.isa = &_NSConcreteStackBlock;
impl.Flags = flags;
impl.FuncPtr = fp;
Desc = desc;
}
};
static void __BlockTest__testBreakRetainCycleBySetNil_block_func_0(struct __BlockTest__testBreakRetainCycleBySetNil_block_impl_0 *__cself) {
__Block_byref_strongSelf_3 *strongSelf = __cself->strongSelf; // bound by ref
NSLog((NSString *)&__NSConstantStringImpl__var_folders_8j_551wkz1d1rg878kt8mg21tg40000gn_T_BlockTest_04791f_mi_25, (strongSelf->__forwarding->strongSelf));
(strongSelf->__forwarding->strongSelf) = __null;
}
static void __BlockTest__testBreakRetainCycleBySetNil_block_copy_0(struct __BlockTest__testBreakRetainCycleBySetNil_block_impl_0*dst, struct __BlockTest__testBreakRetainCycleBySetNil_block_impl_0*src) {_Block_object_assign((void*)&dst->strongSelf, (void*)src->strongSelf, );}
static void __BlockTest__testBreakRetainCycleBySetNil_block_dispose_0(struct __BlockTest__testBreakRetainCycleBySetNil_block_impl_0*src) {_Block_object_dispose((void*)src->strongSelf,);}
static struct __BlockTest__testBreakRetainCycleBySetNil_block_desc_0 {
size_t reserved;
size_t Block_size;
void (*copy)(struct __BlockTest__testBreakRetainCycleBySetNil_block_impl_0*, struct __BlockTest__testBreakRetainCycleBySetNil_block_impl_0*);
void (*dispose)(struct __BlockTest__testBreakRetainCycleBySetNil_block_impl_0*);
} __BlockTest__testBreakRetainCycleBySetNil_block_desc_0_DATA = { 0, sizeof(struct __BlockTest__testBreakRetainCycleBySetNil_block_impl_0), __BlockTest__testBreakRetainCycleBySetNil_block_copy_0, __BlockTest__testBreakRetainCycleBySetNil_block_dispose_0};
static void _I_BlockTest_testBreakRetainCycleBySetNil(BlockTest * self, SEL _cmd) {
__attribute__((__blocks__(byref))) __Block_byref_strongSelf_3 strongSelf = {(void*)0,(__Block_byref_strongSelf_3 *)&strongSelf, 33554432, sizeof(__Block_byref_strongSelf_3), __Block_byref_id_object_copy_131, __Block_byref_id_object_dispose_131, self};
((void (*)(id, SEL, void (*)()))(void *)objc_msgSend)((id)self, sel_registerName("setTestBreakRetainCycleBlockBySetNil:"), ((void (*)())&__BlockTest__testBreakRetainCycleBySetNil_block_impl_0((void *)__BlockTest__testBreakRetainCycleBySetNil_block_func_0, &__BlockTest__testBreakRetainCycleBySetNil_block_desc_0_DATA, (__Block_byref_strongSelf_3 *)&strongSelf, 570425344)));
((void (*(*)(id, SEL))())(void *)objc_msgSend)((id)self, sel_registerName("testBreakRetainCycleBlockBySetNil"))();
}
這種方式可以解決循環(huán)引用：由于定義了block類型的局部變量，會(huì)轉(zhuǎn)換成結(jié)構(gòu)體(結(jié)構(gòu)體內(nèi)部強(qiáng)持有了self)，block函數(shù)內(nèi)部結(jié)構(gòu)體中引用這個(gè)結(jié)構(gòu)體，當(dāng)將block類型的局部變量置為nil之后，強(qiáng)持有關(guān)系就被打斷了
可以看到strongSelf = nil;內(nèi)部是將Block的結(jié)構(gòu)體持有的strongSelf置為空，那么block就沒有強(qiáng)持有self了，引用環(huán)打斷了
(strongSelf->__forwarding->strongSelf) = __null;
4. 將self作為參數(shù)傳遞的方式
@property (copy, nonatomic) void(^testBreakRetainCycleBlockBySetNil)(void);
- (void)testBreakRetainCycleByPassParam {
self.testBreakRetainCycleBlockByPassParam = ^(BlockTest *paramSelf) {
NSLog(@"testBreakRetainCycle:%@", paramSelf);
};
self.testBreakRetainCycleBlockByPassParam(self);
}
Clang Rewrite 查看內(nèi)部實(shí)現(xiàn)
static void _I_BlockTest_testBlockRetainCycle(BlockTest * self, SEL _cmd) {
((void (*)(id, SEL, void (*)()))(void *)objc_msgSend)((id)self, sel_registerName("setTestRetainCycleBlock:"), ((void (*)())&__BlockTest__testBlockRetainCycle_block_impl_0((void *)__BlockTest__testBlockRetainCycle_block_func_0, &__BlockTest__testBlockRetainCycle_block_desc_0_DATA, self, 570425344)));
((void (*(*)(id, SEL))())(void *)objc_msgSend)((id)self, sel_registerName("testRetainCycleBlock"))();
}
struct __BlockTest__testBreakRetainCycleByPassParam_block_impl_0 {
struct __block_impl impl;
struct __BlockTest__testBreakRetainCycleByPassParam_block_desc_0* Desc;
__BlockTest__testBreakRetainCycleByPassParam_block_impl_0(void *fp, struct __BlockTest__testBreakRetainCycleByPassParam_block_desc_0 *desc, int flags=0) {
impl.isa = &_NSConcreteStackBlock;
impl.Flags = flags;
impl.FuncPtr = fp;
Desc = desc;
}
};
static void __BlockTest__testBreakRetainCycleByPassParam_block_func_0(struct __BlockTest__testBreakRetainCycleByPassParam_block_impl_0 *__cself, BlockTest *__strong paramSelf) {
NSLog((NSString *)&__NSConstantStringImpl__var_folders_r0_4tb84bbj15j3kbzrnk8hqdwm0000gn_T_BlockTest_0cf65b_mi_24, paramSelf);
}
static struct __BlockTest__testBreakRetainCycleByPassParam_block_desc_0 {
size_t reserved;
size_t Block_size;
} __BlockTest__testBreakRetainCycleByPassParam_block_desc_0_DATA = { 0, sizeof(struct __BlockTest__testBreakRetainCycleByPassParam_block_impl_0)};
static void _I_BlockTest_testBreakRetainCycleByPassParam(BlockTest * self, SEL _cmd) {
((void (*)(id, SEL, void (*)(BlockTest *__strong)))(void *)objc_msgSend)((id)self, sel_registerName("setTestBreakRetainCycleBlockByPassParam:"), ((void (*)(BlockTest *__strong))&__BlockTest__testBreakRetainCycleByPassParam_block_impl_0((void *)__BlockTest__testBreakRetainCycleByPassParam_block_func_0, &__BlockTest__testBreakRetainCycleByPassParam_block_desc_0_DATA)));
((void (*(*)(id, SEL))(BlockTest *__strong))(void *)objc_msgSend)((id)self, sel_registerName("testBreakRetainCycleBlockByPassParam"))(self);
}
rewrite之后，查看傳參的方式，內(nèi)部不會(huì)捕獲（持有）這個(gè)參數(shù)，當(dāng)block執(zhí)行完畢參數(shù)就被釋放了
這種傳參的方式，不會(huì)有循環(huán)引用問題：由于傳參的方式，block內(nèi)部不會(huì)去捕獲持有這個(gè)參數(shù)，只是一個(gè)臨時(shí)變量的強(qiáng)引用，當(dāng)block執(zhí)行完之后，參數(shù)就會(huì)被釋放
