(一)Block的常見(jiàn)問(wèn)題
首先我們先看下面幾個(gè)問(wèn)題：
block的原理是怎樣的？本質(zhì)是什么？
__block的作用是什么？有什么使用注意點(diǎn)？
block的屬性修飾詞為什么是copy？使用block有哪些使用注意？
block在修改NSMutableArray，需不需要添加__block？
如果上面的不能完整的回答，說(shuō)明block的知識(shí)需要進(jìn)行深刻的學(xué)習(xí)了
(二)block的本質(zhì)及底層結(jié)構(gòu)
- block本質(zhì)上也是一個(gè)OC對(duì)象，它內(nèi)部也有個(gè)isa指針
- block是封裝了函數(shù)調(diào)用以及函數(shù)調(diào)用環(huán)境的OC對(duì)象
下面我們通過(guò)多個(gè)方法去印證。
(1)block的底層結(jié)構(gòu)
我們看下面的代碼：
int main(int argc, const char * argv[]) {
@autoreleasepool {
^{
NSLog(@"hello,world!");
}();
int age = 10;
void(^block)(int,int) = ^(int a,int b){
NSLog(@"this is a block == %d" , age);
NSLog(@"this is a block a == %d b == %d" , a, b);
};
block(20,30);
}
return 0;
}
通過(guò)clang編譯器轉(zhuǎn)化為C++代碼后（截取主要代碼）：
//struct __block_impl結(jié)構(gòu)
struct __block_impl {
void *isa;//isa指針
int Flags;
int Reserved;
void *FuncPtr;
};
//第一個(gè)block的底層結(jié)構(gòu)
struct __main_block_impl_0 {
struct __block_impl impl;
struct __main_block_desc_0* Desc;
//構(gòu)造函數(shù)(C++)
__main_block_impl_0(void *fp, struct __main_block_desc_0 *desc, int flags=0) {
impl.isa = &_NSConcreteStackBlock;
impl.Flags = flags;
impl.FuncPtr = fp;
Desc = desc;
}
};
//第一個(gè)block調(diào)用函數(shù)
static void __main_block_func_0(struct __main_block_impl_0 *__cself) {
NSLog((NSString *)&__NSConstantStringImpl__var_folders_f9__q8q2xr131vf4h1cy7837sm00000gn_T_main_26fdd2_mi_0);
}
//第一個(gè)block描述信息
static struct __main_block_desc_0 {
size_t reserved;
size_t Block_size;
} __main_block_desc_0_DATA = { 0, sizeof(struct __main_block_impl_0)};
//第二個(gè)block的底層結(jié)構(gòu)（包含變量age）
struct __main_block_impl_1 {
struct __block_impl impl;
struct __main_block_desc_1* Desc;
int age;
__main_block_impl_1(void *fp, struct __main_block_desc_1 *desc, int _age, int flags=0) : age(_age) {
impl.isa = &_NSConcreteStackBlock;
impl.Flags = flags;
impl.FuncPtr = fp;
Desc = desc;
}
};
//第二個(gè)block調(diào)用函數(shù)
static void __main_block_func_1(struct __main_block_impl_1 *__cself, int a, int b) {
int age = __cself->age; // bound by copy
NSLog((NSString *)&__NSConstantStringImpl__var_folders_f9__q8q2xr131vf4h1cy7837sm00000gn_T_main_26fdd2_mi_1 , age);
NSLog((NSString *)&__NSConstantStringImpl__var_folders_f9__q8q2xr131vf4h1cy7837sm00000gn_T_main_26fdd2_mi_2 , a, b);
}
//第二個(gè)block描述信息
static struct __main_block_desc_1 {
size_t reserved;
size_t Block_size;
} __main_block_desc_1_DATA = { 0, sizeof(struct __main_block_impl_1)};
int main(int argc, const char * argv[]) {
/* @autoreleasepool */ { __AtAutoreleasePool __autoreleasepool;
((void (*)())&__main_block_impl_0((void *)__main_block_func_0, &__main_block_desc_0_DATA))();
//簡(jiǎn)化后 &__main_block_impl_0(__main_block_func_0,&__main_block_desc_0_DATA)
int age = 10;
void(*block)(int,int) = ((void (*)(int, int))&__main_block_impl_1((void *)__main_block_func_1, &__main_block_desc_1_DATA, age));
//簡(jiǎn)化后 block = &__main_block_impl_1(__main_block_func_1, &__main_block_desc_1_DATA, age)
((void (*)(__block_impl *, int, int))((__block_impl *)block)->FuncPtr)((__block_impl *)block, 20, 30);
//簡(jiǎn)化后 (block -> FuncPtr)(block, 20, 30)
}
return 0;
}
我們通過(guò)轉(zhuǎn)化后的代碼可以證實(shí)。另外，我們通過(guò)，類(lèi)型強(qiáng)轉(zhuǎn)也可以查看block的底層結(jié)構(gòu)
通過(guò)類(lèi)型強(qiáng)轉(zhuǎn)查看底層結(jié)構(gòu)
image.png
總結(jié)
block的底層結(jié)構(gòu)圖解
(2)block的本質(zhì)
int main(int argc, const char * argv[]) {
@autoreleasepool {
int age = 10;
void(^block)(int,int) = ^(int a,int b){
NSLog(@"this is a block == %d" , age);
NSLog(@"this is a block a == %d b == %d" , a, b);
};
block(20,30);
}
return 0;
}
通過(guò)clang編譯器轉(zhuǎn)化為C++代碼后（截取主要代碼）：
struct __block_impl {
void *isa;
int Flags;
int Reserved;
void *FuncPtr;
};
struct __main_block_impl_0 {
struct __block_impl impl;
struct __main_block_desc_0* Desc;
int age;
//構(gòu)造函數(shù)（類(lèi)似于init方法）
__main_block_impl_0(void *fp, struct __main_block_desc_0 *desc, int _age, int flags=0) : age(_age) {
impl.isa = &_NSConcreteStackBlock;
impl.Flags = flags;
impl.FuncPtr = fp;
Desc = desc;
}
};
//將block的內(nèi)部執(zhí)行邏輯封裝到__main_block_func_0函數(shù)
static void __main_block_func_0(struct __main_block_impl_0 *__cself, int a, int b) {
int age = __cself->age; // bound by copy
NSLog((NSString *)&__NSConstantStringImpl__var_folders_f9__q8q2xr131vf4h1cy7837sm00000gn_T_main_142399_mi_1 , age);
NSLog((NSString *)&__NSConstantStringImpl__var_folders_f9__q8q2xr131vf4h1cy7837sm00000gn_T_main_142399_mi_2 , a, b);
}
static struct __main_block_desc_0 {
size_t reserved;
size_t Block_size;
} __main_block_desc_0_DATA = { 0, sizeof(struct __main_block_impl_0)};
int main(int argc, const char * argv[]) {
/* @autoreleasepool */ { __AtAutoreleasePool __autoreleasepool;
int age = 10;
//定義block變量
void(*block)(int,int) = ((void (*)(int, int))&__main_block_impl_0((void *)__main_block_func_0, &__main_block_desc_0_DATA, age));
//簡(jiǎn)化：block = &__main_block_impl_0(__main_block_func_0, &__main_block_desc_0_DATA, age) 參數(shù)1:函數(shù)地址 參數(shù)2：block的描述 參數(shù)3：age
//執(zhí)行block內(nèi)部代碼
((void (*)(__block_impl *, int, int))((__block_impl *)block)->FuncPtr)((__block_impl *)block, 20, 30);
// //簡(jiǎn)化：(block->FuncPtr)(block, 20, 30) 函數(shù)調(diào)用
}
return 0;
}
(2)block的本質(zhì)
int main(int argc, const char * argv[]) {
@autoreleasepool {
int age = 10;
void(^block)(int,int) = ^(int a,int b){
NSLog(@"this is a block a == %d" , age);
};
block(20,30);
NSLog(@"%@",[block class]);//__NSMallocBlock__
NSLog(@"%@",[[block class] superclass]);//__NSMallocBlock
NSLog(@"%@",[[[block class] superclass] superclass]);//NSBlock
NSLog(@"%@",[[[[block class] superclass] superclass] superclass]);//NSObject
NSLog(@"%@",[[[[[block class] superclass] superclass] superclass] superclass]);//(null)
}
return 0;
}
從輸出結(jié)果來(lái)看，block是一個(gè)OC對(duì)象，基類(lèi)是NSObject，下一章我們對(duì)block的類(lèi)型進(jìn)行進(jìn)一步的學(xué)習(xí)。
