这是一篇比较长的博文，前部分是block的测试题目，中间是block的语法、特性，block讲解block内部实现和block存储位置，请读者耐心阅读。具备block基础的同学，直接调转到block的实现
下面列出了五道题，看看能否答对两三个。主要涉及block栈上、还是堆上、怎么捕获变量。答案在博文最后一行
//-----------第一道题：--------------
void exampleA() {
char a = 'A';
^{ printf("%c\n", a);};
}
A.始终能够正常运行 B.只有在使用ARC的情况下才能正常运行
C.不使用ARC才能正常运行 D.永远无法正常运行
//-----------第二道题：答案同第一题--------------
void exampleB_addBlockToArray(NSMutableArray *array) {
char b = 'B';
[array addObject:^{printf("%c\n", b);}];
}
void exampleB() {
NSMutableArray *array = [NSMutableArray array];
exampleB_addBlockToArray(array);
void (^block)() = [array objectAtIndex:0];
block();
}
//-----------第三道题：答案同第一题--------------
void exampleC_addBlockToArray(NSMutableArray *array) {
[array addObject:^{printf("C\n");}];
}
void exampleC() {
NSMutableArray *array = [NSMutableArray array];
exampleC_addBlockToArray(array);
void (^block)() = [array objectAtIndex:0];
block();
}
//-----------第四道题：答案同第一题--------------
typedef void (^dBlock)();
dBlock exampleD_getBlock() {
char d = 'D';
return ^{printf("%c\n", d);};
}
void exampleD() {
exampleD_getBlock()();
}
//-----------第五道题：答案同第一题--------------
typedef void (^eBlock)();
eBlock exampleE_getBlock() {
char e = 'E';
void (^block)() = ^{printf("%c\n", e);};
return block;
}
void exampleE() {
eBlock block = exampleE_getBlock();
block();
}
注：以上题目摘自：CocoaChina论坛点击打开链接
int main(){
void (^blk)(void) = ^{printf("block\n");};
blk();
return 0;
}struct __block_impl{
void *isa;
int Flags;
int Reserved;
void *FuncPtr;
};
static struct __main_block_desc_0{
unsigned long reserved;
unsigned long Block_size
}__main_block_desc_0_DATA = { 0, sizeof(struct __main_block_impl_0)};
struct __main_block_impl_0{
struct __block_impl impl;
struct __main_block_desc_0 *Desc;
}
static struct __main_block_func_0(struct __main_block_impl_0 *__cself)
{
printf("block\n");
}
int main(){
struct __main_block_impl_0 *blk = &__main_block_impl_0(__main_block_func_0,&__main_block_desc_0_DATA);
(*blk->impl.FuncPtr)(blk);
}__main_block_impl_0{
void *isa;
int Flags;
int Reserved;
void *FuncPtr;
struct __main_block_desc_0 *Desc;
}int val = 10;
void (^blk)(void) = ^{printf("val=%d\n",val);};
val = 2;
blk();
上面这段代码，输出值是：val = 10.而不是2.block截获自动变量的瞬时值。因为block保存了自动变量的值，所以在执行block语法后，即使改写block中使用的自动变量的值也不会影响block执行时自动变量的值。
__main_block_impl_0{
void *isa;
int Flags;
int Reserved;
void *FuncPtr;
struct __main_block_desc_0 *Desc;
int val；
}int main(){
struct __main_block_impl_0 *blk = &__main_block_impl_0(__main_block_func_0,&__main_block_desc_0_DATA,val);
}注意函数调用最后一个参数，即val参数。static struct __main_block_func_0(struct __main_block_impl_0 *__cself)
{
printf("val=%d\n"，__cself-val);
}所以，block捕获变量更像是：函数按值传递。__block int val = 10;
void (^blk)(void) = ^{val = 1;};struct __block_byref_val_0{
void *__isa;
__block_byref_val_0 *__forwarding;
int _flags;
int __size;
int val;
}typedef int (^blk_t)(int);
for(...){
blk_t blk = ^(int count) {return count;};
}-(id) getBlockArray{
int val =10;
return [[NSArray alloc]initWithObjects:
^{NSLog(@"blk0:%d",val);},
^{NSLog(@"blk1:%d",val);},nil];
}
id obj = getBlockArray();
typedef void (^blk_t)(void);
blk_t blk = (blk_t){obj objectAtIndex:0};
blk();typedef int (^blkt1)(void) ;
-(void) stackOrHeap{
__block int val =10;
int *valPtr = &val;//使用int的指针，来检测block到底在栈上，还是堆上
blkt1 s= ^{
NSLog(@"val_block = %d",++val);
return val;};
s();
NSLog(@"valPointer = %d",*valPtr);
}调用copy之后的结果呢：
-(void) stackOrHeap{
__block int val =10;
int *valPtr = &val;//使用int的指针，来检测block到底在栈上，还是堆上
blkt1 s= ^{
NSLog(@"val_block = %d",++val);
return val;};
blkt1 h = [s copy];
h();
NSLog(@"valPointer = %d",*valPtr);
}
----------------在ARC下>>>>>>>>>>>无效果。 val_block = 11 valPointer = 10
static void __main_block_copy_0(struct __main_block_impl_0 *dst, struct __main_block_impl_0 *src){
_Block_objct_assign(&dst->val,src->val,BLOCK_FIELD_IS_BYREF);
}
static void __main_block_dispose_0(struct __main_block_impl_0 *src){
_block_object_dispose(src->val,BLOCK_FIELD_IS_BYREF);
}int val = 10;
void (^blk)(void) = ^{printf("val=%d\n",val);};
val = 2;
blk();
上面这段代码，输出值是：val = 10.而不是2.block截获自动变量的瞬时值。因为block保存了自动变量的值，所以在执行block语法后，即使改写block中使用的自动变量的值也不会影响block执行时自动变量的值。
OC高级编程——深入block，如何捕获变量，如何存储在堆上,布布扣,bubuko.com
OC高级编程——深入block，如何捕获变量，如何存储在堆上
原文地址：http://blog.csdn.net/hherima/article/details/38586101
