什么是Block
-
Block是将函数及其执行上下文封装起来的对象。
#import "MCBlock.h" @implementation MCBlock - (void)method { static int multiplier = 6; int(^Block)(int) = ^int(int num) { return num * multiplier; }; Block(2); } @end经过命令行重写
clang -rewrite-objc MCBlock.m,生成文件MCBlock.cpp//I代表实例方法 static void _I_MCBlock_method(MCBlock * self, SEL _cmd) { int multiplier = 6; int(*Block)(int) = ((int (*)(int))&__MCBlock__method_block_impl_0((void *)__MCBlock__method_block_func_0, &__MCBlock__method_block_desc_0_DATA, multiplier)); ((int (*)(__block_impl *, int))((__block_impl *)Block)->FuncPtr)((__block_impl *)Block, 2); }struct __MCBlock__method_block_impl_0 { struct __block_impl impl; struct __MCBlock__method_block_desc_0* Desc; int multiplier; __MCBlock__method_block_impl_0(void *fp, struct __MCBlock__method_block_desc_0 *desc, int _multiplier, int flags=0) : multiplier(_multiplier) { impl.isa = &_NSConcreteStackBlock; impl.Flags = flags; impl.FuncPtr = fp; Desc = desc; } }; static int __MCBlock__method_block_func_0(struct __MCBlock__method_block_impl_0 *__cself, int num) { int multiplier = __cself->multiplier; // bound by copy return num * multiplier; }struct __block_impl { void *isa;//isa指针，Block是对象的标志 int Flags; int Reserved; void *FuncPtr;//函数指针 }; Block的调用即是函数的调用
Block截获变量
//滴滴面试
{
static int multiplier = 6;
int(^Block)(int) = ^int(int num)
{
return num * multiplier;
};
multiplier = 4;
NSLog(@"result is %d",Block(2));//result is 12
}
- 局部变量
- 基本数据类型
- 对象类型
- 静态局部变量
- 全局变量
- 静态全局变量
关于Block的截获特性你是否有了解，以及Block的截获特性又是怎样的？
变量的截获方式
- 对于基本数据类型的局部变量截获其值。
- 对于对象类型的局部变量连同所有权修饰符一起截获。
- 以指针形式截获局部静态变量。
- 不截获全局变量、静态全局变量。
#import "MCBlock.h"
@implementation MCBlock
//全局变量
int global_var = 4;
//静态全局变量
static int static_global_var = 5;
- (void)method
{
//基本数据类型的局部变量
int var = 1;
//对象类型的局部变量
__unsafe_unretained id unsafe_obj = nil;
__strong id strong_obj = nil;
static int static_var = 3;
void(^Block)(void) = ^{
NSLog(@"局部变量<基本数据类型> var %d", var);
NSLog(@"局部变量<__unsafe_unretained 对象类型> var %@", unsafe_obj);
NSLog(@"局部变量<__strong 对象类型> var %@", strong_obj);
NSLog(@"静态变量 %d", static_var);
NSLog(@"全局变量 %d", global_var);
NSLog(@"静态全局变量 %d", static_global_var);
};
Block();
}
@end
clang -rewrite-objc -fobjc-arc MCBlock.m
// @implementation MCBlock
int global_var = 4;
static int static_global_var = 5;
struct __MCBlock__method_block_impl_0 {
struct __block_impl impl;
struct __MCBlock__method_block_desc_0* Desc;
//截获局部变量的值
int var;
//连同所有权修饰符一起截获
__unsafe_unretained id unsafe_obj;
__strong id strong_obj;
//以指针形式截获局部变量
int *static_var;
//对全局变量、静态全局变量不截获
__MCBlock__method_block_impl_0(void *fp, struct __MCBlock__method_block_desc_0 *desc, int _var, __unsafe_unretained id _unsafe_obj, __strong id _strong_obj, int *_static_var, int flags=0) : var(_var), unsafe_obj(_unsafe_obj), strong_obj(_strong_obj), static_var(_static_var) {
impl.isa = &_NSConcreteStackBlock;
impl.Flags = flags;
impl.FuncPtr = fp;
Desc = desc;
}
};
static void __MCBlock__method_block_func_0(struct __MCBlock__method_block_impl_0 *__cself) {
int var = __cself->var; // bound by copy
__unsafe_unretained id unsafe_obj = __cself->unsafe_obj; // bound by copy
__strong id strong_obj = __cself->strong_obj; // bound by copy
int *static_var = __cself->static_var; // bound by copy
NSLog((NSString *)&__NSConstantStringImpl__var_folders_hk_552cymzd23v5b0db528k_jbr0000gn_T_MCBlock_a2d5da_mi_0, var);
NSLog((NSString *)&__NSConstantStringImpl__var_folders_hk_552cymzd23v5b0db528k_jbr0000gn_T_MCBlock_a2d5da_mi_1, unsafe_obj);
NSLog((NSString *)&__NSConstantStringImpl__var_folders_hk_552cymzd23v5b0db528k_jbr0000gn_T_MCBlock_a2d5da_mi_2, strong_obj);
NSLog((NSString *)&__NSConstantStringImpl__var_folders_hk_552cymzd23v5b0db528k_jbr0000gn_T_MCBlock_a2d5da_mi_3, (*static_var));
NSLog((NSString *)&__NSConstantStringImpl__var_folders_hk_552cymzd23v5b0db528k_jbr0000gn_T_MCBlock_a2d5da_mi_4, global_var);
NSLog((NSString *)&__NSConstantStringImpl__var_folders_hk_552cymzd23v5b0db528k_jbr0000gn_T_MCBlock_a2d5da_mi_5, static_global_var);
}
static void __MCBlock__method_block_copy_0(struct __MCBlock__method_block_impl_0*dst, struct __MCBlock__method_block_impl_0*src) {_Block_object_assign((void*)&dst->unsafe_obj, (void*)src->unsafe_obj, 3/*BLOCK_FIELD_IS_OBJECT*/);_Block_object_assign((void*)&dst->strong_obj, (void*)src->strong_obj, 3/*BLOCK_FIELD_IS_OBJECT*/);}
static void __MCBlock__method_block_dispose_0(struct __MCBlock__method_block_impl_0*src) {_Block_object_dispose((void*)src->unsafe_obj, 3/*BLOCK_FIELD_IS_OBJECT*/);_Block_object_dispose((void*)src->strong_obj, 3/*BLOCK_FIELD_IS_OBJECT*/);}
static struct __MCBlock__method_block_desc_0 {
size_t reserved;
size_t Block_size;
void (*copy)(struct __MCBlock__method_block_impl_0*, struct __MCBlock__method_block_impl_0*);
void (*dispose)(struct __MCBlock__method_block_impl_0*);
} __MCBlock__method_block_desc_0_DATA = { 0, sizeof(struct __MCBlock__method_block_impl_0), __MCBlock__method_block_copy_0, __MCBlock__method_block_dispose_0};
static void _I_MCBlock_method(MCBlock * self, SEL _cmd) {
int var = 1;
__attribute__((objc_ownership(none))) id unsafe_obj = __null;
__attribute__((objc_ownership(strong))) id strong_obj = __null;
static int static_var = 3;
void(*Block)(void) = ((void (*)())&__MCBlock__method_block_impl_0((void *)__MCBlock__method_block_func_0, &__MCBlock__method_block_desc_0_DATA, var, unsafe_obj, strong_obj, &static_var, 570425344));
((void (*)(__block_impl *))((__block_impl *)Block)->FuncPtr)((__block_impl *)Block);
}
// @end
__block修饰符
-
一般情况下，对被截获变量进行赋值操作需添加
__block修饰符 - 对静态局部变量，全局变量，静态全局变量进行赋值时候，不需要
__block修饰符
{
NSMutableArray *array = [NSMutableArray array];
void(^Block)(void) = ^{
[array addObject:@123];
};
Block();
}
上面的array是否需要__block修饰？
上面只是对array进行了使用，而不是赋值操作，只有赋值操作才需要
__block修饰的变量变成了对象
{
__block int multiplier = 6;
int(^Block)(int) = ^int(int num)
{
return num * multiplier;
};
multiplier = 4;
Block(2); //8
}
__block修饰.png
栈区__block.png
所以上面的代码multiplier = 4;实际上变成了(multiplier._forwarding->multiplier) = 4;
Block的类型
- _NSConcreteGlobalBlock
- _NSConcreteStackBlock
- _NSConcreteMallocBlock
Block位置.png
Block的Copy操作
假如声明一个对象的成员变量是Block，而在栈上去创建Block，同时赋值给成员变量的Block，如果成员变量的Block没有使用copy关键字的话，当我们具体通过成员变量去访问对应的Block，可能就由于栈上的Block被释放而引起崩溃。
栈上Block的销毁
block的销毁.png
栈上Block的copy
栈上Block的copy.png
在MRC环境，假如对栈上Block进行copy操作，Block会引起内存泄漏，因为Block copy之后，堆上Block没有额外的成员变量去指向它，就像对一个对象进行alloc操作之后，没有调用对应的release的效果是一样的,产生内存泄漏。
栈上Block的copy02.png
栈上Block进行copy之后，
__forwarding指针是指向堆上的__block变量，堆上的Block__forwarding指针指向自身。
__forwarding总结
//百度面试题
{
__block int multiplier = 10;
_blk = ^int(int num) {
return num * multiplier;
};
multiplier = 6;
[self executeBlock];
}
- (void)executeBlock
{
int result = _blk(4);
NSLog(@"result is %d", result); //result is 24
}
__forwarding存在的意义
不论在任何内存位置，都可以顺利的访问同一个__block变量。
Block的引用循环
__block的引用循环
{
__block MCBlock *blockSelf = self;
_blk = ^int(int num) {
//var = 2
return num * blockSelf.var;
};
_blk(3);
}
- 在MRC下，不会产生引用循环。
- 在ARC下，会产生循环引用，引起内存泄漏。ß
__block的循环引用.png
{
__block MCBlock *blockSelf = self;
_blk = ^int(int num) {
//var = 2
int result = num * blockSelf.var;
blockSelf = nil;
return result;
};
_blk(3);//如果Block长时间等不到调用，则引用循环会存在。
}
