iOS开发之Block原理探究
2019-04-18 本文已影响91人
it_Xiong
- Block概述
- Block本质
- Block调用
- Block分类
- Block循环引用
- Block原理探究
- Block截获变量
Block概述
在iOS中,block的使用非常方便,我们一般用来进行值的传递,与其他的传递方式(例如delegate,通知,KVO)不同,Block的调用可以使代码更加紧凑,阅读性更好.
Block本质
那么Block到底是什么呢?为了弄清楚Block的本质,我们来看看底层源码:
- 终端创建一个block.c文件:
vim block.c
里面写上C代码,一个普通的block调用场景:
#include "stdio.h"
int main(){
int multiplier = 6;
int (^block)(int) = ^int(int num){
return num * multiplier;
};
block(8);
return 0;
}
通过以下终端命令编译为c++文件,就得到了一个block.cpp的c++文件
clang -rewrite-objc block.c -o block.cpp
一路翻到最底下,我把比较重要的部分截取出来
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
};
struct __main_block_impl_0 {
struct __block_impl impl; //block结构体
struct __main_block_desc_0* Desc; //block相关描述结构体
int multiplier; //block使用变量
/*
__main_block_impl_0 c++中结构体构造函数的声明
fp 函数指针
desc block描述
_multiplier block使用变量
flags 标记
multiplier(_multiplier) _multiplier赋值给上面的int multiplier
*/
__main_block_impl_0(void *fp, struct __main_block_desc_0 *desc, int _multiplier, int flags=0) : multiplier(_multiplier) {
impl.isa = &_NSConcreteStackBlock; //isa赋值
impl.Flags = flags; //标记位赋值
impl.FuncPtr = fp; //指针赋值
Desc = desc; //block描述赋值
}
};
static int __main_block_func_0(struct __main_block_impl_0 *__cself, int num) {
int multiplier = __cself->multiplier; // bound by copy 只是值传递
return num * multiplier;
}
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 multiplier = 6;
/*
int (^block)(int) = ^int(int num){
return num * multiplier;
};
*/
/*
__main_block_impl_0 结构体
__main_block_func_0 (void*)函数指针
&__main_block_desc_0_DATA block相关描述的结构体
multiplier block中所使用户的变量
*/
int (*block)(int) = ((int (*)(int))&__main_block_impl_0((void *)__main_block_func_0, &__main_block_desc_0_DATA, multiplier));
/*
block(8);
*/
((int (*)(__block_impl *, int))((__block_impl *)block)->FuncPtr)((__block_impl *)block, 8);
return 0;
}
通过以上源码可以验证:
Block就是一个将函数及其执行上下文封装起来的对象.
Block调用
block的调用方式和普通的Objective-C方法调用基本一致,这里我们来看下block调用到底是什么?我们还是来看源码:
/*
((__block_impl *)block)->FuncPtr): 强制类型转换为__block_impl类型,取出当中的成员变量 FuncPtr
((__block_impl *)block, 8): 函数的参数,block是block本身,8是我们block传进来的参数
*/
((int (*)(__block_impl *, int))((__block_impl *)block)->FuncPtr)((__block_impl *)block, 8);
图1
图2
图3
图4
从上面四张图片可以知道,Block的执行过程就是通过__main_block__impl_0函数中传入block函数指针,然后赋值给了 __main_block_impl_0结构体中的 block结构体 impl的属性FuncPtr.
Block调用本质是就是函数的调用.
Block分类
Block分为三种,全局Block,堆Block,栈Block.
1.全局Block
void (^block)(NSString *name) = ^(NSString *name) {
NSLog(@"%@",name);
};
NSLog(@"%@",block);
控制台打印结果:
<__NSGlobalBlock__: 0x1005c1100>
也就是说是一个全局Block
2.堆Block
int a = 10;
void (^block1)(void) = ^{
NSLog(@"%d",a);
};
NSLog(@"%@",block1);
控制台打印结果:
<NSMallocBlock: 0x60000028ba50>
也就是说是一个堆Block
3.栈Block
int a = 10;
NSLog(@"%@",^{
NSLog(@"%d",a);
});
控制台打印结果:
<NSStackBlock: 0x7ffee5a78298>
也就是说是一个栈Block
Block循环引用
循环引用的产生原因我就不多说了,下面直接说我们常用的解决方法:
- __weak
__weak typeof(self) weakSelf = self;
self.oneBlock = ^(NSString *name){
//出了作用域自动释放
weakSelf.name = name;
};
self.oneBlock(@"jack");
- __block
__block TwoViewController *blockSelf = self;
self.oneBlock = ^(NSString *name){
blockSelf.name = name;
blockSelf = nil; //防止blockSelf变成野指针
};
self.oneBlock(@"jack");
3.把self当做参数传入block
self.twoBlock = ^(TwoViewController *vc) {
vc.name = @"jack";
};
self.twoBlock(self);
Block原理探究
- 终端创建一个block.c文件:
vim block.c
里面写上C代码,一个最简单的无参无返回值block:
#include "stdio.h"
int main(){
void (^block)(void) = ^{
printf("hello block");
};
block();
return 0;
}
编译为c++文件,就得到了一个block.cpp的c++文件
clang -rewrite-objc block.c -o block.cpp
一路翻到最底下,我把比较重要的部分截取出来
//block结构体
struct __block_impl {
void *isa;
int Flags;
int Reserved;
void *FuncPtr;
};
//block函数结构体
struct __main_block_impl_0 {
//block结构体
struct __block_impl impl;
struct __main_block_desc_0* Desc;
__main_block_impl_0(void *fp, struct __main_block_desc_0 *desc, int flags=0) {
impl.isa = &_NSConcreteStackBlock; //栈block 创建出来未使用就是在栈区
impl.Flags = flags;
impl.FuncPtr = fp;
Desc = desc;
}
};
static void __main_block_func_0(struct __main_block_impl_0 *__cself) {
printf("hello block");
}
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(){
void (*block)(void) = ((void (*)())&__main_block_impl_0((void *)__main_block_func_0, &__main_block_desc_0_DATA));
/*block函数,把前面的类型去掉
void (*block)(void) = __main_block_impl_0(__main_block_func_0, __main_block_desc_0_DATA));
*/
//调用block函数
((void (*)(__block_impl *))((__block_impl *)block)->FuncPtr)((__block_impl *)block);
return 0;
}
impl.isa = &_NSConcreteStackBlock; //栈block
由此,可以印证上面栈block 创建出来未使用就是在栈区.
2.现在我们block.c中代码改为如下所示
#include "stdio.h"
int main(){
int a = 1;
void (^block)(void) = ^{
printf("%d",a);
};
block();
return 0;
}
继续编译为c++文件:
//多了个 int 参数
struct __main_block_impl_0 {
struct __block_impl impl;
struct __main_block_desc_0* Desc;
int a;
__main_block_impl_0(void *fp, struct __main_block_desc_0 *desc, int _a, int flags=0) : a(_a) {
impl.isa = &_NSConcreteStackBlock;
impl.Flags = flags;
impl.FuncPtr = fp;
Desc = desc;
}
};
static void __main_block_func_0(struct __main_block_impl_0 *__cself) {
int a = __cself->a; // bound by copy
//把传进来的a copy一份, 赋值给了临时变量a,只是值传递
printf("%d",a);
}
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 a = 1;
void (*block)(void) = ((void (*)())&__main_block_impl_0((void *)__main_block_func_0, &__main_block_desc_0_DATA, a));
((void (*)(__block_impl *))((__block_impl *)block)->FuncPtr)((__block_impl *)block);
return 0;
}
在block里面打印a的值,只是把a当成一个参数传入block内部,而block内部把传入的a赋值给了一个同名的临时变量a,只是一个值传递
3.我们把block.c中的代码再次改动一下,
#include "stdio.h"
int main(){
int a = 1;
void (^block)(void) = ^{
a = 2;
};
block();
return 0;
}
再次编译为c++文件,这时终端直接报错
/var/folders/rx/4mhmzqfd0m7brd5j4hwv__xc0000gn/T/block1-18d4f2.i:441:11: error:
variable is not assignable (missing __block type specifier)
a = 2;
~ ^
1 error generated.
终端报错:变量不可赋值（缺少__block类型说明符）
那么我们这次加上 __block,
#include "stdio.h"
int main(){
__block int a = 1;
void (^block)(void) = ^{
a = 2;
};
block();
return 0;
}
这次编译之后,c++文件:
struct __Block_byref_a_0 {
void *__isa;
__Block_byref_a_0 *__forwarding;
int __flags;
int __size;
int a;
};
struct __main_block_impl_0 {
struct __block_impl impl;
struct __main_block_desc_0* Desc;
__Block_byref_a_0 *a; // by ref //截获了外部变量a
__main_block_impl_0(void *fp, struct __main_block_desc_0 *desc, __Block_byref_a_0 *_a, int flags=0) : a(_a->__forwarding) {
impl.isa = &_NSConcreteStackBlock; //堆block
impl.Flags = flags;
impl.FuncPtr = fp;
Desc = desc;
}
};
static void __main_block_func_0(struct __main_block_impl_0 *__cself) {
__Block_byref_a_0 *a = __cself->a; // bound by ref
//这里直接拿到了外部变量a的地址,通过地址把a的值改为了2
(a->__forwarding->a) = 2;
}
static void __main_block_copy_0(struct __main_block_impl_0*dst, struct __main_block_impl_0*src) {_Block_object_assign((void*)&dst->a, (void*)src->a, 8/*BLOCK_FIELD_IS_BYREF*/);}
static void __main_block_dispose_0(struct __main_block_impl_0*src) {_Block_object_dispose((void*)src->a, 8/*BLOCK_FIELD_IS_BYREF*/);}
static struct __main_block_desc_0 {
size_t reserved;
size_t Block_size;
void (*copy)(struct __main_block_impl_0*, struct __main_block_impl_0*);
void (*dispose)(struct __main_block_impl_0*);
} __main_block_desc_0_DATA = { 0, sizeof(struct __main_block_impl_0), __main_block_copy_0, __main_block_dispose_0};
int main(){
__attribute__((__blocks__(byref))) __Block_byref_a_0 a = {(void*)0,(__Block_byref_a_0 *)&a, 0, sizeof(__Block_byref_a_0), 1};
void (*block)(void) = ((void (*)())&__main_block_impl_0((void *)__main_block_func_0, &__main_block_desc_0_DATA, (__Block_byref_a_0 *)&a, 570425344));
((void (*)(__block_impl *))((__block_impl *)block)->FuncPtr)((__block_impl *)block);
return 0;
}
static void __main_block_func_0(struct __main_block_impl_0 *__cself) {
__Block_byref_a_0 *a = __cself->a; // bound by ref //地址拷贝
(a->__forwarding->a) = 2;
}
通过上面的代码可以知道,这里直接拿到了外部变量a的地址,通过地址把a的值改为了2,只是地址传递,所以 __block截获变量,是把变量地址截获,也就是把变量本身传入block内部对其改变,变量本身也会改变.
Block截获变量
这次我们用MACOS写上一段.m代码
#import <Foundation/Foundation.h>
//全局变量
int global_a = 10;
//静态全局变量
static int staic_global_a = 20;
int main(int argc, const char * argv[]) {
@autoreleasepool {
//基本数据类型的局部变量
int a = 5;
//对象类型的局部变量
__unsafe_unretained id unsafe_objc = nil;
__strong id strong_objc = nil;
//局部静态变量
static int staic_a = 6;
void(^block)(void) = ^{
NSLog(@"局部变量.基本数据类型 %d",a);
NSLog(@"局部变量.__unsafe_unretained.对象类型 %@",unsafe_objc);
NSLog(@"局部变量.__strong.对象类型 %@",strong_objc);
NSLog(@"局部静态变量 %d",staic_a);
NSLog(@"全局变量 %d",global_a);
NSLog(@"静态全局变量 %d",staic_global_a);
};
block();
}
return 0;
}
然后在终端用命令 clang -rewrite-objc -fobjc-arc main.m编译为c++代码,这次我们只看block的函数部分:
//对全局变量,静态全局变量不截获
int global_a = 10;
static int staic_global_a = 20;
struct __main_block_impl_0 {
struct __block_impl impl;
struct __main_block_desc_0* Desc;
//截获局部变量的值
int a;
//连同所有权s修饰符一起截获
__unsafe_unretained id unsafe_objc;
__strong id strong_objc;
//以指针形式截获静态局部变量
int *staic_a;
__main_block_impl_0(void *fp, struct __main_block_desc_0 *desc, int _a, __unsafe_unretained id _unsafe_objc, __strong id _strong_objc, int *_staic_a, int flags=0) : a(_a), unsafe_objc(_unsafe_objc), strong_objc(_strong_objc), staic_a(_staic_a) {
impl.isa = &_NSConcreteStackBlock;
impl.Flags = flags;
impl.FuncPtr = fp;
Desc = desc;
}
};
通过以上的代码,我们可以得出结论:
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
一般情况下,对被截获变量进行赋值操作 需要添加__block修饰符,但是它对不同类型的变量却又有不同:
需要用到__block修饰符的: 局部变量(基本数据类型,对象)
不需要用到__block修饰符的: 局部静态变量 (直接操作指针), 全局变量,静态全局变量,而后面两个不涉及截获,直接操作本身
