iOS Block详解

2019-02-27 本文已影响351人 num_one

学习Block的章节

章节

Block介绍（源码解析）

什么是Block

Block的本质

代码被编译后：

static void _I_MCBlock_method(MCBlock * self, SEL _cmd) {
    // int multiplier = 6;
    // 此处的变量可以证实block对执行block函数的上下文做了数据截获。
    int multiplier = 6;
    /*
     int(^Block)(int) = ^int(int num)
     {
        return num * multiplier;
     };
     */
    int(*Block)(int) = ((int (*)(int))&__MCBlock__method_block_impl_0((void *)__MCBlock__method_block_func_0, &__MCBlock__method_block_desc_0_DATA, multiplier));
    // Block(2);
    ((int (*)(__block_impl *, int))((__block_impl *)Block)->FuncPtr)((__block_impl *)Block, 2);
}

__MCBlock__method_block_impl_0 解析：

struct __MCBlock__method_block_impl_0 {
  struct __block_impl impl; // 包含isa指针
  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;
  }
};
// 由此可见，block确实可以被认为是一个对象。
struct __block_impl {
  void *isa;
  int Flags;
  int Reserved;
  void *FuncPtr;
};

__MCBlock__method_block_func_0 解析：

// block内部函数
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;
}

block截获变量（源码解析）

观察哪些类型的变量

OC代码：

@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;
    
    // 局部静态变量
    static int static_var = 2;
    
    // 对象类型的局部变量
    __unsafe_unretained id unsafe_obj = nil;
    __strong id strong_obj = nil;
    
    void (^MYBlock)(void) = ^{
        NSLog(@"1:%d",global_var);
        NSLog(@"2:%d",static_global_var);
        NSLog(@"3:%d",var);
        NSLog(@"4:%d",static_var);
        NSLog(@"5:%@",unsafe_obj);
        NSLog(@"6:%@",strong_obj);
    };
    MYBlock();
}

@end

终端执行
clang -rewrite-objc -fobjc-arc MCBlock.m

源码解析：

// @implementation MCBlock

int global_var = 4;

static int static_global_var = 5;

// 1.先看此处，method的方法实现变异后：
static void _I_MCBlock_method(MCBlock * self, SEL _cmd) {
    
    int var = 1;
    
    static int static_var = 2;
    
    __attribute__((objc_ownership(none))) id unsafe_obj = __null;
    __attribute__((objc_ownership(strong))) id strong_obj = __null;
    
    void (*MYBlock)(void) = ((void (*)())&__MCBlock__method_block_impl_0((void *)__MCBlock__method_block_func_0, &__MCBlock__method_block_desc_0_DATA, var, &static_var, unsafe_obj, strong_obj, 570425344));
    
    ((void (*)(__block_impl *))((__block_impl *)MYBlock)->FuncPtr)((__block_impl *)MYBlock);
}

// 2. MYBlock 的本质代码 __MCBlock__method_block_impl_0
struct __MCBlock__method_block_impl_0 {
    struct __block_impl impl;
    struct __MCBlock__method_block_desc_0* Desc;
    
    // 没有截取 全局变量 global_var，静态全局变量 static_global_var
    // block截取了函数上下文的变量 var（截取值）, *static_var（截取地址）, unsafe_obj（截取对象包括类型）, strong_obj（截取对象包括类型）
    int var;
    int *static_var;
    __unsafe_unretained id unsafe_obj;
    __strong id strong_obj;
    
    __MCBlock__method_block_impl_0(void *fp, struct __MCBlock__method_block_desc_0 *desc, int _var, int *_static_var, __unsafe_unretained id _unsafe_obj, __strong id _strong_obj, int flags=0) : var(_var), static_var(_static_var), unsafe_obj(_unsafe_obj), strong_obj(_strong_obj) {
        impl.isa = &_NSConcreteStackBlock;
        impl.Flags = flags;
        impl.FuncPtr = fp;
        Desc = desc;
    }
};

// 3.MYBlock 函数的内部实现
static void __MCBlock__method_block_func_0(struct __MCBlock__method_block_impl_0 *__cself) {
    int var = __cself->var; // bound by copy
    int *static_var = __cself->static_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
    
    // 全局变量 global_var，静态全局变量 static_global_var
    NSLog((NSString *)&__NSConstantStringImpl__var_folders_sl_ywy8bht94jx4jvy2k65zlrqc0000gn_T_MCBlock_b5dde7_mi_0,global_var);
    NSLog((NSString *)&__NSConstantStringImpl__var_folders_sl_ywy8bht94jx4jvy2k65zlrqc0000gn_T_MCBlock_b5dde7_mi_1,static_global_var);
    
    // var（截取值）, *static_var（截取地址）
    NSLog((NSString *)&__NSConstantStringImpl__var_folders_sl_ywy8bht94jx4jvy2k65zlrqc0000gn_T_MCBlock_b5dde7_mi_2,var);
    NSLog((NSString *)&__NSConstantStringImpl__var_folders_sl_ywy8bht94jx4jvy2k65zlrqc0000gn_T_MCBlock_b5dde7_mi_3,(*static_var));
    
    // unsafe_obj（截取对象包括类型）, strong_obj（截取对象包括类型）
    NSLog((NSString *)&__NSConstantStringImpl__var_folders_sl_ywy8bht94jx4jvy2k65zlrqc0000gn_T_MCBlock_b5dde7_mi_4,unsafe_obj);
    NSLog((NSString *)&__NSConstantStringImpl__var_folders_sl_ywy8bht94jx4jvy2k65zlrqc0000gn_T_MCBlock_b5dde7_mi_5,strong_obj);
}

// 暂时不看这些代码
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};

// @end