《Objective-C高級編程》（二）Block

一、Block簡介

帶有自動變量的匿名函數(shù)，實質(zhì)就是Objective-C的對象。

1、Block語法

^ returnType paramType expression
^ int (int count) {return count++;}
^ (int count) {return count++;}
^ void (void)  {printf("Hello Block");}
^ {printf("Hello Block");}

2、Block聲明

// 函數(shù)指針
int (*funcptr) (int)
// Block聲明
int (^block) (int)
// Block類型作為函數(shù)參數(shù)
void func (int (^block) (int))
// Block類型作為屬性
@property (nonatomic, copy) void(^block)(void);
// typedef
typedef int (^block) (int)

二、Block實質(zhì)

1、轉(zhuǎn)換源代碼

// 轉(zhuǎn)換源代碼指令
clang -rewrite-objc [fileName]
xcrun -sdk iphoneos clang -rewrite-objc ViewController.m
xcrun -sdk iphonesimulator clang -rewrite-objc ViewController.m

源代碼

#import <Foundation/Foundation.h>

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        void (^JDPBlock) (void) = ^{printf("Hello, Block!");};
        JDPBlock();
    }
    return 0;
}

轉(zhuǎn)換

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;
  __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;
  }
};
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(int argc, const char * argv[]) {
    /* @autoreleasepool */ { __AtAutoreleasePool __autoreleasepool; 
        void (*JDPBlock) (void) = ((void (*)())&__main_block_impl_0((void *)__main_block_func_0, &__main_block_desc_0_DATA));
        ((void (*)(__block_impl *))((__block_impl *)JDPBlock)->FuncPtr)((__block_impl *)JDPBlock);
    }
    return 0;
}
static struct IMAGE_INFO { unsigned version; unsigned flag; } _OBJC_IMAGE_INFO = { 0, 2 };

可理解為

struct A
struct B, init b
function f(x)

struct X {
A,
B,
constructor(F,B, Flag)
}

x = X(f, b)
x.f(x)

2、Block截獲自動變量

源代碼

#import <Foundation/Foundation.h>

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        NSInteger integer = 1;
        void (^JDPBlock1) (void) = ^{printf("%lu\n", integer);};
        integer = 2;
        JDPBlock1();
    }
    return 0;
}

轉(zhuǎn)換

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;
  NSInteger integer;
  __main_block_impl_0(void *fp, struct __main_block_desc_0 *desc, NSInteger _integer, int flags=0) : integer(_integer) {
    impl.isa = &_NSConcreteStackBlock;
    impl.Flags = flags;
    impl.FuncPtr = fp;
    Desc = desc;
  }
};
static void __main_block_func_0(struct __main_block_impl_0 *__cself) {
  NSInteger integer = __cself->integer; // bound by copy
printf("%d", integer);}

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; 
        NSInteger integer = 1;
        void (*JDPBlock1) (void) = ((void (*)())&__main_block_impl_0((void *)__main_block_func_0, &__main_block_desc_0_DATA, integer));
        integer = 2;
        ((void (*)(__block_impl *))((__block_impl *)JDPBlock1)->FuncPtr)((__block_impl *)JDPBlock1);
    }
    return 0;
}
static struct IMAGE_INFO { unsigned version; unsigned flag; } _OBJC_IMAGE_INFO = { 0, 2 };

理解為

struct A
struct B, init b
function f(x) {
i = x.i
}

struct X {
A,
B,
I,
constructor(F, B, I, Flag)
}

i = 1
x = X(f, b, i)
i = 2;
x.f(x)

1、Block語法表達式中使用的自動變量被作為成員變量被添加到結(jié)構(gòu)體中。
2、Block語法表達式中沒有使用的自動變量不會被追加。
3、Block截獲自動變量值，意味著在定義Block時，Block語法表達式所使用的自動變量值被保存到Block的結(jié)構(gòu)體實例中。
4、Block定義后，自動變量值的修改不會影響到Block內(nèi)的成員變量的值。

3、__block變量

源代碼

#import <Foundation/Foundation.h>

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        __block NSInteger integer = 1;
        void (^JDPBlock1) (void) = ^{
            integer = 3;
            printf("%lu\n", integer);
        };
        integer = 2;
        JDPBlock1();
    }
    return 0;
}

3

轉(zhuǎn)換

struct __Block_byref_integer_0 {
  void *__isa;
__Block_byref_integer_0 *__forwarding;
 int __flags;
 int __size;
 NSInteger integer;
};

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;
  __Block_byref_integer_0 *integer; // by ref
  __main_block_impl_0(void *fp, struct __main_block_desc_0 *desc, __Block_byref_integer_0 *_integer, int flags=0) : integer(_integer->__forwarding) {
    impl.isa = &_NSConcreteStackBlock;
    impl.Flags = flags;
    impl.FuncPtr = fp;
    Desc = desc;
  }
};
static void __main_block_func_0(struct __main_block_impl_0 *__cself) {
  __Block_byref_integer_0 *integer = __cself->integer; // bound by ref

            (integer->__forwarding->integer) = 3;
            printf("%lu\n", (integer->__forwarding->integer));
        }
static void __main_block_copy_0(struct __main_block_impl_0*dst, struct __main_block_impl_0*src) {_Block_object_assign((void*)&dst->integer, (void*)src->integer, 8/*BLOCK_FIELD_IS_BYREF*/);}

static void __main_block_dispose_0(struct __main_block_impl_0*src) {_Block_object_dispose((void*)src->integer, 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(int argc, const char * argv[]) {
    /* @autoreleasepool */ { __AtAutoreleasePool __autoreleasepool; 
        __attribute__((__blocks__(byref))) __Block_byref_integer_0 integer = {(void*)0,(__Block_byref_integer_0 *)&integer, 0, sizeof(__Block_byref_integer_0), 1};
        void (*JDPBlock1) (void) = ((void (*)())&__main_block_impl_0((void *)__main_block_func_0, &__main_block_desc_0_DATA, (__Block_byref_integer_0 *)&integer, 570425344));
        (integer.__forwarding->integer) = 2;
        ((void (*)(__block_impl *))((__block_impl *)JDPBlock1)->FuncPtr)((__block_impl *)JDPBlock1);
    }
    return 0;
}
static struct IMAGE_INFO { unsigned version; unsigned flag; } _OBJC_IMAGE_INFO = { 0, 2 };

理解為

struct Q
struct A
struct B, init b
function f(x) {
q = x.q
q.i = 3
}

struct X {
A,
B,
Q,
constructor(F, B, Q, flag)
}

q = constructor(0, &q, 0, size(Q), 1)
x = X(f, b, q)
q.i = 2;
x.f(x)

三、Block類型

1、Block和__block變量的實質(zhì)是棧上生成的結(jié)構(gòu)體實例。
2、Block的類型有_NSConcreteGlobalBlock、_NSConcreteStackBlock、_NSConcreteMallocBlock，分別在數(shù)據(jù)區(qū)、棧和堆中。
3、Block不截獲自動變量時，就可以將Block設(shè)置在程序的數(shù)據(jù)區(qū)中。
4、__block結(jié)構(gòu)體，成員變量__forwarding可以實現(xiàn)無論__block變量配置在棧還是堆上時都能夠正確地訪問同一個__block變量。
5、ARC有效時，編譯器自動將Block從棧復制到堆，何時自動？
5.1、調(diào)用Block的copy實例方法時
5.2、Block作為函數(shù)返回值返回時
5.3、將Block賦值給附有__strong修飾符的變量時
5.4、在方法名中含有usingBlock的Cocoa框架方法或者GCD的API中傳遞Block時
6、向方法或函數(shù)的參數(shù)中傳遞Block時，編譯器不能判斷是否自動拷貝，Cocoa usingBlock和GCD除外。

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        void (^JDPBlock) (void) = ^{printf("Hello, Block!");};
        NSLog(@"%@", JDPBlock);
    }
    return 0;
}

<__NSGlobalBlock__: 0x100001050>

#import <Foundation/Foundation.h>

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        NSInteger i = 1;
        void (^JDPBlock) (void) = ^{printf("%lu", i);};
        NSLog(@"%@", JDPBlock);
    }
    return 0;
}

// MRC
<__NSStackBlock__: 0x7ffeefbff588>
// ARC
<__NSMallocBlock__: 0x1005969b0>

四、Block&對象

1、Block截獲對象

源代碼

#import <Foundation/Foundation.h>

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        NSMutableArray *array = [[NSMutableArray alloc] init];
        void (^JDPBlock) (void) = ^{NSLog(@"%@", array);};
        JDPBlock();
    }
    return 0;
}

轉(zhuǎn)換

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;
  NSMutableArray *array;
  __main_block_impl_0(void *fp, struct __main_block_desc_0 *desc, NSMutableArray *_array, int flags=0) : array(_array) {
    impl.isa = &_NSConcreteStackBlock;
    impl.Flags = flags;
    impl.FuncPtr = fp;
    Desc = desc;
  }
};
static void __main_block_func_0(struct __main_block_impl_0 *__cself) {
  NSMutableArray *array = __cself->array; // bound by copy
NSLog((NSString *)&__NSConstantStringImpl__var_folders_yp_b0w71dps58s5rncj_t1znym80000gn_T_main_6313b8_mi_0, array);}
static void __main_block_copy_0(struct __main_block_impl_0*dst, struct __main_block_impl_0*src) {_Block_object_assign((void*)&dst->array, (void*)src->array, 3/*BLOCK_FIELD_IS_OBJECT*/);}

static void __main_block_dispose_0(struct __main_block_impl_0*src) {_Block_object_dispose((void*)src->array, 3/*BLOCK_FIELD_IS_OBJECT*/);}

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(int argc, const char * argv[]) {
    /* @autoreleasepool */ { __AtAutoreleasePool __autoreleasepool; 
        NSMutableArray *array = ((NSMutableArray *(*)(id, SEL))(void *)objc_msgSend)((id)((NSMutableArray *(*)(id, SEL))(void *)objc_msgSend)((id)objc_getClass("NSMutableArray"), sel_registerName("alloc")), sel_registerName("init"));
        void (*JDPBlock) (void) = ((void (*)())&__main_block_impl_0((void *)__main_block_func_0, &__main_block_desc_0_DATA, array, 570425344));
        ((void (*)(__block_impl *))((__block_impl *)JDPBlock)->FuncPtr)((__block_impl *)JDPBlock);
    }
    return 0;
}
static struct IMAGE_INFO { unsigned version; unsigned flag; } _OBJC_IMAGE_INFO = { 0, 2 };

理解為

struct A
struct B, init b
function f(x) {
NSMutableArray *array = x.array;
NSLog(@"%@", array);
}

struct X {
A,
B,
NSMutableArray *array;
constructor(F, B, NSMutableArray *array, Flag)
}

NSMutableArray *array = [[NSMutableArray alloc] init];
x = X(f, b, array)
x.f(x)

2、__block修飾對象

源代碼

#import <Foundation/Foundation.h>

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        __block NSMutableArray *array = [[NSMutableArray alloc] init];
        void (^JDPBlock) (void) = ^{NSLog(@"%@", array);};
        JDPBlock();
    }
    return 0;
}

轉(zhuǎn)換

struct __Block_byref_array_0 {
  void *__isa;
__Block_byref_array_0 *__forwarding;
 int __flags;
 int __size;
 void (*__Block_byref_id_object_copy)(void*, void*);
 void (*__Block_byref_id_object_dispose)(void*);
 NSMutableArray *array;
};

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;
  __Block_byref_array_0 *array; // by ref
  __main_block_impl_0(void *fp, struct __main_block_desc_0 *desc, __Block_byref_array_0 *_array, int flags=0) : array(_array->__forwarding) {
    impl.isa = &_NSConcreteStackBlock;
    impl.Flags = flags;
    impl.FuncPtr = fp;
    Desc = desc;
  }
};
static void __main_block_func_0(struct __main_block_impl_0 *__cself) {
  __Block_byref_array_0 *array = __cself->array; // bound by ref
NSLog((NSString *)&__NSConstantStringImpl__var_folders_yp_b0w71dps58s5rncj_t1znym80000gn_T_main_b13376_mi_0, (array->__forwarding->array));}
static void __main_block_copy_0(struct __main_block_impl_0*dst, struct __main_block_impl_0*src) {_Block_object_assign((void*)&dst->array, (void*)src->array, 8/*BLOCK_FIELD_IS_BYREF*/);}

static void __main_block_dispose_0(struct __main_block_impl_0*src) {_Block_object_dispose((void*)src->array, 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(int argc, const char * argv[]) {
    /* @autoreleasepool */ { __AtAutoreleasePool __autoreleasepool; 
        __attribute__((__blocks__(byref))) __Block_byref_array_0 array = {(void*)0,(__Block_byref_array_0 *)&array, 33554432, sizeof(__Block_byref_array_0), __Block_byref_id_object_copy_131, __Block_byref_id_object_dispose_131, ((NSMutableArray *(*)(id, SEL))(void *)objc_msgSend)((id)((NSMutableArray *(*)(id, SEL))(void *)objc_msgSend)((id)objc_getClass("NSMutableArray"), sel_registerName("alloc")), sel_registerName("init"))};
        void (*JDPBlock) (void) = ((void (*)())&__main_block_impl_0((void *)__main_block_func_0, &__main_block_desc_0_DATA, (__Block_byref_array_0 *)&array, 570425344));
        ((void (*)(__block_impl *))((__block_impl *)JDPBlock)->FuncPtr)((__block_impl *)JDPBlock);
    }
    return 0;
}
static struct IMAGE_INFO { unsigned version; unsigned flag; } _OBJC_IMAGE_INFO = { 0, 2 };

理解為

struct Q
struct A
struct B, init b
function f(x) {
q = x.q
NSLog(@"%@", x.array);
}

struct X {
A,
B,
Q;
constructor(F, B, Q, Flag)
}

NSMutableArray *array = [[NSMutableArray alloc] init];
q = Q(0, &q, 0, sizeof(Q), array);
x = X(f, b, q)
x.f(x)

五、Block循環(huán)引用

MRC中，__block修飾符被用來避免Block中的循環(huán)引用。這是由于當Block從棧復制到堆時，若Block使用的變量為附有__block修飾符的id類型或?qū)ο箢愋偷淖詣幼兞?，不會被retain。若Block使用的變量為沒有__block修飾符的id類型或?qū)ο箢愋偷淖詣幼兞?，則被retain。

最后編輯于 ：
?著作權(quán)歸作者所有,轉(zhuǎn)載或內(nèi)容合作請聯(lián)系作者
【社區(qū)內(nèi)容提示】社區(qū)部分內(nèi)容疑似由AI輔助生成，瀏覽時請結(jié)合常識與多方信息審慎甄別。
平臺聲明：文章內(nèi)容（如有圖片或視頻亦包括在內(nèi)）由作者上傳并發(fā)布，文章內(nèi)容僅代表作者本人觀點，簡書系信息發(fā)布平臺，僅提供信息存儲服務(wù)。

相關(guān)閱讀更多精彩內(nèi)容

友情鏈接更多精彩內(nèi)容