iOS Block 底層原理

Block 結構體寫法

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;
  int age;
  __main_block_impl_0(void *fp, struct __main_block_desc_0 *desc, int _age, int flags=0) : age(_age) {
    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, int b) {
  int age = __cself->age; // bound by copy

            NSLog((NSString *)&__NSConstantStringImpl__var_folders_2r__m13fp2x2n9dvlr8d68yry500000gn_T_main_3f4c4a_mi_0, age);
            NSLog((NSString *)&__NSConstantStringImpl__var_folders_2r__m13fp2x2n9dvlr8d68yry500000gn_T_main_3f4c4a_mi_1);
            NSLog((NSString *)&__NSConstantStringImpl__var_folders_2r__m13fp2x2n9dvlr8d68yry500000gn_T_main_3f4c4a_mi_2);
            NSLog((NSString *)&__NSConstantStringImpl__var_folders_2r__m13fp2x2n9dvlr8d68yry500000gn_T_main_3f4c4a_mi_3);
        }

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; 

        int age = 10;

        // - (void(*)(int,int)) 是強制類型轉換
        /* 本質
         block = &__main_block_impl_0(  __main_block_func_0,
                                        &__main_block_desc_0_DATA,
                                        age));

         */
        void (*block)(int, int) = ((void(*)(int,int))&__main_block_impl_0(
                                (void*)__main_block_func_0,
                                &__main_block_desc_0_DATA,
                                age));

        // - (void (*)(__block_impl *, int, int)) 是強制類型轉換
        /* 本質
            block->FuncPtr(block, 10, 10);
         */
        ((void (*)(__block_impl *, int, int))((__block_impl *)block)->FuncPtr)(
                                                                               (__block_impl *)block,
                                                                               10,
                                                                               10);
    }
    return 0;
}

block 中的參數類型

// - 有參數的 block 的數據類型的源碼(以上的 block 的內部結構是clang編譯的結果 具有一定的參考意義 下邊的是逆向工程中的block 的使用)

#import <Foundation/Foundation.h>

// -  block 在內存中的結構
typedef struct  {
    unsigned long int reserved;
    unsigned long int size;
    void (*copy_helper)(void *dst, void *src);
    void (*dispose_helper)(void *src);
    const char *signature;
}QGBlockDescriptor;

// -  block 在內存中的結構
typedef struct  {
    void *isa;
    int flags;
    int reserved;
    void (*invoke)(void *, ...);
    QGBlockDescriptor *descriptor;
} QGBlockLiteral;

@interface QGBlockTool : NSObject

/** 輸出逆向的 block 的說明 */
+ (NSMethodSignature *)printBlockDescriptor:(QGBlockLiteral *)blockStruct;

@end

#import "QGBlockTool.h"

@implementation QGBlockTool
// - 獲取 block 的返回值和參數的簽名和 block 的實現函數指針
/** 輸出逆向的 block 的說明 */
+ (NSMethodSignature *)printBlockDescriptor:(QGBlockLiteral *)blockStruct{
    
    /** 判斷 block 是否有簽名 */
    BOOL hasSign = ((blockStruct -> flags  & (1 << 30)) != 0);
    if (!hasSign) return nil;
    
    // - block 的函數地址
    NSLog(@"調用地址 : %p", blockStruct->invoke);
    
    // - block 的函數指針
    QGBlockDescriptor *descriptor = blockStruct->descriptor;
    NSLog(@"descriptor : %@", [NSString stringWithUTF8String:descriptor -> signature]);
    NSMethodSignature *methodSignature = [NSMethodSignature signatureWithObjCTypes:descriptor -> signature];
    return methodSignature;
}
@end

block 對變量的捕獲 (考慮作用域的問題，需要跨函數訪問，爲了保證block內部能夠正常訪問外部的變量，block有個變量捕獲機制){
1. auto自動變量可能會銷燬的，內存可能會消失，不採用指針訪問；static變量一直保存在內存中，指針訪問即可
2. block不需要對全局變量捕獲，都是直接採用取值的
  }

[示例如下]:
block 對局部變量的捕獲 (a 爲局部變量, b 爲靜態局部變量)

block 對成員變量和 self 的捕獲(以爲 self 和成員變量屬於方法的參數和需要方法的參數查找的變量, 所以也是屬於捕獲局部變量, 需要捕獲)
block 對全局變量的捕獲 (a 爲全局變量, b 爲靜態全局變量)

[結論]:

block 對變量的引用操作(爲什麼有__weak typeof(self) weakSelf = self; 和 __strong typeof(self) strongSelf = weakSelf;)
因爲 block 是一個代碼塊, 是由程序員自己調用的, 而程序員調用 block 時候, 可能 block 內部訪問的局部對象已經釋放, 爲了防止這樣已經被釋放的變量再次被訪問而出錯, 所以 block 會對內部的訪問的變量有一次強引用操作, 如下:
代碼:

// - OC 代碼
__block __weak int age = 10;

// - 修改對象類型 也必須要加 __block
__block NSObject *obj = [[NSObject alloc]init];
MJBlock block1 = ^{
    __strong int myage = age;
    age = 20;
    obj = nil;
    NSLog(@"age is %d", age);
};

MJBlock block2 = ^{
    age = 30;
    NSLog(@"age is %d", age);
};

block1();
block2();


// - 訪問成員變量的 block 的內部實現代碼 
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;
  NSObject *p;
  __Block_byref_age_0 *age; // by ref
  __main_block_impl_0(void *fp, struct __main_block_desc_0 *desc, NSObject *_p, __Block_byref_age_0 *_age, int flags=0) : p(_p), age(_age->__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_age_0 *age = __cself->age; // bound by ref
  NSObject *p = __cself->p; // bound by copy

            (age->__forwarding->age) = 20;
            NSLog((NSString *)&__NSConstantStringImpl__var_folders_2r__m13fp2x2n9dvlr8d68yry500000gn_T_main_e2457b_mi_0, p);
        }
static void __main_block_copy_0(struct __main_block_impl_0*dst, struct __main_block_impl_0*src) {
    _Block_object_assign((void*)&dst->age, (void*)src->age, 8/*BLOCK_FIELD_IS_BYREF*/);
    _Block_object_assign((void*)&dst->p, (void*)src->p, 3/*BLOCK_FIELD_IS_OBJECT*/);}

static void __main_block_dispose_0(struct __main_block_impl_0*src) {
    _Block_object_dispose((void*)src->age, 8/*BLOCK_FIELD_IS_BYREF*/);
    _Block_object_dispose((void*)src->p, 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};

block 對局部變量的捕捉和引用問題

如果block在棧空間，不管外部變量是強引用還是弱引用，block都會弱引用訪問對象
如果block在堆空間，如果外部強引用，block內部也是強引用；如果外部弱引用，block內部也是弱引用

[結論 : ]

棧block
a. 如果block是在棧上，將不會對auto變量產生強引用
b. 棧上的block隨時會被銷燬，也沒必要去強引用其他對象
堆block
a. 如果block被拷貝到堆上：
1. 會調用block內部的copy函數
2. copy函數內部會調用_Block_object_assign函數
3. _Block_object_assign函數會根據auto變量的修飾符（__strong、__weak、__unsafe_unretained）做出相應的操作，形成強引用（retain）或者弱引用
b. 如果block從堆上移除
1. 會調用block內部的dispose函數
2. dispose函數內部會調用_Block_object_dispose函數
3. _Block_object_dispose函數會自動釋放引用的auto變量（release）

//  -  測試 (一)
-(void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event{
    Person *person = [[Person alloc] init];
    person.age = 10;
    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(2.0 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
        NSLog(@"age:%d",person.age);
    });
      NSLog(@"touchesBegan");
}
[打印] : touchesBegan   age:10    Person-dealloc

//  -  測試 (二)
-(void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event{
    Person *person = [[Person alloc] init];
    person.age = 10;
    __weak Person *weakPerson = person;
    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(2.0 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
        NSLog(@"age:%p",weakPerson);
    });
    NSLog(@"touchesBegan");
}
[打印] : touchesBegan   Person-dealloc    age:0x0

//  -  測試 (三)
-(void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event{
    Person *person = [[Person alloc] init];
    person.age = 10;
    __weak Person *weakPerson = person;
    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(4.0 * NSEC_PER_SEC)),
                   dispatch_get_main_queue(), ^{  
        dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(3.0 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
            NSLog(@"2-----age:%p",person);
        });
        NSLog(@"1-----age:%p",weakPerson);
    });
    NSLog(@"touchesBegan");
}
[打印] : touchesBegan   1-----age:0x604000015eb0    2-----age:0x604000015eb0    Person-dealloc

//  -  測試 (四)
-(void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event{
    Person *person = [[Person alloc] init];
    person.age = 10;
    __weak Person *weakPerson = person;
    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(4.0 * NSEC_PER_SEC)),
                   dispatch_get_main_queue(), ^{          
        dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(3.0 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
            NSLog(@"2-----age:%p",weakPerson);
        });
        NSLog(@"1-----age:%p",person);
    });
    NSLog(@"touchesBegan");
}
[打印] : touchesBegan   1-----age:0x6000000178d0   Person-dealloc   2-----age:0x0

__block 修飾的變量

// - __block 的使用
int b = 20;
__block int age = 10;

MJBlock block = ^{
    age = 20;
    NSLog(@"age is %d", age);
};
struct __main_block_impl_0 *blockImpl = (__bridge struct __main_block_impl_0 *)block;
((void (*)(void *))blockImpl->impl.FuncPtr)(blockImpl);


// - __block 修飾的變量的內部的代碼  如下 age 被包裝成一個對象
struct __Block_byref_age_0 {
    void *__isa;
    struct __Block_byref_age_0 *__forwarding;
    int __flags;
    int __size;
    int age;
};

struct __main_block_desc_0 {
    size_t reserved;
    size_t Block_size;
    void (*copy)(void);
    void (*dispose)(void);
};

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;
    struct __Block_byref_age_0 *age;
};

[__block 的使用] :

__block可以用於解決block內部無法修改auto變量值的問題(auto修飾變量，block無法修改，因爲block使用的時候是內部創建了變量來保存外部的變量的值，block只有修改內部自己變量的權限，無法修改外部變量的權限。)
__block不能修飾全局變量、靜態變量（static）(因爲全局變量, block 內部可以直接訪問到,不用捕獲; static 變量, 是指針傳遞, 也是可以直接修改的)
__block 修飾的變量, 內部的實質是將 block 內部要訪問的那個變量,包裝成爲一個對象, 這樣就可以隨意賦值了;
當block被copy到堆時,copy函數內部會調用_Block_object_assign函數, _Block_object_assign函數會對__block變量形成強引用（retain）對於__block 修飾的變量 assign函數對其強引用；對於外部對象 assign函數根據外部如何引用而引用;
block 的使用(少見的 block 的定義屬性的方法)

// - 定義 block
typedef  QGLabel *(^Block3)(UIColor *color);
@interface QGLabel : UILabel
@property (nonatomic, copy) Block3 block2;
@property (nonatomic, copy) Block3 block3;
@property (nonatomic, copy) NSString *(^block)(NSNumber *);

@end

@implementation QGLabel
#pragma mark - 設置初始化數據
/** 設置數據 */
-(void)setupData{
 self.h = ^NSString *(NSNumber *num) {
        NSLog(@"%@", num);
    };

    __weak typeof(self) weakSelf = self;
    self.block2 = ^QGLabel *(UIColor *color) {
        weakSelf.textColor = color;
        return weakSelf;
    };
}
    
-(Block3)block3{
    __weak typeof(self) weakSelf = self;
    return ^QGLabel *(UIColor *color) {
        weakSelf.backgroundColor = color;
        return weakSelf;
    };
}

/** 測試調用 */
-(void)test{
    // - setter
    self.block = ^NSString *(NSNumber *num) {
        return [NSString stringWithFormat:@"%@", num];
    };
    
    // - getter
    self.block(@(1));
}

/** block 的 setter */
-(void)setBlock:(NSString *(^)(NSNumber *num))block{
    
    NSString *str =  [NSString stringWithFormat:@"block 的 setter  %@", block(@1)];
    NSLog(@"str : %@", str);
}

/** block 的 getter */
-(NSString *(^)(NSNumber *))block{
    return ^NSString *(NSNumber *num){
        NSString *str = [NSString stringWithFormat:@"block 的 getter  %@", num];
        NSLog(@"str : %@", str);
        return str;
    };
}

@end

// - block中傳遞 self ,使self及時釋放
- (void)asyncRun:(void(^)(MLVBLiveRoom *self))block {
    __weak __typeof(self) wself = self;
    dispatch_async(_queue, ^{
        __strong __typeof(wself) self = wself;
        if (self) {
            block(self);
        }
    });
}

block 中使用 __weak

// - 視圖控制器
- (void)viewDidLoad {
    [super viewDidLoad];
    self.redView = [[RedView alloc]initWithFrame:CGRectMake(90, 90, 90, 90)];
    self.redView.backgroundColor = [UIColor redColor];
    [self.view addSubview:self.redView];
}

-(void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event{
    [self.redView removeFromSuperview];
    self.redView = nil;
}

/*  以下代碼中 (在 property的聲明中可以不用__weak 修飾)
// - 聲明中可以不寫 __weak; 即:  
	@property (nonatomic, copy) void(^block)( id xx); 但是後邊的賦值,必須寫__weak,否則不會及時釋放
	@property (nonatomic, copy) void(^block)(__weak id xx);
	self.block = ^(__weak id xx) {
            [NSTimer scheduledTimerWithTimeInterval:10 repeats:NO block:^(NSTimer * 		_Nonnull timer) {
                NSLog(@"xx : %@", xx);
            }];
        };
*/
1. 如果 block 中的 xx 變量, 如果沒有使用__weak 修飾, touch時候, 則不會立即 dealloc ,需要計時器 10s 後回調block打印 xx : <RedView: 0x7faabc411830; frame = (90 90; 90 90); layer = <CALayer: 0x6000028d8920>> 之後纔會 dealloc
2. 如果 block 中的 xx 變量, 如果有__weak 修飾, touch時候, 會立即 dealloc ,然後計時器 10s 後打印 xx : (null)

// -  redView
@interface RedView()
@property (nonatomic, copy) void(^block)(__weak id xx);
@end

@implementation RedView
- (instancetype)initWithFrame:(CGRect)frame{
    if (self = [super initWithFrame:frame]) {
        self.block = ^(__weak id xx) {
            [NSTimer scheduledTimerWithTimeInterval:10 repeats:NO block:^(NSTimer * _Nonnull timer) {
                NSLog(@"xx : %@", xx);
            }];
        };
        self.block(self);
    }
    return self;
}

-(void)dealloc{
    NSLog(@"dealloc --- [%s]", __func__);
}
@end

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