iOS - block - 序

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参考

block

https://blog.csdn.net/u014600626/article/details/78697535

http://www.cocoachina.com/cms/wap.php?action=article&id=23147

https://www.jianshu.com/p/d96d27819679

block 简介

block 是能够捕获外部变量的匿名函数, 在 iOS4 中引入, 是对C语言的扩充, C语言本身不存在这样的匿名函数。

block 本质

block 本质上是封装了 函数调用(函数指针) 以及 函数调用环境(捕获到的参数) 的 OC对象。既然是OC对象, 那就是一个内部有isa指针的结构体。

block 属性策略

MRC下block属性的建议写法

@property (copy, nonatomic) void (^block)(void);

ARC下block属性的建议写法

@property (strong, nonatomic) void (^block)(void);
@property (copy, nonatomic) void (^block)(void);

代码存放在堆区时, 就需要特别注意, 因为堆区不像代码区不变化, 堆区是不断变化的(不断创建销毁)。因此当没有强指针指向时, 代码有可能会被销毁, 如果这时再访问此段代码则会程序崩溃。
因此, 对于这种情况, 我们在定义一个block属性时应指定为strong, 或copy:

@property (nonatomic, strong) void (^myBlock)(void); // 这样就有强指针指向它
@property (nonatomic, copy)  void (^myBlock)(void); 

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block 相关源码

Block.h

BLOCK_EXPORT void *_Block_copy(const void *aBlock);

BLOCK_EXPORT void _Block_release(const void *aBlock);

#define Block_copy(...) ((__typeof(__VA_ARGS__))_Block_copy((const void *)(__VA_ARGS__)))

#define Block_release(...) _Block_release((const void *)(__VA_ARGS__))

Block_private.h

// (非完整)
enum {
    /* See function implementation for a more complete description of these fields and combinations */
    BLOCK_FIELD_IS_OBJECT   =  3,  /* id, NSObject, __attribute__((NSObject)), block, ... */
    BLOCK_FIELD_IS_BLOCK    =  7,  /* a block variable */
    BLOCK_FIELD_IS_BYREF    =  8,  /* the on stack structure holding the __block variable */
    BLOCK_FIELD_IS_WEAK     = 16,  /* declared __weak, only used in byref copy helpers */
    BLOCK_BYREF_CALLER      = 128  /* called from __block (byref) copy/dispose support routines. */
};

/* Runtime entry point called by compiler when assigning objects inside copy helper routines */
BLOCK_EXPORT void _Block_object_assign(void *destAddr, const void *object, const int flags);
    /* BLOCK_FIELD_IS_BYREF is only used from within block copy helpers */

/* runtime entry point called by the compiler when disposing of objects inside dispose helper routine */
BLOCK_EXPORT void _Block_object_dispose(const void *object, const int flags);

runtime.c

/* Copy, or bump refcount, of a block.  If really copying, call the copy helper if present. */
static void *_Block_copy_internal(const void *arg, const int flags) {
    struct Block_layout *aBlock;
    const bool wantsOne = (WANTS_ONE & flags) == WANTS_ONE;

    //printf("_Block_copy_internal(%p, %x)\n", arg, flags); 
    if (!arg) return NULL;
    
    
    // The following would be better done as a switch statement
    aBlock = (struct Block_layout *)arg;
    if (aBlock->flags & BLOCK_NEEDS_FREE) {
        // latches on high
        latching_incr_int(&aBlock->flags);
        return aBlock;
    }
    else if (aBlock->flags & BLOCK_IS_GC) {
        // GC refcounting is expensive so do most refcounting here.
        if (wantsOne && ((latching_incr_int(&aBlock->flags) & BLOCK_REFCOUNT_MASK) == 1)) {
            // Tell collector to hang on this - it will bump the GC refcount version
            _Block_setHasRefcount(aBlock, true);
        }
        return aBlock;
    }
    else if (aBlock->flags & BLOCK_IS_GLOBAL) {
        return aBlock;
    }

    // Its a stack block.  Make a copy.
    if (!isGC) {
        struct Block_layout *result = malloc(aBlock->descriptor->size);
        if (!result) return (void *)0;
        memmove(result, aBlock, aBlock->descriptor->size); // bitcopy first
        // reset refcount
        result->flags &= ~(BLOCK_REFCOUNT_MASK);    // XXX not needed
        result->flags |= BLOCK_NEEDS_FREE | 1;
        result->isa = _NSConcreteMallocBlock;
        if (result->flags & BLOCK_HAS_COPY_DISPOSE) {
            //printf("calling block copy helper %p(%p, %p)...\n", aBlock->descriptor->copy, result, aBlock);
            (*aBlock->descriptor->copy)(result, aBlock); // do fixup
        }
        return result;
    }
    else {
        // Under GC want allocation with refcount 1 so we ask for "true" if wantsOne
        // This allows the copy helper routines to make non-refcounted block copies under GC
        unsigned long int flags = aBlock->flags;
        bool hasCTOR = (flags & BLOCK_HAS_CTOR) != 0;
        struct Block_layout *result = _Block_allocator(aBlock->descriptor->size, wantsOne, hasCTOR);
        if (!result) return (void *)0;
        memmove(result, aBlock, aBlock->descriptor->size); // bitcopy first
        // reset refcount
        // if we copy a malloc block to a GC block then we need to clear NEEDS_FREE.
        flags &= ~(BLOCK_NEEDS_FREE|BLOCK_REFCOUNT_MASK);   // XXX not needed
        if (wantsOne)
            flags |= BLOCK_IS_GC | 1;
        else
            flags |= BLOCK_IS_GC;
        result->flags = flags;
        if (flags & BLOCK_HAS_COPY_DISPOSE) {
            //printf("calling block copy helper...\n");
            (*aBlock->descriptor->copy)(result, aBlock); // do fixup
        }
        if (hasCTOR) {
            result->isa = _NSConcreteFinalizingBlock;
        }
        else {
            result->isa = _NSConcreteAutoBlock;
        }
        return result;
    }
}

// API entry point to release a copied Block
void _Block_release(void *arg) {
    struct Block_layout *aBlock = (struct Block_layout *)arg;
    int32_t newCount;
    if (!aBlock) return;
    newCount = latching_decr_int(&aBlock->flags) & BLOCK_REFCOUNT_MASK;
    if (newCount > 0) return;
    // Hit zero
    if (aBlock->flags & BLOCK_IS_GC) {
        // Tell GC we no longer have our own refcounts.  GC will decr its refcount
        // and unless someone has done a CFRetain or marked it uncollectable it will
        // now be subject to GC reclamation.
        _Block_setHasRefcount(aBlock, false);
    }
    else if (aBlock->flags & BLOCK_NEEDS_FREE) {
        if (aBlock->flags & BLOCK_HAS_COPY_DISPOSE)(*aBlock->descriptor->dispose)(aBlock);
        _Block_deallocator(aBlock);
    }
    else if (aBlock->flags & BLOCK_IS_GLOBAL) {
        ;
    }
    else {
        printf("Block_release called upon a stack Block: %p, ignored\n", (void *)aBlock);
    }
}

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block 相关 struct

block 在 clang编译后的结构体:

struct __block_impl {
    void *isa; // 指向block的具体类型
    int Flags;
    int Reserved;
    void *FuncPtr; // 函数指针 FunctionPointer, 指向block的实现 __funcName_block_func_0
};

block 在 Block_private.h 中的声明:

struct Block_layout {
    void *isa;
    int flags;
    int reserved; 
    void (*invoke)(void *, ...);
    struct Block_descriptor *descriptor;
    /* Imported variables. */
};

struct Block_descriptor {
    unsigned long int reserved;
    unsigned long int size;
    void (*copy)(void *dst, void *src);
    void (*dispose)(void *);
};

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