检查Objective-C块类型？ [英] Checking Objective-C block type?

问题描述

这主要是一个好奇心，我不确定这是什么的实际用途，但这里。

由于块也是Objective-C对象，它可以检查他们的类型？也就是说，它是否响应 isKindOfClass：消息以及如何使用该消息相对于块？

我的天真想到它可能是这样的：

   - （void）aMethod {
 typedef int（^ BlockA） int x，int y）; 
 id blockVar = ...; //从某处获取块
 if（[blockVar isKindOfClass：BlockA]）{
 BlockA blockVarA = blockVar; 
 int result = blockVarA（1,2）; 
} 
} 
  

上面的代码可能不工作。

解决方案

但是首先，让我们来消除歧义。 - [NSObject isKindOfClass：] 可以告诉你它是一个块，这就是它。例如。我相信这行代码 - 表面上&不幸的是 A BAD IDEA 会对现在的Lion& iOS 5.x：

  [myBlock isKindOfClass：NSClassFromString（@NSBlock）] 
  

这不会帮助你区分块的函数签名。

但它可以通过阻塞来自块的文档内部结构的签名来完成。代码如下是一个示例OS X命令行应用程序，其中许多摘录自Mike Ash的 MABlockClosure （great 详细说明）。 （更新：Github项目 CTObjectiveCRuntimeAdditions 也显然提供了仅用于此目的的库代码。）

  #import< Foundation / Foundation.h> 
 
 struct BlockDescriptor {
 unsigned long reserved; 
 unsigned long size; 
 void * rest [1]; 
}; 
 
 struct Block {
 void * isa; 
 int flags; 
 int reserved; 
 void * invoke; 
 struct BlockDescriptor * descriptor; 
}; 
 
 static const char * BlockSig（id blockObj）
 {
 struct Block * block =（void *）blockObj; 
 struct BlockDescriptor * descriptor = block-> descriptor; 
 
 int copyDisposeFlag = 1<< 25; 
 int signatureFlag = 1<< 30; 
 
 assert（block-> flags& signatureFlag）; 
 
 int index = 0; 
 if（block-> flags& copyDisposeFlag）
 index + = 2; 
 
 return descriptor-> rest [index]; 
} 
 
 int main（int argc，const char * argv []）
 {
 @autoreleasepool {
 
 int ）（NSNumber *）= ^（NSNumber * num）{
 NSLog（@％@％@，NSStringFromClass（[num class]），num） 
 return [num intValue]; 
}; 
 NSLog（@signature％s，BlockSig（block））; 
 NSLog（@retval％d，（int）block（[NSNumber numberWithInt：42]）） 
} 
 return 0; 
} 
  

运行此操作，您应该得到类似的结果：

[58003：403] signature i16 @？0 @ 8
[58003：403] __NSCFNumber 42
[58003：403] retval 42


签名中的数字（我被告知他们是偏移量） code> i @？@ 。

签名位于 @ encode 格式，这是不完美的（例如大多数对象映射到相同 @ ），但应该能够让您在运行时区分具有不同签名的块。

Apple链接，我的测试指向 @？是块类型的代码，这对上面的签名很有意义。我发现一个clang开发者关于这个问题的讨论，似乎支持这个问题。

This is primarily a curiosity, I'm not really sure what's the practical use of this but here goes.

Since blocks are also Objective-C objects, is it possible to check their type? That is, does it respond to the isKindOfClass: message and how to use that message with respect to blocks?

My naive thought that it's probably like this:

-(void) aMethod {
    typedef int (^BlockA)(int x, int y);
    id blockVar = ...; // get a block from somewhere
    if([blockVar isKindOfClass:BlockA]) {
        BlockA blockVarA = blockVar;
        int result = blockVarA(1,2);
    }
}

The code above probably won't work. But if it is possible to check a block's type, what is the correct way to do it?

解决方案

Can do, kinda sorta.

But first, let's disambiguate. -[NSObject isKindOfClass:] can tell you it's a block, and that's about it. E.g. I believe this line of code -- ostensibly & unfortunately A BAD IDEA -- will return YES for blocks on present Lion & iOS 5.x:

[myBlock isKindOfClass:NSClassFromString(@"NSBlock")]

That won't help you distinguish the block's function signature.

But it can be done, by snagging the signature from the block's documented internal struct. Code follows for an example OS X command-line app, much of which ripped from Mike Ash's MABlockClosure (great detailed explanation). (UPDATE: Github project CTObjectiveCRuntimeAdditions also apparently provides library code for just this purpose.)

#import <Foundation/Foundation.h>

struct BlockDescriptor {
    unsigned long reserved;
    unsigned long size;
    void *rest[1];
};

struct Block {
    void *isa;
    int flags;
    int reserved;
    void *invoke;
    struct BlockDescriptor *descriptor;
};

static const char *BlockSig(id blockObj)
{
    struct Block *block = (void *)blockObj;
    struct BlockDescriptor *descriptor = block->descriptor;

    int copyDisposeFlag = 1 << 25;
    int signatureFlag = 1 << 30;

    assert(block->flags & signatureFlag);

    int index = 0;
    if(block->flags & copyDisposeFlag)
        index += 2;

    return descriptor->rest[index];
}

int main(int argc, const char * argv[])
{
    @autoreleasepool {

        int (^block)(NSNumber *) = ^(NSNumber *num) { 
            NSLog(@"%@ %@", NSStringFromClass([num class]), num); 
            return [num intValue]; 
        };
        NSLog(@"signature %s", BlockSig(block));
        NSLog(@"retval %d", (int)block([NSNumber numberWithInt:42]));
    }
    return 0;
}

Run this and you should get something like:

[58003:403] signature i16@?0@8
[58003:403] __NSCFNumber 42
[58003:403] retval 42

The numbers in the signature (I'm told they are offsets) can be stripped for simpler i@?@.

The signature is in the @encode format, which isn't perfect (e.g. most objects map to same @), but should afford you some ability to distinguish blocks with different signatures at runtime.

While it's not documented in the Apple link, my testing points to @? being the code for a block type, which makes sense of the signature above. I found a clang-developers discussion on this issue which seems to back this up.

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