Question

这是此问题的扩展： Is it possible to create a category of the "Block" object in Objective-C

基本上虽然似乎可以通过NSObject或NSBlock在块上创建类别，但我无法理解块如何能够自我评估。上一个问题答案中给出的例子：

- (void) doFoo {
  //do something awesome with self, a block
  //however, you can't do "self()".  
  //You'll have to cast it to a block-type variable and use that
}

意味着有可能以某种方式将self转换为块变量，但是如何执行块本身呢？例如，假设我在NSBlock上做了一个类别，并在方法中做了：

NSBlock* selfAsBlock = (NSBlock*)self;

是否有任何消息可以发送给selfAsBlock以对块进行评估？

Answer 1

意味着可以以某种方式将自我转换为块变量

像这样：

- (void)doFoo {
    // Assume the block receives an int, returns an int,
    // and cast self to the corresponding block type
    int (^selfBlock)(int) = (int (^)(int))self;

    // Call itself and print the return value
    printf("in doFoo: %d\n", selfBlock(42));
}

请注意（在大多数情况下）您需要修复块签名，以便编译器能够根据目标平台ABI设置调用站点。在上面的示例中，签名是返回类型int，类型为int的单个参数。

一个完整的例子是：

#import <Foundation/Foundation.h>
#import <objc/runtime.h>

@interface Foo : NSObject
- (void)doFoo;
@end

@implementation Foo
- (void)doFoo {
    // Assume the block receives an int, returns an int,
    // and cast self to the corresponding block type
    int (^selfBlock)(int) = (int (^)(int))self;

    // Call itself and print the return value
    printf("in doFoo: %d\n", selfBlock(42));
}
@end

int main(void) {
    [NSAutoreleasePool new];

    // From Dave's answer
    Method m = class_getInstanceMethod([Foo class], @selector(doFoo));
    IMP doFoo = method_getImplementation(m);
    const char *type = method_getTypeEncoding(m);
    Class nsblock = NSClassFromString(@"NSBlock");
    class_addMethod(nsblock, @selector(doFoo), doFoo, type);

    // A block that receives an int, returns an int
    int (^doubler)(int) = ^int(int someNumber){ return someNumber + someNumber; };

    // Call the category method which in turn calls itself (the block)
    [doubler doFoo];

    return 0;
}

Answer 2

NSBlock有invoke方法可用于调用块。

NSBlock* b = ^() { /* do stuff */ };
[b invoke];

请注意，这是一种私有的，未记录的方法。

Obj-C Block可以自行执行吗？

2 个答案: