Cocoa触发方法调用的几种方法

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1.SEL触发

SEL就是selector的缩写，它表示Cocoa中的方法选择器，不明白？那请仔细了解Objective_C的运行时机制与Cocoa底层思想。

SEL theSelector = @selector(methodWithInt:andInt:);

[self performSelector:theSelector]
[self performSelector:@selector(methodWithInt:andInt:)];

上面两句代码是同一个道理。

2. IMP触发

IMP其实就是一个函数指针的概念，就可以这么简单理解。

IMP theImplementation = [self methodForSelector:theSelector];

有了这个函数指针后，我们就可以触发方法：

theImplementation(self, theSelector, 30, 5);

第一个是对象，第二个SEL， 后面的传入的参数。

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3. objc_msgSend方法

这是Objc运行时的一个C方法，我们先看一个示列:

objc_msgSend(self, @selector(fly));

[self fly];

4. NSInvocation

NSInvocation相比上面几种方法，可以动态决定传入的参数个数。有了它，我们就可以实现…这样的变参API封装。
说得有点抽像，看代码更清楚:

假如我们实现了一个两个数相加的方法，

-(void)addNSNumber:(NSNumber *)first withNumber:(NSNumber *)second

NSNumber *arg1 = [NSNumber numberWithDouble:15.0]; 
NSNumber *arg2 = [NSNumber numberWithDouble:13.0];
 SEL selector = @selector(addNSNumber:withNumber:); 
NSMethodSignature *sig = [self methodSignatureForSelector:selector]; 
NSInvocation *invocation = [NSInvocation invocationWithMethodSignature:sig]; 
[invocation setTarget:self]; [invocation setSelector:selector]; 
[invocation setArgument:(void *)&arg1 atIndex:2];
 [invocation setArgument:(void *)&arg2 atIndex:3];
 [invocation invoke]; if ([sig methodReturnLength])
 { [invocation getReturnValue:&retval]; return retval; } return nil; }

+ (NSInvocation *)createInvocationOnTarget:(id)target selector:(SEL)selector {
  return [NSObject createInvocationOnTarget:target selector:selector withArguments:nil];
}
 
+ (NSInvocation *)createInvocationOnTarget:(id)target selector:(SEL)selector withArguments:(id)arg1, ... {
  NSMethodSignature *sig = [target methodSignatureForSelector:selector];
  NSInvocation *invocation = [NSInvocation invocationWithMethodSignature:sig];
 
  [invocation setTarget:target];
  [invocation setSelector:selector];
 
 
 
  if(arg1) {
      va_list args;
      va_start(args, arg1);
 
      [invocation setArgument:(void *)&arg1 atIndex:2];
 
      id obj;
      int ct = 3;
 
      while( obj = va_arg(args, id) ) {
          NSLog(@"%@", obj);
          [invocation setArgument:(void *)&obj atIndex:ct];
 
          ct++;
      }
 
      va_end(args);
  }
 
 
  return invocation;
}

NSInvocation *invocation = [NSObject createInvocationOnTarget:mathInstance selector:selector withArguments: arg1, arg2, nil]; 
 
[invocation invoke];
[invocation getReturnValue:&retval];
 
return retval;

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