title: why the instance method that is "methodSignatureForSelector:" can be invoke by the class object
body: <p>What make me doubt is that the class object can invoke instance method.</p>
  
  <p>The method of "<code>methodSignatureForSelector</code> is instance method 
  but when I invoke it by instance object, it go wrong.</p>
  
  <pre><code>NSString *classStr = @"NSInvocationObject";
  
  // 获取class
  Class objClass = NSClassFromString(classStr);
  
  // 获取函数
  SEL selector = NSSelectorFromString(@"classTest");
  
  // 获取对象
  Class stclass = [objClass class];
  // - (NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector OBJC_SWIFT_UNAVAILABLE(""); here is a instance method ,but be invoke by objClass 
  NSMethodSignature *singature = [objClass methodSignatureForSelector:selector];
  
  NSInvocation *invocation = [NSInvocation invocationWithMethodSignature:singature];
  
  invocation.target = objClass;
  invocation.selector = selector;
  [invocation invoke];ere
  </code></pre>