使用另一个类的方法

Question

我尝试使用另一个类的调用方法将图片添加到我的NSView。
我如何在secondClass中初始化firstClass实例？
代码示例：

firstClass.m：

//method for adding picture
-(void) createPoint:(NSPoint)location{
  point = [NSImage imageNamed:@"point3"];
  point.size = NSMakeSize(21, 21);
  pView = [[NSImageView alloc] init];
  [pView setFrameSize:crimePoint.size];
  [pView setImage:crimePoint];
  [pView setFrameOrigin:CGPointMake(location.x-10, location.y-10)];
  [self addSubview:pView];   
}

secondClass.m：

- (IBAction)updateButton:(id)sender {
  [firstC createPoint:NSMakePoint(300, 300)]; // FirstClass* firstC; (alloced and inited)
}

这个看起来一样，但我不明白如何使用这个答案：
Calling Method From Another UIViewController Has No Visible Effect