Swift 2.0回调语法：什么是$ 0？

Question

我正在更新到Swift 2.0。

我的旧代码：

animateButtonsOnScreen(callback: completedImageEditingButtonSwap)

编译器想要的新代码：

animateButtonsOnScreen(callback: { completedImageEditingButtonSwap($0) })

callback:没有参数。这个新的$ 0对象是什么？我没有参数，所以它在这里引用了什么？

注意：编译器说这是我希望我使用这种新语法

  

使用闭包来安全地包装对函数的调用

使用某些代码进行更新

func swapButtonsForImageCapture()
{

    if(DeviceProperties.isIphone4() == true)//iPhone 4s needs smaller icon
    {
        _takePhotoButton.setImage(UIImage(named: "snapimageicon4s"), forState:UIControlState.Normal)
    }else
    {
        _takePhotoButton.setImage(UIImage(named: "snapimageicon"), forState:UIControlState.Normal)
    }
    _takePhotoButton.removeTarget(nil, action: nil, forControlEvents: UIControlEvents.AllEvents)
    _takePhotoButton.addTarget(_controller, action: Selector("takePhoto:"), forControlEvents: UIControlEvents.TouchUpInside)
    let buttonsToAnimateOut:[UIButton] = [_textButton,_filtersButton,_previewButton]
    animateButtonsOffScreen(buttonsToAnimateOut, callback: { self.swapButtonsForImageCapturePart2($0) })
}

func swapButtonsForImageCapturePart2()
{
    let buttonsToAnimateIn:[UIButton] = [_swapCameraButton,_flashButton,_galleryButton]
    animateButtonsOnScreen(buttonsToAnimateIn, callback: { self.completedImageCaptureButtonSwap($0) })
}