使用代理模式的时候需要用到protocol，有的时候会用到optional可选方法
如果直接这样写肯定会报错
```swift
protocol WaterFallFlowLayoutDatasource{
optional func waterFallFlowLayout(numberOfColumnInSection section:Int)-》Int
}
```
报错： 'optional' can only be applied to members of an @objc protocol
这里的解决方法是在protocol前面加上@objc关键字
像这样
```swift
@objc protocol WaterFallFlowLayoutDatasource
```
但是如果要使用自定义结构体作为返回值的时候就就会又出现新的问题：
```swift
public struct EdgeSpace {
public var vertical: CGFloat
public var horizontal: CGFloat
public init(vertical: CGFloat, horizontal: CGFloat){
self.horizontal = horizontal
self.vertical = vertical
}
}
@objc protocol WaterFallFlowLayoutDatasource{
optional func waterFallFlowLayout(insetForSectionOfIndex section:Int)->EdgeSpace
}
```
会报错：Method cannot be a member of an @objc protocol because its result type cannot be represented in Objective-C
这里可以这样来解决
```swift
public struct EdgeSpace {
public var vertical: CGFloat
public var horizontal: CGFloat
public init(vertical: CGFloat, horizontal: CGFloat){
self.horizontal = horizontal
self.vertical = vertical
}
}
protocol WaterFallFlowLayoutDatasource{
//这里是必须实现的方法
func waterFallFlowLayout(viewAtIndexPath indexPath:NSIndexPath)->UIView
// 这里是可选方法
func waterFallFlowLayout(insetForSectionOfIndex section:Int)->EdgeSpace
}
//在这里实现一下可选方法的默认值
extension WaterFallFlowLayoutDatasource{
func waterFallFlowLayout(insetForSectionOfIndex section:Int)->EdgeSpace{return EdgeSpace(vertical: 5, horizontal: 5)}
}
```
这样就解决问题了
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