# Swift的Binding小纸条

我们经常会使用Binding来把一个打通两个View。最常见的一个例子就是Picker:

![image.png](https://cdn.hashnode.com/res/hashnode/image/upload/v1652536204127/IuFQiY7w4.png align="left")

### 自定义Binding

#### 使用Binding实例

如果我们在Picker选择发生变化时想做一些操作，就会发现问题了，因为Picker里不好写.onTapGesture。这时就需要使用自定义Binding变量了。最简单的方法，就是在selection参数处直接定义一个Binding，就像这段代码，一个Binding的实例有了get和set两个匿名函数，你可以完成自己的自定义操作了（联动改变choosed的值）。

```
import SwiftUI
struct MyContentView : View {
    @State private var choosed = "You selected 1"
    @State var choosed_int = 1
    
    var body: some View{
        VStack{
            Text("\(choosed)")
            Picker("Please Choose", 
                   selection: Binding(
                    get: { choosed_int }, 
                    set: { 
                        choosed_int = $0 
                        choosed = "You select \($0)"
                    })) { 
                Text("hello 1").tag(1)
                Text("hello 2").tag(2)
                Text("hello 3").tag(3)
            }.frame(width: 100)
        }
    }
}
```

#### 使用body内的Binding实例

对于在Picker声明时写大段的代码，这确实从代码的可读性上显得不够优雅。所以我们可以在body里声明一个Binding的实例来优雅的完成这段代码：

```
import SwiftUI
struct MyContentView : View {
    @State private var choosed = "You selected 1"
    @State var choosed_int = 1
    
    var body: some View{
        let binding = Binding(
            get: { choosed_int }, 
            set: { 
                choosed_int = $0 
                choosed = "You select \($0)"
            })
        return VStack{
            Text("\(choosed)")
            Picker("Please Choose", 
                   selection: binding ) { 
                Text("hello 1").tag(1)
                Text("hello 2").tag(2)
                Text("hello 3").tag(3)
            }.frame(width: 100)
        }
    }
}
```

### 对View的Binding属性进行自定义

如果对一个已经使用Binding声明的属性，想做自定义的getter和setter也是一个非常方便的事：

```
struct MyContentView : View {
    @State private var choosed = "You selected 1"
    @Binding var value:Int
    var choosed_int : Binding<Int>{
        Binding<Int>(
            get: { 
                return self.value 
            }, 
            set: { 
                self.value = $0 
                choosed = "You select \($0)"
            }
        )
    }
    
    var body: some View{
        VStack{
            Text("\(choosed)")
            Picker("Please Choose", 
                   selection: choosed_int ) { 
                Text("hello 1").tag(1)
                Text("hello 2").tag(2)
                Text("hello 3").tag(3)
            }.frame(width: 100)
        }
    }
}
```