SwiftUI笔记

@Published 标记对象定义的属性变更可以被监听，当对应字段变化时会触发对象 objectWillChange 通知，订阅了该属性的View也会收到值改变的通知

/// A type that publishes a property marked with an attribute.
///
/// Publishing a property with the `@Published` attribute creates a publisher of this type. You access the publisher with the `$` operator, as shown here:
///
///     class Weather {
///         @Published var temperature: Double
///         init(temperature: Double) {
///             self.temperature = temperature
///         }
///     }
///
///     let weather = Weather(temperature: 20)
///     cancellable = weather.$temperature
///         .sink() {
///             print ("Temperature now: \($0)")
///     }
///     weather.temperature = 25
///
///     // Prints:
///     // Temperature now: 20.0
///     // Temperature now: 25.0
///

@ObservedObject 和 @State 类似，需要修饰的属性需要实现了 ObservableObject

父View 以 @State 声明的属性 @State var param: DataType，可以在子View初始化的时候以 $param 给到子View，子View 通过 @Banding 关键字取得 param 数据关联，在值变化时进行相应的改变