body 不是计算后（ computed ）的参数吗，a/b 有默认值的也不用传参数啊

@available(iOS 13.0, macOS 10.15, tvOS 13.0, watchOS 6.0, *)
public protocol View {

/// The type of view representing the body of this view.
///
/// When you create a custom view, Swift infers this type from your
/// implementation of the required ``View/body-swift.property`` property.
associatedtype Body : View

/// The content and behavior of the view.
///
/// When you implement a custom view, you must implement a computed
/// `body` property to provide the content for your view. Return a view
/// that's composed of primitive views that SwiftUI provides, plus other
/// composite views that you've already defined:
///
/// struct MyView: View {
/// var body: some View {
/// Text("Hello, World!")
/// }
/// }