SwiftUI ViewModifier有什么用

• 看了下源码 ViewModifier 是一个 protocol

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

    /// The type of view representing the body.
    associatedtype Body : View

    /// Gets the current body of the caller. 获取调用方的当前主体
    ///
    /// `content` is a proxy for the view that will have the modifier
    ///  content是具有修饰符的视图的代理
    /// represented by `Self` applied to it.
    func body(content: Self.Content) -> Self.Body

    /// The content view type passed to `body()`.
    typealias Content
}

• 新建一个struct实现一下 body方法， 其他的默认就行， 这个方法入参数是当前调用者的主体， 返回值是一个View(协议), 那么是不是入参也应该是一个实现了协议View的对象， 简单理解为UIView

struct Loading: ViewModifier {
    
    func body(content: Content) -> some View {
        ZStack(){
            content.zIndex(0)
            Text("123").zIndex(1)
        }
    }
}

• 既然可以拿到uiview 我们就可以做很多事情了， 像上面一样调整布局， 对content进行一些属性设置等等, 看你的需求了

• 自定义了 ViewModifier 使用

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

    /// Applies a modifier to a view and returns a new view.
    ///
    /// Use this modifier to combine a ``View`` and a ``ViewModifier``, to
    /// create a new view. For example, if you create a view modifier for
    /// a new kind of caption with blue text surrounded by a rounded rectangle:
    ///
    ///     struct BorderedCaption: ViewModifier {
    ///         func body(content: Content) -> some View {
    ///             content
    ///                 .font(.caption2)
    ///                 .padding(10)
    ///                 .overlay(
    ///                     RoundedRectangle(cornerRadius: 15)
    ///                         .stroke(lineWidth: 1)
    ///                 )
    ///                 .foregroundColor(Color.blue)
    ///         }
    ///     }
    ///
    /// You can use ``modifier(_:)`` to extend ``View`` to create new modifier
    /// for applying the `BorderedCaption` defined above:
    ///
    ///     extension View {
    ///         func borderedCaption() -> some View {
    ///             modifier(BorderedCaption())
    ///         }
    ///     }
    ///
    /// Then you can apply the bordered caption to any view:
    ///
    ///     Image(systemName: "bus")
    ///         .resizable()
    ///         .frame(width:50, height:50)
    ///     Text("Downtown Bus")
    ///         .borderedCaption()
    ///
    /// ![A screenshot showing the image of a bus with a caption reading
    /// Downtown Bus. A view extension, using custom a modifier, renders the
    ///  caption in blue text surrounded by a rounded
    ///  rectangle.](SwiftUI-View-ViewModifier.png)
    ///
    /// - Parameter modifier: The modifier to apply to this view.
    @inlinable public func modifier<T>(_ modifier: T) -> ModifiedContent<Self, T>
}

• 通过 modifier方法即可

• 所以还是要多看源码啊