🅿️PreferenceKey

The PreferenceKey protocol enables a way to send data up the view hierarchy. 👉 SwiftOnTap

當一開始佈局時 (layout)，就要子視件回報一些資料給母視件才能順利完成佈局的話，就可以考慮使用 PreferenceKey。

// ------------------------
//     🅿️ PreferenceKey
// ------------------------

public protocol PreferenceKey {
    // value type
    associatedtype Value
    // default value
    static var defaultValue: Self.Value { get }
    // combine values from different children
    static func reduce(
        value    : inout Self.Value, 
        nextValue: () -> Self.Value
    )
} 

// ------------------------
//     View methods
// ------------------------

// 🔸 (child view) set view preference
func preference<K: PreferenceKey>(
    key  : K.Type = K.self, 
    value: K.Value
) -> some View 

// 🔸 .onPreferenceChange()
// (parent view) react to view preference change
func onPreferenceChange<K>(
    _          key: K.Type = K.self, 
    perform action: @escaping (K.Value) -> Void
) -> some View where 
    K       : PreferenceKey, 
    K.Value : Equatable

// 🔸 .overlayPreferenceValue(key:transform:)
func overlayPreferenceValue<Key: PreferenceKey, T: View>(
    _       key: Key.Type = Key.self, 
    _ transform: @escaping (Key.Value) -> T
) -> some View 

// 🔸 .backgroundPreferenceValue(key:transform:)
func backgroundPreferenceValue<Key: PreferenceKey, T: View>(
    _       key: Key.Type = Key.self, 
    _ transform: @escaping (Key.Value) -> T
) -> some View

範例

// 📦 MaxValue<T: FloatingPoint>
public struct MaxValue<T: FloatingPoint>: PreferenceKey {
    // value type
    public typealias Value = T?
    // default value (nil == not set) 
    public static var defaultValue: Value { nil }
    // ⭐️ choose max value
    public static func reduce(value: inout Value, nextValue: () -> Value) {
        // ⭐️ [T?] --(compactMap)--> [T] --(max)--> T?
        value = [value, nextValue()].compactMap{ $0 }.max()
    }
}

// 📦 AllValues<T>
public struct AllValues<T>: PreferenceKey {
    // ⭐️ 收集的資料放在 [T] 裡面
    public typealias Value = [T]
    // ⭐️ 初始值：空陣列
    public static var defaultValue: Value { Value() }
    // ⭐️ 加入新資料的方法：[v1, v2, ...] + [vn]
    public static func reduce(value: inout Value, nextValue: () -> Value) {
        value += nextValue()   // nextValue() == [vn]
    }
}

// 📦 FirstNonNil<T>
public struct FirstNonNil<T>: PreferenceKey {
    public typealias Value = T?
    // default value
    public static var defaultValue: Value { nil }
    // combine values from different child views
    public static func reduce(value: inout Value, nextValue: () -> Value) {
        value = value ?? nextValue()   // nil or first non-nil value
    }
}