Created
June 10, 2019 18:09
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An example of matching up @State values from multiple View trees. SwiftUI probably does something faster/cleaner/nicer, but this is how it could work.
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| import UIKit | |
| // Just hold a value in a class so that we can reference it. | |
| private class XStateStorage<Value> { | |
| var value: Value | |
| init(value: Value) { | |
| self.value = value | |
| } | |
| } | |
| // Something like SwiftUI's View but leaving out the fancy function builder stuff for simplicity | |
| protocol XView { | |
| var body: [XView] { get } | |
| } | |
| // A resulting struct for converting things that conform to XView into an explicit tree | |
| struct XViewTree { | |
| var parent: XView | |
| var children: [XViewTree] | |
| } | |
| class XViewTreeGenerator { | |
| static var current: XViewTreeGenerator? // Could/should be thread-local instead | |
| // Here we store where we are currently rendering the tree, and all of the state in the tree | |
| var currentPath = IndexPath() | |
| var allState: [NSIndexPath : AnyObject] = [:] | |
| // Convert a root XView into an XViewTree. You'd do this on each render. | |
| func generateTree(root: XView) -> XViewTree { | |
| func internalGenerateTree(root: XView) -> XViewTree { | |
| var children: [XViewTree] = [] | |
| var index = 0 | |
| for child in root.body { | |
| currentPath.append(index) | |
| children.append(generateTree(root: child)) | |
| currentPath.removeLast() | |
| index += 1 | |
| } | |
| return XViewTree(parent: root, children: children) | |
| } | |
| XViewTreeGenerator.current = self | |
| let result = internalGenerateTree(root: root) | |
| XViewTreeGenerator.current = nil | |
| return result | |
| } | |
| // For use just by XState property delegates | |
| func getCurrentState() -> AnyObject? { | |
| return allState[currentPath as NSIndexPath] | |
| } | |
| func setCurrentState(_ state: AnyObject) { | |
| allState[currentPath as NSIndexPath] = state | |
| } | |
| } | |
| // XState finds its actual state storage in the current generator based on the current position in the view tree. | |
| // Note that for simplicity, this example implementation only works for a single @XState value per view. Multiple | |
| // values per view would require some additional index path munging and checking. | |
| @propertyDelegate | |
| public struct XState<Value> { | |
| private var initial: Value | |
| private func getStorage() -> XStateStorage<Value> { | |
| guard let generator = XViewTreeGenerator.current else { fatalError("Accessing @XState outside of body") } | |
| if let storage = generator.getCurrentState() as! XStateStorage<Value>? { | |
| print("\(generator.currentPath) matched up to existing storage: \(storage.value)") | |
| return storage | |
| } else { | |
| print("\(generator.currentPath) creating new storage with value: \(initial)") | |
| let storage = XStateStorage(value: initial) | |
| generator.setCurrentState(storage) | |
| return storage | |
| } | |
| } | |
| public var value: Value { | |
| get { getStorage().value } | |
| set { getStorage().value = newValue } | |
| } | |
| public init(initialValue: Value) { | |
| initial = initialValue | |
| } | |
| } | |
| // Here are some sample XView types. | |
| struct XChildView : XView { | |
| @XState var isThing = true | |
| var body: [XView] { | |
| print(isThing) | |
| return [] | |
| } | |
| } | |
| struct XIntermediateView : XView { | |
| @XState var stuff = "String" | |
| var body: [XView] { | |
| print(stuff) | |
| return [XChildView(), XChildView()] | |
| } | |
| } | |
| struct XContentView : XView { | |
| var body: [XView] { | |
| return [XChildView(), XIntermediateView(), XChildView()] | |
| } | |
| } | |
| // See how the state is created on first render, and matched up to the already existing state on re-render, even though the XViews themselves are new. | |
| let g = XViewTreeGenerator() | |
| g.generateTree(root: XContentView()) | |
| g.generateTree(root: XContentView()) |
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