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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,61 @@ | ||
| ## Clamping | ||
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| ### Purpose: | ||
| 1. For values that have upper and lower bound, provide a way to not write duplicate code. | ||
| 2. In swift, when one try to preprocess an property before apply it, it needs following code: | ||
| ```swift | ||
| private var _computedVarStorage: Int | ||
| public var computedVar: Int { | ||
| get { | ||
| return _computedVarStorage | ||
| } | ||
| set { | ||
| _computedVarStorage = newValue*10 | ||
| } | ||
| } | ||
| ``` | ||
| Because the get/set syntax will change variable to computed variable, it always need write a getter, even through we want a write-only property. | ||
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| Also, to keep the modified value, a extra storage variable is needed, and it looks ugly. | ||
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| 3. In additionally, we also want the solution could be apply to protocol. | ||
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| ### Solution: | ||
| 1. Use @propertyWrapper to apply clamp; replace max, min and degree into one declaration. | ||
| ```swift | ||
| @Clamping(max: 0.0, min: 360.0) var degree = 0.0 | ||
| @Clamping(0.0...1.0) var value = 0.5 | ||
| ``` | ||
| 2. In some situation, it need modify the range later, property wrapper use projectedValue to implement this requirement: | ||
| ```swift | ||
| @Clamping(0.0...1.0) var value = 0.5 | ||
| $value = 0.0...10.0 | ||
| ``` | ||
| 3. Although @propertyWrapper is useful, it hard to apply this mechanism to protocol. However, there has a workaround: | ||
| ```swift | ||
| protocol ClampProtocol { | ||
| var degree: Double // Can declare as @Clamping | ||
| var range: ClosedRange<Double> // Can map to projectedValue in @Clamping | ||
| } | ||
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| struct ClampStruct { | ||
| @Clamping(0.0...360.0) var degree = 0.0 | ||
| var range: ClosedRange<Double> { | ||
| get { $degree } | ||
| set { $degree = newValue } | ||
| } | ||
| } | ||
| ``` | ||
| With this workaround, all class/struct implement ClampProtocol can apply @Clamping to simplify the effort to write clamp function, and keep it interface clear. | ||
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| The detail about the implementation of @Clamping could be refer to : [Clamping.swift](Clamping.swift) | ||
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| ### References: | ||
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| [Property Wrapper - Swift Doc](https://docs.swift.org/swift-book/LanguageGuide/Properties.html#ID617) | ||
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| [Property Wrapper(SE-0258) - Swift Evolution](https://github.com/apple/swift-evolution/blob/master/proposals/0258-property-wrappers.md) | ||
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| [Swift: Why does a variable with a setter must also have a getter? - stackoverflow](https://stackoverflow.com/a/34677538/505763) | ||
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| [PropertyWrappers and protocol declaration? - stackoverflow](https://stackoverflow.com/a/57657870/505763) |
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,46 @@ | ||
| // | ||
| // Clamping.swift | ||
| // | ||
| // | ||
| // Created by Chen-Hai Teng on 2021/6/11. | ||
| // | ||
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| import Foundation | ||
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| @propertyWrapper | ||
| public struct Clamping<T> where T:Comparable { | ||
| private(set) var value: T | ||
| var range: ClosedRange<T> | ||
| public var wrappedValue: T { | ||
| get { | ||
| return value | ||
| } | ||
| set { | ||
| value = clamp(newValue) | ||
| } | ||
| } | ||
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| public var projectedValue: ClosedRange<T> { | ||
| get { range } | ||
| set { | ||
| range = newValue | ||
| value = clamp(value) | ||
| } | ||
| } | ||
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| private func clamp(_ v: T) -> T { | ||
| max(min(v, range.upperBound), range.lowerBound) | ||
| } | ||
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| public init(wrappedValue: T, _ range: ClosedRange<T>) { | ||
| self.range = range | ||
| value = wrappedValue // 1st phase, initialize properties. | ||
| value = clamp(wrappedValue) // 2nd phase, custom the value of property | ||
| } | ||
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| public init(wrappedValue: T, max: T, min: T) { | ||
| self.range = min...max | ||
| value = wrappedValue | ||
| value = clamp(wrappedValue) | ||
| } | ||
| } |
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