Question

我尝试在else方法中设置ifLet的默认参数，但遇到错误：Protocol 'View' can only be used as a generic constraint because it has Self or associated type requirements。怎么了

extension View {
    func ifLet<Value, Then: View, Else: View>(
        _ value: Value?,
        then: (Value) -> Then,
        else: () -> View = { EmptyView() }
    ) -> _ConditionalContent<Then, Else> {
        if let value = value {
            return ViewBuilder.buildEither(first: then(value))
        } else {
            return ViewBuilder.buildEither(second: `else`())
        }
    }
}

使用：

struct TestView: View {
    var test: String?

    var body: some View {
        Group {
            ifLet(test) { Text($0) }
            ifLet(test, then: { Text($0) }, else: { Text("Empty") })
        }
    }
}

最好的解决方案，而不使用将来可能会更改或删除的非官方_ConditionalContent结帐here

Answer 1

这是可行的方法。经过测试并可以与Xcode 11.2 / iOS 13.2配合使用。

struct TestingIfLet: View {
    var some: String?
    var body: some View {
        VStack {
            ifLet(some, then: {value in Text("Test1 \(value)") })
            ifLet(some, then: {value in Text("Test2 \(value)") }, 
                else: { Text("Test3") })
        }
    }
}

extension View {

    func ifLet<Value, Then: View>(
        _ value: Value?,
        then: (Value) -> Then
    ) -> _ConditionalContent<Then, EmptyView> {
        if let value = value {
            return ViewBuilder.buildEither(first: then(value))
        } else {
            return ViewBuilder.buildEither(second: EmptyView())
        }
    }

    func ifLet<Value, Then: View, Else: View>(
        _ value: Value?,
        then: (Value) -> Then,
        else: () -> Else
    ) -> _ConditionalContent<Then, Else> {
        if let value = value {
            return ViewBuilder.buildEither(first: then(value))
        } else {
            return ViewBuilder.buildEither(second: `else`())
        }
    }
}

如何在View方法中设置默认Clouse参数？

1 个答案: