3.4.1 Typestate 模式示例

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# 3.4.1 Typestate 模式示例
// SPDX-License-Identifier: Apache-2.0
use std::fmt::Write as _;

#[derive(Default)]
struct Serializer {
    output: String,
}

struct SerializeStruct {
    serializer: Serializer,
}

impl Serializer {
    fn serialize_struct(mut self, name: &str) -> SerializeStruct {
        writeln!(&mut self.output, "{name} {{").unwrap();
        SerializeStruct { serializer: self }
    }

    fn finish(self) -> String {
        self.output
    }
}

impl SerializeStruct {
    fn serialize_field(mut self, key: &str, value: &str) -> Self {
        writeln!(&mut self.serializer.output, "  {key}={value};").unwrap();
        self
    }

    fn finish_struct(mut self) -> Serializer {
        self.serializer.output.push_str("}\n");
        self.serializer
    }
}

fn main() {
    let serializer = Serializer::default()
        .serialize_struct("User")
        .serialize_field("id", "42")
        .serialize_field("name", "Alice")
        .finish_struct();

    println!("{}", serializer.finish());
}