Rust XML处理库sxd-xpath的使用：高效解析与查询XML文档的XPath实现

sxd-xpath是Rust中一个强大的XPath处理库，专门用于解析和查询XML文档。它提供了完整的XPath 1.0实现，能够高效地在XML文档中定位和提取数据。

安装方法

在Cargo.toml中添加依赖：

[dependencies]
sxd-xpath = "0.4"
sxd-document = "0.3"

基本使用方法

1. 解析XML文档

use sxd_document::parser;

let xml = r#"<books>
    <book id="1">
        <title>Rust in Action</title>
        <author>Tim McNamara</author>
        <price>49.99</price>
    </book>
    <book id="2">
        <title>The Rust Programming Language</title>
        <author>Steve Klabnik</author>
        <price>39.99</price>
    </book>
</books>"#;

let package = parser::parse(xml).expect("Failed to parse XML");
let document = package.as_document();

2. 执行XPath查询

use sxd_xpath::{Factory, Value};

let factory = Factory::new();
let xpath = factory.build("//book[price > 40]/title").expect("Invalid XPath");
let xpath = xpath.expect("No XPath was compiled");

let value = xpath.evaluate(&document, document.root()).expect("XPath evaluation failed");

match value {
    Value::Nodeset(nodes) => {
        for node in nodes.document_order() {
            println!("Title: {}", node.string_value());
        }
    },
    _ => println!("Unexpected result type"),
}

高级功能

1. 使用变量

use sxd_xpath::{Factory, Value, Variables};

let factory = Factory::new();
let xpath = factory.build("//book[price > $min_price]/title").expect("Invalid XPath");
let xpath = xpath.expect("No XPath was compiled");

let mut variables = Variables::new();
variables.set("min_price", Value::Number(40.0));

let value = xpath.evaluate(&document, document.root(), Some(&variables)).expect("XPath evaluation failed");

2. 处理命名空间

use sxd_xpath::{Factory, NamespaceResolver, Value};

let xml = r#"<ns:books xmlns:ns="http://example.com/books">
    <ns:book id="1">
        <ns:title>Rust in Action</ns:title>
    </ns:book>
</ns:books>"#;

let package = parser::parse(xml).expect("Failed to parse XML");
let document = package.as_document();

let factory = Factory::new();
let xpath = factory.build("//ns:book/ns:title").expect("Invalid XPath");
let xpath = xpath.expect("No XPath was compiled");

let resolver = NamespaceResolver::new();
resolver.add_namespace("ns", "http://example.com/books");

let value = xpath.evaluate_with_namespace_resolver(
    &document, 
    document.root(), 
    &resolver
).expect("XPath evaluation failed");

3. 自定义函数

use sxd_xpath::{Factory, Value, Function, ExecutionContext};

struct CustomFunction;
impl Function for CustomFunction {
    fn evaluate(&self, _: &ExecutionContext, args: &[Value]) -> Value {
        // 实现自定义逻辑
        Value::String("custom result".to_string())
    }
}

let factory = Factory::new();
factory.set_function("custom-fn", CustomFunction);

let xpath = factory.build("custom-fn()").expect("Invalid XPath");
let xpath = xpath.expect("No XPath was compiled");

let value = xpath.evaluate(&document, document.root()).expect("XPath evaluation failed");

性能提示

1. 重用Factory实例，因为它会缓存编译的XPath表达式
2. 对于重复查询，预编译XPath表达式
3. 当处理大量XML数据时，考虑使用流式解析器

错误处理

match parser::parse(xml) {
    Ok(package) => {
        // 处理成功
    },
    Err(e) => {
        eprintln!("XML解析错误: {}", e);
    }
}

match factory.build(xpath_str) {
    Ok(Some(compiled)) => {
        // XPath编译成功
    },
    Ok(None) => {
        eprintln!("XPath表达式为空");
    },
    Err(e) => {
        eprintln!("XPath语法错误: {}", e);
    }
}

sxd-xpath提供了强大的XPath 1.0功能，适合需要精确查询XML文档的Rust应用场景。通过合理使用，可以高效地提取和处理XML数据。

完整示例代码

下面是一个完整的示例，展示了如何使用sxd-xpath库解析XML文档并执行XPath查询：

use sxd_document::parser;
use sxd_xpath::{Factory, Value, Variables};

fn main() {
    // 示例XML数据
    let xml = r#"<books>
        <book id="1">
            <title>Rust in Action</title>
            <author>Tim McNamara</author>
            <price>49.99</price>
        </book>
        <book id="2">
            <title>The Rust Programming Language</title>
            <author>Steve Klabnik</author>
            <price>39.99</price>
        </book>
        <book id="3">
            <title>Rust for Rustaceans</title>
            <author>Jon Gjengset</author>
            <price>44.99</price>
        </book>
    </books>"#;

    // 解析XML文档
    let package = parser::parse(xml).expect("Failed to parse XML");
    let document = package.as_document();

    // 创建XPath工厂
    let factory = Factory::new();

    // 示例1：基本XPath查询
    println!("=== 所有书籍标题 ===");
    let xpath = factory.build("//book/title").expect("Invalid XPath");
    if let Some(compiled) = xpath {
        let value = compiled.evaluate(&document, document.root()).expect("XPath evaluation failed");
        if let Value::Nodeset(nodes) = value {
            for node in nodes.document_order() {
                println!("Title: {}", node.string_value());
            }
        }
    }

    // 示例2：带条件的XPath查询
    println!("\n=== 价格高于45的书籍 ===");
    let xpath = factory.build("//book[price > 45]").expect("Invalid XPath");
    if let Some(compiled) = xpath {
        let value = compiled.evaluate(&document, document.root()).expect("XPath evaluation failed");
        if let Value::Nodeset(nodes) = value {
            for node in nodes.document_order() {
                let title = node.children()
                    .find(|n| n.element().map(|e| e.name().local_part() == "title").unwrap();
                let price = node.children()
                    .find(|n| n.element().map(|e| e.name().local_part() == "price").unwrap();
                println!("{} - {}", title.string_value(), price.string_value());
            }
        }
    }

    // 示例3：使用变量
    println!("\n=== 使用变量的查询 ===");
    let xpath = factory.build("//book[price > $min_price]").expect("Invalid XPath");
    if let Some(compiled) = xpath {
        let mut variables = Variables::new();
        variables.set("min_price", Value::Number(40.0));
        
        let value = compiled.evaluate(&document, document.root(), Some(&variables))
            .expect("XPath evaluation failed");
        
        if let Value::Nodeset(nodes) = value {
            println!("找到{}本价格高于40的书", nodes.size());
        }
    }
}

这个完整示例展示了：

1. 如何解析XML文档
2. 执行基本的XPath查询
3. 使用条件表达式过滤结果
4. 在XPath查询中使用变量
5. 处理查询结果并提取所需数据

输出结果将会是：

=== 所有书籍标题 ===
Title: Rust in Action
Title: The Rust Programming Language
Title: Rust for Rustaceans

=== 价格高于45的书籍 ===
Rust in Action - 49.99

=== 使用变量的查询 ===
找到2本价格高于40的书