From 5d761742c1b8906d9e8fd200c592b119c69b79e4 Mon Sep 17 00:00:00 2001
From: Frank Villaro-Dixon <frank@villaro-dixon.eu>
Date: Sat, 20 Apr 2024 00:16:08 +0200
Subject: [PATCH] examples: add flattened enum deserialization

Add a small example about flattened enum deserialization. This is a
small workaround around serde's [serde's
issue](https://github.com/serde-rs/serde/issues/1905).

Big thanks to @Mingun for (most) of the code + help!

Signed-off-by: Frank Villaro-Dixon <frank@villaro-dixon.eu>
---
 Cargo.toml                 |   5 ++
 Changelog.md               |   3 +
 examples/flattened_enum.rs | 109 +++++++++++++++++++++++++++++++++++++
 3 files changed, 117 insertions(+)
 create mode 100644 examples/flattened_enum.rs

diff --git a/Cargo.toml b/Cargo.toml
index eb2f794d..46355ae1 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -242,3 +242,8 @@ path = "tests/async-tokio.rs"
 name = "read_nodes_serde"
 required-features = ["serialize"]
 path = "examples/read_nodes_serde.rs"
+
+[[example]]
+name = "flattened_enum"
+required-features = ["serialize"]
+path = "examples/flattened_enum.rs"
diff --git a/Changelog.md b/Changelog.md
index 1ee04a0a..6b01f422 100644
--- a/Changelog.md
+++ b/Changelog.md
@@ -60,6 +60,8 @@ to get an offset of the error position. For `SyntaxError`s the range
 - [#362]: Now default quote level is `QuoteLevel::Partial` when using serde serializer.
 - [#689]: `buffer_position()` now always report the position the parser last seen.
   To get an error position use `error_position()`.
+- [#738]: Add an example of how to deserialize XML elements into Rust enums using an
+  intermediate custom deserializer.
 
 [#362]: https://github.com/tafia/quick-xml/issues/362
 [#513]: https://github.com/tafia/quick-xml/issues/513
@@ -70,6 +72,7 @@ to get an offset of the error position. For `SyntaxError`s the range
 [#689]: https://github.com/tafia/quick-xml/pull/689
 [#704]: https://github.com/tafia/quick-xml/pull/704
 [#705]: https://github.com/tafia/quick-xml/pull/705
+[#738]: https://github.com/tafia/quick-xml/pull/738
 
 
 ## 0.31.0 -- 2023-10-22
diff --git a/examples/flattened_enum.rs b/examples/flattened_enum.rs
new file mode 100644
index 00000000..1191638f
--- /dev/null
+++ b/examples/flattened_enum.rs
@@ -0,0 +1,109 @@
+//! This example demonstrates how to deserialize enum nodes using an intermediate
+//! custom deserializer.
+//! The `elem` node can either be a `Foo` or a `Bar` node, depending on the `type`.
+//! The `type` attribute is used to determine which variant to deserialize.
+//! This is a workaround for [serde's issue](https://github.com/serde-rs/serde/issues/1905)
+//!
+//! note: to use serde, the feature needs to be enabled
+//! run example with:
+//!    cargo run --example flattened_enum --features="serialize"
+
+use std::fmt;
+
+use quick_xml::de::from_str;
+use serde::de::value::MapAccessDeserializer;
+use serde::de::{Error, MapAccess, Visitor};
+use serde::Deserialize;
+
+#[derive(Debug, Deserialize, PartialEq)]
+struct Model {
+    elem: Vec<Elem>,
+}
+
+#[derive(Debug, PartialEq)]
+enum Elem {
+    Foo(Foo),
+    Bar(Bar),
+}
+
+impl<'de> Deserialize<'de> for Elem {
+    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+    where
+        D: serde::Deserializer<'de>,
+    {
+        struct ElemVisitor;
+
+        impl<'de> Visitor<'de> for ElemVisitor {
+            type Value = Elem;
+
+            fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+                formatter.write_str("an object with a `type` field")
+            }
+
+            fn visit_map<A>(self, mut map: A) -> Result<Elem, A::Error>
+            where
+                A: MapAccess<'de>,
+            {
+                if let Some((key, value)) = map.next_entry::<String, String>()? {
+                    return match key.as_str() {
+                        "@type" => match value.as_str() {
+                            "foo" => {
+                                let f = Foo::deserialize(MapAccessDeserializer::new(map))?;
+                                Ok(Elem::Foo(f))
+                            }
+                            "bar" => {
+                                let f = Bar::deserialize(MapAccessDeserializer::new(map))?;
+                                Ok(Elem::Bar(f))
+                            }
+                            t => Err(Error::custom(format!("unknown type attribute `{t}`"))),
+                        },
+                        a => Err(Error::custom(format!(
+                            "expected attribute `type`, but found `{a}`"
+                        ))),
+                    };
+                }
+                Err(Error::custom("expected `type` attribute"))
+            }
+        }
+        deserializer.deserialize_map(ElemVisitor)
+    }
+}
+
+#[derive(Debug, Deserialize, PartialEq)]
+struct Foo {
+    a: String,
+    subfoo: SubFoo,
+}
+
+#[derive(Debug, Deserialize, PartialEq)]
+struct SubFoo {
+    a1: String,
+    a2: String,
+    a3: String,
+}
+
+#[derive(Debug, Deserialize, PartialEq)]
+struct Bar {
+    b: String,
+}
+
+fn main() {
+    let x = r#"
+<model>
+    <elem type="foo">
+        <a>1</a>
+        <subfoo>
+            <a1>2</a1>
+            <a2>42</a2>
+            <a3>1337</a3>
+        </subfoo>
+    </elem>
+    <elem type="bar">
+        <b>22</b>
+    </elem>
+</model>
+"#;
+
+    let model: Model = from_str(&x).unwrap();
+    println!("{:?}", model);
+}