Make some Attrs append instead of replace

src/element.rs

@@ -1,4 +1,4 @@
-use std::collections::{BTreeMap, HashMap};
+use std::collections::{btree_map::Entry, BTreeMap, HashMap};
 
 /// <https://html.spec.whatwg.org/multipage/syntax.html#elements-2>
 #[derive(Clone, Copy, PartialEq, Eq)]
@@ -101,28 +101,76 @@ pub trait ElementComponent {
 pub struct Attr {
     name: String,
     value: String,
+    append_by: Option<String>,
 }
 
 impl Attr {
+    /// Create or replace an attribute.
+    ///
+    /// When this attribute is added to an [`Element`] through
+    /// [`ElementComponent::add_to_element`] and an attribute of the same name
+    /// already exists, it replaces that attribute's value.
     pub fn new(name: impl ToString, value: impl ToString) -> Self {
         Self {
             name: name.to_string(),
             value: value.to_string(),
+            append_by: None,
         }
     }
 
+    /// Create or append to an attribute.
+    ///
+    /// When this attribute is added to an [`Element`] through
+    /// [`ElementComponent::add_to_element`] and an attribute of the same name
+    /// already exists, it appends the separator and then its own value to that
+    /// attribute's value.
+    pub fn append(name: impl ToString, value: impl ToString, separator: impl ToString) -> Self {
+        Self {
+            name: name.to_string(),
+            value: value.to_string(),
+            append_by: Some(separator.to_string()),
+        }
+    }
+
+    /// Create (or replace) a new empty attribute.
+    ///
+    /// `Attr::yes(name)` is equivalent to `Attr::new(name, "").`
+    ///
+    /// When rendering an empty attribute as HTML, the value can be omitted:
+    /// `name=""` is equivalent to just `name`.
     pub fn yes(name: impl ToString) -> Self {
         Self::new(name, "")
     }
 
+    /// Create (or replace) an `id` attribute.
+    ///
+    /// `Attr::id(id)` is equivalent to `Attr::new("id", id)`.
     pub fn id(id: impl ToString) -> Self {
         Self::new("id", id)
     }
 
+    /// Create (or append) to a `class` attribute.
+    ///
+    /// `Attr::class(class)` is equivalent to
+    /// `Attr::append("class", class, " ")`.
     pub fn class(class: impl ToString) -> Self {
-        Self::new("class", class)
+        Self::append("class", class, " ")
     }
 
+    /// Create (or append) to a `style` attribute.
+    ///
+    /// `Attr::style(style)` is equivalent to
+    /// `Attr::append("style", style, ";")`.
+    pub fn style(style: impl ToString) -> Self {
+        Self::append("style", style, ";")
+    }
+
+    /// Create (or replace) a new [`data-*` attribute][mdn].
+    ///
+    /// `Attr::data(name, value)` is equivalent to
+    /// `Attr::new(format!("data-{name}"), value)`.
+    ///
+    /// [mdn]: https://developer.mozilla.org/en-US/docs/Web/HTML/Global_attributes/data-*
     pub fn data(name: impl ToString, value: impl ToString) -> Self {
         Self::new(format!("data-{}", name.to_string()), value)
     }
@@ -133,7 +181,21 @@ impl ElementComponent for Attr {
         if element.kind != ElementKind::Foreign {
             self.name = self.name.to_ascii_lowercase();
         }
-        element.attributes.insert(self.name, self.value);
+        match element.attributes.entry(self.name) {
+            Entry::Vacant(entry) => {
+                entry.insert(self.value);
+            }
+            Entry::Occupied(mut entry) => match self.append_by {
+                None => {
+                    entry.insert(self.value);
+                }
+                Some(sep) => {
+                    let value = entry.get_mut();
+                    value.push_str(&sep);
+                    value.push_str(&self.value);
+                }
+            },
+        }
     }
 }