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This commit implements the first half of [RFC #5] where the `Deref` trait is implemented for all imported types. The target of `Deref` is either the first entry of the list of `extends` attribute or `JsValue`. All examples using `.as_ref()` with various `web-sys` types have been updated to the more ergonomic deref casts now. Additionally the `web-sys` generation of the `extends` array has been fixed slightly to explicitly list implementatoins in the hierarchy order to ensure the correct target for `Deref` is chosen. [RFC #5]: https://github.com/rustwasm/rfcs/blob/master/text/005-structural-and-deref.md
56 lines
2.0 KiB
Markdown
56 lines
2.0 KiB
Markdown
# Inheritance in `web-sys`
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Inheritance between JS classes is the bread and butter of how the DOM works on
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the web, and as a result it's quite important for `web-sys` to provide access to
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this inheritance hierarchy as well! There are few ways you can access the
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inheritance hierarchy when using `web-sys`.
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### Accessing parent classes using `Deref`
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Like smart pointers in Rust, all types in `web_sys` implement `Deref` to their
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parent JS class. This means, for example, if you have a `web_sys::Element` you
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can create a `web_sys::Node` from that implicitly:
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```rust
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let element: &Element = ...;
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element.append_child(..); // call a method on `Node`
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method_expecting_a_node(&element); // coerce to `&Node` implicitly
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let node: &Node = &element; // explicitly coerce to `&Node`
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```
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Using `Deref` allows ergonomic transitioning up the inheritance hierarchy to the
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parent class and beyond, giving you access to all the methods using the `.`
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operator.
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### Accessing parent classes using `AsRef`
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In addition to `Deref`, the `AsRef` trait is implemented for all types in
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`web_sys` for all types in the inheritance hierarchy. For example for the
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`HtmlAnchorElement` type you'll find:
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```rust
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impl AsRef<HtmlElement> for HtmlAnchorElement
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impl AsRef<Element> for HtmlAnchorElement
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impl AsRef<Node> for HtmlAnchorElement
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impl AsRef<EventTarget> for HtmlAnchorElement
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impl AsRef<Object> for HtmlAnchorElement
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impl AsRef<JsValue> for HtmlAnchorElement
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```
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You can use `.as_ref()` to explicitly get a reference to any parent class from
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from a type in `web_sys`. Note that because of the number of `AsRef`
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implementations you'll likely need to have type inference guidance as well.
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### Accessing child clases using `JsCast`
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Finally the `wasm_bindgen::JsCast` trait can be used to implement all manner of
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casts between types. It supports static unchecked casts between types as well as
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dynamic runtime-checked casts (using `instanceof`) between types.
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More documentation about this can be found [on the trait itself][jscast]
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[jscast]: https://docs.rs/wasm-bindgen/0.2/wasm_bindgen/trait.JsCast.html
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