This change introduces class/style reconciliation algorithm for DOM elements.
NOTE: The code is not yet hooked up, it will be used by future style algorithm.
Background:
Styling algorithm currently has [two paths](https://hackmd.io/@5zDGNGArSxiHhgvxRGrg-g/rycZk3N5S)
when computing how the style should be rendered.
1. A direct path which concatenates styling and uses `elemnent.className`/`element.style.cssText` and
2. A merge path which uses internal data structures and uses `element.classList.add/remove`/`element.style[property]`.
The situation is confusing and hard to follow/maintain. So a future PR will remove the merge-path and do everything with
direct-path. This however breaks when some other code adds class or style to the element without Angular's knowledge.
If this happens instead of switching from direct-path to merge-path algorithm, this change provides a different mental model
whereby we always do `direct-path` but the code which writes to the DOM detects the situation and reconciles the out of bound write.
The reconciliation process is as follows:
1. Detect that no one has modified `className`/`cssText` and if so just write directly (fast path).
2. If out of bounds write did occur, switch from writing using `className`/`cssText` to `element.classList.add/remove`/`element.style[property]`.
This does require that the write function computes the difference between the previous Angular expected state and current Angular state.
(This requires a parser. The advantage of having a parser is that we can support `style="width: {{exp}}px" kind of bindings.`)
Compute the diff and apply it in non destructive way using `element.classList.add/remove`/`element.style[property]`
Properties of approach:
- If no out of bounds style modification:
- Very fast code path: Just concatenate string in right order and write them to DOM.
- Class list order is preserved
- If out of bounds style modification detected:
- Penalty for parsing
- Switch to non destructive modification: `element.classList.add/remove`/`element.style[property]`
- Switch to alphabetical way of setting classes.
PR Close#34004
76698d38f7