76698d38f7
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 |
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