BREAKING CHANGE: replae `watch(fn, options?)` with `watchEffect`
The `watch(fn, options?)` signature has been replaced by the new
`watchEffect` API, which has the same usage and behavior. `watch`
now only supports the `watch(source, cb, options?)` signautre.
BREAKING CHANGE: reactive arrays no longer unwraps contained refs
When reactive arrays contain refs, especially a mix of refs and
plain values, Array prototype methods will fail to function
properly - e.g. sort() or reverse() will overwrite the ref's value
instead of moving it (see #737).
Ensuring correct behavior for all possible Array methods while
retaining the ref unwrapping behavior is exceedinly complicated; In
addition, even if Vue handles the built-in methods internally, it
would still break when the user attempts to use a 3rd party utility
functioon (e.g. lodash) on a reactive array containing refs.
After this commit, similar to other collection types like Map and
Set, Arrays will no longer automatically unwrap contained refs.
The usage of mixed refs and plain values in Arrays should be rare in
practice. In cases where this is necessary, the user can create a
computed property that performs the unwrapping.
BREAKING CHANGE: `watch` behavior has been adjusted.
- When using the `watch(source, callback, options?)` signature, the
callback now fires lazily by default (consistent with 2.x
behavior).
Note that the `watch(effect, options?)` signature is still eager,
since it must invoke the `effect` immediately to collect
dependencies.
- The `lazy` option has been replaced by the opposite `immediate`
option, which defaults to `false`. (It's ignored when using the
effect signature)
- Due to the above changes, the `watch` option in Options API now
behaves exactly the same as 2.x.
- When using the effect signature or `{ immediate: true }`, the
intital execution is now performed synchronously instead of
deferred until the component is mounted. This is necessary for
certain use cases to work properly with `async setup()` and
Suspense.
The side effect of this is the immediate watcher invocation will
no longer have access to the mounted DOM. However, the watcher can
be initiated inside `onMounted` to retain previous behavior.
Terser will aggressively inline hot functions in renderer.ts in order
to reduce "function" declarations, but the inlining leads to performance
overhead (small, but noticeable in benchmarks).
Since we cannot control user's minifier options, we have to avoid the
deopt in the source code by using arrow functions in hot paths.
