Chained 'pre' watchers

[skirtles-code]

Version

master at commit 11ed210

The example is using a build of the latest master branch as it relies on a commit that went in yesterday . I'm unclear whether there's a better way to report problems against the latest master.

Reproduction link

https://codesandbox.io/s/holy-sunset-fc8di?file=/index.html

Steps to reproduce

1. Open the console log.
2. Click the button.
3. Note that the watcher for otherValue isn't called until after render, despite being flush: 'pre'.

What is expected?

Changes made by a 'pre' watcher should trigger other 'pre' watchers prior to any rendering.

In Vue 2 this kind of watcher chaining worked fine, see:

https://jsfiddle.net/skirtle/vnkxz35p/9/

What is actually happening?

The second 'pre' watcher isn't called until after render.

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Why do you need a watcher that runs before render?

The example is for illustration only. I've explained in greater detail here why this is important in real applications.

Why not use flush: 'sync' on that second watcher?

In this simple example that would work.

But using 'sync' brings its own problems, such as those discussed in #1763. If the watcher were watching a computed property that also depended on props it wouldn't be viable to use 'sync'.