POLA array.length Atom

[grithin]

Adding an array.length Atom to make observation of array.length as expected.

1751937
This is the change I was recommending in #3988

Performance tests pass. The only tests that don't pass, apart from the tests that fail prior to my change, are those related to Spy changes.

If this is an acceptable change to mobx, I will update the Spy unit tests and send a pull request.

[grithin]

@mweststrate any thoughts on this now that you can see the code?

It makes it so autoruns that are using array.length only respond to changes to length.
For example

function test({makeObservable, observable, action, autorun, toJS}:typeof mobx){
	const do_test_action = (name, cb)=>{
		console.log(`\n==Test: ${name}==`)
		action(cb)()
		console.log(`<==`)
	}
	let observed:any[];
	observed = observable(['homer']) as Array<any>

	autorun(() => {
		console.log("array length observed: ", observed.length)
	})
	do_test_action('change homer', () => observed[0] += ' simpson')
}

test(mobx)
/* ~>
==Test: change homer==
array length observed:  1
<==
*/

test(mobx_modded)
/* ~>
==Test: change homer==
<==
*/

[grithin]

I suppose an issue might be that people are using array.length within an autorun as a method of observing general array modifications instead of observe.