Allow to override turbo fetch response (or export FetchRequest class)

[omohokcoj]

I'm attempting to implement http://instantclick.io functionality (prefetch on link hover) in my turbo app - one of the ways to implement it is via turbo:before-fetch-request event handler with the ability to override turbo fetch response:
8de3c39

https://gist.github.com/omohokcoj/781a18b3615e4e721ae5fa7f5d5c44ec (prefetch on hover implementation on top of the turbo:before-fetch-request event)

Another way to implement it is via the FetchRequest class monkey patch (not exported in turbo currently).

Would appreciate it a lot if any of those two commits (or both) could be merged and released in the next version.

Related issue: #174

[davidalejandroaguilar]

@omohokcoj This PR might be of interest #1000, since it touches the same files as this PR.

[davidalejandroaguilar]

@seanpdoyle @dhh Have you guys looked at this?

Oleksandr and Pete from Docuseal (Open Source Document Signing built with Rails and Turbo) added a Turbo version of Instantclick, and the perceived speed benefits are simply amazing (they can be experienced on their page.

[dhh]

I'd like to see us add instaclick functionality straight into Turbo, if anyone feels like investigating that.

[shiftyp]

Maybe a good place to “start”

turbo/src/core/frames/link_interceptor.js

Lines 7 to 17 in 3d30fe1

	start() {
	this.element.addEventListener("click", this.clickBubbled)
	document.addEventListener("turbo:click", this.linkClicked)
	document.addEventListener("turbo:before-visit", this.willVisit)
	}
	stop() {
	this.element.removeEventListener("click", this.clickBubbled)
	document.removeEventListener("turbo:click", this.linkClicked)
	document.removeEventListener("turbo:before-visit", this.willVisit)
	}

turbo/src/observers/link_click_observer.js

Lines 12 to 24 in 3d30fe1

	start() {
	if (!this.started) {
	this.eventTarget.addEventListener("click", this.clickCaptured, true)
	this.started = true
	}
	}
	stop() {
	if (this.started) {
	this.eventTarget.removeEventListener("click", this.clickCaptured, true)
	this.started = false
	}
	}