add key for rendering items

[linniksa]

it will increase speed of VNode-Dom comparison and patch

for example for list of images when new slots added to the list only these images will requested from server (at this moment all 30 will be requested) also no visual glitches when this images downloaded (rerender from previous image to new)

[tangbc]

Did you make a comparison with it?

[linniksa]

only by hands in chrome with x6 slowdownm, each item contains ~23-25 dom nodes (one of them is img)

default keeps (30) and slow scroll (range change)

before

[Violation] 'scroll' handler took 700-860ms and all 30 images requested from server (all images receives new src and request it from server or cache - this has visual glitch when new image replaces old image)

after
[Violation] 'scroll' handler took 370-460ms and 10 images requested from server and no visual glitch

full replace (.scrollToOffset(0)) almost the same ~800ms in both cases

[tangbc]

Er, this will make chat-room demo bug.

In here:

vue-virtual-scroll-list/example/src/views/chat-room/Main.vue

Lines 99 to 100 in 33eda19

	this.$nextTick(() => {
	const vsl = this.$refs.vsl

Where $nextTick was triggered before item really render into DOM.

I need find out and check performance again.