Implement streaming preview window

[junegunn]

Preliminary implementation of the streaming preview window. Please test and report any bugs. Fix #2212.

# Will start rendering after 200ms, update every 100ms
fzf --preview 'for i in $(seq 100); do echo $i; sleep 0.01; done'

# Should print "Loading preview" message after 500ms
fzf --preview 'sleep 1; for i in $(seq 100); do echo $i; sleep 0.01; done'

# The first line should appear after 200ms
fzf --preview 'date; sleep 2; date'

# Should not render before enough lines for the scroll offset are ready
rg --line-number --no-heading --color=always ^ |
  fzf --delimiter : --ansi --preview-window '+{2}-/2' \
      --preview 'sleep 1; bat --style=numbers --color=always --pager=never --highlight-line={2} {1}'

/ping @p-kolacz @edi9999

[edi9999]

I see some issues on some of my custom scripts.

Only the first line is shown even after the preview has fully loaded.

When I start to scroll to the bottom, then I see the second line and the ones after that.

I'm trying to create a simple example.

[edi9999]

Actually it can be reproduced simply with the following :

FZF_DEFAULT_OPTS="" ./bin/fzf --preview 'echo "1"; sleep 1; echo 2'

[edi9999]

See recording :

[edi9999]

You can also reproduce the same issue (probably), with the following :

FZF_DEFAULT_OPTS="" ./bin/fzf --preview 'for i in $(seq 10); do echo $i; sleep 0.13; done' 

Here, on my computer, only the first 3 numbers are shown (numbers 1 to 3).

with :

FZF_DEFAULT_OPTS="" ./bin/fzf --preview 'for i in $(seq 10); do echo $i; sleep 0.11; done'

it writes number from 1 to 8.

[edi9999]

with FZF_DEFAULT_OPTS="" ./bin/fzf --preview 'for i in $(seq 10); do echo $i; sleep 0.10; done',

everything works fine, so there is something special happening near 1 tenth of a second.

[junegunn]

Nice find, 63444b4 should fix that.

[junegunn]

I'm also seeing an issue where fzf not terminating the running command when the cursor is moved. Need further investigation.

[edi9999]

There's one behavior I'm not sure is intended :

When I run the following :

fzf ./bin/fzf --preview ' yes | head -n 20; sleep 5; echo foo ;  ' --preview-window 'up:nohidden'

It shows the 1/20 while loading, even though the preview does not have a scroll.

After the sleep and the echo foo however, the 1/20 disappears.

I suggest to put only the loading screen in this case, not the 1/20 part.

Maybe to avoid some "jittering", it would make more sense to put the loading on the right of the pagination, or maybe one line below that.

[junegunn]

This is the intended behavior as of now. You can see there's a braille character indicating that the stream is open (i.e. ⠋ 1/20). We don't know if the final number of lines will exceed the height of the window or not at this point.

# `⠋ 1/20` disappears when the stream is closed
fzf --preview ' yes | head -n 20; sleep 5; echo foo ;  ' --preview-window 'up:nohidden'

# `1/1020` remains (spinner on the left gone)
fzf --preview ' yes | head -n 20; sleep 5; seq 1000; sleep 1' --preview-window 'up:nohidden'

One idea is to only print ⠋ without numbers before the content overflows.

[edi9999]

One idea is to only print ⠋ without numbers before the content overflows.

Yes, exactly, that is what I thought would make more sense, but I also would understand that you'd prefer to keep current behavior.

[edi9999]

I have just tested again and it seems to work well for me.

[junegunn]

I'm going to merge this as it all seems to be working fine and test it over the next few days. Please let me know if you find any issues.

[edi9999]

Yes, sure I will !