Avoid refreshing previews for async shell commands

[joelim-work]

This pull request addresses a user complaint where previews are redrawn when opening a file, see #414 (comment) for more details.

I think this issue probably extends to running any other asynchronous shell commands which could take a while to complete (e.g. tar), and when this happens there is no point refreshing the preview immediately because the command would not have completed yet.

[gokcehan]

@joelim-work Thanks a lot for the patch. Since I need to describe this change in the changelog soon, could you provide me the proper context for this in a simple manner? I wasn't able to follow all the changes and discussions about this. From what I understand:

• There was a change made in the on-select feature patch that caused an issue in image preview scripts (released in r28).
• A fix for this issue made it into the master branch but that fix introduced another flickering issue for async commands (not yet released).
• This patch fixes the flickering issue for async commands (not yet released).

Does that sound right?

[joelim-work]

Hi @gokcehan, I agree that the conversation in #414 is a bit hard to follow because there are actually two issues being described:

• The loading... message caused whenever a preview needs to be redrawn
• Whether or not a preview needs to be redrawn when a file is opened

This pull request addresses only the second issue.

When a file is opened, the app.runShell function is called, which in turn calls ui.loadFile, causing the preview to be redrawn. Here is a timeline of the behaviour:

• In r27, the preview will be redrawn when a file is opened (or when any other shell command is called). This behaviour is undesirable, as it causes the preview to flicker.
• In r28, commit e10ef91 introduced a bug where the preview would not be redrawn if ui.loadFile was called and the current file didn't change. This happens to 'fix' the issue, because opening a file is an action which doesn't change the current file, but also causes other problems, like the preview not refreshing when the terminal size changes.
• The commit 4a443ce which I added reverts the bug introduced e10ef91, which reintroduces the issue, prompting the discussion starting in Regression causing flashing lf with each selection #1133 and continuing in Flickering "loading…​" #414.
• Finally, the issue is fixed in this pull request.

For the purposes of the changelog, I suppose you can just say that the preview is no longer redrawn if an async shell command is called.

[ilyagr]

I'm not 100% sure about this, but I was thinking that it might be better to move the refresh to when the command finishes, in the goroutine at

lf/app.go

Lines 612 to 617 in 6ffc4a4

	case "&":
	go func() {
	if err := cmd.Wait(); err != nil {
	log.Printf("running shell: %s", err)
	}
	}()

Let me know if you have any thoughts about this. I'll think if I can come up with an example where it's useful.

[joelim-work]

Hi @ilyagr, the original intention of this pull request was to avoid immediately refreshing the preview when launching an async command, but now that I think about it, it's also quite reasonable to refresh it when the async command actually finishes.

I did notice that running a % command causes a refresh after it's done, so I think we can just do the same for async as well?

--- a/app.go
+++ b/app.go
@@ -614,6 +614,7 @@ func (app *app) runShell(s string, args []string, prefix string) cleanFunc {
                        if err := cmd.Wait(); err != nil {
                                log.Printf("running shell: %s", err)
                        }
+                       app.ui.exprChan <- &callExpr{"load", nil, 1}
                }()
        case "$|":
                return func() {

Also please give me any example use cases you have, they are very important for testing purposes and PR descriptions.