# Waiting and retrying

Source: https://libtmux.org/en/swift/latest/topics/waiting-and-retry/

> Polling a condition instead of guessing a sleep, and tmux's own wait-for signal channel as the alternative to polling.

After sending input or starting a process, wait for the state your next step
requires. [Pane
interaction](https://libtmux.org/en/swift/latest/topics/pane-interaction/#waiting-for-something-to-finish) covers
waiting for screen text. This page covers arbitrary conditions and tmux's named
[`wait-for`](https://libtmux.org/en/tmux/latest/manual/wait-for/) signal channels.

## Polling a condition

Polling checks a condition repeatedly until it succeeds or a deadline expires.
Set a deadline and choose an interval that limits unnecessary tmux commands.

**Helper:** Not listed.

**Where it lives:** a [`waitUntil`](<https://libtmux.org/en/swift/latest/reference/waituntil(within-_-)/>) helper exists only inside the test target's
own support code, not shipped

### Examples

Use a loop with a deadline and interval when a specific wait API does not fit.
[Pane interaction](https://libtmux.org/en/swift/latest/topics/pane-interaction/#waiting-for-something-to-finish)
covers output waits.

## tmux's own [wait-for](https://libtmux.org/en/tmux/latest/manual/wait-for/) channel

Use [`tmux wait-for -S <channel>`](https://libtmux.org/en/tmux/latest/manual/wait-for/) to signal and [`tmux wait-for <channel>`](https://libtmux.org/en/tmux/latest/manual/wait-for/) to
block until signalled. This avoids repeated screen captures when the command can
announce its own completion:

[`Server.signal`](<https://libtmux.org/en/swift/latest/reference/server-signal(_-)/>) signals a channel, and [`Server.wait(for:)`](<https://libtmux.org/en/swift/latest/reference/server-wait(for-)/>) waits
for it. Both calls are async and throwing.

```swift
try await server.signal("built")
try await server.wait(for: "built")
```

tmux remembers a signal sent before a waiter starts. The next wait on that
channel returns immediately, so completion is not lost when the command finishes
first.

A raw [`wait-for`](https://libtmux.org/en/tmux/latest/manual/wait-for/) client can exit zero when the server dies, as well as when the
channel is signalled. Verify server liveness when a lost server must be treated
as a failed task.

[`wait(for:)`](<https://libtmux.org/en/swift/latest/reference/server-wait(for-)/>) checks the server PID before and after waiting to distinguish
server loss from a signal.

Use a channel name specific to the task. A remembered signal can otherwise
satisfy an unrelated later wait.

<details>
<summary>tmux manual and source</summary>

The tmux manual describes [completion channels](https://github.com/tmux/tmux/blob/94796f6b1182507efac8a272fc309a79e22e58a5/tmux.1#L8715).
The [channel implementation](https://github.com/tmux/tmux/blob/94796f6b1182507efac8a272fc309a79e22e58a5/cmd-wait-for.c#L388)
retains an early signal until a waiter consumes it. Use a fresh channel name
for each operation.

</details>