Fix #89 support running a block only once

[PicoJr]

Fix #89

interval = 2  # every 2 seconds

interval = "once" # only update once

# interval not provided, use default value

This PR modifies the Block trait and adds an Update enum:

    /// Forces an update of the internal state of the block.
-    fn update(&mut self) -> Result<Option<Duration>> {
+    fn update(&mut self) -> Result<Option<Update>> {

pub enum Update {
    Every(Duration), // update every <duration> seconds
    Once, // only update once
}

What do you think?

How to try this PR

status.toml:

[[block]]
block = "custom"
command = "date"
interval = 2 # updated every 2 seconds

[[block]]
block = "custom"
command = "date"
interval = "once" # only updated once

[[block]]
block = "custom"
command = "date"
# no interval provided, updated every 10 seconds

./target/debug/i3status-rs test.toml

Then check the first block is updated every 2 seconds, the 2nd block only once and 3rd block every 10 seconds.

TODO before merge

• rebase
• rename Refresh to Update ?
• add doc
• does it make sense to support running once for other blocks?

[ammgws]

does it make sense to support running once for other blocks?

TLDR: "maybe"

Block	Has update interval?	Run once sensible?
Backlight	no	-
Battery	yes	no
Bluetooth	no	-
CPU Utilization	yes	no
Custom	yes	yes
Custom DBus	no	-
Disk Space	yes	maybe
Focused Window	no	-
IBus	no	-
Keyboard Layout	yes	maybe
Load	yes	no
Maildir	yes	no
Memory	yes	no
Music	no	-
Net	yes	no
NetworkManager	no	-
Notmuch	yes	no
Nvidia Gpu	yes	maybe
Pacman	yes	no
Pomodoro	yes	no
Sound	no	-
Speed Test	yes	maybe
Taskwarrior	yes	no
Temperature	yes	no
Time	yes	no
Toggle	yes	yes
Uptime	yes	no
Watson	yes	no
Weather	yes	maybe
Xrandr	yes	maybe

[ammgws]

I was mulling whether we should go with a mixed keyword/numeric option like you've proposed here or just purely numeric based with -1 representing "run once". However I just checked i3blocks and they use mixed so I guess it's OK. If we ever introduce any other interval modes, having a named option would be better than trying to remember what -2 means.

i3blocks interval options:

• <non-zero number>
• -1 or once
• -2 or repeat
• -3 or persist

[PicoJr]

Thanks for your feedback: I'll rebase and resolve conflicts.