Add a command and message for getting terminal size

There may be too much rigamarole around this (it's simpler to get the size
imperatively and synchronously) but adding it for now.

commands.go

@@ -0,0 +1,60 @@
+package boba
+
+import (
+	"os"
+	"time"
+
+	"golang.org/x/crypto/ssh/terminal"
+)
+
+// Every is a command that ticks in sync with the system clock. So, if you
+// wanted to tick with the system clock every second, minute or hour you
+// could use this. It's also handy for having different things tick in sync.
+//
+// Note that because we're ticking with the system clock the tick will likely
+// not run for the entire specified duration. For example, if we're ticking for
+// one minute and the clock is at 12:34:20 then the next tick will happen at
+// 12:35:00, 40 seconds later.
+func Every(duration time.Duration, fn func(time.Time) Msg) Cmd {
+	return func() Msg {
+		n := time.Now()
+		d := n.Truncate(duration).Add(duration).Sub(n)
+		t := time.NewTimer(d)
+		select {
+		case now := <-t.C:
+			return fn(now)
+		}
+	}
+}
+
+// Tick is a command that at an interval independent of the system clock at the
+// given duration. That is, the timer begins when precisely when invoked, and
+// runs for its entire duration.
+func Tick(d time.Duration, fn func(time.Time) Msg) Cmd {
+	return func() Msg {
+		t := time.NewTimer(d)
+		select {
+		case now := <-t.C:
+			return fn(now)
+		}
+	}
+}
+
+// TerminalSizeMsg defines an interface for a message that contains terminal
+// sizing as sent by GetTerminalSize.
+type TerminalSizeMsg interface {
+
+	// Size returns the terminal width and height, in that order
+	Size() (int, int)
+
+	// Error returns the error, if any, received when fetching the terminal size
+	Error() error
+}
+
+// GetTerminalSize is used to get
+func GetTerminalSize(newMsgFunc func(int, int, error) TerminalSizeMsg) Cmd {
+	w, h, err := terminal.GetSize(int(os.Stdout.Fd()))
+	return func() Msg {
+		return newMsgFunc(w, h, err)
+	}
+}