Adding zshell autocomplete script

pyinvoke/autocomplete.zsh

@@ -0,0 +1,34 @@
+# Invoke tab-completion script to be sourced with the Z shell.
+# Known to work on zsh 5.0.x, probably works on later 4.x releases as well (as
+# it uses the older compctl completion system).
+
+_complete_invoke() {
+    # `words` contains the entire command string up til now (including
+    # program name).
+    #
+    # We hand it to Invoke so it can figure out the current context: spit back
+    # core options, task names, the current task's options, or some combo.
+    #
+    # Before doing so, we attempt to tease out any collection flag+arg so we
+    # can ensure it is applied correctly.
+    collection_arg=''
+    if [[ "${words}" =~ "(-c|--collection) [^ ]+" ]]; then
+        collection_arg=$MATCH
+    fi
+    # `reply` is the array of valid completions handed back to `compctl`.
+    # Use ${=...} to force whitespace splitting in expansion of
+    # $collection_arg
+    reply=( $(invoke ${=collection_arg} --complete -- ${words}) )
+}
+
+
+# Tell shell builtin to use the above for completing our given binary name(s).
+# * -K: use given function name to generate completions.
+# * +: specifies 'alternative' completion, where options after the '+' are only
+#   used if the completion from the options before the '+' result in no matches.
+# * -f: when function generates no results, use filenames.
+# * positional args: program names to complete for.
+compctl -K _complete_invoke + -f invoke inv
+
+# vim: set ft=sh :
+