cmd/gio: expand usage document

Signed-off-by: Elias Naur <mail@eliasnaur.com>

cmd/gio/gio.go

@@ -23,10 +23,10 @@ import (
 )
 
 var (
-	target        = flag.String("target", "", "specify target (ios, tvos, android, js)")
+	target        = flag.String("target", "", "specify target (ios, tvos, android, js).\n")
-	archNames     = flag.String("arch", "", "specify architecture(s) to include")
+	archNames     = flag.String("arch", "", "specify architecture(s) to include (arm, arm64, amd64).")
-	buildMode     = flag.String("buildmode", "exe", "specify buildmode: archive or exe")
+	buildMode     = flag.String("buildmode", "exe", "specify buildmode (archive, exe)")
-	destPath      = flag.String("o", "", `output path. Specify a path with the ".app" suffix for iOS simulators.`)
+	destPath      = flag.String("o", "", "output file or directory.\nFor -target ios or tvos, use the .app suffix to target simulators.")
 	appID         = flag.String("appid", "org.gioui.app", "app identifier (for -buildmode=exe)")
 	version       = flag.Int("version", 1, "app version (for -buildmode=exe)")
 	printCommands = flag.Bool("x", false, "print the commands")
@@ -46,7 +46,7 @@ type buildInfo struct {
 
 func main() {
 	flag.Usage = func() {
-		mainUsage(os.Stderr)
+		fmt.Fprintf(os.Stderr, mainUsage)
 	}
 	flag.Parse()
 	if err := mainErr(); err != nil {

cmd/gio/help.go

@@ -0,0 +1,51 @@
+// SPDX-License-Identifier: Unlicense OR MIT
+
+package main
+
+const mainUsage = `The Gio command builds and packages Gio (gioui.org) programs.
+
+Usage:
+
+	gio -target <target> [flags] <package> [run arguments]
+
+The go tool is sufficient to build, install and run Gio programs on platforms
+where a single executable is sufficient. The gio tool can build and package Gio
+programs for platforms where additional metadata or support files are required.
+
+The package argument specifies an import path or a single Go source file to
+package. Any run arguments are appended to os.Args at runtime.
+
+If the package contains an appicon.png file, it is used as the app icon on
+supported platforms.
+
+The mandatory -target flag selects the target platform: ios or android for the
+mobile platforms, tvos for Apple's tvOS, js for WebAssembly/WebGL.
+
+The -arch flag specifies a comma separated list of GOARCHs to include. The
+default is all supported architectures.
+
+The -o flag specifies an output file or directory, depending on the target.
+
+The -buildmode flag selects the build mode. Two build modes are available, exe
+and archive. Buildmode exe outputs an .ipa file for iOS or tvOS, an .apk file
+for Android or a directory with the WebAssembly module and support files for
+a browser.
+
+As a special case for iOS or tvOS, specifying a path that ends with ".app"
+will output an app directory suitable for a simulator.
+
+The other buildmode is archive, which will output an .aar library for Android
+or a .framework for iOS and tvOS.
+
+The -appid flag specifies the package name for Android or the bundle id for
+iOS and tvOS. A bundle id must be provisioned through Xcode before the gio
+tool can use it.
+
+The -version flag specifies the integer version for Android and the last
+component of the 1.0.X version for iOS and tvOS.
+
+The -work flag prints the path to the working directory and suppress
+its deletion.
+
+The -x flag will print all the external commands executed by the gio tool.
+`