Build logic for installing Diffuse as a native macOS app

Tested on macOS 12.5:

  brew install meson python3 py3cairo pygobject3 gtk+3
  meson setup build
  cd build
  meson compile
  meson test
  meson install

After `meson install`, `diffuse` can be used to launch a
native Mac app that is installed into /Applications/Diffuse.app.

These steps could be put into a Homebrew formula, see
https://docs.brew.sh/Formula-Cookbook, as a way to
distribute Diffuse on macOS.

docs/developers/developers-setup.md

@@ -128,6 +128,25 @@ sudo rm -v /usr/local/share/locale/*/LC_MESSAGES/diffuse.mo
 Meson allows to change the default installation directories, see
 [command-line documentation](https://mesonbuild.com/Commands.html#configure).
 
+## Setup on Mac OS
+
+Building on Mac OS is similar to building on Linux. To recap, these are
+the steps needed to build and install Diffuse manually:
+
+```brew install meson python3 py3cairo pygobject3 gtk+3
+  meson setup build
+  cd build
+  meson compile
+  meson test
+  meson install
+```
+
+After `meson install`, the `diffuse` command can be used to launch Diffuse
+as a native Mac app that is installed into `/Applications/Diffuse.app`.
+
+The `diffuse` command is compatible with git. To use Diffuse as git's
+`git difftool` run `git config --global diff.tool diffuse`
+
 ## Setup on Windows
 
 _Note:_ The Windows port is not maintained and would need some love.

src/diffuse/mac-os-app/Info.plist.in

@@ -0,0 +1,22 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE plist PUBLIC "-//Apple Computer//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
+<plist version="1.0">
+<dict>
+    <key>CFBundleDevelopmentRegion</key>
+    <string>English</string>
+    <key>CFBundleExecutable</key>
+    <string>mac_launcher.sh</string>
+    <key>CFBundleIconFile</key>
+    <string>diffuse.icns</string>
+    <key>CFBundleIdentifier</key>
+    <string>io.github.mightycreak.Diffuse</string>
+    <key>CFBundleName</key>
+    <string>Diffuse</string>
+    <key>CFBundleShortVersionString</key>
+    <string>@VERSION@</string>
+    <key>CFBundlePackageType</key>
+    <string>APPL</string>
+    <key>NSAppleScriptEnabled</key>
+    <string>NO</string>
+</dict>
+</plist>

src/diffuse/mac-os-app/diffuse-mac.in

@@ -0,0 +1,14 @@
+#!/bin/bash
+#
+# This is the command-line entry point on Mac.
+#
+# We want to use Mac's `open` command for mainly two reasons;
+# a) open lets us choose our own icon.
+# b) open puts the app on top of the other windows, including the terminal we ran this from.
+#
+# --new lets us open multiple windows.
+# --wait-apps lets Diffuse be a "git difftool", letting Diffuse run before git deletes its tmp files.
+#
+# We pass "pwd" because Mac's `open` command launches processes at '/'.
+# "printf %q" escapes spaces and other characters so the complete dir is passed as one.
+open /Applications/Diffuse.app --new --wait-apps --args $(printf %q "$(pwd)") $@

src/diffuse/mac-os-app/mac_launcher.sh.in

@@ -0,0 +1,7 @@
+#!/bin/bash
+
+# Mac's `open` command resets working dir. This extra script only
+# does `cd` back to the dir from which `diffuse` was launched to
+# allow Python to pick up any relative paths given by `git difftool`.
+cd $1
+@BINDIR@/diffuse_impl ${@:2}

src/diffuse/meson.build

@@ -9,15 +9,42 @@ conf.set('VERSION', meson.project_version())
 conf.set('PKGDATADIR', pkgdatadir)
 conf.set('LOCALEDIR', join_paths(get_option('prefix'), get_option('localedir')))
 conf.set('SYSCONFIGDIR', join_paths(get_option('prefix'), get_option('sysconfdir')))
+conf.set('BINDIR', join_paths(get_option('prefix'), get_option('bindir')))
 
 configure_file(
   input: 'diffuse.in',
-  output: 'diffuse',
+  output: build_machine.system() == 'darwin' ? 'diffuse_impl' : 'diffuse',
   configuration: conf,
   install: true,
   install_dir: get_option('bindir')
 )
 
+if build_machine.system() == 'darwin'
+  install_subdir('mac-os-app/Diffuse.app', install_dir: '/Applications', strip_directory: false)
+
+  configure_file(
+    input: 'mac-os-app/diffuse-mac.in',
+    output: 'diffuse',
+    configuration: conf,
+    install: true,
+    install_dir: get_option('bindir')
+  )
+  configure_file(
+    input: 'mac-os-app/mac_launcher.sh.in',
+    output: 'mac_launcher.sh',
+    configuration: conf,
+    install: true,
+    install_dir: '/Applications/Diffuse.app/Contents/MacOS'
+  )
+  configure_file(
+    input: 'mac-os-app/Info.plist.in',
+    output: 'Info.plist',
+    configuration: conf,
+    install: true,
+    install_dir: '/Applications/Diffuse.app/Contents'
+  )
+endif
+
 diffuse_sources = [
   '__init__.py',
   'constants.py',

utils/makemacicon.sh

@@ -0,0 +1,20 @@
+#!/usr/bin/env bash
+
+# Use this tool if you need to re-create
+# Diffuse.app/Contents/Resources/diffuse.icns
+# in case the icon changes (unlikely).
+
+sizes=(16 32 64 128 256 512)
+for s in "${sizes[@]}"; do
+  echo $s
+  rsvg-convert -h $s "$1" > "icon_${s}x$s.png"
+done
+
+cp 'icon_32x32.png'     'icon_16x16@2x.png'
+cp 'icon_64x64.png'     'icon_32x32@2x.png'
+cp 'icon_256x256.png'   'icon_128x128@2x.png'
+cp 'icon_512x512.png'   'icon_256x256@2x.png'
+
+mkdir icon.iconset
+mv icon_*x*.png icon.iconset
+iconutil -c icns icon.iconset