[wpigui] Add platform-specific preferences save

wpigui/src/main/native/cpp/wpigui.cpp

@@ -140,6 +140,8 @@ bool gui::Initialize(const char* title, int width, int height) {
   iniHandler.WriteAllFn = IniWriteAll;
   ImGui::GetCurrentContext()->SettingsHandlers.push_back(iniHandler);
 
+  io.IniFilename = gContext->iniPath.c_str();
+
   for (auto&& initialize : gContext->initializers) {
     if (initialize) initialize();
   }
@@ -331,6 +333,25 @@ void gui::SetStyle(Style style) {
 
 void gui::SetClearColor(ImVec4 color) { gContext->clearColor = color; }
 
+void gui::ConfigurePlatformSaveFile(const std::string& name) {
+  gContext->iniPath = name;
+#if defined(_MSC_VER)
+  const char* env = std::getenv("APPDATA");
+  if (env) gContext->iniPath = env + std::string("/" + name);
+#elif defined(__APPLE__)
+  const char* env = std::getenv("HOME");
+  if (env)
+    gContext->iniPath = env + std::string("/Library/Preferences/" + name);
+#else
+  const char* xdg = std::getenv("XDG_CONFIG_HOME");
+  const char* env = std::getenv("HOME");
+  if (xdg)
+    gContext->iniPath = xdg + std::string("/" + name);
+  else if (env)
+    gContext->iniPath = env + std::string("/.config/" + name);
+#endif
+}
+
 void gui::EmitViewMenu() {
   if (ImGui::BeginMenu("View")) {
     if (ImGui::BeginMenu("Style")) {

wpigui/src/main/native/include/wpigui.h

@@ -8,6 +8,7 @@
 #pragma once
 
 #include <functional>
+#include <string>
 
 #include <imgui.h>
 
@@ -123,6 +124,15 @@ void SetStyle(Style style);
  */
 void SetClearColor(ImVec4 color);
 
+/**
+ * Configures a save file (.ini) in a platform specific location. On Windows,
+ * the .ini is saved in %APPDATA%; on macOS the .ini is saved in
+ * ~/Library/Preferences; on Linux the .ini is stored in $XDG_CONFIG_HOME or
+ * ~/.config if the former is not defined. This must be called before
+ * gui::Initialize().
+ */
+void ConfigurePlatformSaveFile(const std::string& name);
+
 /**
  * Emits a View menu (e.g. for a main menu bar) that allows setting of
  * style and zoom.  Internally starts with ImGui::BeginMenu("View").

wpigui/src/main/native/include/wpigui_internal.h

@@ -56,6 +56,8 @@ struct Context : public SavedSettings {
 
   int fontScale = 2;  // updated by main loop
   std::vector<Font> fonts;
+
+  std::string iniPath = "imgui.ini";
 };
 
 extern Context* gContext;