feat(sys): expose canGoBack for the chrome that offers the control

lua/api/core/sys.lua

@@ -36,19 +36,23 @@ function sys.getAppDataPath() end
 ---@param title string
 function sys.setAppTitle(title) end
 
----Launches /.lua/apps/<path>/main.lua and pushes the current route.
----@param path string App-relative directory path; traversal is rejected.
----@param arg? string Passed to init(arg).
+---Launches a route, which main.lua resolves, and pushes the current one.
+---@param path string App-relative route; traversal is rejected.
+---@param arg? string Passed to main.start(route, arg).
 function sys.launch(path, arg) end
 
----Launches an app path without retaining the current route.
----@param path string App-relative directory path; traversal is rejected.
----@param arg? string Passed to init(arg).
+---Launches a route without retaining the current one.
+---@param path string App-relative route; traversal is rejected.
+---@param arg? string Passed to main.start(route, arg).
 function sys.replace(path, arg) end
 
 ---Returns to the previous app, or the launcher when history is empty.
 function sys.back() end
 
+---Whether sys.back() would return somewhere rather than land on the launcher, which is what chrome needs to decide whether to offer a back control.
+---@return boolean
+function sys.canGoBack() end
+
 ---Returns heap statistics.
 ---@return integer freeBytes
 ---@return integer totalBytes

native/src/bindings/core/sys.cpp

@@ -50,6 +50,10 @@ int back(lua_State* state) {
   Runtime::from(state)->requestBack();
   return 0;
 }
+int canGoBack(lua_State* state) {
+  lua_pushboolean(state, Runtime::from(state)->canGoBack());
+  return 1;
+}
 
 int getMemory(lua_State* state) {
   const MemoryInfo memory = Runtime::from(state)->sys().memory();
@@ -88,16 +92,22 @@ const luaL_Reg FUNCTIONS[] = {
     // --- Changes the running app's display title.
     // @param title string
     {"setAppTitle", setAppTitle},
-    // --- Launches /.lua/apps/<path>/main.lua and pushes the current route.
-    // @param path string App-relative directory path; traversal is rejected.
-    // @param arg string|nil Passed to init(arg).
+    // --- Launches a route, which main.lua resolves, and pushes the current
+    // one.
+    // @param path string App-relative route; traversal is rejected.
+    // @param arg string|nil Passed to main.start(route, arg).
     {"launch", launch},
-    // --- Launches an app path without retaining the current route.
-    // @param path string App-relative directory path; traversal is rejected.
-    // @param arg string|nil Passed to init(arg).
+    // --- Launches a route without retaining the current one.
+    // @param path string App-relative route; traversal is rejected.
+    // @param arg string|nil Passed to main.start(route, arg).
     {"replace", replace},
     // --- Returns to the previous app, or the launcher when history is empty.
     {"back", back},
+    // --- Whether sys.back() would return somewhere rather than land on the
+    // launcher, which is what chrome needs to decide whether to offer a back
+    // control.
+    // @return boolean
+    {"canGoBack", canGoBack},
     // --- Returns heap statistics.
     // @return integer freeBytes
     // @return integer totalBytes