gears.object: Implement read-only properties

If a getter exists, but there is no setter, then this means that the property is
read-only.

Signed-off-by: Uli Schlachter <psychon@znc.in>

lib/gears/object.lua

@@ -148,8 +148,12 @@ local function set_miss(self, key, value)
         if changed then
             self:emit_signal("property::"..key, value)
         end
-    else
+    elseif (not rawget(self, "get_"..key))
+        and not (class and class["get_"..key]) then
         return rawset(self, key, value)
+    else
+        error("Cannot set '" .. tostring(key) .. "' on " .. tostring(self)
+                .. " because it is read-only")
     end
 end
 

spec/gears/object_spec.lua

@@ -140,7 +140,6 @@ describe("gears.object", function()
     it("dynamic property disabled", function()
         local class = {}
         function class:get_foo() return "bar" end
-        function class:set_foo() end
 
         local obj2 = object{class=class}
 
@@ -152,11 +151,12 @@ describe("gears.object", function()
     it("dynamic property disabled", function()
         local class = {}
         function class:get_foo() return "bar" end
-        function class:set_foo() end
 
         local obj2 = object{class=class, enable_properties = true}
 
-        obj2.foo = 42
+        assert.has_error(function()
+            obj2.foo = 42
+        end)
 
         assert.is.equal(obj2.foo, "bar")
     end)