gears.object: Add :weak_connect_signal()

Connecting to a signal weakly has the same effect as connecting to it strongly,
but it allows the garbage collector to disconnect the signal in case nothing
else references this function.

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

lib/gears/object.lua

@@ -38,7 +38,10 @@ function object:add_signal(name)
     check(self)
     assert(type(name) == "string", "name must be a string, got: " .. type(name))
     if not self._signals[name] then
-        self._signals[name] = {}
+        self._signals[name] = {
+            strong = {},
+            weak = setmetatable({}, { __mode = "k" })
+        }
     end
 end
 
@@ -48,7 +51,19 @@ end
 function object:connect_signal(name, func)
     assert(type(func) == "function", "callback must be a function, got: " .. type(func))
     local sig = find_signal(self, name, "connect to")
-    sig[func] = func
+    assert(sig.weak[func] == nil, "Trying to connect a strong callback which is already connected weakly")
+    sig.strong[func] = true
+end
+
+--- Connect to a signal weakly. This allows the callback function to be garbage
+-- collected and automatically disconnects the signal when that happens.
+-- @param name The name of the signal
+-- @param func The callback to call when the signal is emitted
+function object:weak_connect_signal(name, func)
+    assert(type(func) == "function", "callback must be a function, got: " .. type(func))
+    local sig = find_signal(self, name, "connect to")
+    assert(sig.strong[func] == nil, "Trying to connect a weak callback which is already connected strongly")
+    sig.weak[func] = true
 end
 
 --- Disonnect to a signal
@@ -56,7 +71,8 @@ end
 -- @param func The callback that should be disconnected
 function object:disconnect_signal(name, func)
     local sig = find_signal(self, name, "disconnect from")
-    sig[func] = nil
+    sig.weak[func] = nil
+    sig.strong[func] = nil
 end
 
 --- Emit a signal
@@ -67,7 +83,10 @@ end
 --   that are given to emit_signal()
 function object:emit_signal(name, ...)
     local sig = find_signal(self, name, "emit")
-    for func in pairs(sig) do
+    for func in pairs(sig.strong) do
+        func(self, ...)
+    end
+    for func in pairs(sig.weak) do
         func(self, ...)
     end
 end

spec/gears/object_spec.lua

@@ -12,13 +12,19 @@ describe("gears.object", function()
         obj:add_signal("signal")
     end)
 
-    it("connect non-existent signal", function()
+    it("strong connect non-existent signal", function()
         assert.has.errors(function()
             obj:connect_signal("foo", function() end)
         end)
     end)
 
-    it("disconnect non-existent signal", function()
+    it("weak connect non-existent signal", function()
+        assert.has.errors(function()
+            obj:weak_connect_signal("foo", function() end)
+        end)
+    end)
+
+    it("strong disconnect non-existent signal", function()
         assert.has.errors(function()
             obj:disconnect_signal("foo", function() end)
         end)
@@ -30,16 +36,27 @@ describe("gears.object", function()
         end)
     end)
 
-    it("connecting and emitting signal", function()
+    it("strong connecting and emitting signal", function()
         local called = false
-        obj:connect_signal("signal", function()
+        local function cb()
             called = true
-        end)
+        end
+        obj:connect_signal("signal", cb)
         obj:emit_signal("signal")
         assert.is_true(called)
     end)
 
-    it("connecting, disconnecting and emitting signal", function()
+    it("weak connecting and emitting signal", function()
+        local called = false
+        local function cb()
+            called = true
+        end
+        obj:weak_connect_signal("signal", cb)
+        obj:emit_signal("signal")
+        assert.is_true(called)
+    end)
+
+    it("strong connecting, disconnecting and emitting signal", function()
         local called = false
         local function cb()
             called = true
@@ -50,13 +67,64 @@ describe("gears.object", function()
         assert.is_false(called)
     end)
 
+    it("weak connecting, disconnecting and emitting signal", function()
+        local called = false
+        local function cb()
+            called = true
+        end
+        obj:weak_connect_signal("signal", cb)
+        obj:disconnect_signal("signal", cb)
+        obj:emit_signal("signal")
+        assert.is_false(called)
+    end)
+
     it("arguments to signal", function()
         obj:connect_signal("signal", function(arg1, arg2)
             assert.is.equal(obj, arg1)
             assert.is.same(42, arg2)
         end)
+        obj:weak_connect_signal("signal", function(arg1, arg2)
+            assert.is.equal(obj, arg1)
+            assert.is.same(42, arg2)
+        end)
         obj:emit_signal("signal", 42)
     end)
+
+    it("strong non-auto disconnect", function()
+        local called = false
+        obj:connect_signal("signal", function()
+            called = true
+        end)
+        collectgarbage("collect")
+        obj:emit_signal("signal")
+        assert.is_true(called)
+    end)
+
+    it("weak auto disconnect", function()
+        local called = false
+        obj:weak_connect_signal("signal", function()
+            called = true
+        end)
+        collectgarbage("collect")
+        obj:emit_signal("signal")
+        assert.is_false(called)
+    end)
+
+    it("strong connect after weak connect", function()
+        local function cb() end
+        obj:weak_connect_signal("signal", cb)
+        assert.has.errors(function()
+            obj:connect_signal("signal", cb)
+        end)
+    end)
+
+    it("weak connect after strong connect", function()
+        local function cb() end
+        obj:connect_signal("signal", cb)
+        assert.has.errors(function()
+            obj:weak_connect_signal("signal", cb)
+        end)
+    end)
 end)
 
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