Intercept provided values

callback.go

@@ -20,6 +20,8 @@
 
 package dig
 
+import "reflect"
+
 // CallbackInfo contains information about a provided function or decorator
 // called by Dig, and is passed to a [Callback] registered with
 // [WithProviderCallback] or [WithDecoratorCallback].
@@ -32,6 +34,11 @@ type CallbackInfo struct {
 	// function, if any. When used in conjunction with [RecoverFromPanics],
 	// this will be set to a [PanicError] when the function panics.
 	Error error
+
+	// Values contains all values constructed by the [Callback]'s
+	// associated function. These are the actual values inside the container:
+	// modifying them may result in undefined behaviour.
+	Values []reflect.Value
 }
 
 // Callback is a function that can be registered with a provided function

constructor.go

@@ -160,12 +160,15 @@ func (n *constructorNode) Call(c containerStore) (err error) {
 		}
 	}
 
+	var values []reflect.Value
+
 	if n.callback != nil {
 		// Wrap in separate func to include PanicErrors
 		defer func() {
 			n.callback(CallbackInfo{
-				Name:  fmt.Sprintf("%v.%v", n.location.Package, n.location.Name),
-				Error: err,
+				Name:   fmt.Sprintf("%v.%v", n.location.Package, n.location.Name),
+				Error:  err,
+				Values: values,
 			})
 		}()
 	}
@@ -192,6 +195,9 @@ func (n *constructorNode) Call(c containerStore) (err error) {
 	// the rest of the graph to instantiate the dependencies of this
 	// container.
 	receiver.Commit(n.s)
+
+	values = n.resultList.Values(results)
+
 	n.called = true
 	return nil
 }

decorate.go

@@ -121,12 +121,15 @@ func (n *decoratorNode) Call(s containerStore) (err error) {
 		}
 	}
 
+	var values []reflect.Value
+
 	if n.callback != nil {
 		// Wrap in separate func to include PanicErrors
 		defer func() {
 			n.callback(CallbackInfo{
-				Name:  fmt.Sprintf("%v.%v", n.location.Package, n.location.Name),
-				Error: err,
+				Name:   fmt.Sprintf("%v.%v", n.location.Package, n.location.Name),
+				Error:  err,
+				Values: values,
 			})
 		}()
 	}
@@ -146,6 +149,8 @@ func (n *decoratorNode) Call(s containerStore) (err error) {
 	if err = n.results.ExtractList(n.s, true /* decorated */, results); err != nil {
 		return err
 	}
+	values = n.results.Values(results)
+
 	n.state = decoratorCalled
 	return nil
 }

dig_test.go

@@ -1685,6 +1685,14 @@ func TestRecoverFromPanic(t *testing.T) {
 
 func giveInt() int { return 5 }
 
+type providedInterface interface {
+	Start()
+}
+
+type providedStruct struct{}
+
+func (providedStruct) Start() {}
+
 func TestCallback(t *testing.T) {
 	t.Run("no errors", func(t *testing.T) {
 		var (
@@ -1698,6 +1706,9 @@ func TestCallback(t *testing.T) {
 			dig.WithProviderCallback(func(ci dig.CallbackInfo) {
 				assert.Equal(t, "go.uber.org/dig_test.giveInt", ci.Name)
 				assert.NoError(t, ci.Error)
+				assert.Len(t, ci.Values, 1)
+				assert.True(t, ci.Values[0].CanInt())
+				assert.EqualValues(t, 5, ci.Values[0].Int())
 				provideCallbackCalled = true
 			}),
 		)
@@ -1706,6 +1717,9 @@ func TestCallback(t *testing.T) {
 			dig.WithDecoratorCallback(func(ci dig.CallbackInfo) {
 				assert.Equal(t, "go.uber.org/dig_test.TestCallback.func1.2", ci.Name)
 				assert.NoError(t, ci.Error)
+				assert.Len(t, ci.Values, 1)
+				assert.True(t, ci.Values[0].CanInt())
+				assert.EqualValues(t, 10, ci.Values[0].Int())
 				decorateCallbackCalled = true
 			}),
 		)
@@ -1727,6 +1741,7 @@ func TestCallback(t *testing.T) {
 			dig.WithProviderCallback(func(ci dig.CallbackInfo) {
 				assert.Equal(t, "go.uber.org/dig_test.TestCallback.func2.1", ci.Name)
 				assert.ErrorContains(t, ci.Error, "terrible callback sadness")
+				assert.Nil(t, ci.Values)
 				called = true
 			}),
 		)
@@ -1747,6 +1762,7 @@ func TestCallback(t *testing.T) {
 			dig.WithDecoratorCallback(func(ci dig.CallbackInfo) {
 				assert.Equal(t, "go.uber.org/dig_test.TestCallback.func3.1", ci.Name)
 				assert.ErrorContains(t, ci.Error, "terrible callback sadness")
+				assert.Nil(t, ci.Values)
 				called = true
 			}),
 		)
@@ -1766,6 +1782,7 @@ func TestCallback(t *testing.T) {
 				var pe dig.PanicError
 				assert.True(t, errors.As(ci.Error, &pe))
 				assert.ErrorContains(t, ci.Error, "panic: \"unreal misfortune\"")
+				assert.Nil(t, ci.Values)
 				called = true
 			}),
 		)
@@ -1786,6 +1803,7 @@ func TestCallback(t *testing.T) {
 				var pe dig.PanicError
 				assert.True(t, errors.As(ci.Error, &pe))
 				assert.ErrorContains(t, ci.Error, "panic: \"unreal misfortune\"")
+				assert.Nil(t, ci.Values)
 
 				called = true
 			}),
@@ -1794,6 +1812,179 @@ func TestCallback(t *testing.T) {
 		c.Invoke(func(int) {})
 		assert.True(t, called)
 	})
+	t.Run("callback receives primitives", func(t *testing.T) {
+		var providerCallbackCalled bool
+
+		c := digtest.New(t)
+
+		c.RequireProvide(giveInt, dig.WithProviderCallback(func(ci dig.CallbackInfo) {
+			assert.Equal(t, "go.uber.org/dig_test.giveInt", ci.Name)
+			assert.Nil(t, ci.Error)
+			assert.Len(t, ci.Values, 1)
+			assert.True(t, ci.Values[0].CanInt())
+			assert.EqualValues(t, 5, ci.Values[0].Int())
+			providerCallbackCalled = true
+		}))
+
+		c.RequireInvoke(func(a int) {
+			assert.Equal(t, 5, a)
+		})
+
+		assert.True(t, providerCallbackCalled)
+	})
+
+	t.Run("callback works with value groups", func(t *testing.T) {
+		var providerCallbackCalledTimes int
+
+		c := digtest.New(t)
+
+		c.RequireProvide(giveInt, dig.Group("test"), dig.WithProviderCallback(func(ci dig.CallbackInfo) {
+			assert.NotEmpty(t, ci.Name)
+			assert.Nil(t, ci.Error)
+			assert.Len(t, ci.Values, 1)
+			assert.True(t, ci.Values[0].CanInt())
+			assert.EqualValues(t, 5, ci.Values[0].Int())
+			providerCallbackCalledTimes++
+		}))
+		c.RequireProvide(func() int { return 6 }, dig.Group("test"), dig.WithProviderCallback(func(ci dig.CallbackInfo) {
+			assert.NotEmpty(t, ci.Name)
+			assert.Nil(t, ci.Error)
+			assert.Len(t, ci.Values, 1)
+			assert.True(t, ci.Values[0].CanInt())
+			assert.EqualValues(t, 6, ci.Values[0].Int())
+			providerCallbackCalledTimes++
+		}))
+
+		type params struct {
+			dig.In
+
+			Value []int `group:"test"`
+		}
+
+		c.RequireInvoke(func(a params) {
+			assert.ElementsMatch(t, []int{5, 6}, a.Value)
+		})
+
+		assert.Equal(t, 2, providerCallbackCalledTimes)
+	})
+
+	t.Run("callback works with interfaces", func(t *testing.T) {
+		var providerCallbackCalled bool
+
+		var gave providedInterface
+
+		c := digtest.New(t)
+
+		c.RequireProvide(
+			func() providedInterface {
+				gave = &providedStruct{}
+				return gave
+			},
+			dig.WithProviderCallback(func(ci dig.CallbackInfo) {
+				assert.NotEmpty(t, ci.Name)
+				assert.Nil(t, ci.Error)
+				assert.Len(t, ci.Values, 1)
+				assert.True(t, ci.Values[0].CanInterface())
+				_, ok := ci.Values[0].Interface().(providedInterface)
+				assert.True(t, ok)
+				providerCallbackCalled = true
+			}),
+		)
+
+		c.RequireInvoke(func(got providedInterface) {
+			assert.Equal(t, gave, got)
+		})
+
+		assert.True(t, providerCallbackCalled)
+	})
+
+	t.Run("callback works with provider returning multiple values", func(t *testing.T) {
+		var providerCallbackCalled bool
+
+		c := digtest.New(t)
+
+		c.RequireProvide(
+			func() (string, int) {
+				return "five", 5
+			},
+			dig.WithProviderCallback(func(ci dig.CallbackInfo) {
+				assert.NotEmpty(t, ci.Name)
+				assert.Nil(t, ci.Error)
+				assert.Len(t, ci.Values, 2)
+				assert.EqualValues(t, "five", ci.Values[0].String())
+				assert.EqualValues(t, 5, ci.Values[1].Int())
+				providerCallbackCalled = true
+			}),
+		)
+
+		c.RequireInvoke(func(s string, i int) {
+			assert.Equal(t, "five", s)
+			assert.Equal(t, 5, i)
+		})
+
+		assert.True(t, providerCallbackCalled)
+	})
+
+	t.Run("callback does not receive nil error value with providers that can fail", func(t *testing.T) {
+		var providerCallbackCalled bool
+
+		c := digtest.New(t)
+
+		c.RequireProvide(
+			func() (string, int, error) {
+				return "five", 5, nil
+			},
+			dig.WithProviderCallback(func(ci dig.CallbackInfo) {
+				assert.NotEmpty(t, ci.Name)
+				assert.Nil(t, ci.Error)
+				assert.Len(t, ci.Values, 2)
+				assert.EqualValues(t, "five", ci.Values[0].String())
+				assert.EqualValues(t, 5, ci.Values[1].Int())
+				providerCallbackCalled = true
+			}),
+		)
+
+		c.RequireInvoke(func(s string, i int) {
+			assert.Equal(t, "five", s)
+			assert.Equal(t, 5, i)
+		})
+
+		assert.True(t, providerCallbackCalled)
+	})
+
+	t.Run("object values are exploded into single values", func(t *testing.T) {
+		var containerCallbackCalled bool
+
+		c := digtest.New(t)
+
+		type out struct {
+			dig.Out
+
+			String string
+			Int    int
+		}
+
+		c.RequireProvide(
+			func() (out, error) {
+				return out{String: "five", Int: 5}, nil
+			},
+			dig.WithProviderCallback(func(ci dig.CallbackInfo) {
+				assert.NotEmpty(t, ci.Name)
+				assert.Nil(t, ci.Error)
+				assert.Len(t, ci.Values, 2)
+				assert.EqualValues(t, "five", ci.Values[0].String())
+				assert.EqualValues(t, 5, ci.Values[1].Int())
+				containerCallbackCalled = true
+			}),
+		)
+
+		c.RequireInvoke(func(s string, i int) {
+			assert.Equal(t, "five", s)
+			assert.Equal(t, 5, i)
+		})
+
+		assert.True(t, containerCallbackCalled)
+	})
 }
 
 func TestProvideConstructorErrors(t *testing.T) {

result.go

@@ -45,6 +45,9 @@ type result interface {
 	// This MAY panic if the result does not consume a single value.
 	Extract(containerWriter, bool, reflect.Value)
 
+	// GetValues returns all values contained in a result.
+	GetValues(reflect.Value) []reflect.Value
+
 	// DotResult returns a slice of dot.Result(s).
 	DotResult() []*dot.Result
 }
@@ -259,6 +262,21 @@ func (rl resultList) ExtractList(cw containerWriter, decorated bool, values []re
 	return nil
 }
 
+func (rl resultList) GetValues(values reflect.Value) []reflect.Value {
+	digerror.BugPanicf("resultList.GetValues() must never be called")
+	panic("") // Unreachable, as BugPanicf above will panic.
+}
+
+func (rl resultList) Values(values []reflect.Value) []reflect.Value {
+	result := make([]reflect.Value, 0)
+	for i, v := range values {
+		if resultIdx := rl.resultIndexes[i]; resultIdx >= 0 {
+			result = append(result, rl.Results[resultIdx].GetValues(v)...)
+		}
+	}
+	return result
+}
+
 // resultSingle is an explicit value produced by a constructor, optionally
 // with a name.
 //
@@ -336,6 +354,10 @@ func (rs resultSingle) Extract(cw containerWriter, decorated bool, v reflect.Val
 	}
 }
 
+func (rs resultSingle) GetValues(v reflect.Value) []reflect.Value {
+	return []reflect.Value{v}
+}
+
 // resultObject is a dig.Out struct where each field is another result.
 //
 // This object is not added to the graph. Its fields are interpreted as
@@ -388,6 +410,14 @@ func (ro resultObject) Extract(cw containerWriter, decorated bool, v reflect.Val
 	}
 }
 
+func (ro resultObject) GetValues(v reflect.Value) []reflect.Value {
+	res := make([]reflect.Value, len(ro.Fields))
+	for i, f := range ro.Fields {
+		res[i] = v.Field(f.FieldIndex)
+	}
+	return res
+}
+
 // resultObjectField is a single field inside a dig.Out struct.
 type resultObjectField struct {
 	// Name of the field in the struct.
@@ -533,3 +563,7 @@ func (rt resultGrouped) Extract(cw containerWriter, decorated bool, v reflect.Va
 		cw.submitGroupedValue(rt.Group, rt.Type, v.Index(i))
 	}
 }
+
+func (rt resultGrouped) GetValues(v reflect.Value) []reflect.Value {
+	return []reflect.Value{v}
+}