Add test & documentation for PropertiesToApply

Document PropertiesToApply expectations about props, and suggest it
should generally be generated via CreateProperties.

Test: go soong tests
Test: m nothing
Change-Id: I7cc2590db96865382ad6e0da333d4a4e2c697f45

android/soongconfig/modules_test.go

@@ -17,6 +17,8 @@ package soongconfig
 import (
 	"reflect"
 	"testing"
+
+	"github.com/google/blueprint/proptools"
 )
 
 func Test_CanonicalizeToProperty(t *testing.T) {
@@ -247,3 +249,72 @@ func Test_createAffectablePropertiesType(t *testing.T) {
 		})
 	}
 }
+
+type properties struct {
+	A *string
+	B bool
+}
+type soongConfigVariables struct {
+	Bool_var       properties
+	Other_bool_var properties
+}
+
+type soongConfigProps struct {
+	Soong_config_variables soongConfigVariables
+}
+
+func Test_PropertiesToApply(t *testing.T) {
+
+	mt := &ModuleType{
+		BaseModuleType:  "foo",
+		ConfigNamespace: "bar",
+		Variables: []soongConfigVariable{
+			newBoolVariable("bool_var"),
+			newBoolVariable("other_bool_var"),
+		},
+		affectableProperties: []string{
+			"a",
+			"b",
+		},
+	}
+	props := soongConfigProps{
+		Soong_config_variables: soongConfigVariables{
+			Bool_var: properties{
+				A: proptools.StringPtr("a"),
+				B: true,
+			},
+			Other_bool_var: properties{
+				A: proptools.StringPtr("other"),
+				B: false,
+			},
+		},
+	}
+
+	testCases := []struct {
+		config    SoongConfig
+		wantProps []interface{}
+	}{
+		{
+			config: Config(map[string]string{}),
+		},
+		{
+			config:    Config(map[string]string{"bool_var": "y"}),
+			wantProps: []interface{}{props.Soong_config_variables.Bool_var},
+		},
+		{
+			config:    Config(map[string]string{"other_bool_var": "y"}),
+			wantProps: []interface{}{props.Soong_config_variables.Other_bool_var},
+		},
+	}
+
+	for _, tc := range testCases {
+		gotProps, err := PropertiesToApply(mt, reflect.ValueOf(&props), tc.config)
+		if err != nil {
+			t.Errorf("Unexpected error in PropertiesToApply: %s", err)
+		}
+
+		if !reflect.DeepEqual(gotProps, tc.wantProps) {
+			t.Errorf("Expected %s, got %s", tc.wantProps, gotProps)
+		}
+	}
+}