bind: Fix setter name generation for ObjC

The generation for getters and setters of Go
exported fields that followed the accroynyms
rule, i.e. for a field named `OSName` the
resulting getter and settere were `osName` and
`setOSName` respectively, however the Obj-C rules
defined here:

  * https://developer.apple.com/library/archive/documentation/Cocoa/Conceptual/ProgrammingWithObjectiveC/EncapsulatingData/EncapsulatingData.html

expected for the names to be the following: `osName`
and `setOsName`.

This commit updates setter name generation by capitalizing
the field name and leaves the getter name unchaged, this
also updates the golden files.

Fix golang/go#32008

bind/bind_test.go

@@ -650,6 +650,29 @@ func TestLowerFirst(t *testing.T) {
 	}
 }
 
+func Test_objcCapitalize(t *testing.T) {
+	testCases := []struct {
+		in, want string
+	}{
+		{"", ""},
+		{"hello", "Hello"},
+		{"helloGopher", "HelloGopher"},
+		{"ID", "Id"},
+		{"IDOrName", "IdOrName"},
+		{"OSName", "OsName"},
+		{"a00name", "A00name"},
+
+		{"γειαΣας", "ΓειαΣας"},
+	}
+
+	for _, tc := range testCases {
+		if got := objcCapitalize(tc.in); got != tc.want {
+			t.Errorf("capitalize(%q) = %q; want %q", tc.in, got, tc.want)
+		}
+	}
+
+}
+
 // Test that typeName work for anonymous qualified fields.
 func TestSelectorExprTypeName(t *testing.T) {
 	e, err := parser.ParseExprFrom(fset, "", "struct { bytes.Buffer }", 0)

bind/genobjc.go

@@ -10,6 +10,8 @@ import (
 	"go/types"
 	"math"
 	"strings"
+	"unicode"
+	"unicode/utf8"
 
 	"golang.org/x/mobile/internal/importers/objc"
 )
@@ -668,7 +670,7 @@ func (g *ObjcGen) genGetter(oName string, f *types.Var) {
 func (g *ObjcGen) genSetter(oName string, f *types.Var) {
 	t := f.Type()
 
-	g.Printf("- (void)set%s:(%s)v {\n", f.Name(), g.objcType(t))
+	g.Printf("- (void)set%s:(%s)v {\n", objcCapitalize(f.Name()), g.objcType(t))
 	g.Indent()
 	g.Printf("int32_t refnum = go_seq_go_to_refnum(self._ref);\n")
 	g.genWrite("v", f.Type(), modeRetained)
@@ -1410,6 +1412,46 @@ func isObjcType(t types.Type) bool {
 	return typePkgFirstElem(t) == "ObjC"
 }
 
+// objcCapitalize ...
+func objcCapitalize(s string) string {
+	switch utf8.RuneCount([]byte(s)) {
+	case 0:
+		return s
+
+	case 1:
+		r, _ := utf8.DecodeRuneInString(s)
+		return string(unicode.ToTitle(r))
+
+	default:
+		var sb strings.Builder
+		sb.Grow(len(s))
+
+		// title case first
+		r, n := utf8.DecodeRuneInString(s)
+		sb.WriteRune(unicode.ToTitle(r))
+		s = s[n:]
+
+		// lower case second
+		r, n = utf8.DecodeRuneInString(s)
+		sb.WriteRune(unicode.ToLower(r))
+		s = s[n:]
+
+		// just copy the rest
+		for len(s) > 0 {
+			r, n = utf8.DecodeRuneInString(s)
+			if r == utf8.RuneError {
+				return sb.String()
+			}
+
+			sb.WriteRune(r)
+
+			s = s[n:]
+		}
+
+		return sb.String()
+	}
+}
+
 var objcNameReplacer = newNameSanitizer([]string{
 	"bool", "bycopy", "byref", "char", "const", "double", "float",
 	"id", "in", "init", "inout", "int", "long", "nil", "oneway",

bind/testdata/doc.objc.m.golden

@@ -54,7 +54,7 @@
 	return _r0;
 }
 
-- (void)setSF:(NSString* _Nonnull)v {
+- (void)setSf:(NSString* _Nonnull)v {
 	int32_t refnum = go_seq_go_to_refnum(self._ref);
 	nstring _v = go_seq_from_objc_string(v);
 	proxydoc_S_SF_Set(refnum, _v);