Reduce string allocations in mimetype code

The code handling mimetypes was using strings.Split() to split the string and that allocates. We can use strings.IndexByte() to get an index and subslice instead. I also refactored some code to avoid duplicating mimetype split logic.

The new internal/repository/mime package exists to avoid cycling imports.

NOTE: The split logic is slightly different because a second / would be includes as part of the subtype now but as far as I can see from reading the spec (https://specifications.freedesktop.org/shared-mime-info-spec/shared-mime-info-spec-0.21.html) it should not be a problem as the format is "mainType/subType;weight" so there should not be any more separators.

internal/repository/mime/mime.go

@@ -0,0 +1,18 @@
+package mime
+
+import (
+	"strings"
+)
+
+// Split spits the mimetype into its main type and subtype.
+func Split(mimeTypeFull string) (mimeType, mimeSubType string) {
+	// Replace with strings.Cut() when Go 1.18 is our new base version.
+	separatorIndex := strings.IndexByte(mimeTypeFull, '/')
+	if separatorIndex == -1 || mimeTypeFull[separatorIndex+1:] == "" {
+		return "", "" // Empty or only one part. Ignore.
+	}
+
+	mimeType = mimeTypeFull[:separatorIndex]
+	mimeSubType = mimeTypeFull[separatorIndex+1:]
+	return
+}

internal/repository/mime/mime_test.go

@@ -0,0 +1,21 @@
+package mime
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestSplitMimeType(t *testing.T) {
+	main, sub := Split("text/plain")
+	assert.Equal(t, "text", main)
+	assert.Equal(t, "plain", sub)
+
+	main, sub = Split("text/")
+	assert.Empty(t, main)
+	assert.Empty(t, sub)
+
+	main, sub = Split("")
+	assert.Empty(t, main)
+	assert.Empty(t, sub)
+}

storage/filter.go

@@ -4,6 +4,7 @@ import (
 	"strings"
 
 	"fyne.io/fyne/v2"
+	"fyne.io/fyne/v2/internal/repository/mime"
 )
 
 // FileFilter is an interface that can be implemented to provide a filter to a file dialog.
@@ -42,18 +43,21 @@ func NewExtensionFileFilter(extensions []string) FileFilter {
 
 // Matches returns true if a file URI has one of the filtered mimetypes.
 func (mt *MimeTypeFileFilter) Matches(uri fyne.URI) bool {
-	mimeType, mimeSubType := splitMimeType(uri)
+	mimeType, mimeSubType := mime.Split(uri.MimeType())
 	for _, mimeTypeFull := range mt.MimeTypes {
-		mimeTypeSplit := strings.Split(mimeTypeFull, "/")
-		if len(mimeTypeSplit) <= 1 {
+		mType, mSubType := mime.Split(mimeTypeFull)
+		if mType == "" || mSubType == "" {
 			continue
 		}
-		mType := mimeTypeSplit[0]
-		mSubType := strings.Split(mimeTypeSplit[1], ";")[0]
-		if mType == mimeType {
-			if mSubType == mimeSubType || mSubType == "*" {
-				return true
-			}
+
+		// Replace with strings.Cut() when Go 1.18 is our new base version.
+		subTypeSeparatorIndex := strings.IndexByte(mSubType, ';')
+		if subTypeSeparatorIndex != -1 {
+			mSubType = mSubType[:subTypeSeparatorIndex]
+		}
+
+		if mType == mimeType && (mSubType == mimeSubType || mSubType == "*") {
+			return true
 		}
 	}
 	return false
@@ -64,16 +68,3 @@ func (mt *MimeTypeFileFilter) Matches(uri fyne.URI) bool {
 func NewMimeTypeFileFilter(mimeTypes []string) FileFilter {
 	return &MimeTypeFileFilter{MimeTypes: mimeTypes}
 }
-
-func splitMimeType(uri fyne.URI) (mimeType, mimeSubType string) {
-	mimeTypeFull := uri.MimeType()
-	mimeTypeSplit := strings.Split(mimeTypeFull, "/")
-	if len(mimeTypeSplit) <= 1 {
-		mimeType, mimeSubType = "", ""
-		return
-	}
-	mimeType = mimeTypeSplit[0]
-	mimeSubType = mimeTypeSplit[1]
-
-	return
-}

storage/repository/uri.go

@@ -30,7 +30,6 @@ func (u *uri) Name() string {
 }
 
 func (u *uri) MimeType() string {
-
 	mimeTypeFull := mime.TypeByExtension(u.Extension())
 	if mimeTypeFull == "" {
 		mimeTypeFull = "text/plain"
@@ -50,7 +49,12 @@ func (u *uri) MimeType() string {
 		}
 	}
 
-	return strings.Split(mimeTypeFull, ";")[0]
+	// Replace with strings.Cut() when Go 1.18 is our new base version.
+	semicolonIndex := strings.IndexByte(mimeTypeFull, ';')
+	if semicolonIndex == -1 {
+		return mimeTypeFull
+	}
+	return mimeTypeFull[:semicolonIndex]
 }
 
 func (u *uri) Scheme() string {

widget/fileicon.go

@@ -1,10 +1,9 @@
 package widget
 
 import (
-	"strings"
-
 	"fyne.io/fyne/v2"
 	"fyne.io/fyne/v2/canvas"
+	"fyne.io/fyne/v2/internal/repository/mime"
 	"fyne.io/fyne/v2/internal/widget"
 	"fyne.io/fyne/v2/storage"
 	"fyne.io/fyne/v2/theme"
@@ -99,7 +98,8 @@ func (i *FileIcon) lookupIcon(uri fyne.URI) fyne.Resource {
 		return theme.FolderIcon()
 	}
 
-	switch splitMimeType(uri) {
+	mainMimeType, _ := mime.Split(uri.MimeType())
+	switch mainMimeType {
 	case "application":
 		return theme.FileApplicationIcon()
 	case "audio":
@@ -201,11 +201,3 @@ func trimmedExtension(uri fyne.URI) string {
 	}
 	return ext
 }
-
-func splitMimeType(uri fyne.URI) string {
-	mimeTypeSplit := strings.Split(uri.MimeType(), "/")
-	if len(mimeTypeSplit) <= 1 {
-		return ""
-	}
-	return mimeTypeSplit[0]
-}