First pass for iOS file save

Working for 'On this Phone' file locations.
Seems to have an issue with DropBox export.
Fixes #2076

go.mod

@@ -7,7 +7,7 @@ require (
 	github.com/akavel/rsrc v0.8.0 // indirect
 	github.com/fredbi/uri v0.0.0-20181227131451-3dcfdacbaaf3
 	github.com/fsnotify/fsnotify v1.4.9
-	github.com/fyne-io/mobile v0.1.3-0.20210312180903-f9a21000f5dc
+	github.com/fyne-io/mobile v0.1.3-0.20210318200029-09e9c4e13a8f
 	github.com/go-gl/gl v0.0.0-20190320180904-bf2b1f2f34d7
 	github.com/go-gl/glfw/v3.3/glfw v0.0.0-20200625191551-73d3c3675aa3
 	github.com/godbus/dbus/v5 v5.0.3

go.sum

@@ -14,8 +14,8 @@ github.com/fsnotify/fsnotify v1.4.9 h1:hsms1Qyu0jgnwNXIxa+/V/PDsU6CfLf6CNO8H7IWo
 github.com/fsnotify/fsnotify v1.4.9/go.mod h1:znqG4EE+3YCdAaPaxE2ZRY/06pZUdp0tY4IgpuI1SZQ=
 github.com/fyne-io/glfw/v3.3/glfw v0.0.0-20201123143003-f2279069162d h1:WfVxpuVm+5Gr3ipAoWrxV8lJFYkaBWoEwFRrWThWRSU=
 github.com/fyne-io/glfw/v3.3/glfw v0.0.0-20201123143003-f2279069162d/go.mod h1:tQ2UAYgL5IevRw8kRxooKSPJfGvJ9fJQFa0TUsXzTg8=
-github.com/fyne-io/mobile v0.1.3-0.20210312180903-f9a21000f5dc h1:A5hFL3tVUfFHhVpjmLGPs8SdVIcSkTRpFMm1Vsio5+k=
-github.com/fyne-io/mobile v0.1.3-0.20210312180903-f9a21000f5dc/go.mod h1:/kOrWrZB6sasLbEy2JIvr4arEzQTXBTZGb3Y96yWbHY=
+github.com/fyne-io/mobile v0.1.3-0.20210318200029-09e9c4e13a8f h1:rguJ/t99j/6zRSFzsBKlsmmyl+vOvCeTJ+2uTBvuXFI=
+github.com/fyne-io/mobile v0.1.3-0.20210318200029-09e9c4e13a8f/go.mod h1:/kOrWrZB6sasLbEy2JIvr4arEzQTXBTZGb3Y96yWbHY=
 github.com/go-gl/gl v0.0.0-20190320180904-bf2b1f2f34d7 h1:SCYMcCJ89LjRGwEa0tRluNRiMjZHalQZrVrvTbPh+qw=
 github.com/go-gl/gl v0.0.0-20190320180904-bf2b1f2f34d7/go.mod h1:482civXOzJJCPzJ4ZOX/pwvXBWSnzD4OKMdH4ClKGbk=
 github.com/godbus/dbus/v5 v5.0.3 h1:ZqHaoEF7TBzh4jzPmqVhE/5A1z9of6orkAe5uHoAeME=

internal/driver/gomobile/file_ios.go

@@ -10,10 +10,10 @@ package gomobile
 
 void* iosParseUrl(const char* url);
 const void* iosReadFromURL(void* url, int* len);
+const int iosWriteToURL(void* url, const void* bytes, int len);
 */
 import "C"
 import (
-	"errors"
 	"io"
 	"unsafe"
 
@@ -64,6 +64,31 @@ func (s *secureReadCloser) Close() error {
 	return nil
 }
 
+type secureWriteCloser struct {
+	url    unsafe.Pointer
+	closer func()
+
+	offset int
+}
+
+// Declare conformity to WriteCloser interface
+var _ io.WriteCloser = (*secureWriteCloser)(nil)
+
+func (s *secureWriteCloser) Write(p []byte) (int, error) {
+	count := int(C.iosWriteToURL(s.url, C.CBytes(p), C.int(len(p))))
+	s.offset += count
+
+	return count, nil
+}
+
+func (s *secureWriteCloser) Close() error {
+	if s.closer != nil {
+		s.closer()
+	}
+	s.url = nil
+	return nil
+}
+
 func nativeFileOpen(f *fileOpen) (io.ReadCloser, error) {
 	if f.uri == nil || f.uri.String() == "" {
 		return nil, nil
@@ -79,7 +104,17 @@ func nativeFileOpen(f *fileOpen) (io.ReadCloser, error) {
 }
 
 func nativeFileSave(f *fileSave) (io.WriteCloser, error) {
-	return nil, errors.New("Currently unsupported.")
+	if f.uri == nil || f.uri.String() == "" {
+		return nil, nil
+	}
+
+	cStr := C.CString(f.uri.String())
+	defer C.free(unsafe.Pointer(cStr))
+
+	url := C.iosParseUrl(cStr)
+
+	fileStruct := &secureWriteCloser{url: url, closer: f.done}
+	return fileStruct, nil
 }
 
 func registerRepository(d *mobileDriver) {

internal/driver/gomobile/file_ios.m

@@ -16,3 +16,14 @@
     *len = data.length;
     return data.bytes;
 }
+
+const int iosWriteToURL(void* urlPtr, const void* bytes, int len) {
+    NSURL* url = (NSURL*)urlPtr;
+    NSData *data = [NSData dataWithBytes:bytes length:len];
+    BOOL ok = [data writeToURL:url atomically:YES];
+
+    if (!ok) {
+        return 0;
+    }
+    return data.length;
+}

vendor/modules.txt

@@ -12,7 +12,7 @@ github.com/davecgh/go-spew/spew
 github.com/fredbi/uri
 # github.com/fsnotify/fsnotify v1.4.9
 github.com/fsnotify/fsnotify
-# github.com/fyne-io/mobile v0.1.3-0.20210312180903-f9a21000f5dc
+# github.com/fyne-io/mobile v0.1.3-0.20210318200029-09e9c4e13a8f
 github.com/fyne-io/mobile/app
 github.com/fyne-io/mobile/app/internal/callfn
 github.com/fyne-io/mobile/event/key