break out just effectiveStartingDir

This implements just the effectiveStartingDir() and a basic test from PR fyne-io#1108.
It does provide the ability to set a custom starting dir.

dialog/file.go

@@ -262,6 +262,35 @@ func (f *fileDialog) setSelected(file *fileDialogItem) {
 	}
 }
 
+// effectiveStartingDir calculates the directory at which the file dialog
+// should open, based on the values of  CWD, home, and any error conditions
+// which occur.
+//
+// Order of precedence is:
+//
+// * os.Getwd()
+// * os.UserHomeDir()
+// * "/" (should be filesystem root on all supported platforms)
+func (f *FileDialog) effectiveStartingDir() string {
+
+	// Try to use CWD
+	var err error = nil
+	dir, err := os.Getwd()
+	if err == nil {
+		return dir
+	}
+	fyne.LogError("Could not load CWD", err)
+
+	// fail over to home dir
+	dir, err = os.UserHomeDir()
+	if err == nil {
+		return dir
+	}
+	fyne.LogError("Could not load user home dir", err)
+
+	return "/"
+}
+
 func showFile(file *FileDialog) *fileDialog {
 	d := &fileDialog{file: file}
 	ui := d.makeUI()

dialog/file_test.go

@@ -15,6 +15,51 @@ import (
 	"fyne.io/fyne/widget"
 )
 
+// comparePaths compares if two file paths point to the same thing, and calls
+// t.Fatalf() if there is an error in performing the comparison.
+//
+// Returns true of both paths point to the same thing.
+//
+// You should use this if you need to compare file paths, since it explicitly
+// normalizes the paths to a stable canonical form. It also nicely
+// abstracts out the requisite error handling.
+//
+// You should only call this function on paths that you expect to be valid.
+func comparePaths(t *testing.T, p1, p2 string) bool {
+	a1, err := filepath.Abs(p1)
+	if err != nil {
+		t.Fatalf("Failed to normalize path '%s'", p1)
+	}
+
+	a2, err := filepath.Abs(p2)
+	if err != nil {
+		t.Fatalf("Failed to normalize path '%s'", p2)
+	}
+
+	return a1 == a2
+}
+
+func TestEffectiveStartingDir(t *testing.T) {
+	wd, err := os.Getwd()
+	if err != nil {
+		t.Skipf("os.Getwd() failed, cannot run this test on this system (error getting ./), error was '%s'", err)
+	}
+
+	dialog := &FileDialog{}
+
+	// test that we get ./ when running with the default struct values
+	res := dialog.effectiveStartingDir()
+	expect := wd
+	if err != nil {
+		t.Skipf("os.Getwd() failed, cannot run this test on this system, error was '%s'", err)
+	}
+	if !comparePaths(t, res, expect) {
+		t.Errorf("Expected effectiveStartingDir() to be '%s', but it was '%s'",
+			expect, res)
+	}
+
+}
+
 func TestShowFileOpen(t *testing.T) {
 	var chosen fyne.URIReadCloser
 	var openErr error