do not reposition cursor to end of number on change

restore cursor position after update.
cursor position must be restored after update.
when non number chars are deleted or inserted move cursor position for -1 or 1 respectively

Signal/src/view controllers/RegisterViewController.m

@@ -381,7 +381,7 @@
 
 #pragma mark - UITextFieldDelegate
 
-- (NSUInteger) recalculateLocation:(NSUInteger)location {
+- (NSUInteger) calculateLocationOffset:(NSUInteger)location {
     uint offset = 0, phonenumberposition = 0;
     for (uint i=0;i<location;i++) {
         if ([_phoneNumberTextField.text characterAtIndex:i] != [_enteredPhoneNumber characterAtIndex:phonenumberposition])
@@ -395,20 +395,32 @@
 
 - (BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range
                                                        replacementString:(NSString *)string {
-    
-    range.location -= [self recalculateLocation:range.location];
+    uint offset = [self calculateLocationOffset:range.location];
+    unsigned short currentPosCharacter = 0;
+    range.location -= offset;
     BOOL handleBackspace = range.length == 1;
     if (handleBackspace) {
+        if ((range.location + offset) <  _phoneNumberTextField.text.length)
+            currentPosCharacter = [_phoneNumberTextField.text characterAtIndex:(range.location + offset)];
+        
+        if (currentPosCharacter < '0' || currentPosCharacter > '9')
+            range.location --;
         NSRange backspaceRange = NSMakeRange(range.location, 1);
         [_enteredPhoneNumber replaceCharactersInRange:backspaceRange withString:string];
+        range.location ++;
     } else {
         NSString* sanitizedString = [[string componentsSeparatedByCharactersInSet:[[NSCharacterSet decimalDigitCharacterSet ] invertedSet]] componentsJoinedByString:@""];
         NSMutableString* mutablePhoneNumber = [NSMutableString stringWithString:_enteredPhoneNumber];
         [mutablePhoneNumber insertString:sanitizedString atIndex:range.location];
         _enteredPhoneNumber = mutablePhoneNumber;
+        if ((range.location + offset +1) <  _phoneNumberTextField.text.length)
+            currentPosCharacter = [_phoneNumberTextField.text characterAtIndex:(range.location + offset +1)];
+        
+        if ((currentPosCharacter < '0' || currentPosCharacter > '9') && currentPosCharacter != 0)
+            range.location++;
     }
 
-    [self updatePhoneNumberFieldWithString:_enteredPhoneNumber cursorposition:range.location+1];
+    [self updatePhoneNumberFieldWithString:_enteredPhoneNumber cursorposition:range.location+offset];
     return NO;
 }
 
@@ -416,7 +428,11 @@
                           cursorposition:(NSUInteger)cursorpos {
     NSString* result = [PhoneNumber bestEffortFormatPartialUserSpecifiedTextToLookLikeAPhoneNumber:_enteredPhoneNumber
                                                                     withSpecifiedCountryCodeString:_countryCodeLabel.text];
+    cursorpos += result.length - _phoneNumberTextField.text.length;
     _phoneNumberTextField.text = result;
+    UITextPosition *position = [_phoneNumberTextField positionFromPosition:[_phoneNumberTextField beginningOfDocument]
+                                                 offset:(NSInteger)cursorpos];
+    [_phoneNumberTextField setSelectedTextRange:[_phoneNumberTextField textRangeFromPosition:position toPosition:position]];
 }
 
 @end