Detects when a text field's current editor is the parent window's first responder (the "view" here is an NSTableCellView subclass)

FirstResponderRAC.m

RACSignal *textFieldDidFocus = [[[[view rac_valuesForKeyPath:@keypath(view, textField.window.firstResponder) observer:view] 
    takeUntil:view.rac_willDeallocSignal] filter:^BOOL(id value) {
        @strongify(view);
        return [view.textField.currentEditor isEqualTo:value];
    }] 
    mapReplace:view.textField];