按下键盘返回键时，UITextView不记得字体样式

I have a UITextView in storyboard (text property is attributed, and Allow Editing Attributes, selected, editable are checked). When the user press a button, I want to "activate" bold font. So, whatever the user types from there must be in bold. And when the return key is pressed, I want to go to the next line and the UITextView should remember the attributes of all text. The problem is that after I press the return key, all character is changing to bold. I have a UITextView in storyboard (text propert