React Native _this2.refs.myinput.focus is not a function

This is because focus is a method of TextInput, and it is not present in your extended version.

You can add a focus method to TextBox.js as below:

focus() {
    this.refs.textInput.focus();
},

and add a ref to the TextInput

render() {
  return (
  <TextInput
    {...this.props}
    ref={'textInput'}
    style={styles.textBox}/>
  );
}

If you wanted to do this in the modern React Version use

ref = { ref => this._randomName = ref }

If you want to access the method use

this._randomName.anyMethod()