Difference in using functions in react-native

The difference depends really how and when onLayout is triggered.

• onLayout={this._someFunc()}

  Each time your component is rendered the onLayout prop will get the return value from the this._someFunc() function. In other words, the function will run on every render.

• onLayout={() => {return this._someFunc()}} or onLayout={() => this._someFunc()}

  Each time your component is rendered the onLayout prop will bind the reference to an anonymous function which contains the call to the _someFunc() function. The latter function doesn't get executed unless onLayout is somehow triggered.

For example, assume the following component:

class MyApp extends React.Component {

  _someFunc = () => {
    return "bar";
  }

  render() {
    return(
      <div>
        <A foo={this._someFunc()} />
        <B foo={() => {return this._someFunc()}} />
      </div>
    );
  }
}

const A = ({foo}) => {
  console.log(foo);  //prints "bar"
  return <div>A</div>;
}

const B = ({foo}) => {
  console.log(foo);  //prints the reference to _someFunc()
  console.log(foo());  //prints "bar"
  return <div>B</div>;
}

ReactDOM.render(<MyApp />, document.getElementById("app"));

<script src="https://cdnjs.cloudflare.com/ajax/libs/react/15.1.0/react.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/15.1.0/react-dom.min.js"></script>
<div id="app"></div>

In component A the foo prop will get the value of "bar".

In component B the foo prop will be a reference to a function that you can later call. That function will in turn call _someFunc which will return "bar". So if in component B you want to get the value, you need to call it with this.props.foo().