when using native base toast has error fontFamily "Roboto_medium" is not a system font and has not been loaded through Font.loadAsync.

was able to solve the issue by adding this to your Font.loadasync

 Roboto: require("native-base/Fonts/Roboto.ttf"),
 Roboto_medium: require("native-base/Fonts/Roboto_medium.ttf"),
 
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import { Ionicons } from '@expo/vector-icons';
import * as Font from 'expo-font';
import * as SplashScreen from 'expo-splash-screen';
import * as React from 'react';

export default function useCachedResources() {
  const [isLoadingComplete, setLoadingComplete] = React.useState(false);

  // Load any resources or data that we need prior to rendering the app
  React.useEffect(() => {
    async function loadResourcesAndDataAsync() {
      try {
        SplashScreen.preventAutoHideAsync();

        // Load fonts
        await Font.loadAsync({
          ...Ionicons.font,
          'space-mono': require('../assets/fonts/SpaceMono-Regular.ttf'),
          Roboto: require("native-base/Fonts/Roboto.ttf"),
          Roboto_medium: require("native-base/Fonts/Roboto_medium.ttf"),
        });
     
      } catch (e) {
        // We might want to provide this error information to an error reporting service
        console.warn(e);
      } finally {
        setLoadingComplete(true);
        SplashScreen.hideAsync();
      }
    }

    loadResourcesAndDataAsync();
  }, []);

  return isLoadingComplete;
}