react-native-maps: fitToElements not working

in componentDidMount i set fitToElements(true) but the map doesn’t fit all markers to screen. Here my code:

componentDidMount(){
		let _markers = [];
		for (let i = 0; i < this.props.locations.length; i++) {
			let _placeMarker = {
				coordinate : {
					latitude: this.props.locations[i].latitude,
					longitude: this.props.locations[i].longitude
				},
				title: this.props.locations[i].name,
				color: 'blue',
				key: i + 1
			};
			_markers.push(_placeMarker);
		};
		this.setState({
      region: {
        latitude: this.props.locations[0].latitude,
        longitude: this.props.locations[0].longitude,
        latitudeDelta: LATITUDE_DELTA,
        longitudeDelta: LONGITUDE_DELTA
      },
      markers: _markers
    });
    this.refs.map.fitToElements(true);
}

And in the render()

                           <View>
   		      	<MapView
   		        	ref="map"
   		          provider={this.props.provider}
   		          style={styles.map}
   		          initialRegion={this.state.region}
   		          loadingEnabled={true}
   		          onPress={(e) => this.onMapPress(e)}
   		        >
   		          {this.state.markers.map(marker => (
   		            <MapView.Marker
   		              title={marker.title} 
   		              key={marker.key}
   		              coordinate={marker.coordinate}
   		              pinColor={marker.color}
   		            />
   		          ))}
   		        </MapView>
   		      </View>

But my map screen only show the initialRegion marker, i must zoom out the map to show all markers I want to the map look like: