render() {
    const { username,password } = this.state;

    return (
        <View style={styles.container}>
            <View style={styles.group}>
                <Text style={styles.text}>Username:</Text>
                <TextInput
                    style={styles.input}
                    onChangeText={this.handleUserNameChange.bind(this)}
                    value={username}
                    underlineColorAndroid="transparent"
                />
            </View>
            <View style={styles.group}>
                <Text style={styles.text}>Password:</Text>
                <TextInput
                    style={styles.input}
                    secureTextEntry={true}
                    onChangeText={this.handlePasswordChange.bind(this)}
                    value={password}
                    underlineColorAndroid="transparent"
                />
            </View>
            <TouchableHighlight
                style={styles.button}
                onPress={this.handleLogin.bind(this)}>
                <Text style={styles.buttonText}>logon</Text>
            </TouchableHighlight>
        </View>
    );
}

现有款式：

const styles = StyleSheet.create({
    container: {
        flex: 1,justifyContent: 'flex-start',alignItems: 'center',backgroundColor: '#F5FCFF',marginTop: 60
    },group: {
        alignItems: 'flex-start',padding: 10
    },input: {
        width: 150,padding: 2,paddingLeft: 5,borderColor: 'gray',borderWidth: 1
    },text: {
        padding: 0
    },button: {
        width: 150,backgroundColor: '#2299F2',padding: 15,marginTop: 20,borderRadius: 5
    },buttonText: {
        textAlign: 'center',color: '#fff',fontSize: 24
    },});

我需要一个ActivityIndi​​cator到上面的View，覆盖视图，并使ActivityIndi​​cator居中。