handlePress = (event) => {
    console.log(event.target.key)
}

<TouchableOpacity key={key} onPress={this.handlePress} />

handlePress = (key) => {
    console.log(key)
}

<TouchableOpacity key={key} onPress={() => this.handlePress(key)} />

handlePress(key) {
    console.log(key)
}

<TouchableOpacity key={key} onPress={() => this.handlePress(key)} />