完全に覚書き。Reactも書いたことねえからセットステイト周りで積んでた。一ヶ月ぶりに再チャレンジでできるようになった。dbからチャットの履歴を取得した程で。

import React from 'react';
import { StyleSheet, Text, View } from 'react-native';
import { GiftedChat } from 'react-native-gifted-chat';

export default class App extends React.Component {


  state = {

        messages: 
         [
            {
              _id: 1,
              text: 'Hello developer',
              createdAt: new Date(),
              user: 
              {
                _id: 2,
                name: 'React Native',
                avatar: 'https://placeimg.com/140/140/any',
              },
            },
        ],
  }

  componentWillMount() {

    let texts = [
                  {
                    _id: 3,
                    text: 'Hello developer 3',
                    createdAt: new Date(),
                    user: {
                      _id: 1,
                      name: 'React Native',
                      avatar: 'https://placeimg.com/140/140/any',
                    },
                  },

                  {
                    _id: 4,
                    text: 'Hello developer 4',
                    createdAt: new Date(),
                    user: {
                      _id: 2,
                      name: 'React Native',
                      avatar: 'https://placeimg.com/140/140/any',
                    },
                  },
                ];

    let texts2 = [
                  {
                    _id: 6,
                    text: 'Hello developer 6',
                    createdAt: new Date(),
                    user: {
                      _id: 1,
                      name: 'React Native',
                      avatar: 'https://placeimg.com/140/140/any',
                    },
                  },
                ];

      texts = texts.concat(texts2);


      var i;
      var ii;
      for (i = 7; i < 23; i++) { 


          //奇数の場合
          if( ( i % 2 ) != 0 ) {

            ii = 1
          }

          //偶数の場合
          if( ( i % 2 ) == 0 ) {

            ii = 2

          }

          let texts3 = [
                        {
                          _id: i,
                          text: 'Hello developer 6',
                          createdAt: new Date(),
                          user: {
                            _id: ii,
                            name: 'React Native',
                            avatar: 'https://placeimg.com/140/140/any',
                          },
                        },
                      ];

            texts = texts.concat(texts3);

      }

       this.setState({ 

            messages: this.state.messages.concat(
              texts 
              )
            })
       }

  onSend(messages = []) {
    this.setState(previousState => ({
      messages: GiftedChat.append(previousState.messages, messages),
    }))
  }

  render() {
    return (
      <GiftedChat
        messages={this.state.messages}
        onSend={messages => this.onSend(messages)}
        user={{
          _id: 1,
        }}
      />
    )
  }
}