I want to change client = {state:0}
You can access it by using this.client.state

I also have a child which contains a button.
I'm trying to change this 'state' variable when you press the button.

For some reason everything I find everything on the internet not working for me.
I've been stuck at it for 5 hours and I think it's time to ask help myself

import React from 'react';

import { StyleSheet, Text, View, Image, TouchableOpacity } from 'react-native';

import Home from './Components/Home';

import Activity from './Components/Activity';



export default class App extends React.Component {

client = {state:0}



  render() {

    if(this.client.state == 0){

        return(

            <View style={{flex: 1}}>

                <Home />

                <Child />

            </View>

        );

    } else {

        return(

            <View style={{flex: 1}}>

                <Activity />

                <Child />

            </View>

        );

    }

There are different ways of doing this. It could be done with Redux for example, but let's take a simpler approach.

Also note that it can't be done by props, because a child component cannot update its parents' props.

Also note that the way you are using the state seems rather strange. It should be set on the class level (or component level).

class MyComponent extends React.Component {

    constructor(props) {

        super(props);

        this.state = {myProperty: 0};

    }

}

You could pass a callback method to the Child React Component.

<Child callback={this.onButtonClick} />

On the Client Component, create that callback method:

onButtonClick() {

    this.setState({buttonClicked: true});

}

To keep it clean, define the initial value in the constructor. You'll also have to bind the function to have a correct this parameter, otherwise the this variable will be from the event instead of the class you're expecting.

constructor(props) {

    super(props);

    this.state = {buttonClicked: false};

    this.onButtonClick = this.onButtonClick.bind(this);

}

That's it for the Client component.

Now on the Child Component, you'll need to trigger this callback method when possible.

Imagine the Child has the following button, add an event handler on the child component as well, onChildButtonClick. You'll also have to bind in the constructor.

constructor(props) {

    super(props);

    // bind this for access to this class' data

    this.onChildButtonClick = this.onChildButtonClick.bind(this);

}



onChildButtonClick() {

    // Might want to do a typeof is function check here too

    if (this.props.callback) {

        // Trigger the callback on the parent component, letting it know the button was triggered

        this.props.callback();

    }

}



render() {

    return <button onClick={this.onChildButtonClick}>Click me</button>;

}

During initialisation, the Parent component sends a callback method to the child component. Whenever the button is clicked on the child component, the child component triggers the function (callback) given by the parent, essentially running a piece of code on the parent component, which then updates the state with the requested value (could be a string, or anything).

Redux

Redux is another way of doing it, which basically keeps a sort of tracked database that can be used from any component, by pageload - however, that would require an entire tutorial.