Added example usage of react-admin Admin component (#619)

* Added example usage of react-admin Admin component instead of HydraAdmin and AdminBuilder

* Update getting-started.md

Author: helios-ag

admin/getting-started.md

@@ -228,3 +228,170 @@ export default class extends Component {
   }
 }
 ```
+
+### Using the Hydra Data Provider Directly with react-admin
+
+By default, the `HydraAdmin` component shipped with API Platform Admin will generate a convenient admin interface for every resources and every properties exposed by the API. But sometimes, you may prefer having full control over the generated admin.
+
+To do so, an alternative approach is [to configure every react-admin components manually](https://marmelab.com/react-admin/Tutorial.html) instead of letting the library generating it, but to still leverage the built-in Hydra [data provider](https://marmelab.com/react-admin/DataProviders.html):
+
+```javascript
+// admin/src/App.js
+
+import React, { Component } from 'react';
+import { Admin, Resource } from 'react-admin';
+import parseHydraDocumentation from '@api-platform/api-doc-parser/lib/hydra/parseHydraDocumentation';
+import { hydraClient, fetchHydra as baseFetchHydra  } from '@api-platform/admin';
+import authProvider from './authProvider';
+import { Redirect } from 'react-router-dom';
+import { createMuiTheme } from '@material-ui/core/styles';
+import Layout from './Component/Layout';
+import { UserShow } from './Components/User/Show';
+import { UserEdit } from './Components/User/Edit';
+import { UserCreate } from './Components/User/Create';
+import { UserList } from './Components/User/List';
+
+const theme = createMuiTheme({
+    palette: {
+        type: 'light'
+    },
+});
+
+const entrypoint = process.env.REACT_APP_API_ENTRYPOINT;
+const fetchHeaders = {'Authorization': `Bearer ${window.localStorage.getItem('token')}`};
+const fetchHydra = (url, options = {}) => baseFetchHydra(url, {
+    ...options,
+    headers: new Headers(fetchHeaders),
+});
+const dataProvider = api => hydraClient(api, fetchHydra);
+const apiDocumentationParser = entrypoint => parseHydraDocumentation(entrypoint, { headers: new Headers(fetchHeaders) })
+    .then(
+        ({ api }) => ({api}),
+        (result) => {
+            switch (result.status) {
+                case 401:
+                    return Promise.resolve({
+                        api: result.api,
+                        customRoutes: [{
+                            props: {
+                                path: '/',
+                                render: () => <Redirect to={`/login`}/>,
+                            },
+                        }],
+                    });
+
+                default:
+                    return Promise.reject(result);
+            }
+        },
+    );
+
+export default class extends Component {
+    state = { api: null };
+
+    componentDidMount() {
+        apiDocumentationParser(entrypoint).then(({ api }) => {
+            this.setState({ api });
+        }).catch((e) => {
+            console.log(e);
+        });
+    }
+
+    render() {
+        if (null === this.state.api) return <div>Loading...</div>;
+        return (
+            <Admin api={ this.state.api }
+                   apiDocumentationParser={ apiDocumentationParser }
+                   dataProvider= { dataProvider(this.state.api) }
+                   theme={ theme }
+                   appLayout={ Layout }
+                   authProvider={ authProvider }          
+            >                
+                <Resource name="users" list={ UserList } create={ UserCreate } show={ UserShow } edit={ UserEdit } title="Users"/>
+            </Admin>
+        )
+    }
+}
+```
+
+And accordingly create files `Show.js`, `Create.js`, `List.js`, `Edit.js`
+in the `admin/src/Component/User` directory:
+
+```javascript
+// admin/src/Component/User/Create.js
+
+import React from 'react';
+import { Create, SimpleForm, TextInput, email, required } from 'react-admin';
+
+export const UserCreate = (props) => (
+    <Create { ...props }>
+        <SimpleForm>
+            <TextInput source="email" label="Email" validate={ email() } />
+            <TextInput source="plainPassword" label="Password" validate={ required() } />
+            <TextInput source="name" label="Name"/>
+            <TextInput source="phone" label="Phone"/>
+        </SimpleForm>
+    </Create>
+);
+
+```
+
+```javascript
+// admin/src/Component/User/Edit.js
+
+import React from 'react';
+import { Edit, SimpleForm, DisabledInput, TextInput, DateInput, email } from 'react-admin';
+
+export const UserEdit = (props) => (
+    <Edit {...props}>
+        <SimpleForm>
+            <DisabledInput source="originId" label="ID"/>
+            <TextInput source="email" label="Email" validate={ email() } />
+            <TextInput source="name" label="Name"/>
+            <TextInput source="phone" label="Phone"/>
+            <DateInput disabled source="createdAt" label="Date"/>
+        </SimpleForm>
+    </Edit>
+);
+```
+
+```javascript
+// admin/src/Component/User/List.js
+
+import React from 'react';
+import { List, Datagrid, TextField, EmailField, DateField, ShowButton, EditButton } from 'react-admin';
+import { CustomPagination } from '../Pagination/CustomPagination';
+
+export const UserList = (props) => (
+    <List {...props} title="Users" pagination={ <CustomPagination/> }  perPage={ 30 }>
+        <Datagrid>
+            <TextField source="originId" label="ID"/>
+            <EmailField source="email" label="Email" />
+            <TextField source="name" label="Name"/>
+            <TextField source="phone" label="Phone"/>
+            <DateField source="createdAt" label="Date"/>
+            <ShowButton />
+            <EditButton />
+        </Datagrid>
+    </List>
+);
+```
+
+```javascript
+// admin/src/Component/User/Show.js
+import React from 'react';
+import { Show, SimpleShowLayout, TextField, DateField, EmailField, EditButton } from 'react-admin';
+
+export const UserShow = (props) => (
+    <Show { ...props }>
+        <SimpleShowLayout>
+            <TextField source="originId" label="ID"/>
+            <EmailField source="email" label="Email" />
+            <TextField source="name" label="Name"/>
+            <TextField source="phone" label="Phone"/>
+            <DateField source="createdAt" label="Date"/>
+            <EditButton />
+        </SimpleShowLayout>
+    </Show>
+);
+```