Server-side pagination using Apex on large datasets

LoadRecordsCtrl.cls

public class LoadRecordsCtrl {

    @RemoteAction
    public static Map<String, Object> fetchRecords(String sObjectName, String lastKnownId, Integer pageSize, Integer direction) {
        Map<String, Object> result = new Map<String, Object>();
        try {
            SObject sObj = (SObject) Type.forName(sObjectName).newInstance();
            Integer totalRecords = SObjectQueryService.getTotalCount(sObjectName);
            Integer totalPages = (Integer) Math.ceil((Decimal) totalRecords / pageSize);
            Integer maxResults = pageSize;
            if (String.isBlank(lastKnownId) && direction == -1) {
                // when querying the last page, get the number of records on last page
                maxResults = SObjectQueryService.getLastPageRecordCount(totalRecords, pageSize);
            }
            List<SObject> records = SObjectQueryService.queryRecords(sObjectName, lastKnownId, maxResults, direction);
            // remove extra record and set next / previous state
            result.putAll(SObjectQueryService.getPageState(records, lastKnownId, totalPages, pageSize, direction));
            // sort and set records
            if (direction == -1) records.sort();
            result.put('totalRecords', totalRecords);
            result.put('records', records);
            result.put('status', true);
        } catch (Exception ex) {
            result.put('status', false);
            result.put('error', ex.getMessage());
        }
        return result;
    }
}

SObjectQueryService.cls

public with sharing class SObjectQueryService {

    public static Integer getTotalCount(String sObjectName) {
        Integer totalCount = 0;
        String query = 'select count(Id) totalCount from ' + sObjectName;
        List<AggregateResult> res = (List<AggregateResult>) Database.query(query);
        if (!res.isEmpty()) {
            totalCount = (Integer) res.get(0).get('totalCount');
        }
        return totalCount;
    }
    
    public static Integer getLastPageRecordCount(Integer totalRecords, Integer pageSize) {
        Integer totalPages = (Integer) Math.ceil((Decimal) totalRecords / pageSize);
        Integer recordCount = totalRecords - ((totalPages - 1) * pageSize);
        return recordCount;
    }
    
    public static List<SObject> queryRecords(String sObjectName, String lastKnownId, Integer maxResults, Integer direction) {
        List<String> fields = describeSObjectFields(sObjectName);
        String query = 'select ' + String.join(fields, ',');
        query += ' from ' + sObjectName;
        if (String.isNotBlank(lastKnownId)) {
            query += ' where Id ' + (direction == 1 ? '>' : '<') + ' \'' + lastKnownId + '\' ';
        }
        if (maxResults != null) {
            // query one extra record
            maxResults =  maxResults + 1;
            query += ' order by Id ' + (direction == 1 ? 'asc' : 'desc') + ' limit :maxResults';
        }
        return Database.query(query);
    }
    
    public static Map<String, Object> getPageState(List<SObject> records, String lastKnownId, Integer totalPages, Integer pageSize, Integer direction) {
        Map<String, Object> pageState = new Map<String, Object>();
        if (String.isBlank(lastKnownId)) {
            if (records.size() > pageSize) {
                records.remove(records.size() - 1);
            }
            if (direction == -1) {
                // loading last page
                pageState.put('hasPrevious', totalPages > 1);
                pageState.put('hasNext', false);
            } else {
                // loading first page
                pageState.put('hasPrevious', false);
                pageState.put('hasNext', totalPages > 1);
            }
        } else if (records.size() > pageSize) {
            records.remove(records.size() - 1);
            pageState.put('hasPrevious', true);
            pageState.put('hasNext', true);
        } else {
            if (direction == -1) {
                // reached first page while navigating backwards
                pageState.put('hasPrevious', false);
                pageState.put('hasNext', true);
            } else {
                // reached last page while navigating forwards
                pageState.put('hasPrevious', true);
                pageState.put('hasNext', false);
            }
        }
        if (!records.isEmpty()) {
            // set first and last Id on the page
            if (direction == -1) {
                pageState.put('firstId', records.get(records.size() - 1).get('Id'));
                pageState.put('lastId', records.get(0).get('Id'));
            } else {
                pageState.put('firstId', records.get(0).get('Id'));
                pageState.put('lastId', records.get(records.size() - 1).get('Id'));
            }
        }
        return pageState;
    }
    
    private static List<String> describeSObjectFields(String sObjectName) {
        List<String> fields = new List<String>();
        SObject sObj = (SObject) Type.forName(sObjectName).newInstance();
        DescribeSObjectResult objResult = sObj.getSObjectType().getDescribe();
        Map<String, SObjectField> fieldsMap = objResult.fields.getMap();
        for (String field : fieldsMap.keySet()) {
            DescribeFieldResult fieldResult = fieldsMap.get(field).getDescribe();
            if (fieldResult.isAccessible() && fieldResult.getType() != Schema.DisplayType.BASE64) {
                fields.add(fieldResult.getName());
                if (fieldResult.getType() == Schema.DisplayType.REFERENCE) {
                    List<SObjectType> sObjectTypes = fieldResult.getReferenceTo();
                    SObjectType referenceTo = sObjectTypes.get(0);
                    String relName = fieldResult.getRelationshipName();
                    String nameField = getNameField(referenceTo);
                    fields.add(relName + '.' + nameField);
                }
            }
        }
        return fields;
    }
    
    private static String getNameField(SObjectType sObjType) {
        String fieldName = null;
        DescribeSObjectResult relObjResult = sObjType.getDescribe();
        Map<String, SObjectField> fieldsMap = relObjResult.fields.getMap();
        for (String field : fieldsMap.keySet()) {
            DescribeFieldResult fieldResult = fieldsMap.get(field).getDescribe();
            if (fieldResult.isNameField()) {
                fieldName = fieldResult.getName();
                break;
            }
        }
        return fieldName;
    }
}