Sending an email notification to Admin whenever a field is created or deleted for a custom object in salesforce

S.No	Component
1	Create New Object Singular Label: ObjectsExistingFieldsInfo Plural Label: ObjectsExistingFieldsInfos Object Name: ObjectsExistingFieldsInfo Standard Field Field Label: ObjectsExistingFieldsInfo Data Type: Auto Number Display Format: ObjInfo-{000}
2	Create New Field Object Name: ObjectsExistingFieldsInfo Field Label: Field API Name Field Name: Field_API_Name Data Type: Text (255)
3	Create New Field Object Name: ObjectsExistingFieldsInfo Field Label: Fields Count Field Name: Fields_Count Data Type: Number (18,0)
4	Create New Field Object Name: ObjectsExistingFieldsInfo Field Label: Fields Info Field Name: Fields_Info Data Type: Long Text Area

	global class FieldsValidator implements Database.Batchable<Sobject> {
	global Database.queryLocator start(Database.BatchableContext bc) {
	return Database.getQueryLocator('select name, fields_info__c, fields_count__c from ObjectsExistingFieldsInfo__c');
	}
	global void execute(Database.BatchableContext bc, List<SObject> sobjLst) {
	String[] types = new String[]{'Account','Contact'};
	String emailBody = '';
	// Make the describe call
	Schema.DescribeSobjectResult[] results = Schema.describeSObjects(types);
	Map<String,List<Field>> objFieldsDesMap = new Map<String,List<Field>>();
	// For each returned result, get some info
	for(Schema.DescribeSobjectResult res : results) {
	Map<String, Schema.SObjectField> objectFields = res.fields.getMap();
	for(Schema.SObjectField fDes : objectFields.values()) {
	if(fDes.getDescribe().isCustom()) {
	system.debug(fDes.getDescribe().getName()+'@@@'+fDes.getDescribe().getType());
	String fieldAPIName = String.valueOf(fDes.getDescribe().getName());
	String fieldDataType = String.valueOf(fDes.getDescribe().getType());
	if(objFieldsDesMap.containsKey(res.getName())) {
	objFieldsDesMap.get(res.getName()).add(new Field(fieldAPIName,fieldDataType));
	}
	else {
	objFieldsDesMap.put(res.getName(),new List<Field>{new Field(fieldAPIName,fieldDataType)});
	}
	}
	}
	}
	Map<String, ObjectsExistingFieldsInfo__c> objectInfoMap = new Map<String, ObjectsExistingFieldsInfo__c>();
	for(ObjectsExistingFieldsInfo__c objInfo : [select Field_API_Name__c , Fields_Count__c, Fields_Info__c from
	ObjectsExistingFieldsInfo__c where Field_API_Name__c in: types]) {
	objectInfoMap.put(objInfo.Field_API_Name__c,objInfo);
	}
	for(String type : types) {
	if(objectInfoMap.get(type).fields_count__c == null) {
	objectInfoMap.get(type).fields_count__c = objFieldsDesMap.get(type).size();
	objectInfoMap.get(type).fields_info__c = JSON.serialize(objFieldsDesMap.get(type));
	}
	else {
	System.debug('entering to else condition...');
	System.debug('entering to for loop..');
	List<Field> exstingFields = new List<Field>();
	List<Field> newFields = new List<Field>();
	Map<String,Field> bkupexstingFields = new Map<String,Field>();
	Map<String,Field> bkupnewFields = new Map<String,Field>();
	exstingFields = (List<Field>)JSON.deserialize(objectInfoMap.get(type).fields_info__c, List<Field>.class);
	System.debug('exstingFields: '+exstingFields);
	newFields.addAll(objFieldsDesMap.get(type));
	System.debug('objFieldsDesMap: '+objFieldsDesMap);
	System.debug('newFields: '+newFields);
	for(Field f : exstingFields) {
	bkupexstingFields.put(f.apiName,f);
	}
	for(Field f : newFields) {
	bkupnewFields.put(f.apiName,f);
	}
	for(Field f : exstingFields) {
	bkupnewFields.remove(f.apiName);
	}
	for(Field f : newFields) {
	bkupexstingFields.remove(f.apiName);
	}
	System.debug(bkupnewFields+'@@@'+bkupexstingFields);
	emailBody += type + ' : ';
	//for new fields
	for(Field f :bkupnewFields.values()) {
	emailBody += f.apiName + ' with datatype '+f.dataType+' is newly created.';
	}
	objectInfoMap.get(type).fields_count__c = objFieldsDesMap.get(type).size();
	objectInfoMap.get(type).fields_info__c = JSON.serialize(objFieldsDesMap.get(type));
	}
	}
	/*** Sending Email ***/
	Messaging.SingleEmailMessage mail = new Messaging.SingleEmailMessage();
	mail.setToAddresses(new List<String>{system.label.Dev_Admin});
	mail.setSubject('Fields Update');
	mail.setPlainTextBody(emailBody);
	update objectInfoMap.values();
	Messaging.sendEmail(new Messaging.SingleEmailMessage[]{mail});
	}
	global void finish(Database.BatchableContext bc) {
	}
	public class Field {
	String apiName, dataType;
	public Field(String apiName, String dataType) {
	this.apiName = apiName;
	this.dataType = dataType;
	}
	}
	}

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Notes-

• We cannot create trigger on 'Object' or 'Field'.
• Alternative is creating a Batch Class and schedule it as per the requirement.
• To store the history of the fields we can user either list custom settings or custom object.
• List Custom Settings is not supporting Text Area Long.
• To store the history of the fields we need Text Area Long (255 character for text area data type is not sufficient.) field which is possible with the custom object.