Current Opportunity details are not coming in Email using apex trigger

Question

I have created a Apex trigger on opportunity when the available__C checkbox unchecked then mail needs to go to opportunity Owners manager.. I have written below trigger.Here mail is getting sent but no opp details are coming in mail..I have used Text template with merge field(like opp.name) which are correct

    trigger OnOpportunity on Opportunity (after insert,after update) { 
    set<id>oppId=new set<id>(); 
    map<id,opportunity> OppMap=new map<id,opportunity>(); 
    list<id> userid=new list<id>(); 
    List<Messaging.SingleEmailMessage> emails = new List<Messaging.SingleEmailMessage>(); 
    if (trigger.isinsert && trigger.isAfter){ 
        for(opportunity opp:trigger.new){ 
            oppmap.put(opp.OwnerId,opp);  
        }
    }
    if (trigger.isupdate && trigger.isAfter){ 
        for(opportunity opp:trigger.new){ 
            if(opp.Avialable__c==false && opp.Avialable__c!=trigger.oldmap.get(opp.id).Avialable__c){
                oppmap.put(opp.OwnerId,opp); 
            } 
        } 
    }     
    Map<Id,User> userMap = new Map<Id,User>([select id,manager.email,managerid from user where id in:oppmap.keySet()]);
    for (opportunity opp:trigger.new){ 
        if(userMap.containsKey(opp.ownerid)){ 
            Messaging.SingleEmailMessage mail = new Messaging.SingleEmailMessage(); 
            mail.setTargetObjectId(userMap.get(opp.ownerid).managerid); 
            mail.setTemplateId('00X28000001Fiuv');     
            mail.setSaveAsActivity(false); 

            Messaging.sendEmail(new Messaging.SingleEmailMessage[] { mail }); 
        } 
    } 

}

Answer 1

in setWhatId method pass Opportunity Id.

List<contact> conList = [select id from contact limit 1];

    Messaging.SingleEmailMessage mail = new Messaging.SingleEmailMessage(); 
    mail.setTargetObjectId(conList[0].Id); 
    mail.setToAddresses(new List<string>{userMap.get(opp.ownerid).manager.email}); 
    mail.setTreatTargetObjectAsRecipient(false);
    mail.setTemplateId('00X28000001Fiuv'); 
    mail.setWhatid(Opp.Id);    
    mail.setSaveAsActivity(false); 

It will populate the values.

I am passing contact Id because we are using email template.