//Handler Class
public class InvoiceClass {
    public static void invoiceProduct(List<Invoice_Product__c> invProduct){
        for(Invoice_Product__c inv : invProduct){
            inv.Total_Price__c = inv.Price__c * inv.Quantity__c;
        }
    }
    public static void invoiceAmountUpdate(List<Invoice_Product__c> invProduct ){
        Set<id> invoiceIds = new Set<id>();
        for(Invoice_Product__c invPro : invProduct){
            if(invPro.InvoiceLookUp__c != Null){
                invoiceIds.add(invPro.InvoiceLookUp__c);   
            }
        }
        List<Invoice__c> invoices = [Select id, Total_Amount__c, (Select id, Total_Price__c
                                                                  From Invoice_Products__r)
                                     From Invoice__c where id in : invoiceIds];
        for(Invoice__c inv: invoices){
            inv.Total_Amount__c = 0;
            for(Invoice_Product__c invProd : inv.Invoice_Products__r){
                inv.Total_Amount__c += invProd.Total_Price__c;
            }
        }
        if(invoices != Null){
            update invoices;
        }
    }
}
//trigger
trigger InvoiceProductTrigger on Invoice_Product__c (before insert, before update, after insert, after update, after delete) {
    if((Trigger.isInsert || Trigger.isUpdate) && Trigger.isBefore){
        InvoiceClass.invoiceProduct(Trigger.new);
    }
    if(Trigger.isAfter && (Trigger.isInsert || Trigger.isUpdate)){
        InvoiceClass.invoiceAmountUpdate(Trigger.new);
    } 
    if(Trigger.isAfter && Trigger.isDelete){
        InvoiceClass.invoiceAmountUpdate(Trigger.old);
    }
    if(Trigger.isAfter && Trigger.isUpdate){
        InvoiceClass.invoiceAmountUpdate(Trigger.old);
    }
}