How to send sales transaction emails?

Question

I like to send emails for my sales order.

1. If the order is placed status as "Processing". (email send automatically) it's okay
2. Processing to "Dispatch" (not having the option to set dispatch)
3. Dispatch to "Shipments" (this too I assigned email with tracking details)
4. Shipments to "Delivered" (not having option to set delivered)
5. For all this order I need to send emails to customers.

Answer 1

For that you have to use event sales_order_state_change_before and check previous state/status and current state/status. According to this pass data in template variable and use in email template.

<?php

namespace Vendor\Namespace\Observer;

use Magento\Framework\Translate\Inline\StateInterface;
use Magento\Framework\Escaper;
use Magento\Framework\Mail\Template\TransportBuilder;

class Statechange implements \Magento\Framework\Event\ObserverInterface {

    protected $inlineTranslation;
    protected $escaper;
    protected $transportBuilder;
    protected $logger;

    public function __construct(        
        StateInterface $inlineTranslation,
        Escaper $escaper,
        TransportBuilder $transportBuilder,
        \Psr\Log\LoggerInterface $logger
    ) {

        $this->inlineTranslation = $inlineTranslation;
        $this->escaper = $escaper;
        $this->transportBuilder = $transportBuilder;
        $this->logger = $logger;

    }

    public function execute(\Magento\Framework\Event\Observer $observer)
    {
        $order = $observer->getEvent()->getOrder();
        $transport = $observer->getEvent()->getTransport();
        $previousState = $order->getState();
        $currentState = $transport->getState();
        if ($order instanceof \Magento\Framework\Model\AbstractModel) {
           if($previousState == 'processing ' && $currentState == 'complete') {
                // processing to complete logic here
                $this->sendEmail($order,$previousState,$currentState);
           }else if($previousState == 'new' && $currentState == 'cancel') {
                // new to cance logic here
                $this->sendEmail($order,$previousState,$currentState);
           }
        }
        return $this;
    }
    public function sendEmail($order,$previousState,$currentState)
    {
            try {
                $this->inlineTranslation->suspend();
                $sender = [
                    'name' => $this->escaper->escapeHtml('Test'),
                    'email' => $this->escaper->escapeHtml('abc@gmail.com'),
                ];
                $transport = $this->transportBuilder
                    ->setTemplateIdentifier('email_demo_template')
                    ->setTemplateOptions(
                        [
                            'area' => \Magento\Framework\App\Area::AREA_FRONTEND,
                            'store' => \Magento\Store\Model\Store::DEFAULT_STORE_ID,
                        ]
                    )
                    ->setTemplateVars([
                        'order'  => $order,
                        'previousState'  => $previousState,
                        'currentState'  => $currentState,
                    ])
                    ->setFrom($sender)
                    ->addTo($order->getCustomerEmail())
                    ->getTransport();
                $transport->sendMessage();
                $this->inlineTranslation->resume();
            } catch (\Exception $e) {
                $this->logger->debug($e->getMessage());
            }
    }
}