Magento 2 – Using this.getUrl() in JavaScript

As you pointed out, when you click "Ship here" a HTTP POST request is dispatched to "/V1/carts/mine/estimate-shipping-methods-by-address-id" REST API (from module-quote). If you take a look at module-quote/etc/webapi.xml you will find the url:

<route url="/V1/carts/mine/estimate-shipping-methods-by-address-id" method="POST">
  <service class="Magento\Quote\Api\ShippingMethodManagementInterface" method="estimateByAddressId"/>
  <resources>
    <resource ref="self" />
  </resources>
  <data>
    <parameter name="cartId" force="true">%cart_id%</parameter>
  </data>
</route>

You can notice that under the <route> element there is <service> element with the class="Magento\Quote\Api\GuestShipmentEstimationInterface" and method="estimateByExtendedAddress". Now obviously, the estimateByAddressId method cannot be instantiated from an interface.

Here comes in scene the magento 2 dependency injection. Look at module-quote/etc/di.xml file that maps an interface(Magento\Quote\Api\ShippingMethodManagementInterface) dependency to a preferred implementation class(Magento\Quote\Model\ShippingMethodManagement).

<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
    <preference for="Magento\Quote\Api\ShippingMethodManagementInterface" type="Magento\Quote\Model\ShippingMethodManagement" />
    ...................
</config>

This is how estimateByAddressId method is called.

Useful links:

Magento 2 Web APIs:
http://devdocs.magento.com/guides/v2.0/get-started/bk-get-started-api.html
http://devdocs.magento.com/guides/v2.0/extension-dev-guide/service-contracts/service-to-web-service.html

Magento 2 Dependency injection:
http://devdocs.magento.com/guides/v2.0/extension-dev-guide/depend-inj.html
http://magento-quickies.alanstorm.com/post/68129858943/magento-2-injecting-interfaces

Okay, I have figured out how to set the shipping method programmatically:

Firstly, all my dependencies:

define(
    [
        'ko',
        'jquery',
        'Magento_Ui/js/form/form',
        'uiRegistry',
        'Magento_Checkout/js/model/quote',
        'Magento_Checkout/js/model/resource-url-manager',
        'Magento_Checkout/js/action/create-shipping-address',
        'Magento_Checkout/js/model/error-processor',
        'Magento_Checkout/js/model/shipping-service',
        'Magento_Checkout/js/model/cart/totals-processor/default',
        'Magento_Checkout/js/model/shipping-rate-registry',
        'Magento_Checkout/js/action/select-shipping-method',
        'Magento_Checkout/js/checkout-data'
    ],
    function (
        ko,
        $,
        Component,
        uiRegistry,
        quote,
        resourceUrlManager,
        createShippingAddress,
        errorProcessor,
        shippingService,
        totalsDefaultProvider,
        rateRegistry,
        selectShippingMethodAction,
        checkoutData
    ) { 

    ...

Then, the magic promise:

                setShippingMethod: function (carrier, method, title) {
                    return new Promise((resolve, reject) => {
                        let data = {
                            carrier_code: carrier,
                            method_code: method,
                            carrier_title: title,
                            method_title: title
                        };
                        selectShippingMethodAction(data);
                        checkoutData.setSelectedShippingRate(code + '_' + method);
                        if (quote.shippingMethod()) {
                            resolve(quote.shippingMethod());
                        } else {
                            reject(quote.shippingMethod());
                        }
                    });
                }

So then in your event (or whatever):

                    setShippingMethod(
                        methodData['carrier_code'], methodData['warehouse_id'], methodData['title']
                    ).then(
                        (shippingMethod) => { 
                             totalsDefaultProvider.estimateTotals(quote.shippingAddress());
                        }
                    ).catch(
                        (err) => failHandlerMethod(err)
                    );

Optional, you can also add the shipping address:

                    let payload = JSON.stringify({
                        addressInformation: {
                            address: {
                                countryId: "country",
                                postcode: "postcode",
                                regionId: "region"
                            },
                            shipping_carrier_code: "carrier_code",
                            shipping_method_code: "method_code"
                        }
                    });
                    let addr = createShippingAddress(payload);
                    rateRegistry.set(address.getCacheKey(), addr);