SOURCE: https://www.youtube.com/watch?v=QUzzepdALY4
Okay, so in this lecture, we're ready to move on. And our next task is to load this entire content. This main content of the front page to load this whole content dynamically. Now first of all, why do we want to do that? The reason we want to do it is because we're trying to move towards what's called an SPA, or SPA and stands for single page application. So a single page web application. And besides for being a pretty popular web coding technique nowadays, since SPAs heavy rely on Ajax, this whole content right here is going to be inserted dynamically. But the rest of the content is going to stay. So we're going to basically update the least amount of the page possible to make it functional. And that's very advantageous because obviously it's going to be faster and also is not going to refresh the page for the user. It's going to be a much, much nicer user experience. So let's go ahead and go to our code editor. And right now, we're located in Lecture60 in the after folder and we're looking at index.html. Well, for that purpose in order to make the entire content, the main content the dynamic, we actually need to wipe it out and this is exactly what we did. We don't have any more in the index.html the main content. There's only just a placeholder for the main content with id main-content. Now, where did all of that stuff go? Well, we have a new folder here called snippets. And if we open it up, there is a file called homesnippet.html. So if we open that up, the entire contents of our homepage are really contained right here. So you can see there's our jumbo tron, home tiles and so on. We did change a couple of links, cause you notice the menu item here is no longer just a link, it's actually a link to basically nowhere with an onclick event handler. And we'll talk about that a little bit later, but we just kind of prepped it for the next step. because obviously we're going to have to display our menu category page and then individual single category page and we're still going to have to do this, using this spot technique or using Ajax. That is, we'll need to take that content and dynamically insert it into the index dot html right here at the place holder in the content. But for this lecture all we need to do is figure out how to insert the previous mint page content inside the main content placeholder. Let's go to our script.gs, where I have prewritten some code, and let's go over that. So first of all, you could see this code up top. This is what we just wrote in a previous lecture. This is just to make the collapsible button collapse the menu when it loses focus. Okay, so what we're going to do is first of all we're going to set up an immediately invoked functional expression. As you can see here we're starting with prints global and going all the way down here we're executing this function passing it to window object. So our global right here Is really just the window object. And we're setting up the namespace dc, just probably for David Chu's. That's how I named it. And as you could see, we're exposing this thing right before the end of this call function. We're saying global, which is really window. .$dc, that's that where we want to expose this as and dc our internal theme space. So whatever we attach to the dc as a property is going to get exposed to the global object in order for us to use it in some other page or some other script. Okay, so let's take a look as to what we're doing here. First of all we're setting up our home HTML. That's nothing more than just the URL as to where the snippet is going to sit. And then we're going to have a convenience method here so we don't have to write this from scratch every time because we're going to have to do this several times. Is if you give me a selector and you give me the HTML, I will go ahead and take, select that selector. Take that argument, take that element I should say, grab that element, the target element, and set its inner HTML, to whatever the HTML string that you send me. So this is just a convenience method to say, insertHtml, pass it a selector, and the HTML string, and it will insert it into the target element.
Another function that we need is show loading icon, it's this showLoading here. So obviously an HS request is going to be done asynchronously and it will go out to the server and bring us back some content. But meanwhile, what is going to be displayed to the user? Well, we obviously don't want to display nothing. We don't want to display blank. We would rather display some sort of an animated icon that is going to show the user that something is going on. Well, ajax-loader.gif is an animated GIF. And I'm going to tell you in a minute where I got that from. But let me go over the code first. So showLoading basically says give me a selector to which I should attach this loading icon. And I'm going to set up a very simple div. I'm going to put an image inside of that div and close the div. So this is just a, building up an HTML string. And it's pointing to this images/ajax-loader.gif. And then I'm reusing my insertHtml that I just defined, and basically inserting that into this particular selector. So I'm able to then display this. Rotating or whatever it is Ajax loader icon inside this selective past send. Now where did I get this index or ajax-loader.gif. Let me go back to the browser for a second. There's a great website called ajaxload.info and it allows you to basically choose different types of loading icons that you could choose and then set whatever background color you want and whatever foreground color you want and generate that icon for yourself, and there it is. And then you could click to download that GIF, and then you could use it in your web site and absolutely free. So it's a great web site for these type of activities. But it's not the only one, there are others out there, and definitely google for it and you'll see some others that are perhaps a little bit more creative but not as straight forward. So let's go back to our code editor, and keep going here, so. So we want to set up an on-page load. What we're doing here is we're waiting till the dom content has been loaded. And in that case, once it is loaded, we're able to start executing things and basically executing queries. The selector quarries that will actually find things since we need HTML to be loaded at that point in order to execute query selectors. And here what we're about to do is we're about to make that Ajax call we've seen before. But before we make the Ajax call we want to go ahead and turn on that showLoading. And that showLoading is going to insert in the main Content ID that element is going to insert our rotating Ajax loader so it's going to place it right here as the inner HTML of this element.
Once it's done it's going to issue our ajax request. The Ajax request is going to be a fairly simple one it's going to give us the home URL or the home HTML URL. And that's something we've defined right here, previously. That's just the URL as to where that snippet is sitting. And since we're passing false as the last argument, as you remember, false means I don't want you to pre-process this as JSON. And we don't want this as JSON, because, all it is is just an HTML snippet. What is going to come back in our handler function is just some text to actually just a response HTML. And once it does come back we're going to select our element with ID main-content and set its innerHTML to whatever the responseText came back to be. And that's about it. So once we're done with this, we can save this, go to our browser, and switch to David Chu's China Bistro through our browser sync. And we if we actually reload this right now, it should very quickly get loaded so the entire content is here. So let's actually take a look at The Chrome developer tools to make sure that we think is going on is in fact going on. Let's go ahead and open it up and leave it on XHR, which stands for XMLHttpRequest that's the Ajax request, so we don't want to see all the requests. We just want to see the Ajax only. So let's go ahead and refresh the browser that's going to give us a whole bunch of requests here because we have Google Maps embedded in as well. So there's definitely a lot of things going on here. But one of the things you see is there is an Ajax Request for home snippet.html. And that's exactly what we get, we get that little snippet over here. And that happens once the page loads and kicks off this whole Ajax process, which then goes ahead and fills in the content, the main content of our page. So if we take a look right now and try to click on this Menu button right here, nothing's really going to happen, it's just going to sit right here. But if we open the Chrome Developer Tools and look at our console, we going to see there's an error, the dc.loadMenuCategories is not a function. Well, that's true it's not a function because we haven't defined it yet. That function is supposed to swap the content of our HTML page, the main content, to the MenuCategoires content and that's what we're going to do in the next lecture.
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