Você já percebeu que a única funcionalidade de uma aplicação web com a qual os usuários interagem diretamente é através da página web? Se você implementá-la de uma maneira pobre, o lado do servidor passa a ser irrelevante! Os usuários de hoje esperam muito das páginas web: tem que carregar rápido, exibir o serviço desejado, e ser confortável para ser visualizada em todos os dispositivos, desde desktops a tablets e smartphones. Neste curso, vamos aprender as ferramentas básicas que todos os desenvolvedores web precisam conhecer. Vamos começar do início e evoluir até aprender como implementar páginas web modernas com HTML e CSS. Depois, vamos avançar para aprender como codificar nossas páginas para que seus componentes se rearrangem e alterem seus tamanhos automaticamente, baseado no tamanho da tela do uuário. Você será capaz de codificar uma página que será tão útil num telefone celular quanto num desktop. Nada de aumentar o zoom! e por último, mas não menos importante, teremos uma introdução da linguagem web mais presente, mais popular e incrivelmente poderosa: Javascript. Utilizando JavaScript, você será capaz de construir um aplicação web completamente funcional que utiliza Ajax para expor funcionalidade do laod do servidor e dados ao usuário final.
Lesson 57, Part 2: Ajax Basics
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Do curso por Johns Hopkins University
HTML, CSS e Javascript para Desenvolvedores Web
4009 classificações
Johns Hopkins University
4009 classificações
Curso 4 de 6 em Specialization Desenvolvimento Web com Ruby on Rails
Na lição
Using Javascript to Build Web Applications
In this module, we are going to take all those newly learned Javascript language skills and learn how to utilize them within the context of a web page. We’ll start by learning how to properly manipulate the web page components using the Javascript Document Object Model API. We will then move on to learning one of the most popular ways of serving up data to a web site - Ajax. We'll learn about the protocol that the language of the web speaks in (HTTP), how to set up and handle Ajax requests and responses, as well as how to process JSON data. We'll finish the module by connecting our restaurant web site from Module 3 to a real backend service that will allow us to pull the data for the restaurant menu dynamically from the server without having to reload the entire page.
Conheça os instrutores
Yaakov ChaikinAdjunct Professor, Graduate Computer Science
Whiting School of Engineering
[SOUND] In part
one of this lecture, we spoke about the theory behind the Ajax call to the server.
In this part, we're going to speak about the actual practice of it, and
show you some code to do it.
So right now, I'm located in lecture 57, so that's lecture folder 57, and
I'm looking at actually quite a few files here.
And, there's a couple of new files that I want you to pay attention to.
Number one is, ajaxutils.js.
And this is the file which is going to give us the JavaScript,
in order to make those Ajax calls.
So we kind of separated it into a separate file, so we could see the Ajax procedure
in its entirety, and then just use it inside the script.gs.
Notice, that in html, the index.html,
I place the Ajax utils.gs above, js/script.gs is
because script.gs uses things inside of ajaxutils.js.
Which means that I actually have them,
I have to have them loaded in the browser already, before I start using them inside.
So that's why I, inside script.js.
That's why I placed them, above that.
Now unlike previous examples, where we had the web page click and say hello,
to whoever was, the name that was in the text box, this time, I want to click and
say hello, and get the name, of the person I want to say hello to, from the server.
And for that I actually have something a little data file on the server,
it's sitting inside a data folder in name.txc.
And all it is is just says Yaakov Chaikin there's nothing else to it,
it's just simple text file.
And I want to verify that I can access or grab that file or
the data from that file through the browser while my browser sync is
running by simply putting the URL in and saying data.
That's the data folder.
And it's name.txt.
If I say press Enter and I see that that's exactly what get's returned.
It's a very simple two-word strain.
So we could close up for now.
Okay, so let's close the file explorer in sublime text and
let's go to our ajaxutils.js.
So the first thing I want you to notice is that I am using the iifi, the immediately
invoked function expression, in order to wrap my entire code.
So it comes from all the way here, and if I scroll down, all the way here.
And I'm passing it the window object, that's my global object, and
that's how we remember it right here, global.
And then what I'm doing is I'm setting up a name space for my utility.
So, this is ajax utils, and all it is is just an empty object.
So I can then attach things that I want to expose to the outside world.
And you can see the very first function here is not attached to my ajax utils.
Which means, that when I expose ajax utils, to the outside world,
meaning outside of this iifi.
This function will not be directly available to the user of ajaxUtils.
Okay, so let's go over what this function does, first of all.
Well, it says it returns an HTTP request object.
And what it does is it checks what type of object is available to us.
Now the very first if right here is the most
most important one because this is the most current object or
the most current version of the Ajax object there is.
The rest of them are kind of for the old browser, so window.ActiveXObject that's
something old Internet Explorer browsers used and really is not used anymore and
if you wanted to you could really probably take out this whole thing out of here.
Very few Microsoft Explorer browsers out there that are that old.
And the last thing is just, just in case there's a browser out there
that's going to access this that does not support Ajax at all,
which means this object will not be available.
Neither will this object be available.
We're going to go ahead and pop up a message,
an alert here on window object which is what this global is pointing to.
And we'll just say Ajax is not supported,
not necessarily the best message to return but it's good enough.
It's probably never going to happen, anyway.
And instead of the object, we'll just return null.
So point is, this is that object that we could use to make a request.
And we're creating here a new XML HTTP request object.
And this is what we're going to use in our main function that we're going to
write in a minute, we're going to see in a minute.
And the reason we really pulled it out into a separate function is because it's
just easier to put all these if statements in a separate place so
we don't have to deal with it right in line with everything else.
Okay, so let's take a look at the main function that we really,
really care about.
So that function is attached to ajaxUtils,
as you can see I'm defining it as ajaxUtils.sendGetRequest, and what we're
going to do in this function is we're going to send a GET request to the server.
So here's how this function is defined, there's the function right there.
And I can see that I really need to put actually a semi-colon at the end because
this is just an expression, right?
Expression of the function.
Okay, so let's see what of this function is expecting as parameters.
Number one is expecting the request URL.
Well, that makes sense.
We need to know where to request, where to go,
to make that get request on the server.
So that's the url.
And, we'll need a response handler function.
So after the server response, the response handler function
is what is going to handle the result of what the server returns.
Okay so those are the two arguments we expect to be passed into this function.
The first I'm going to do is I'm going to get that request object and
this is just that function we just spoke about this request object right here.
So we basically are going to get this new XML HttpRequest object.
So once we're done with that, once we got this request object,
we're saving it in this local variable.
And then we're setting this request object onreadystatechange to another function.
So onreadystatechange is different stages in the network communication
between the browser and the server, and we're going to see that in a minute.
So what we're doing in this function is we're calling this handleResponse, and
the handleResponse function is passed a request, this request object, and
has passed the response handler as well.
So when the server comes back with a response, this is the function that's
going to get called every time there's a change in communication state including
the very final one that we're completely ready, and here's your final response.
This is Is the function that's going to get called.
But we're not done setting up the request.
The other thing we need to do is actually go ahead and
make the open command, with a get request.
So that's the type of request we want, that's the method of our request.
We're going to pass it the URL, and we're going to make sure to say true here.
And if you pass false here, it will make this request a synchronous request,
which means the browser will freeze, and will wait for
the response before doing anything else.
And we don't want that, we want this request to be asynchronous, in other words
the second this request is made, we want the browser to continue operating.
So and the last step here is actually to go ahead and send the request.
So all of these lines right here were really just to set up the parameters of
the request, what it's going to be like, and this line right here,
the last line is actually the one executes the request and sends it to the server.
The reason we're passing a null here is because if this were a post request
our request parameters would not be part of the request URL, as you remember from
a previous lecture but it would actually be part of the body of the request.
And this is where you put the body of the request.
So if you wanted to have named value pairs for the request parameters,
you would put that string right here, or probably save it in an object and
then kind of paste that whole thing in here.
Now, let's take a look at our handle response malfunction that we
provided here.
The handle response function as you can see takes our original request object and
the response handler function that our client, meaning our user whoever is using
this whole library, this whole ajaxUtil library is providing for us.
So we're kind of doing some,
kind of nitty gritty work that they wouldn't have to do anymore.
Number one is, request that ready state,
we want to make sure there's four states in this request already.
Number one, we want to make sure we're in the last state, so it is ready to go.
We don't have ay other lower level network communication's going on.
And what they are is really not important but what is important is that you have to
check whether or not it's really straight forward.
That's number one and
number two is we want to check that our request.status is 200.
And this 200 is that response status code we spoke about previously.
So 200 means everything is fine and here comes your response.
And if those two conditions are true only then should we then take the response
handler, that is the function that the user of this library provided for us.
And then we could go ahead and pass it the request that the user of this library
will start pulling out of this request the response of the server.
So that is how this response handler is being used.
The last but not least is we want to expose this utility to global objects so
we can actually use it.
And what we're going to do is we're going to change the name of it a little bit,
we'll say global dot and we'll use the dollar sign, because we want to be all
fancy obviously, dollar sign ajaxUtils, so just like jQuery uses dollar sign,
we'll also use dollar sign as power of our variable.
So dollar sign ajaxUtils, that's what's going to be exposed to the global object,
is going to be equal to our ajaxUtils that is the local objects right here
in this immediately invoked function expression.
Now I hope you're wondering, at least a little bit,
as to why this crazy setup is being made right here.
Onreadystatechange, we're passing it a function expression, so
we're not executing this function.
We're just passing the value of this function.
Not the return value, but just the value of the function object, and
we're setting it to this property onreadystatechange.
And inside of that, we're calling handleResponse, and this
is what is going to be executed, since you can see the parenthesis around it.
Why not simply erase this and happily call, I'm not going to
be able to fit in on one line here, but happily call this function right here?
Why not have onreadystatechange equal to handleResponse?
Well first of all, we need to equal it to a function value,
not the return of a function, so this will have to go away.
You can't really pass parameters into a functional value.
Only when you're executing a function, can you pass parameters into it.
And we're not quite ready to execute this function, we just want its value.
Well when we did that, this function right here,
wouldn't really have the request, or the response handler available.
Well, one way we could take care of it, and unfortunately, and you'll see why I'm
saying unfortunately, a lot of people do take care of it, is by doing this.
We can just take this line out of here, and make it a little bit more global.
And now our request is available everywhere.
So it doesn't have to be part of these parameters, part of these arguments.
We could just pull it out of the global part, outside of this function.
And obviously we need to take care of this as well, to make it a valid syntax.
So let's take a look as to what we've done.
What will happen now?
Let's say somebody on the page clicks some button that invokes this send-get request.
This will go ahead and
use the request that already has been done right here which it seems pretty good.
We'll assign handleResponse to our onreadState change,
that's that function and I go ahead and make the request.
The only thing is this handle response also really wants, at least in our case,
wants the response handler function and we're not passing that.
But that's easy to remedy by just saying var myhandle response or
myhandler is equal to, and we will just equal it to this, right?
So now to do that, we could just say this is equal to null.
And once we're done, we could just say myhandler = responseHandler.
Okay, so now it's all complete.
So at this point this handler function, handleResponse,
we'll be able to get the request and will also be able to get the response handler.
So we really don't have to have these arguments this function.
The problem is is that this whole thing is being called asynchronously.
Remember what asynchronous means?
It means that more than one thing could be executing at a time.
Which means once we send this request off, and the server is waiting to respond,
it could wait two, three seconds before it responds.
Well meanwhile, either the user or
some other JavaScript process can call this again, and if it calls it again,
it's going to end up using the same request object, as we use for the previous
request and therefore they will get crisscrossed get confused and the response
you're going to get is probably not the response that you are waiting for.
And what you will have is what is called in computer science a race condition.
In other words there is some code that is going to be erasing another piece of code
and whoever wins is going to over ride this with its data.
But the second one will never be able to get to it,
it will give you the wrong data.
So this is really going to be a gigantic mess, so let's go ahead and
do a bunch of undoes.
And put it back the way it was.
So now we're ready to see how we're actually going to
use this library in our script.gs.
And that's what we're going to do in part three of this lecture.
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