Preface
Due to the characteristics of the development model of Sencha Touch 2, its native data interaction behavior can almost only be achieved through AJAX.
Of course, by calling the powerful PhoneGap plug-in and then packaging it, you can achieve 100% Socket communication and local database functions, or you can also achieve communication with the server and server push functions through HTML5 WebSocket. However, both methods have their limitations. The former requires PhoneGap support, and the latter requires that the user device must support WebSocket. Therefore, neither can be regarded as a native solution for ST2. The only native one is AJAX.
When it comes to AJAX, we will inevitably face two questions. The first is what format does AJAX use to exchange data? The second is how to solve cross-domain needs? There are currently different solutions to these two problems. For example, data can be described with custom strings or XML, and cross-domain problems can be solved through server-side proxies.
But so far, the most recommended or preferred solution is to use JSON to transmit data and rely on JSONP to cross domains. And that’s what this article is about.
Although there is only one letter difference between JSON (JavaScript Object Notation) and JSONP (JSON with Padding), they are actually not the same thing at all: JSON is a data exchange format, and JSONP is a dependency An unofficial cross-domain data exchange protocol created by the ingenuity of developers. Let's use the recent popular spy movie as an analogy. JSON is the "code" used by underground parties to write and exchange information, while JSONP is the connection method used to transmit information written in code to their comrades. Did you see that? One is to describe the format of the information, and the other is the method agreed upon by both parties for transmitting the information.
Since we are just chatting, we will no longer use a dogmatic way to tell it, but focus on helping developers understand whether they should choose to use it and how to use it.
What is JSON
As briefly mentioned before, JSON is a text-based data exchange method, or data description format. Should you choose it first? Definitely pay attention to the advantages it has. 
Advantages of JSON:
1. Based on plain text, cross-platform transmission is extremely simple;
2. Javascript native support, background Almost all languages are supported;
3. Lightweight data format, occupying very few characters, especially suitable for Internet transmission;
4. Strong readability, although not as good as XML It is clear at a glance, but it is still easy to identify after reasonable indentation;
5. Easy to write and parse, of course, the premise is that you need to know the data structure;
Of course, there are also disadvantages of JSON, But in the author's opinion, it is really insignificant, so I won't explain it separately.
JSON format or rules:
JSON can describe the data structure in a very simple way. It can do everything XML can do, so it is cross-platform The two are completely indistinguishable from each other.
1. JSON has only two data type descriptors, curly brackets {} and square brackets []. The remaining English colons are mapping characters, English commas are delimiters, and English double quotes "" are delimiters. .
2. Curly brackets {} are used to describe a set of "different types of unordered key-value pair sets" (each key-value pair can be understood as an OOP attribute description), and square brackets [] are used to describe A set of "ordered data collections of the same type" (which can correspond to OOP arrays).
3. If there are multiple sub-items in the above two sets, they should be separated by commas.
4. The key-value pairs are separated by English colon:, and it is recommended that the key names be added with English double quotes "" to facilitate the analysis of different languages.
5. Commonly used data types within JSON are nothing more than strings, numbers, Boolean, dates, and null. Strings must be enclosed in double quotes, and the rest are not used. The date type is special. Here is I won’t go into details, but I just suggest that if the client does not have the function requirement of sorting by date, then just pass the date and time directly as a string, which can save a lot of trouble.
JSON example:
What is JSONP
Let’s first talk about how JSONP was generated: 
In fact, there are many explanations about JSONP on the Internet, but they are all the same and vague. It is a bit difficult for many people who are new to it to understand. It is not a small thing, but I try to explain this problem in my own way. See if that helps.
1. A well-known problem, Ajax direct request for ordinary files has the problem of cross-domain unauthorized access. Regardless of whether you are a static page, dynamic web page, web service, or WCF, as long as it is a cross-domain request, it is not allowed;
2. However, we also found that when calling js files on a Web page, it is not affected by whether it is cross-domain (not only that, we also found that all tags with the "src" attribute have cross-domain capabilities, such as < ;script>, ,
