source: rtos_arduino/trunk/arduino_lib/libraries/Milkcocoa_Arduino_SDK/aJson/README.md@ 144

  aJson v1.0
================
  Copyright (c) 2010, Interactive Matter, Marcus Nowotny

  Based on the cJSON Library, Copyright (C) 2009 Dave Gamble

  Permission is hereby granted, free of charge, to any person obtaining a copy
  of this software and associated documentation files (the "Software"), to deal
  in the Software without restriction, including without limitation the rights
  to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
  copies of the Software, and to permit persons to whom the Software is
  furnished to do so, subject to the following conditions:

  The above copyright notice and this permission notice shall be included in
  all copies or substantial portions of the Software.

  THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
  IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
  FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
  AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
  LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
  OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
  THE SOFTWARE.

 Welcome to aJson.
================

aJson is the attempt to port a complete JSON implementation to Arduino. It is based on the cJSON implementation, reduced in size and removing one or two features:

- The code has very limited support on ATmega168 - there is just not enough memory and 
  memory fragmentation is a serious problem
- Arrays and Lists are max 255 elements big
- There is no proper Unicode handling in this code
- There is an internal buffer eating up 256 bytes of ram

Most of the limitation will be gone in one of the future releases.

JSON is described best here: http://www.json.org/
It's like XML, but fat-free. You use it to move data around, store things, or just
generally represent your program's state.
JSON is especially useful to exchange data efficiently with e.g. JavaScript, Java, C++,
Processing or anything else

aJson is a library to receive, understand, create or modify JSON strings directly in the 
Arduino. JSON is quite a standard, so that is perfect for exchanging data with other 
applications. I combination with HTTP it is suitable to implement REST Web Services.

aJson provides functions to parse JSON strings to object models. Handle, search and
create and modify JSON Object structures.

This is some JSON from this page: http://www.json.org/fatfree.html

```javascript
{
    "name": "Jack (\"Bee\") Nimble", 
    "format": {
        "type":       "rect", 
        "width":      1920, 
        "height":     1080, 
        "interlace":  false, 
        "frame rate": 24
    }
}

```

Parsing JSON
================

To parse such a structure with aJson you simply convert it to a object tree:

```c
aJsonObject* jsonObject = aJson.parse(json_string);
```

(assuming you got the JSON string in the variable json_string - as a char*)

This is an object. We're in C. We don't have objects. But we do have structs.
Therefore the objects are translated into structs, with all the drawbacks it brings.s

Now we can e.g. retrieve the value for name:

```c
 aJsonObject* name = aJson.getObjectItem(root, "name");
```

The value of name can be retrieved via:

```c
 Serial.println(name->valuestring);
```

Note that the aJsonObject has a union which holds all possible value types as
overlays - you can get only useful data for the type which you have at hand. You can get
the type as

```c
 name->type
```

which can be either aJson_False, aJson_True, aJson_NULL, aJson_Number, aJson_String, aJson_Array
or aJson_Object. For aJson_Number you can use value.number.valueint or value.number.valuedouble, for aJson_String 
you can use value.valuestring, for True or False, you can use value.valuebool.

To render the object back to a string you can simply call

```c
 char *json_String=aJson.print(jsonObject);
```

Finished? Delete the root (this takes care of everything else).

```c
 aJson.deleteItem(root);
```

This deletes the objects and all values referenced by it.

Parsing streams
--------------

As you can see this will eat up lots of memory. Storing the original string and the JSON object is a bit too much 
for your Arduino - it will most likely use up all the memory. Therefore it is better to parse streams instead of strings.
A stream in C is a FILE* - on Arduino there are some special streams, but later adapters will be provided.
So if you for example read from a FILE* stream you can simply call
   
```c
 aJsonObject* jsonObject = aJson.parse(file);
```
 
By that you will not have to store the JSON string in memory.

Filtering while parsing
--------------

Any JSON respond can have object name/value pairs your code either does not understand or is not interested in.
To avoid those values going into your memory you can simply add filters to your parsing request.
A set of filter is just a list of names you are interested in, ended by a null value. If you are 
only interested in "name", "format", "height" and "width" in the above example you can do it like:

```c
char** jsonFilter = {"name,"format","height","width",NULL};
 aJsonObject* jsonObject = aJson.parse(json_string,jsonFilter);
```
(assuming you got the JSON string in the variable json_string - as a char*)

By that only the following structure is parsed - the rest will be ignored:

```javascript
{
    "name": "Jack (\"Bee\") Nimble", 
    "format": {
        "width":      1920, 
        "height":     1080, 
    }
}
```

It is good practice to always use the filtering feature to parse JSON answers, to avoid unknown objects swamping your
memory.

Creating JSON Objects from code
================

If you want to see how you'd build this struct in code?

```c
        aJsonObject *root,*fmt;
        root=aJson.createObject();      
        aJson.addItemToObject(root, "name", aJson.createItem("Jack (\"Bee\") Nimble"));
        aJson.addItemToObject(root, "format", fmt = aJson.createObject());
        aJson.addStringToObject(fmt,"type",             "rect");
        aJson.addNumberToObject(fmt,"width",            1920);
        aJson.addNumberToObject(fmt,"height",           1080);
        aJson.addFalseToObject (fmt,"interlace");
        aJson.addNumberToObject(fmt,"frame rate",       24);
```

The root object has: Object Type and a Child
The Child has name "name", with value "Jack ("Bee") Nimble", and a sibling:
Sibling has type Object, name "format", and a child.
That child has type String, name "type", value "rect", and a sibling:
Sibling has type Number, name "width", value 1920, and a sibling:
Sibling has type Number, name "height", value 1080, and a sibling:
Sibling hs type False, name "interlace", and a sibling:
Sibling has type Number, name "frame rate", value 24

If you want to create an array it works nearly the same way:
 
```c
  aJsonObject* root = aJson.createArray();

  aJsonObject* day;
  day=aJson.createItem("Monday");
  aJson.addItemToArray(root, day);
  day=aJson.createItem("Tuesday");
  aJson.addItemToArray(root, day);
  day=aJson.createItem("Wednesday");
  aJson.addItemToArray(root, day);
  day=aJson.createItem("Thursday");
  aJson.addItemToArray(root, day);
  day=aJson.createItem("Friday");
  aJson.addItemToArray(root, day);
  day=aJson.createItem("Saturday");
  aJson.addItemToArray(root, day);
  day=aJson.createItem("Sunday");
  aJson.addItemToArray(root, day);
```


The whole library (nicely provided by cJSON) is optimized for easy usage. You can create and modify
the object as easy as possible.

aJson Data Structures
================

aJson stores JSON objects in struct objects: 

```c
// The aJson structure:
typedef struct aJsonObject {
        char *name; // The item's name string, if this item is the child of, or is in the list of subitems of an object.
        struct aJsonObject *next, *prev; // next/prev allow you to walk array/object chains. Alternatively, use GetArraySize/GetArrayItem/GetObjectItem
        struct aJsonObject *child; // An array or object item will have a child pointer pointing to a chain of the items in the array/object.

        char type; // The type of the item, as above.

        union {
                char *valuestring; // The item's string, if type==aJson_String
                char valuebool; //the items value for true & false
                int valueint; // The item's number, if type==aJson_Number
                float valuefloat; // The item's number, if type==aJson_Number
        };
} aJsonObject;
```

By default all values are 0 unless set by virtue of being meaningful.

Note that the aJsonObject has a union 'value' which holds all possible value types as
overlays - you can get only useful data for the type which you have at hand. You can get
the type as

```c
 name->type
```

which can be either aJson_False, aJson_True, aJson_NULL, aJson_Number, aJson_String, aJson_Array
or aJson_Object. For aJson_Number you can use value.number.valueint or value.number.valuedouble. 
If you're expecting an int, read valueint, if not read valuedouble. For aJson_String 
you can use value.valuestring, for True or False, you can use value.valuebool. 

next/prev is a doubly linked list of siblings. next takes you to your sibling,
prev takes you back from your sibling to you.
Only objects and arrays have a "child", and it's the head of the doubly linked list.
A "child" entry will have prev==0, but next potentially points on. The last sibling has next=0.
The type expresses Null/True/False/Number/String/Array/Object, all of which are #defined in
aJson.h

Any entry which is in the linked list which is the child of an object will have a "string"
which is the "name" of the entry. When I said "name" in the above example, that's "string".
"string" is the JSON name for the 'variable name' if you will.

Now you can trivially walk the lists, recursively, and parse as you please.
You can invoke aJson.parse to get aJson to parse for you, and then you can take
the root object, and traverse the structure (which is, formally, an N-tree),
and tokenise as you please. 

Lists in aJson
================

Lists are easily handled in aJson, to create a list you can simply use the provided API functions:
 
```c
 aJson.create<TYPE>Array(objects,24);
```

You simply pass a array of the respective type: char*[], int[] and so on.

aJSON doesn't make any assumptions about what order you create things in.
You can attach the objects, as above, and later add children to each
of those objects with
 
```c
 aJson.addItemToArray()
```

or remove them with 

```c
 aJson.deleteItemFromArray() - which also deletes the objects, or
 aJson.detachItemFromArray() - which does not free the memory
```

As soon as you call aJson.print(), it renders the structure to text.


Have Fun!