How to parse nested JSON object in Delphi XE2? - json

Question

I'm new to JSON and I have this project on my hands that require me to parse a JSON and display some of its contents in a ListView. The problem is that the documentation I've read by now dealt with JSON objects containing JSON arrays, while my case involves dealing with nested objects. To cut the story short, here's the summary: I'm using Delphi XE2 with DBXJSON. I post some values to a server and it replies with a JSON object that looks like that:
{
"products": {
"Men's Sneakers": {
"instock": false,
"size": "423",
"manufacturer": "Adidas",
"lastcheck": "20120529"
},
"Purse": {
"instock": true,
"size": "not applicable",
"manufacturer": "Prada",
"lastcheck": "20120528"
},
"Men's Hood": {
"instock": false,
"size": "M",
"manufacturer": "Generic",
"lastcheck": "20120529"
}
},
"total": 41,
"available": 30
}
What I wanted to achieve was to have each item (i.e. Purse) parsed and added as caption in a listview, along with one subitem (manufacturer). I created a procedure that takes the JSON string as argument, created the JSON object, but I don't know how to parse the nested objects any further.
procedure TForm1.ParseString(const AString: string);
var
json : TJSONObject;
jPair : TJSONPair;
jValue : TJSONValue;
jcValue : TJSONValue;
l,i : Integer;
begin
json := TJSONObject.ParseJSONValue(TEncoding.ASCII.GetBytes(AString),0) as TJSONObject;
try
//get the pair to evaluate in this case the index is 1
jPair := json.Get(1);
{further process the nested objects and adding them to the listview}
finally
json.Free;
end;
end;
Any suggestions would be highly appreciated. Lost quite some time trying to get the ins and outs of JSON in Delphi with no avail.
Thanks,
sphynx

Answer 1

Try this sample
{$APPTYPE CONSOLE}
{$R *.res}
uses
DBXJSON,
System.SysUtils;
Const
StrJson=
'{'+
' "products": {'+
' "Men''s Sneakers": {'+
' "instock": false,'+
' "size": "423",'+
' "manufacturer": "Adidas",'+
' "lastcheck": "20120529"'+
' },'+
' "Purse": {'+
' "instock": true,'+
' "size": "not applicable",'+
' "manufacturer": "Prada",'+
' "lastcheck": "20120528"'+
' },'+
' "Men''s Hood": {'+
' "instock": false,'+
' "size": "M",'+
' "manufacturer": "Generic",'+
' "lastcheck": "20120529"'+
' }'+
' },'+
' "total": 41,'+
' "available": 30'+
'}';
procedure ParseJson;
var
LJsonObj : TJSONObject;
LJPair : TJSONPair;
LProducts : TJSONValue;
LProduct : TJSONValue;
LItem : TJSONValue;
LIndex : Integer;
LSize : Integer;
begin
LJsonObj := TJSONObject.ParseJSONValue(TEncoding.ASCII.GetBytes(StrJson),0) as TJSONObject;
try
LProducts:=LJsonObj.Get('products').JsonValue;
LSize:=TJSONArray(LProducts).Size;
for LIndex:=0 to LSize-1 do
begin
LProduct := TJSONArray(LProducts).Get(LIndex);
LJPair := TJSONPair(LProduct);
Writeln(Format('Product Name %s',[LJPair.JsonString.Value]));
for LItem in TJSONArray(LJPair.JsonValue) do
begin
if TJSONPair(LItem).JsonValue is TJSONFalse then
Writeln(Format(' %s : %s',[TJSONPair(LItem).JsonString.Value, 'false']))
else
if TJSONPair(LItem).JsonValue is TJSONTrue then
Writeln(Format(' %s : %s',[TJSONPair(LItem).JsonString.Value, 'true']))
else
Writeln(Format(' %s : %s',[TJSONPair(LItem).JsonString.Value, TJSONPair(LItem).JsonValue.Value]));
end;
end;
finally
LJsonObj.Free;
end;
end;
begin
try
ParseJson;
except
on E: Exception do
Writeln(E.ClassName, ': ', E.Message);
end;
Readln;
end.
This will return
Product Name Men's Sneakers
instock : false
size : 423
manufacturer : Adidas
lastcheck : 20120529
Product Name Purse
instock : true
size : not applicable
manufacturer : Prada
lastcheck : 20120528
Product Name Men's Hood
instock : false
size : M
manufacturer : Generic
lastcheck : 20120529

Answer 2

This site describes the type TJSONValue in more detail. If your data is an object, it will have the type TJSONObject, so check its API to see how to proceed.
I believe the first thing you need it to iterate over its pairs (use GetEnumerator if you don't know the key names, otherwise just use the overloaded Get - passing a string instead of a number). For each pair, the key will be a simple string (type TJSONString) and the value may be any TJSONValue. Repeat until you reach the leaves.
Example:
products := jPair.Get('products');
purse := products.GetJsonValue().Get('Purse');
purseManuf := purse.GetJsonValue().Get('manufacturer');
...
Or if you don't know what the products are:
products := jPair.Get('products');
for prodPair in products.GetEnumerator() do
begin
prodName := prodPair.GetJsonString();
prodObj := prodPair.GetJsonValue();
...

Answer 3

This blog post shows a very modern and simple way to convert JSON:
uses REST.JSON; // Also new System.JSON
procedure TForm1.Button1Click(Sender: TObject);
var
Foo: TFoo;
begin
Foo := TFoo.Create;
try
Foo.Foo := 'Hello World';
Foo.Fee := 42;
Memo1.Lines.Text := TJson.ObjectToJsonString(Foo);
finally
Foo.Free;
end;
Foo := TJson.JsonToObject<TFoo>(Memo1.Lines.Text);
try
Foo.Fee := 100;
Memo1.Lines.Add(TJson.ObjectToJsonString(Foo));
finally
Foo.Free;
end;
end;