ezjson - JSON serialize / deserialize with attributes

[mr-highball]

Hey everyone, hope everyone had a good start to the new year 
The past few days I've been working on JSON serialization / deserialization using custom attributes. These work by attaching two types of decorators to either your classes / interfaces or to properties of classes / interfaces.
Each decorator simply takes a name to be used when when serializing

JsonObject('objectName')
JsonProperty('propertyName')

Once you have your decorated objects, you can then call one of two generic methods,
EZSerialize<T>(...) (outputs json)
EZDeSerialize<T>(...) (input json)

An example is worth a 1000 words, so here you go,

Code: Pascal  [Select][+][-]

1.   { TTestDecorated }
2.   (*
3.     a simple object decorated with a custom name to be used when serializing and
4.     deserializing, as well as some simple typed properties to include in
5.     serialization
6.   *)
7.   [JsonObject('myTestObject')]
8.   TTestDecorated = class(TObject)
9.   private
10.     FTest : String;
11.     FTestInt : Integer;
12.   published
13.     [JsonProperty('test')]
14.     property Test : String read FTest write FTest;
15.  
16.     [JsonProperty('testInteger')]
17.     property TestInt : Integer read FTestInt write FTestInt;
18.   end;
19.  
20.   ...
21.  
22. procedure TestSimple;  
23. var
24.   LTest : TTestDecorated;
25.   LJSON,
26.   LError : String;
27. begin
28.   //setting a value on our test
29.   LTest := TTestDecorated.Create;
30.   LTest.Test := 'a test';
31.  
32.   //calling serialize
33.   if not (EZSerialize<TTestDecorated>(LTest, LJSON, LError)) then
34.     raise Exception.Create(LError) //failed to serialize
35.   else
36.     WriteLn(LJSON); //success
37. end;
38.  

the output from above is below,
{ "myTestObject" : { "test" : "a value", "testInteger" : 0 } }

Nested object or interface properties can also be used in conjunction with standard types, here's another example (minus the calling code for brevity)

Code: Pascal  [Select][+][-]

1. TTestComplex = class(TObject)
2.   private
3.     FDecorated: TTestDecorated;
4.     FNonDecorated: TTestNonDecorated;
5.   public
6.     constructor Create;
7.     destructor Destroy; override;
8.   published
9.     [JsonProperty('decorated')]
10.     property Decorated : TTestDecorated read FDecorated;
11.  
12.     [JsonProperty('nonDecorated')]
13.     property NonDecorated : TTestNonDecorated read FNonDecorated;
14.   end;
15.  

and here's the output when serialized,

{ "TestComplex" : { "decorated" : { "test" : "a value", "testInteger" : 0 }, "nonDecorated" : { "test" : "" } } }

I'm actively working on this, and right now only serialization has been implemented (still working on deserialize  ) but I thought someone might find some use or could provide a second set of eyes.

here's the repo, https://github.com/mr-highball/ezjson

Cheers 🍻

[mr-highball]

running into some rtti issues, that are stalling finishing this library up reported here -> https://bugs.freepascal.org/view.php?id=36651

serialize "should" be ok, and deserialize is in a rough draft state but isn't properly working (either misunderstanding on my part or bug)

the bug report has my test cases and descriptions of each,
if anyone can help out and look over that would be nice

[mr-highball]

Shoutout to @Pascaldragon for helping me with this and for pointing out my wrong usage of typeinfo method 

Pushed a new commit which seems to be working for both serializing and deserializing. Deserializing code is currently on written for standard types (not objects / interfaces) but support for that will be added soon now that I have a path forward.

[Slawek]

Hi,
I noticed you are using JsonObject and JsonProperty in your class like [JsonObject('myTestObject')] and [JsonProperty('test')].
But in Lazarus I can't compile it.
I am getting a syntax error: Fatal: Syntax error, "IMPLEMENTATION" expected but "[" found
What does the compiler need to accept this syntax?
Regards, Slawek

[mr-highball]

Hi @slawek, first if you're trying to use the ezjson decorators you'd need to git clone my repo and add the src path to your project "other units". If you're just trying to play with custom attributes you'd need the latest fpc/lazarus since they were a new addition.
https://wiki.freepascal.org/Custom_Attributes

Hope this helps!

[Slawek]

Hi,
Thank you for the quick reply. I'm just learning JSON in Lazarus. I have Lazarus 2.0.10 with FPC 3.2.0 64bit. Revision of SVN 63526. Date 07-07-2020.
I found this article first: https://community.idera.com/developer-tools/programming-languages/f/delphi-language/69467/json---converting-array-of-custom-records-to-json-text
I used the {$modeswitch prefixedattributes} directive, but that doesn't work either.

[mr-highball]

That version should be new enough, but since ezjson is non-standard, are you certain you have pulled down the git repo and included it in the search path? Also, please post a bit of code so I can look over your syntax and try it locally

[Slawek]

I haven't used ezjson yet.
I just wanted to compile sample programs from https://wiki.freepascal.org/Custom_Attributes
Unfortunately, I get errors like:
tcustomattr.lpr(4,2) Warning: Illegal compiler switch "PREFIXEDATTRIBUTES+"
tcustomattr.lpr(7,25) Error: Identifier not found "TCustomAttribute"
tcustomattr.lpr(7,41) Error: class type expected, but got "<erroneous type>"
tcustomattr.lpr(15,4) Fatal: Syntax error, "BEGIN" expected but "[" found
testattributes.lpr(19,31) Error: Identifier not found "TCustomAttribute"
testattributes.lpr(19,47) Error: class type expected, but got "<erroneous type>"
testattributes.lpr(38,4) Fatal: Syntax error, "BEGIN" expected but "[" found

I also have FPC 3.2.0 on Linux (Fedora 32) and I get the same errors as on Windows.

Thought you could help me since you use these attributes. I still don't understand why it doesn't work for me.

[PascalDragon]

I also have FPC 3.2.0 on Linux (Fedora 32) and I get the same errors as on Windows.

Custom attributes are only available in the development version 3.3.1.

[Slawek]

Yes, I just found this information too. Thanks for your confirmation.

[PascalDragon]

I have now highlighted that in the wiki article you linked.

[mr-highball]

Ahh sorry was misunderstanding. We'll for getting the latest versions of fpc/lazarus a very easy tool is fpcupdeluxe (I use it all of the time)
https://github.com/LongDirtyAnimAlf/fpcupdeluxe

Just go to the "releases" tab and select the version for your OS and with a few clicks you can have a separate installation for trunk.