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Accessing the DOM from WebAssembly

General architecture

Create pascal units, containing ‘proxy’ classes: calling a method on a proxy class will call the corresponding class in JS. The proxy classes can be generated by adapting the existing webidl2pas tool.

Problem: data

JS/Webassembly interface only supports passing integers & floats, not objects.

solution: Every object is stored in an array with ID ID is used to pass references to object between JS and Webassembly Lifetime is controlled from WebAssembly. By using interfaces, the lifetime of objects can be controlled by compiler. Methods can be called using an invoke mechanism. Due to limited type support in Javascript, only a handful of types must be supported by invoke.

Problem: Callbacks

An event handler in WebAssembly must be callable from Javascript. The AddEventListener has a single method signature, so a single exported function from webassembly can be used for this:

all that is needed is to pass the object pointer & method pointer (both integers), plus the ID of the event object. Pointers to methods & instances can be passed between JS and webassembly, this can be used. Alternatively: Using the FPC dispatchstr mechanism, the correct method can be called in Webassembly. To be checked.

Advantage of this method is that only a couple of webassembly and Javascript exports are needed.

Implementation details

Here are some technical notes describing the various architectural decisions.

A tool is created to generate an interface from the .webidl files. These files for example exist in the mozilla firefox repo on github: WebIDL

IElement = interface ['someawfulGUID'] (IJSObject);
  function childElementCount : Integer;
  function firstElementChild : IElement;
  // all other

Only the interfaces are exposed in the API to access the DOM.

In implementation, the following kind of code can be found:

// Hand crafted in e.g. JSObject unit
TJSObject = class(TInterfacedObject)
  constructor CreateFromID(aID: NativeInt);
  destructor destroy; override;
  Property ObjectID : NativeInt;
  function InvokeJSNativeIntResult(aName : string; Const args : Array of const) : NativeInt;
  function InvokeJSStringResult(aName : string; Const args : Array of const) :  String;
  function InvokeJSObjResult(aName : string; aResultClass: TDOMOBjectClass; Const args : Array of const) : TDDOMOBject;

The various Invoke* functions encode the arguments in a memory block so they can be read on the JS side, then calls a JSInvokeNNN function which lives in Javascript, and which is imported from the browser.

That function does the actual call: it uses ObjectID to look for the Self object in an array:

  • Negative IDs are special: window, document.
  • positive IDs use a wasmObjects['id'] to look for the object.

The Invoke* function decodes the arguments and uses TJSFunction.apply to execute the requested function. The result is put in a memory block, encoded in the same way as incoming arguments.

If the result is an object, an ID is generated (simple counter), the result value is stored in the array wasmObjects['id'] .

The ID is returned to the webassembly, which will use the ID to create a TJSObject descendent.

The destructor of TJSObject calls a releaseObject function in javascript if the ObjectID is positive. The releaseObject function simply sets wasmObjects['id'] to null. (so the browser also releases it)

The above is a basic invoke mechanism for Javascript code.

This basic mechanism is then used by a modified version of the webidl program to generate proxy definitions. For each object in Javascript, 2 definitions are generated:

  • The interface (see above for an example)
  • An implementation object as below, descendent of TJSObject
// Generated from webIDL in jsweb/jsdom unit.
TElementImpl = class(TJSObject,IElement)
  function childElementCount : Integer;
  function firstElementChild : IElement;
  // all other
function TElementImpl.childElementCount : Integer;
function TElementImpl.firstElementChild : IElement;
  Result:=InvokeJSObjResult('firstElementChild',TElementImpl,[]) as IElement;