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1 Introduction

Inspired by the idea that was present in AngularJS, Trapped is an Amber library that creates bidirectional data-binding between elements in HTML page (the view side) and a data object (the model side).

Similarly to AngularJS, you write expressions specifying what data to bind to and how to transform it directly in HTML in a data-trap attribute, and changes to those data are automagically communicated there and back.

2 Quick jump in

Easy way (but you cannot save and reload with new code):

Visit the classic Counter example at or simple Todo similar to one in AngularJS page at

Hard way (but you can save and reload with new code):

Clone this repo. Run bower install to get amber, which is a dependency. Get the cli to start amber server: npm -g install amber-cli. Then start the server: amber serve from the project root directory. It starts on port 4000. Visit http://localhost:4000/example-counter/counter.html (similarly for Todo example) in your browser. Page opens, dialog to open IDE appears.

3 Big picture

Trapped is closest to the MVVM variant of MVC. MVVM splits application into Model (the application logic and data loaded / saved from external sources), View (the presentation facilities, not containg any application logic) and ViewModel (the subset of data that is going to be presented at any point of time).

Trapped fills the place of the View and the ViewModel (View being synchronized with ViewModel bidirectionally), leaving you to build your Model any way you wish and abstracting presentation details and user events away from you by letting the Model just observe and manipulate contents of the ViewModel.

More precisely:

  1. ViewModel. You provide the class for ViewModel (TrappedCounter / TrappedTodo in the examples). You can build it any way you wish, it has to be able to hold all the data ViewModel may need to hold. It can also hold some methods for manipulation of these data, which is good for the example, but in real project this should be the responsibility of Model.
  2. anApp start. You should set up the blackboard in app startup method. (CounterApp >> start / TodoApp >> start in the examples). Trapped uses Axxord to implement a blackboard pattern - in which many external observers (called specialists) observe the data object (called blackboard), make partial changes and react to them. To do that, you must send axxord: to ViewModel instance with an instance of Axon (a blackboard subscription manager) as an argument. Elements of View are observers of the ViewModel blackboard, as should be the parts of the Model (this way, Model and View are completely decoupled and both see only changes to the blackboard, that is, the ViewModel). In Todo example, one trivial specialist is created as well, which observes for changes in todos and updates their remaining number. In real world, it should be in Model with other specialists.
  3. index.html. You write HTML and annotate the elements with the data-binding expressions (attribute data-trap) in which you describe what data to bind to (path) and how to process it in the way to the user or from the user (processors).
  4. App package. Since you will likely use some non-trivial processors in HTML, (or in code via API which is also possible), your app package should import 'trapped/Trapped-Processors'.
  5. anApp start. When initializing the page, you must call Trapped start: anArray where anArray should contain instances of all blackboards (most often you will only have one, viewModel in the examples) that are intended to be used (Trapped start: { viewModel } final statement of start method in the examples).
  6. model From that point on, you should only modify or watch data of the blackboard using Axxord API. For the example, try this is Counter example: (Trapped current byName: 'TrappedCounter') axes: #((value)) transform: [ :old | console log: old. 2 * old ] or this in the Todo example: (Trapped current byName: 'TrappedTodo') axes: #((todos) 1 done) transform: [ :state | state not ]. Of course, in real app, the model would be passed the view model instance and use it directly, not using Trapped >> byName: to read its blackboard registry.

4 data-trap attribute

All data binding and processing info for an HTML element is contained in an attribute data-trap.


It consists of several statements separated by dot (.).

Each statement consists of several expressions, separated by colon (:).

The first expression is called path. It describes the location of the data the expression is processing, relative to actual position, similarly to how relative paths work in file systems.

The second expression is the processing chain. It describes the series of transformations the data goes through in its way to the view or back. If not present, the default contents is used.

The role of other expressions is not defined in the moment.

All expressions represent the array of strings, numbers or sub-expressions and have same syntax: they consists of series of space-separated strings (without reserved characters), whole numbers (float ones use decimal dot, which is used as delimiter), or a sub-expression within parentheses (()).

The syntax is resembling Smalltalk literal array syntax very closely - so similar or same expressions can be used in API in code as well as in data-trap attribute. For example paths #((value)) and #((todos) 1 done) from API examples in previous chapter could be written in data-trap as '(value)' and '(todos) 1 done'.

Syntactic sugar: as (foo) happens often in data-trap expressions, it can be written equivalently as ~foo, to improve readability. So above paths would likely be written ~value and ~todos 1 done instead.

Semantics of path

! This section is common to data-trap paths and in-code trapped paths !

The Trapped data-path is an array of elements: either strings, numbers or a sub-arrays. These are used to denote the (relative) location of a piece of data in a Trapped blackboard, and is used to read or write from / to this position.

Elements of a path are equivalent to elements of paths in classic file systems: each elements is one step deeper in a tree hierarchy. Thus, to read a data denoted by a path, Trapped starts from actual position, reads the contents denoted by first element, use the result to read the contents denoted by second elements etc. until the end. To write the data, the algorithm is similar to reading one, byt the last element is used to write the data instead.

  • if string path element is read from foo, foo at: aString is performed;
  • if string path element is written to foo, foo at: aString put: value is performed;
  • if number path element is read from foo, foo at: aNumber is performed;
  • if number path element is written to foo, foo at: aNumber put: value is performed;
  • if subarray path element #(bar) is read from foo, foo bar is performed;
  • if subarray path element #(bar) is written to foo, foo bar: value is performed.

In addition, these operation are error-tolerant - if any data in the path is nil, selectors do not exist, indexes are out of bounds etc., the result of the whole expression is nil.

So, the blackboard modify: #((todos) 1 done) do: [ :state | state not ] example from previous chapter essentially does

| x |
x := blackboard todos at: 1.
x at: 'done' put: (x at: 'done') not

Plus, of course, all the bookkeeping of the blackboard.

Semantics of processing chain

The Trapped processing chain descriptor is an array of elements: either strings, numbers or a sub-arrays. These are used to describe the transformations and operations that happen with a piece of data in the way from view-model to a view or back.

The descriptor describes the way how a processing elements in a chain are created (which ones, and with what parameters). These then process the data (see TrappedProcessor class). These elements are created by sending a message to the factory object. In the moment, TrappedProcessor class itself serves as a factory.

  • if string processing element description foo occurs, aFactory foo is used to create processing element;
  • using number processing element description is useless and produces an error (currently silent);
  • if one-element array processing element description (foo) occurs, aFactory foo is used to create processing element;
  • if even-element array processing element description (foo something bar anotherThing) occurs, aFactory foo: something bar: anotherThing is used to create processing element, the odd elements representing the keywords of the message, the even elements representing the arguments (which can be string, number or array);
  • using odd-element array processing element description other than one-element is useless and produces an error (currently silent).

Note: thus, in processing chain descriptors, contents, ~contents and (contents) describe the same one-element processing chain where the lone processing element is created inside Trapped by running TrappedProcessor contents.

Another example: (signal increment) whenClicked from counter example describes two-element processing chain, with first element created by TrappedProcessor signal: 'increment' and the second one by TrappedProcessor whenClicked. All mentioned methods (contents, signal: and whenClicked are factory methods that create instance of appropriate processor class, subclass of TrappedProcessor).

5 Processing chain

The elements created as described above are used in sequence to process data. This includes not only transforming, but also reading/writing it from/to DOM element (contents writes data to TagBrush via contents:), reading/writing from/to blackboard and other bookkeeping.

Data-binding chain

The processing chain that is data-binding (contains certain elements that switch on databinding, contents being one of them), observes a blackboard at the position given in path.

If a data changes in blackboard, the event is queued and the data is eventually taken, filled in a TrappedDataCarrier instance and processed by elements in a chain, in order, beginning with the first element, by calling toView: aDataCarrier. This method may read (value) or write (value:) the piece of data carried. It must explicitly ask to proceed (proceed) to push the data carrier to processing by next element. It may also choose not to send proceed and stop the processing chain for this piece of data.

Processing elements can also subscribe to events in a DOM and start moving data from DOM into a blackboard. In that case, data from DOM are fed into another TrappedDataCarrier instance, and it is processed by every element of the chain, in backward order, by calling toModel: aDataCarrier. Again, this methods reads or writes the piece of data and proceeds to preceding element or stops the chain. If the data travels to the beginning of the chain, it is written to the blackboard at the position denoted in path.

Meta-processing chain

If the processing chain is not data-binding (it does not contain any element that responds true to isExpectingModelData), it is meta-processing. In that case, it does not observe the blackboard; but it can use path for other purposes.

Since meta-processing chain is not observing blackboard, the data event can not start it. It is instead started once, immediately after creating it (that is, after parsing and processing data-trap or when trap:processors: is called directly in code), running the toView: chain and setting true as the "data" in the data carrier.

Often, meta-processing chain contains control structures (guards, loops), so it will actually subscribe the blackboard itself for later processing - it just does it itself, not using automatic blackboard subscription via path.

Example: (signal increment) whenClicked from Counter example ++ button is meta-processing: it does not observe any data in the blackboard. Instead, whenClicked installs click event handler on the element, which starts toModel: processing when button is clicked, feeding it with true as the data. whenClicked's toModel: passes the data unchanged, and signal increment's toModel: does aBlackboard modify: aPath do: [ :value | value increment ], that is, signal processor modifies blackboard at path by sending a message supplied in its parameter. You can look at the code of both of these processors yourself: find their respective factory methods (signal:, whenClicked) in a factory (TrappedProcessor class), see what classes are they instantiating and look into those classes.

6 Processors

All processing elements (also called processors) are instances of some subclass of TrappedProcessor.

Basic processor contents which is used as a default when no processing chain is specified, is contained within Trapped package; all the other processors supplied by Trapped which will be described later, are contained in Trapped-Processors package. If you look at the factory (TrappedProcessor class), you see that their factory methods are in category *Trapped-Processors). The processor implementations are in one package with their factory methods, which are extending the factory.

This is by design - for your application, you may need to create your own processor, which you do easily: create your subclass of TrappedProcessor, and extend the factory with its creating method. The keywords of this creating method will need to be used inside data-trap or in call to trap:processors: API. The name of the class is not used in the descriptors, only the name of factory method is important.

You may end up with library of processors - this is what Trapped-Processors package is, the library of processors included in Trapped itself.

(Reference to all supplied processors: TBD)