Dunno Why I Avoid Strategy So

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I now have my Dart-based Hipster MVC framework able to query and update both local storage and remote data stores (via Ajax). But to do so, I have two different branches of my framework. Backbone.js has the nifty little Backbone.sync method, through which all queries and updates run. I hope to be able to replicate that in Dart.

I suspect that I will eventually succumb to typing various parts of this, but, for now, I will stick closer to Backbone's Javascript than idiomatic Dart. But immediately, I can think of a problem with this approach--it is not possible to redefine methods in Dart. By default, Backbone.sync() performs CRUD over Ajax. To change this behavior, one redefines Backbone.sync (i.e. not overriding it in a sub-class). Complicating matters is that both the model and collection classes need to have access to this overridden sync() method. This seems difficult, at best, if I cannot rewrite a single method.

My first attempt is to define a function that can be source'd into both the model and collection. For example, it would be nice if I could simply define a sync method:

// sync.dart
sync() {
  print("[sync]");
}

And then source() it in my collection sub-class:

#library('Collection class to describe my comic book collection');

#import('HipsterCollection.dart');
#import('Models.ComicBook.dart');

#source('sync.dart');

class Comics extends HipsterCollection {
  get url() => '/comics';
  modelMaker(attrs) => new ComicBook(attrs);
}

Back in my base class, I then try to replace my local storage version of fetch() with an attempt to invoke this sync() function:

class HipsterCollection implements Collection<HipsterModel> {
  // ...
  fetch() {
    try {
      sync();
    }
    catch (NoSuchMethodException e) {
      print("*** time to define a default sync");
    }
  }
  // ...
}

Unfortunately, this will not work—the NoSuchMethodException clause is always executed. It actually works as expected if I source() the sync.dart code directly into the HipsterCollection base class. This is because the source'd code is available to the enclosing library, but nowhere else (not even the base class of the enclosing library).

I need some way for the concrete class to communicate the strategy for syncing back in a way that the base class can manipulate it. So I am stuck defining a "syncer" (that really does not look right) class:

#library('Sync layer for the Hipster MVC');

class HipsterSync {
  sync() {
    print("[sync]");
  }
}

I can then specify this in my Comics concrete collection class:

class Comics extends HipsterCollection {
  get url() => '/comics';
  modelMaker(attrs) => new ComicBook(attrs);
  get syncer() => new HipsterSync();
}

And use it in the base class:

  fetch() {
    if (syncer != null) {
      syncer.sync();
    }
    else {
      // be ajax- like
    }
  }

That seems to work just fine. I do not see any other options besides supplying a syncing object. It is not as nice as Backbone.sync, with which I only need to define the sync strategy once. In my Dart MVC, I will have to supply the syncing strategy for each model and class in the application. That is less than ideal, but hopefully I can find ways to mitigate the pain.

Starting tomorrow.


Day #295