Conditional imports / code for Dart packages

问题

Is there any way to conditionally import libraries / code based on environment flags or target platforms in Dart? I'm trying to switch out between dart:io's ZLibDecoder / ZLibEncoder classes and zlib.js based on the target platform.

There is an article that describes how to create a unified interface, but I'm unable to visualize that technique not creating duplicate code and redundant tests to test that duplicate code. game_loop employs this technique, but uses separate classes (GameLoopHtml and GameLoopIsolate) that don't seem to share anything.

My code looks a bit like this:

class Parser {
  Layer parse(String data) {
    List<int> rawBytes = /* ... */;
    /* stuff you don't care about */
    return new Layer(_inflateBytes(rawBytes));
  }
  String _inflateBytes(List<int> bytes) {
    // Uses ZLibEncoder on dartvm, zlib.js in browser
  }
}

I'd like to avoid duplicating code by having two separate classes -- ParserHtml and ParserServer -- that implement everything identically except for _inflateBytes.

EDIT: concrete example here: https://github.com/radicaled/citadel/blob/master/lib/tilemap/parser.dart. It's a TMX (Tile Map XML) parser.

回答1:

You could use mirrors (reflection) to solve this problem. The pub package path is using reflection to access dart:io on the standalone VM or dart:html in the browser.

The source is located here. The good thing is, that they use @MirrorsUsed, so only the required classes are included for the mirrors api. In my opinion the code is documented very good, it should be easy to adopt the solution for your code.

Start at the getters _io and _html (stating at line 72), they show that you can load a library without that they are available on your type of the VM. Loading just returns false if the library it isn't available.

/// If we're running in the server-side Dart VM, this will return a
/// [LibraryMirror] that gives access to the `dart:io` library.
///
/// If `dart:io` is not available, this returns null.
LibraryMirror get _io => currentMirrorSystem().libraries[Uri.parse('dart:io')];

// TODO(nweiz): when issue 6490 or 6943 are fixed, make this work under dart2js.
/// If we're running in Dartium, this will return a [LibraryMirror] that gives
/// access to the `dart:html` library.
///
/// If `dart:html` is not available, this returns null.
LibraryMirror get _html =>
  currentMirrorSystem().libraries[Uri.parse('dart:html')];

Later you can use mirrors to invoke methods or getters. See the getter current (starting at line 86) for an example implementation.

/// Gets the path to the current working directory.
///
/// In the browser, this means the current URL. When using dart2js, this
/// currently returns `.` due to technical constraints. In the future, it will
/// return the current URL.
String get current {
  if (_io != null) {
    return _io.classes[#Directory].getField(#current).reflectee.path;
  } else if (_html != null) {
    return _html.getField(#window).reflectee.location.href;
  } else {
    return '.';
  }
}

As you see in the comments, this only works in the Dart VM at the moment. After issue 6490 is solved, it should work in Dart2Js, too. This may means that this solution isn't applicable for you at the moment, but would be a solution later.

The issue 6943 could also be helpful, but describes another solution that is not implemented yet.

回答2:

Conditional imports are possible based on the presence of dart:html or dart:io, see for example the import statements of resource_loader.dart in package:resource.

I'm not yet sure how to do an import conditional on being on the Flutter platform.

来源：https://stackoverflow.com/questions/19525433/conditional-imports-code-for-dart-packages

标签

dart

dart-mirrors