Nate Biggs
773cf6b1d5
The attached bug shows an issue users have been encountering where a constructor seems to be undefined. It turns out this is because DDC is trying to read the constructor from the wrong library. This is happening because both 'package:dio' and a sister package 'package:dio_web_adapter' both contain a library with the same path: 'src/adapter.dart'. 'BrowserHttpClientAdapter' the class they are trying to reference is defined in 'package:dio/src/adapter.dart'. However, due to a naming collision, their import is referencing 'package:dio_web_adapter/src/adapter.dart'. This naming collision happens because of the logic in '_jsLibraryAlias'. By truncating the start of the import URI (i.e. 'dio/' and 'dio_web_adapter/') the two libraries map to the same alias. This alias is then used to as the key in the AMD module export object and since both libraries are in the same module, only the second one gets exported. This code may have been written with the assumption that libraries from different packages would always be in different modules (in which case the shortened paths shouldn't collide) but this is not the case. The fix is to use the full import URI including the package name. In writing the attached modular test I discovered another issue that only affects es6 imports. The ScopedId resolver was not considering NameSpecifier as a declaration point for variables. This lead to a similar name collision since the import alias's name was also being derived from a truncated import URI. In the test, both 'f1/foo.dart' and 'f2/foo.dart' were being imported 'as foo'. Now one is 'as foo' and the other is 'as foo$'. The first issue affects both AMD and es6 while the second issue only affects es6. The modular tests run with es6 so the new test fails if either of these fixes is not in place. The new DDC module system is not affected by either issue since it doesn't use NameSpecifiers and it uses the full import URI as a string to register libraries rather than a shortened alias. Tested on TGP and with a local Flutter application. Bug: https://github.com/dart-lang/sdk/issues/56498 Change-Id: I5bdb945cfbe615874b40e2fc4ebba31b661cf3b7 Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/410260 Reviewed-by: Nicholas Shahan <nshahan@google.com> Commit-Queue: Nate Biggs <natebiggs@google.com>
The Dart Dev Compiler (DDC) is a fast, modular compiler that generates modern JavaScript (EcmaScript 6). Its primary use today is to support fast, iterative development of Dart web applications for Chrome and other modern browsers.
Support
DDC is meant to be used by build systems like bazel, build_web_compilers and
flutter_tools under the hood. This compiler is not meant to be used by
application developers directly.
While at times the code generated by this compiler may be readable, the representation is not meant to be stable and can break with time. For that reason we do not recommend using this compiler to export Dart as a JavaScript module.
The recommended approach to compile Dart to JavaScript is to use dart compile js instead. If you intend to make a public JavaScript API based on a Dart
implementation, such API should be declared explicitly using the standard
Dart-JSInterop mechanisms.
Implementation details
Modularity
Unlike Dart2JS, DDC does not require an entire Dart application. Instead, it operates modularly: it compiles a set of Dart files into a JavaScript module. A DDC compilation step requires a set of input Dart files and a set of summaries of dependencies. It performs modular type checking as part of this compilation step, and, if the input type checks, it generates a JavaScript module. The browser (i.e., the JavaScript runtime) loads and links the generated modules when running the application. During development, a compilation step only needs to be rerun if the Dart files or summaries it relies upon change. For most changes, only a very small part of your code will require recompilation. Moreover, modules that are unchanged can be cached in the browser.
Representation
Currently Dart classes are mapped to ES6 classes, Dart fields to ES6 properties, Dart getters/setters to ES6 getters/setters, Dart methods to ES6 methods, and so on. Often names are preserved and calling conventions are natural JavaScript ones.
Some Dart concepts don't map directly:
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Libraries. Multiple Dart libraries are mapped to a single JS module. Each library appears as a first class object in the generated JS module, with its top-level symbols as members. We currently use a heuristic (based upon file paths) to ensure unique naming of generated library objects.
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Generics. Dart generics are reified, i.e., they are preserved at runtime. Generic classes are mapped to factories that, given one or more type parameters, return an actual ES6 class (e.g.,
HashMap$(core.String, core.int)produces a class that represents a HashMap from strings to ints). Similarly, generic methods are mapped to factories that, given one or more type parameters, return a method. -
Dynamic. DDC supports dynamically typed code (i.e., Dart's
dynamictype), but it will typically generate less readable and less efficient ES6 output as many type checks must be deferred to runtime. All dynamic operations are invoked via runtime helper code. -
Constructors. Dart supports multiple, named and factory constructors for a given class with a different initialization order for fields. Today, these are mapped to instance or static methods on the generated ES6 class.
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Private members. Dart maps private members (e.g., private fields or methods) to ES6 symbols. For example,
a._xmay map toa[_x]where_xis a symbol only defined in the scope of the generated library. -
Scoping. Dart scoping rules and reserved words are slightly different than JavaScript. While we try to preserve names wherever possible, in certain cases, we are required to rename.
In general, the current conventions (i.e., the Application Binary Interface or ABI in compiler terminology) should not be considered stable. We reserve the right to change these in the future.
Browser support
DDC currently supports Chrome stable (though users have had success running on FireFox and Safari).