Build.dart and the Dart Editor Build System

Build.dart is a simple build script convention that lets you add behavior to the Dart Editor build system. If the root of a Dart Editor project has a script named build.dart, that script is invoked during a build with information about the changed files. You can use build.dart to post-process changed files, generate other files, or drive other aspects of the build system.


Dart Editor can invoke build.dart with any of the following command-line flags:

Specifies a file that changed and should be rebuilt. One instance of the --changed flag is passed in for every changed file.
Specifies a file that was removed and might affect the build. One instance of the --removed flag is passed in for every deleted file.
Requests that the build.dart script delete all generated files and reset any saved state. No other flags are passed in, except for --machine.
Requests a full build; no incremental information is available.
Requests the build.dart script to print rich JSON error messages to the standard output (stdout), so that Dart Editor can interpret the messages.
Creates a directory containing the files needed to deploy this application to a server. Used only for the Polymer.dart linter and build tool. You can find more information in the polymer library file builder.dart.

Currently, the editor always passes the --machine flag into the build script. The other flags include either a single --full flag, a single --clean flag, or one or more --changed and --removed flags.

If a build script returns a non-zero exit code, all the stdout and stderr produced by the script is displayed in the Console view. Zero exit codes do not display any output; build.dart is usually called many times a minute, and having the Console view always scrolling output would be distracting to the user.

Any changes that build.dart makes within a directory named out are not fed back into build.dart using --changed flags. Any other changed or created files are fed back.

An example

Here is a sample build.dart file that processes all .foo files into .foobar files.

import 'dart:io';

void main(List<String> args) {
  for (String arg in args) {
    if (arg.startsWith('--changed=')) {
      String file = arg.substring('--changed='.length);

      if (file.endsWith('.foo')) {

void _processFile(String file) {
  String contents = new File(file).readAsStringSync();

  if (contents != null) {
    File outFile = new File('${file}bar');
    outFile.writeAsStringSync('// processed from ${file}:\n${contents}');

The machine interface

Sometimes the build.dart script needs to get information back to the editor. The general mechanism for doing this is for the editor to scrape the stdout from build.dart, looking for well-formed messages. When Dart Editor starts build.dart, it passes in a --machine flag indicating that the editor is looking for well-formed messages.

Well-formed messages follow these rules:

  • Each message is on its own line.
  • Each message is formed as a JSON RPC command/method call.
  • Each message is wrapped in square brackets.

The next sections give examples of two kinds of messages:

  • Errors, warnings, and informational messages
  • File mappings

Errors, warnings, and informational messages

As build.dart processes input files, it might call out some input files as badly formed, using error or warning messages. You can also generate informational (info) messages—for example, scraping TODO messages from user code. The editor uses these messages to create markers for the given file and line combination, and it displays the messages in the Problems view.

Here is an example of stdout that contains error and warning messages:

[{"method":"error","params":{"file":"foo.html","line":23,"message":"no ID found"}}]

[{"method":"warning","params":{"file":"foo.html","line":23,"message":"no ID found"}}]

[{"method":"error","params":{"file":"foo.html","line":23,"message":"no ID found","charStart":123,"charEnd":130}}]

The JSON RPC method name for these messages can be error, warning, or info. The parameters are:

An absolute or script relative file path
The line with the error or warning (1-based)
A message to display to the user
The starting position of the error from the beginning of the file (0-based)
The ending position of the error from the beginning of the file (0-based)

The charStart and charEnd parameters are optional but encouraged. Including them allows the editor to provide nice tooltip help for the error or warning.

File mappings

When a build.dart script generates an output file from an input file, the script can tell the editor about this mapping.

When you specify a file mapping, Dart Editor redirects HTTP requests and replaces launch attempts so that they use the mapped-to file. For example, if you map index.html to out/index.html, Dart Editor redirects HTTP requests from index.html to out/index.html, and it replaces the user’s attempt to launch index.html with a launch of out/index.html.

Here’s an example of mapping foo.html to out/foo.html:


The JSON RPC method name for a file mapping is mapping. The parameters are:

An absolute or script relative file path
An absolute or script relative file path

Another example

You might find the following to be helpful: