IDE support blog post.

Fixes #31.

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+---
+layouts: posts
+title: [IDE support]
+---
+One of Bazel’s longest-standing feature requests is integration with IDEs.
+With the 0.3 release, we finally have all machinery in place that allows
+implementing integration with Bazel in IDEs. Simultaneous with that
+Bazel release we are also making public two IDE plugins:
+
+*   [Tulsi](http://tulsi.bazel.io): Bazel support for Xcode.
+*   [e4b](https://github.com/bazelbuild/e4b): a sample Bazel plugin for Eclipse.
+
+In this post, we will look into how Bazel enables IDE integration
+and how an IDE plugin integrating with Bazel can be implemented.
+
+
+## Principles of Bazel IDE support
+
+Bazel BUILD files provide a description of a project’s source code: what
+source files are part of the project, what artifacts (targets) should be
+built from those files, what the dependencies between those files are, etc.
+Bazel uses this information to perform a build, that is, it figures out the set
+of actions needed to produce the artifacts (such as running a compiler or
+linker) and executes those actions. Bazel accomplishes this by constructing a
+_dependency graph_ between targets and visiting this graph to collect
+those actions.
+
+IDEs (as well as other tools working with source code) also need the same
+information about the set of sources and their roles; but instead of building
+the artifacts, IDEs use it to provide code navigation, autocompletion and
+other code-aware features.
+
+In the 0.3.0 Bazel release, we are adding a new concept to Bazel -
+[_aspects_](/docs/skylark/aspects.html).
+Aspects allow augmenting build dependency graphs with additional information
+and actions. Applying an aspect to a build target creates a "shadow
+dependency graph" reflecting all transitive depenedencies of that target,
+and the aspect's implementation determines the actions that Bazel executes
+while traversing that graph.
+The [documentation on aspects](/docs/skylark/aspects.html) explains this in more
+detail.
+
+## Architecture of a Bazel IDE plugin.
+
+As an example of how aspects are useful for IDE integration, we will take
+a look at a sample
+[Eclipse plugin for Bazel support, e4b](https://github.com/bazelbuild/e4b).
+
+e4b includes an aspect, defined in a file
+[e4b_aspect.bzl](https://github.com/bazelbuild/e4b/blob/master/com.google.devtools.bazel.e4b/resources/tools/must/be/unique/e4b_aspect.bzl),
+that when
+applied to a particular target, generates a small JSON file with information
+about that target relevant to Eclipse. Those JSON files are then consumed
+by the e4b plugin inside Eclipse to build [Eclipse's representation
+of a project](https://github.com/bazelbuild/e4b/blob/master/com.google.devtools.bazel.e4b/src/com/google/devtools/bazel/e4b/classpath/BazelClasspathContainer.java),
+[IClasspathContainer](http://help.eclipse.org/juno/index.jsp?topic=%2Forg.eclipse.jdt.doc.isv%2Freference%2Fapi%2Forg%2Feclipse%2Fjdt%2Fcore%2FIClasspathContainer.html):
+
+![e4bazel workflow](e4b-workflow.png)
+
+Through the e4b plugin UI, the user specifies an initial set of targets
+(typically a java or android binary, a selection of tests, all targets
+in certain packages, etc). E4b plugin then invokes bazel as follows:
+
+```
+bazel build //java/com/company/example:main \
+--aspect e4b_aspect.bzl%e4b_aspect \
+--output_groups ide-info
+```
+
+(some details are omitted for clarity; see
+[e4b source](https://github.com/bazelbuild/e4b/blob/master/com.google.devtools.bazel.e4b/src/com/google/devtools/bazel/e4b/command/BazelCommand.java) for complete
+invocation)
+
+The "--aspect" flag directs Bazel to apply e4b_aspect, exported from
+e4bazel.bzl Skylark extension, to target //java/com/company/example:main.
+
+The aspect is then applied transitively to the dependencies of the specified
+targets, producing `.e4b-build.json` files for each target in the transitive
+closure of dependencies. The e4b plugin reads those outputs and provides
+a Classpath for Eclipse core to consume. If the input BUILD files change
+so that a project model needs to be re-synced, the plugin still invokes
+the exact same command: Bazel will rebuild only those files that are affected
+by the change, so the plugin need only reexamine only those newly built
+`.e4b-build.json` files. `ide-info` is an output group defined by e4b_aspect;
+the `--output_groups` flag ensures that only the artifacts belonging to that
+group (and hence only to the aspect) are built, and therefore that no
+unnecessary build steps are performed.
+
+The aspect uses the
+['java' provider](/docs/skylark/lib/JavaSkylarkApiProvider.html) on the targets
+it applies to to access a variety of information about Java targets.
+
+