Skip to main content
Bazel emits a stream of structured events during every build — target outcomes, test results, output file paths, timing, and more. AXL tasks can subscribe to this stream and act on individual events as they arrive. Create an iterator with bazel.build_events.iterator(), pass it to ctx.bazel.build() (or ctx.bazel.test()) via the build_events parameter, and then iterate the handle with for event in events:. Events are delivered synchronously (Starlark is single-threaded), so the iterator blocks until the next event is ready and unblocks when the build finishes. Each event has a kind string that identifies the event type, and a payload whose fields depend on that kind. The full set of event kinds and payload types is defined in Bazel’s Build Event Stream protocol.
Build Event Stream reference

​
Collecting output files

A common use case is collecting the output files that Bazel produced. Files arrive in named_set_of_files events: 
def impl(ctx: TaskContext) -> int:
    events = bazel.build_events.iterator()
    build = ctx.bazel.build(
        build_events = [events],
        *ctx.args.target_pattern,
    )

    outputs = []
    for event in events:
        if event.kind == "named_set_of_files":
            for f in event.payload.files:
                outputs.append(f.name)

    status = build.wait()
    if status.code == 0:
        for name in outputs:
            ctx.std.io.stdout.write(name + "\n")
    return status.code

list_outputs = task(
    implementation = impl,
    args = {
        "target_pattern": args.positional(default = ["..."], maximum = 512),
    },
)
The File payload exposes name, digest, length, and path_prefix directly. The URI is wrapped in a file oneof (uri, contents, or symlink_target_path) — most consumers want name, which is the file’s logical identifier (for example bazel-bin/path/to/target.out).

​
Filtering by event kind

To skip irrelevant events, pass kinds= to iterator(). Only the listed event kinds are delivered: 
events = bazel.build_events.iterator(kinds = ["test_result", "build_finished"])

​
Reacting to test results

test_result events fire once per test target. The label of the test target is on the event’s id, and the status comes from the payload: 
events = bazel.build_events.iterator(kinds = ["test_result"])
build = ctx.bazel.test(build_events = [events], "//...")

for event in events:
    label = event.id.test_result.label
    passed = event.payload.status == "PASSED"
    ctx.std.io.stdout.write("{}: {}\n".format(label, "PASS" if passed else "FAIL"))
See the test_result payload reference for the full list of fields.