diff --git a/src/passa/cli/_base.py b/src/passa/cli/_base.py
index 69e2683..04ef5eb 100644
--- a/src/passa/cli/_base.py
+++ b/src/passa/cli/_base.py
@@ -10,10 +10,16 @@
 def build_project(root):
     # This is imported lazily to reduce import overhead. Not evey command
     # needs the project instance.
-    from passa.internals.projects import Project
+    from passa.projects import Project
     return Project(os.path.abspath(root))
 
 
+def configure_reporter():
+    # This is imported lazily to reduce import overhead.
+    from passa.reporters import configure_reporter
+    configure_reporter("stdout")
+
+
 class BaseCommand(object):
     """A CLI command.
     """
@@ -51,4 +57,5 @@ def add_arguments(self):
     def main(self, options):
         # This __dict__ access is needed for Python 2 to prevent Python from
         # wrapping parsed_main into an unbounded method.
+        configure_reporter()
         return type(self).__dict__["parsed_main"](options)
diff --git a/src/passa/cli/lock.py b/src/passa/cli/lock.py
index 67b1d11..7961a5c 100644
--- a/src/passa/cli/lock.py
+++ b/src/passa/cli/lock.py
@@ -15,15 +15,27 @@ def main(options):
     if not success:
         return
 
-    project._l.write()
-    print("Written to project at", project.root)
+    if options.dry_run:
+        print(project._l.dumps())
+    else:
+        project._l.write()
+        print("Written to project at", project.root)
 
 
 class Command(BaseCommand):
+
     name = "lock"
     description = "Generate Pipfile.lock."
     parsed_main = main
 
+    def add_arguments(self):
+        super(Command, self).add_arguments()
+        self.parser.add_argument(
+            "--dry-run",
+            action="store_true", default=False,
+            help="run locking on Pipfile, but do not write to Pipfile.lock",
+        )
+
 
 if __name__ == "__main__":
     Command.run_current_module()
diff --git a/src/passa/internals/lockers.py b/src/passa/internals/lockers.py
index 4f7a722..b1b842f 100644
--- a/src/passa/internals/lockers.py
+++ b/src/passa/internals/lockers.py
@@ -9,11 +9,12 @@
 import resolvelib
 import vistir
 
+from passa import reporters
+
 from .caches import HashCache
 from .hashes import get_hashes
 from .metadata import set_metadata
 from .providers import BasicProvider, EagerUpgradeProvider, PinReuseProvider
-from .reporters import StdOutReporter
 from .traces import trace_graph
 from .utils import identify_requirment
 
@@ -100,10 +101,6 @@ def __repr__(self):
     def get_provider(self):
         raise NotImplementedError
 
-    def get_reporter(self):
-        # TODO: Build SpinnerReporter, and use this only in verbose mode.
-        return StdOutReporter(self.requirements)
-
     def lock(self):
         """Lock specified (abstract) requirements into (concrete) candidates.
 
@@ -116,14 +113,20 @@ def lock(self):
         * Populate markers based on dependency specifications of each
           candidate, and the dependency graph.
         """
+        reporters.report("lock-starting", {"requirements": self.requirements})
+
         provider = self.get_provider()
-        reporter = self.get_reporter()
-        resolver = resolvelib.Resolver(provider, reporter)
+        resolver = resolvelib.Resolver(
+            provider, reporters.get_reporter().build_for_resolvelib(),
+        )
 
         with vistir.cd(self.project.root):
             state = resolver.resolve(self.requirements)
 
         traces = trace_graph(state.graph)
+        reporters.report("lock-trace-ended", {
+            "state": state, "traces": traces,
+        })
 
         hash_cache = HashCache()
         for r in state.mapping.values():
diff --git a/src/passa/internals/reporters.py b/src/passa/internals/reporters.py
deleted file mode 100644
index 4fe6c0b..0000000
--- a/src/passa/internals/reporters.py
+++ /dev/null
@@ -1,90 +0,0 @@
-# -*- coding=utf-8 -*-
-
-from __future__ import absolute_import, print_function, unicode_literals
-
-import resolvelib
-
-from .traces import trace_graph
-
-
-def print_title(text):
-    print('\n{:=^84}\n'.format(text))
-
-
-def print_requirement(r, end='\n'):
-    print('{:>40}'.format(r.as_line(include_hashes=False)), end=end)
-
-
-def print_dependency(state, key):
-    print_requirement(state.mapping[key], end='')
-    parents = sorted(
-        state.graph.iter_parents(key),
-        key=lambda n: (-1, '') if n is None else (ord(n[0].lower()), n),
-    )
-    for i, p in enumerate(parents):
-        if p is None:
-            line = '(user)'
-        else:
-            line = state.mapping[p].as_line(include_hashes=False)
-        if i == 0:
-            padding = ' <= '
-        else:
-            padding = ' ' * 44
-        print('{pad}{line}'.format(pad=padding, line=line))
-
-
-class StdOutReporter(resolvelib.BaseReporter):
-    """Simple reporter that prints things to stdout.
-    """
-    def __init__(self, requirements):
-        super(StdOutReporter, self).__init__()
-        self.requirements = requirements
-
-    def starting(self):
-        self._prev = None
-        print_title(' User requirements ')
-        for r in self.requirements:
-            print_requirement(r)
-
-    def ending_round(self, index, state):
-        print_title(' Round {} '.format(index))
-        mapping = state.mapping
-        if self._prev is None:
-            difference = set(mapping.keys())
-            changed = set()
-        else:
-            difference = set(mapping.keys()) - set(self._prev.keys())
-            changed = set(
-                k for k, v in mapping.items()
-                if k in self._prev and self._prev[k] != v
-            )
-        self._prev = mapping
-
-        if difference:
-            print('New pins: ')
-            for k in difference:
-                print_dependency(state, k)
-        print()
-
-        if changed:
-            print('Changed pins:')
-            for k in changed:
-                print_dependency(state, k)
-        print()
-
-    def ending(self, state):
-        print_title(" STABLE PINS ")
-        path_lists = trace_graph(state.graph)
-        for k in sorted(state.mapping):
-            print(state.mapping[k].as_line(include_hashes=False))
-            paths = path_lists[k]
-            for path in paths:
-                if path == [None]:
-                    print('    User requirement')
-                    continue
-                print('   ', end='')
-                for v in reversed(path[1:]):
-                    line = state.mapping[v].as_line(include_hashes=False)
-                    print(' <=', line, end='')
-                print()
-        print()
diff --git a/src/passa/operations/lock.py b/src/passa/operations/lock.py
index 200735a..d2e3d84 100644
--- a/src/passa/operations/lock.py
+++ b/src/passa/operations/lock.py
@@ -4,25 +4,15 @@
 
 from resolvelib import NoVersionsAvailable, ResolutionImpossible
 
-from passa.internals.reporters import print_requirement
+from passa import reporters
 
 
 def lock(locker):
     success = False
     try:
         locker.lock()
-    except NoVersionsAvailable as e:
-        print("\nCANNOT RESOLVE. NO CANDIDATES FOUND FOR:")
-        print("{:>40}".format(e.requirement.as_line(include_hashes=False)))
-        if e.parent:
-            line = e.parent.as_line(include_hashes=False)
-            print("{:>41}".format("(from {})".format(line)))
-        else:
-            print("{:>41}".format("(user)"))
-    except ResolutionImpossible as e:
-        print("\nCANNOT RESOLVE.\nOFFENDING REQUIREMENTS:")
-        for r in e.requirements:
-            print_requirement(r)
+    except (NoVersionsAvailable, ResolutionImpossible) as e:
+        reporters.report("lock-failed", {"exception": e})
     else:
         success = True
     return success
diff --git a/src/passa/internals/projects.py b/src/passa/projects.py
similarity index 100%
rename from src/passa/internals/projects.py
rename to src/passa/projects.py
diff --git a/src/passa/reporters/__init__.py b/src/passa/reporters/__init__.py
new file mode 100644
index 0000000..dffe532
--- /dev/null
+++ b/src/passa/reporters/__init__.py
@@ -0,0 +1,31 @@
+# -*- coding=utf-8 -*-
+
+from __future__ import absolute_import, print_function, unicode_literals
+
+from .base import BaseReporter
+
+
+_REPORTER = BaseReporter()
+
+
+def _get_stdout_reporter():
+    from .stdout import Reporter
+    return Reporter()
+
+
+def configure_reporter(name):
+    global _REPORTER
+    _REPORTER = {
+        None: BaseReporter,
+        "stdout": _get_stdout_reporter,
+    }[name]()
+
+
+def get_reporter():
+    return _REPORTER
+
+
+def report(event, context=None):
+    if context is None:
+        context = {}
+    _REPORTER.report(event, context)
diff --git a/src/passa/reporters/base.py b/src/passa/reporters/base.py
new file mode 100644
index 0000000..66a432c
--- /dev/null
+++ b/src/passa/reporters/base.py
@@ -0,0 +1,52 @@
+# -*- coding=utf-8 -*-
+
+from __future__ import absolute_import, print_function, unicode_literals
+
+import resolvelib
+
+
+class ResolveLibReporter(resolvelib.BaseReporter):
+    """Implementation of a ResolveLib reporter that bridge messages.
+    """
+    def __init__(self, parent):
+        super(ResolveLibReporter, self).__init__()
+        self.parent = parent
+
+    def starting(self):
+        self.parent.report("resolvelib-starting", {"child": self})
+
+    def ending_round(self, index, state):
+        self.parent.report("resolvelib-ending-round", {
+            "child": self, "index": index, "state": state,
+        })
+
+    def ending(self, state):
+        self.parent.report("resolvelib-ending", {
+            "child": self, "state": state,
+        })
+
+
+class BaseReporter(object):
+    """Basic reporter that does nothing.
+    """
+    def build_for_resolvelib(self):
+        """Build a reporter for ResolveLib.
+        """
+        return ResolveLibReporter(self)
+
+    def report(self, event, context):
+        """Report an event.
+
+        The default behavior is to look for a "handle_EVENT" method on the
+        class to execute, or do nothing if there is no such method.
+
+        :param event: A string to indicate the event.
+        :param context: A mapping containing appropriate data for the handling
+            function.
+        """
+        handler_name = "handle_{}".format(event.replace("-", "_"))
+        try:
+            handler = getattr(self, handler_name)
+        except AttributeError:
+            return
+        handler(context or {})
diff --git a/src/passa/reporters/stdout.py b/src/passa/reporters/stdout.py
new file mode 100644
index 0000000..2042337
--- /dev/null
+++ b/src/passa/reporters/stdout.py
@@ -0,0 +1,106 @@
+# -*- coding=utf-8 -*-
+
+from __future__ import absolute_import, print_function, unicode_literals
+
+from resolvelib import NoVersionsAvailable, ResolutionImpossible
+
+from .base import BaseReporter
+
+
+def _print_title(text):
+    print('\n{:=^84}\n'.format(text))
+
+
+def _print_requirement(r, end='\n'):
+    print('{:>40}'.format(r.as_line(include_hashes=False)), end=end)
+
+
+def _print_dependency(state, key):
+    _print_requirement(state.mapping[key], end='')
+    parents = sorted(
+        state.graph.iter_parents(key),
+        key=lambda n: (-1, '') if n is None else (ord(n[0].lower()), n),
+    )
+    for i, p in enumerate(parents):
+        if p is None:
+            line = '(user)'
+        else:
+            line = state.mapping[p].as_line(include_hashes=False)
+        if i == 0:
+            padding = ' <= '
+        else:
+            padding = ' ' * 44
+        print('{pad}{line}'.format(pad=padding, line=line))
+
+
+class Reporter(BaseReporter):
+    """A reporter implementation that prints messages to stdout.
+    """
+    def handle_resolvelib_starting(self, context):
+        context["child"]._prev_mapping = None
+
+    def handle_resolvelib_ending_round(self, context):
+        _print_title(' Round {} '.format(context["index"]))
+        mapping = context["state"].mapping
+        if context["child"]._prev_mapping is None:
+            difference = set(mapping.keys())
+            changed = set()
+        else:
+            prev = context["child"]._prev_mapping
+            difference = set(mapping.keys()) - set(prev.keys())
+            changed = set(
+                k for k, v in mapping.items()
+                if k in prev and prev[k] != v
+            )
+        context["child"]._prev_mapping = mapping
+
+        if difference:
+            print('New pins: ')
+            for k in difference:
+                _print_dependency(context["state"], k)
+        print()
+
+        if changed:
+            print('Changed pins:')
+            for k in changed:
+                _print_dependency(context["state"], k)
+        print()
+
+    def handle_lock_starting(self, context):
+        _print_title(' User requirements ')
+        for r in context["requirements"]:
+            _print_requirement(r)
+
+    def handle_lock_trace_ended(self, context):
+        _print_title(" STABLE PINS ")
+        mapping = context["state"].mapping
+        for k in sorted(mapping):
+            print(mapping[k].as_line(include_hashes=False))
+            paths = context["traces"][k]
+            for path in paths:
+                if path == [None]:
+                    print('    User requirement')
+                    continue
+                print('   ', end='')
+                for v in reversed(path[1:]):
+                    line = mapping[v].as_line(include_hashes=False)
+                    print(' <=', line, end='')
+                print()
+        print()
+
+    def handle_lock_failed(self, context):
+        e = context["exception"]
+        if isinstance(e, ResolutionImpossible):
+            print("\nCANNOT RESOLVE.\nOFFENDING REQUIREMENTS:")
+            for r in e.requirements:
+                _print_requirement(r)
+        elif isinstance(e, NoVersionsAvailable):
+            print("\nCANNOT RESOLVE. NO CANDIDATES FOUND FOR:")
+            print("{:>40}".format(e.requirement.as_line(include_hashes=False)))
+            if e.parent:
+                line = e.parent.as_line(include_hashes=False)
+                print("{:>41}".format("(from {})".format(line)))
+            else:
+                print("{:>41}".format("(user)"))
+        else:
+            raise