Spy

MobX so far has been somewhat of a black-box, nicely hiding the details of how its reactive system works internally. Some have seen this as "too much magic" and not sure of how/why things work. The problem exacerbates when you have runtime exceptions. The amount of detail exposed on the error may not be enough. It also lacks the complete trace of events that led to the failure.

Spying exposes all these details and shows what is happening inside MobX. It can be summarized like so:

It gives better visibility of MobX internals
Enables better developer experience while debugging
Paves the way for integrating with other tools

Let us Spy

Using the spy() method of a ReactiveContext, you can get notified of all activities happening inside MobX. Most of the time you are dealing with the mainContext, so you could just do:

import 'package:mobx/mobx.dart';

void main() {
  mainContext.config = mainContext.config.clone(
    isSpyEnabled: true,
  );

  mainContext.spy((event) { /* ... */ });

  runApp(MyApp());
}

In this case, we are setting up a spy right in the top-level main() method. This gives us visibility into MobX, right from the get-go.

Notice that before we spy on events, we must first make sure that spying is enabled. Otherwise, we won't be able to capture any events.

`ReactiveContext.spy()`

Dispose spy(SpyListener listener);

typedef Dispose = void Function();
typedef SpyListener = void Function(SpyEvent event);

Setting up a spy gives you a disposer that can be called anytime to turn-off. This is useful if you want to specifically setup a spy around an action, observable or reaction. The details are present in SpyEvent, which are more specifically captured in its sub-classes:

ObservableValueSpyEvent
ComputedValueSpyEvent
ReactionSpyEvent
ReactionErrorSpyEvent
ReactionDisposedSpyEvent
ActionSpyEvent
EndedSpyEvent

Simple logging

Let us setup the spy to do some logging of the MobX activities.

This is part of the mobx_examples code.

import 'package:mobx/mobx.dart';

void main() {
  mainContext.config = mainContext.config.clone(
    isSpyEnabled: true,
  );

  mainContext.spy(print);

  runApp(MyApp());
}

Running the counter example in the simulator gives the following output:

flutter: reaction(START) Observer
#3      CounterExampleState.build (package:mobx_examples/counter/counter_widgets.dart:28:15)
flutter: reaction(END after 0ms) Observer
#3      CounterExampleState.build (package:mobx_examples/counter/counter_widgets.dart:28:15)
flutter: action(START) _Counter.increment
flutter: observable(START) _Counter.value=1, previously=0
flutter: observable(END) _Counter.value
flutter: action(END after 1ms) _Counter.increment
flutter: reaction(START) Observer
#3      CounterExampleState.build (package:mobx_examples/counter/counter_widgets.dart:28:15)
flutter: reaction(END after 0ms) Observer
#3      CounterExampleState.build (package:mobx_examples/counter/counter_widgets.dart:28:15)
flutter: reaction-dispose Observer
#3      CounterExampleState.build (package:mobx_examples/counter/counter_widgets.dart:28:15)

Looking at this log, we can trace the events that led to the new state:

The Observer (reaction) is setup.
When we tap on the Increment button, the _Counter.increment (action) gets fired.
As part of that action, the _Counter.value (observable) gets updated from 0 to 1.
Since the _Counter.value has changed, the Observer gets rebuilt, resulting in displaying the new value.
Once we leave the Counter example, the Observer reaction gets disposed.

Even in this simple example, we can see that a trace of events makes the whole process more meaningful. The magic of MobX can now be unraveled with the use of spy()!