The event system

The event module provides a system for properties and events, to let different components of an application react to each-other and to user input.

In short:

  • The Component class provides a base class which can be subclassed to create the different components of an app.
  • Each component has properties to reflect the state of the component.
  • Properties can only be mutated by actions. Calling (i.e. invoking) an action will not apply the action at once; actions are processed in batches.
  • When properties are modified (i.e. the state is changed), corresponding reactions will be invoked. The reactions are processed when all pending actions are done. This means that during processing reactions, the state never changes, which is a great thing to rely on!
  • Reactions can also react to events generated by emitters, such as mouse events.
  • The event loop object is responsible for scheduling actions and reactions and can be used by the user to e.g. make a function be called later. It intergrates with Python’s own asyncio loop.

The asynchronous nature of actions combined with the fact that the state does not change during processing reactions, makes it easy to reason about cause and effect. The information flows in one direction. This concept was gratefully taken from modern frameworks such as React/Flux and Veux.

One might suggest that the information flow is still circular, because there is an arrow going from reactions to actions. This is true, but note that actions invoked from reactions are not directly executed; they are pended and will be executed only after all reactions are done.

Relation to other parts of Flexx

This event system and its Component class form the basis for app.PyComponent, app.JsComponent and the UI system in flexx.ui. It can be used in both Python and JavaScript and works exactly the same in both languages.

Other than that, this is a generic event system that could drive any system that is based on asyncio.

Event object

An event is something that has occurred at a certain moment in time, such as the mouse being pressed down or a property changing its value. In Flexx, events are represented with dictionary objects that provide information about the event (such as what button was pressed, or the old and new value of a property). A custom Dict class is used that inherits from dict but allows attribute access, e.g. ev.button as an alternative to ev['button'].

Each event object has at least two attributes: source, a reference to the Component object emitting the event, and type, a string indicating the type of the event.

The Component class

The Component class provides a base class for objects that have properties, actions, reactions and emitters. E.g. flexx.ui.Widget inherits from, which inherits from flexx.event.Component.

class MyObject(event.Component):
    ...  # attributes/properties/actions/reactions/emitters go here

    def init(self):

It is common to implement the init() method of the component class. It gets automatically called by the component, at a moment when all properties have been initialized, but no events have been emitted yet. This is a good time to further initialize the component, and/or to instantiate sub components. One rarely needs to implement the __init__() method.

When the init() is called, the component is the currently “active” component, which can be used to e.g. descrive a hierarchy of objects, as is done with widgets. It also implies that mutations are allowed and that actions on the component itself have a direct effect (invoking actions of other components is still asynchronous though).

Properties represent state

Properties can be defined using one of the several property classes. For example:

class MyObject(event.Component):

    foo = event.AnyProp(8, settable=True, doc='can have any value')
    bar = event.IntProp()

Properties accept one positional arguments to set the default value. If not given, a sensible default value is used that depends on the type of property. The foo property above is marked as settable, so that the class will have a set_foo() action. Docs can be added too. Note that properties are readonly: they can can only be mutated by actions.

Property values can be initialized when a component is created (also non-settable properties):

c = MyComponent(foo=42)

One can also set the initial value of a property to a function object. This creates an auto-reaction that sets the property, and makes it possible to hook things up in a very concise manner. In the example below, the label text will be automatically updated when the username property changes:

c = UiLabel(text=lambda: self.username)

An event is emitted every time that a property changes. This event has attributes old_value and new_value (except for in-place array mutations, as explained below). At initialization, a component sends out an event for each property, which has the same value for old_value and new_value.

Component classes can also have Attributes, which are read-only (usually static) non-observable values (e.g.

Actions can mutate properties

Actions can be defined to mutate properties:

class MyObject(event.Component):

    foo = event.AnyProp(8, settable=True, doc='can have any value')
    bar = event.IntProp()

    def increase_bar(self):
        self._mutate_bar( + 1)
        # shorthand for self._mutate('bar', + 1)

Actions can have any number of (positional) arguments. Note that actions are asynchronous, i.e. calling an action will not apply it immediately, unless it is called from another action.

Since actions are asynchronous, their inner function should not return a value. Invoking (i.e. calling) an action always returns the component itself, which allows chainging action invokations, e.g. t.scale(3).translate(3, 4)

Mutations are done via the _mutate method, or by the auto-generated _mutate_xx() methods. Mutations can only be done from an action. Trying to do so otherwise will result in an error. This may seem limiting at first, but it greatly helps keeping it easy to reason about information flowing through your application, even as it scales.

Mutations to array-like properties

The above shows the simple and most common use of mutations. For list properties, mutations can also be done in-place:

class MyObject(event.Component):

    items = event.ListProp()

    def add_item(self, item):
        self._mutate_items([item], 'insert', len(self.items))

This allows more fine-grained control over state updates, which can also be handled by reactions in much more efficient ways. The types of mutations are ‘set’ (the default), ‘insert’, ‘replace’, and ‘remove’. In the latter, the provided value is the number of elements to remove. For the others it must be a list of elements to set/insert/replace at the specified index.

Emitters create events

Emitters make it easy to generate events. Similar to actions, they are created with a decorator.

class MyObject(event.Component):

    def pointer_down(self, js_event):
        ''' Event emitted when the mouse/touchpad/screen is pressed.
        return dict(button=js_event.button)

Emitters can have any number of arguments and should return a dictionary, which will get emitted as an event, with the event type matching the name of the emitter.

Note that stricly speaking emitters are not necessary as Component.emit() can be used to generate an event. However, they provide a mechanism to generate an event based on certain input data, and also document the events that a component may emit.


Reactions are used to react to events and changes in properties, using an underlying handler function:

class MyObject(event.Component):

    first_name = event.StringProp(settable=True)
    last_name = event.StringProp(settable=True)

    @event.reaction('first_name', 'last_name')
    def greet(self, *events):
        print('hi', self.first_name, self.last_name)

    def handle_foo(self, *events):
        for ev in events:

This example demonstrates multiple concepts. Firstly, the reactions are connected via connection-strings that specify the types of the event; in this case the greeter reaction is connected to “first_name” and “last_name”, and handle_foo is connected to the event-type “foo” of the object. This connection-string can also be a path, e.g. “sub.subsub.event_type”. This allows for some powerful mechanics, as discussed in the section on dynamism.

One can also see that the reaction-function accepts *events argument. This is because reactions can be passed zero or more events. If a reaction is called manually (e.g. ob.handle_foo()) it will have zero events. When called by the event system, it will have at least 1 event. When e.g. a property is set twice, the function will be called just once, but with multiple events. If all events need to be processed individually, use for ev in events: ....

In most cases, you will connect to events that are known beforehand, like those corresponding to properties and emitters. If you connect to an event that is not known (like “foo” in the example above) Flexx will display a warning. Use '!foo' as a connection string (i.e. prepend an exclamation mark) to suppress such warnings.

Another useful feature of the event system is that a reaction can connect to multiple events at once, as the greet reaction does.

The following is less common, but it is possible to create a reaction from a normal function, by using the Component.reacion() method:

c = MyComponent()

# Using a decorator
@c.reaction('foo', 'bar')
def handle_func1(self, *events):

# Explicit notation
def handle_func2(self, *events):
c.reaction(handle_func2, 'foo', 'bar')
# this is fine too: c.reaction('foo', 'bar', handle_func2)

Greedy and automatic reactions

Each reaction operates in a certain “mode”. In mode “normal”, the event system ensures that all events are handled in the order that they were emitted. This is often the most useful approach, but this implies that a reaction can be called multiple times during a single event loop iteration, with other reactions called in between to ensure the consisten event order.

If it is preferred that all events targeted at a reaction are handled with a single call to that reaction, it can be set to mode “greedy”. Cases where this makes sense is when all related events must be processed simultenously, or simply when performance matters a lot and order matters less.

Reactions with mode “auto” are automatically triggered when any of the properties that the reaction uses is changed. Such reactions can be created by specifying the mode argument, or simply by creating a reaction with zero connections strings. We refer to such reactions as “auto reactions” or “implicit reactions”. This is a convenient feature, but should probably be avoided when a lot (say hundreds) of properties are accessed.

class MyObject(event.Component):

    first_name = event.StringProp(settable=True)
    last_name = event.StringProp(settable=True)

    def greet(self):
        print('hi', self.first_name, self.last_name)

A similar useful feature is to assign a property (at initialization) using a function. In such a case, the function is turned into an implicit reaction. This can be convenient to easily connect different parts of an app.

class MyObject(event.Component):

    first_name = event.StringProp(settable=True)
    last_name = event.StringProp(settable=True)

person = MyObject()
label = UiLabel(text=lambda: person.first_name)

Reacting to in-place mutations

In-place mutations to lists or arrays can be reacted to by processing the events one by one:

class MyComponent(event.Component):

    def track_array(self, *events):
        for ev in events:
            if ev.mutation == 'set':
                self.items[:] = ev.objects
            elif ev.mutation == 'insert':
                self.items[ev.index:ev.index] = ev.objects
            elif ev.mutation == 'remove':
                self.items[ev.index:ev.index+ev.objects] = []  # objects is int here
            elif ev.mutation == 'replace':
                self.items[ev.index:ev.index+len(ev.objects)] = ev.objects
                assert False, 'we cover all mutations'

For convenience, the mutation can also be “replicated” using the flexx.event.mutate_array() and flexx.event.mutate_dict() functions.

Connection string syntax

The strings used to connect events follow a few simple syntax rules:

  • Connection strings consist of parts separated by dots, thus forming a path. If an element on the path is a property, the connection will automatically reset when that property changes (a.k.a. dynamism, more on this below).
  • Each part can end with one star (‘*’), indicating that the part is a list and that a connection should be made for each item in the list.
  • With two stars, the connection is made recursively, e.g. “children**” connects to “children” and the children’s children, etc.
  • Stripped of ‘*’, each part must be a valid identifier (ASCII).
  • The total string optionally has a label suffix separated by a colon. The label itself may consist of any characters.
  • The string can have a “!” at the very start to suppress warnings for connections to event types that Flexx is not aware of at initialization time (i.e. not corresponding to a property or emitter).

An extreme example could be "!foo.children**.text:mylabel", which connects to the “text” event of the children (and their children, and their children’s children etc.) of the foo attribute. The “!” is common in cases like this to suppress warnings if not all children have a text event/property.


Labels are a feature that makes it possible to infuence the order by which reactions are called, and provide a means to disconnect specific (groups of) handlers.

class MyObject(event.Component):

    def given_foo_handler(*events):

    def my_foo_handler(*events):
        # This one is called first: 'aa' < 'given_f...'

When an event is emitted, any connected reactions are scheduled in the order of a key, which is the label if present, and otherwise the name of the name of the reaction.

The label can also be used in the disconnect() method:

def handle_foo(*events):


h.disconnect('foo:mylabel')  # don't need reference to handle_foo


Dynamism is a concept that allows one to connect to events for which the source can change. For the following example, assume that Node is a Component subclass that has properties parent and children.

main = Node()
main.parent = Node()
main.children = Node(), Node()

def parent_foo_handler(*events):

def children_foo_handler(*events):

The parent_foo_handler gets invoked when the “foo” event gets emitted on the parent of main. Similarly, the children_foo_handler gets invoked when any of the children emits its “foo” event. Note that in some cases you might also want to connect to changes of the parent or children property itself.

The event system automatically reconnects reactions when necessary. This concept makes it very easy to connect to the right events without the need for a lot of boilerplate code.

Note that the above example would also work if parent would be a regular attribute instead of a property, but the reaction would not be automatically reconnected when it changed.

Implicit dynamism

Implicit reactions are also dynamic, maybe even more so! In the example below, the reaction accesses the children property, thus it will be called whenever that property changes. It also connects to the visible event of all children, and to the foo event of all children that are visible.

 def _implicit_reacion():
     for child in main.children:
         if child.visible:

This mechanism is powerful, but one can see how it can potentially access (and thus connect to) many properties, especially if the reaction calls other functions that access more properties. Also keep in mind that implicit reactions have more overhead (because they fully reconnect every time after they are called). One should probably avoid them for properties that change 100 times per second.


The event system presented here is quite flexible and designed to cover the needs of a variety of event/messaging mechanisms. This section discusses how this system relates to some common patterns, and how these can be implemented.

Observer pattern

The idea of the observer pattern is that observers keep track (the state of) of an object, and that an object is agnostic about what it’s tracked by. For example, in a music player, instead of writing code to update the window-title inside the function that starts a song, there would be a concept of a “current song”, and the window would listen for changes to the current song to update the title when it changes.

In flexx.event, a Component object keeps track of its observers (reactions) and notifies them when there are changes. In our music player example, there would be a property “current_song”, and a reaction to take action when it changes.

As is common in the observer pattern, the reactions keep track of the objects that they observe. Therefore both Reaction and Component objects have a dispose() method for cleaning up.

Signals and slots

The Qt GUI toolkit makes use of a mechanism called “signals and slots” as an easy way to connect different components of an application. In flexx.event signals translate to properties and assoctated setter actions, and slots to the reactions that connect to them.

Although signals and slots provide a convenient mechanism, they make it easy to create “spaghetti apps” where the information flows all over the place, which is exactly what frameworks like Flux, Veux and Flexx try to overcome.

Overloadable event handlers

In Qt, the “event system” consists of methods that handles an event, which can be overloaded in subclasses to handle an event differently. In flexx.event, actions and reactions can similarly be re-implemented in subclasses, and these can call the original handler using super() if needed.

Publish-subscribe pattern

In pub-sub, publishers generate messages identified by a ‘topic’, and subscribers can subscribe to such topics. There can be zero or more publishers and zero or more subscribers to any topic.

In flexx.event a Component object can play the role of a broker. Publishers can simply emit events. The event type represents the message topic. Subscribers are represented by handlers.