Dart2: (keypress) expects (Event e) not (KeyEvent e)

[matanlurey]

This fails at the DDC with errors enabled:

https://github.com/dart-lang/angular/blob/f55fea736cb25040be640e75541acfbcd34e0329/_tests/test/core/change_detection/detect_host_changes_test.dart#L67

... I'm not sure why, yet. The temporary fix is to type this Event.

[leonsenft]

@matanlurey What's the actual error?

[matanlurey]

"Expected KeyEvent e got Event e"

[leonsenft]

So the issue arises from the following generated line:

_el_0.addEventListener('keypress', eventHandler1(_SomeDirective_0_5.handleKeyPress));

Element.addEventListener() expects a listener of type void Function(Event), which prevents a user from writing a handler with a specialized event type. Normally you'd write one that accepts Event, then cast (if you're certain of the type) or promote the event. For example, if you're listening to the click event, you know the event type will be ClickEvent.

I tried to do this work for the user if they specified a type with the following code:

https://github.com/dart-lang/angular/blob/f55fea736cb25040be640e75541acfbcd34e0329/angular/lib/src/core/linker/app_view.dart#L629-L634

My thinking is that when this function is invoked directly as a parameter of EventTarget.addEventListener(), the parameter type requires E to be Event, thus F is restricted to a type that's assignable to Event. We then cast this Event to F in the returned closure which itself has type void Function(Event).

Is there a mistake in my reasoning? Is inference not intended to be capable of handling this? I looked at the output of DDC and found this.

this[_el_0][$addEventListener](
  'keypress',
  this.eventHandler1(
    html.Event,
    html.Event,
    EventTovoid()._check(dart.dload(this[_SomeDirective_0_5], 'handleKeyPress')),
  ),
);

By the looks of the parameters (which I assume are the generic type arguments), inference has typed eventHandler1<E, F extends E> as eventHandler<Event, Event>. This is why we see the error cast fail from (KeyEvent) => void to (Event) => void, since the parameter isn't expecting a void Function(KeyEvent) as it should, but rather a void Function(Event).

So is this a DDC bug, or am I misunderstanding the type system? @vsmenon

[vsmenon]

@leonsenft The inference will only work if we know the static type of _SomeDirective_0_5.handleKeyPress. The dload in the generated code means that it doesn't. What's the static type of _SomeDirective_0_5?

[leonsenft]

Of course! The field has type dynamic. I'm wondering why this is the case, since a directive's type should be well known (you have to provide it in the metadata directly). I'll double check if all directives have dynamic types, or if this is just a special case since the directive is being provided by the component hosting it.

Thanks!

[matanlurey]

I am going to assume this is a bug (?) in the compiler, and assign to you for now.