Graphics.UI.Gtk.Gdk.EventM

gtk3-0.14.5: Binding to the Gtk+ 3 graphical user interface library

Maintainer | gtk2hs-users\@lists.sourceforge.net
Stability | provisional
Portability | portable (depends on GHC)
Safe Haskell | None
Language | Haskell98

Graphics.UI.Gtk.Gdk.EventM

Contents

- Detail
- Event monad and type tags
- Accessor functions for event information
- Auxilliary Definitions

Description

Types and accessors to examine information in events.

Synopsis

- type EventM t = ReaderT (Ptr t) IO
- data EAny
- data EKey
- data EButton
- data EScroll
- data EMotion
- data EExpose
- data EVisibility
- data ECrossing
- data EFocus
- data EConfigure
- data EProperty
- data EProximity
- data EWindowState
- data EOwnerChange
- data EGrabBroken
- eventWindow :: EventM any DrawWindow
- eventSent :: EventM any Bool
- eventCoordinates :: HasCoordinates t => EventM t (Double, Double)
- eventRootCoordinates :: HasRootCoordinates t => EventM t (Double, Double)
- eventModifier :: HasModifier t => EventM t [Modifier]
- eventModifierAll :: HasModifier t => EventM t [Modifier]
- eventModifierMouse :: HasModifier t => EventM t [Modifier]
- eventTime :: HasTime t => EventM t TimeStamp
- eventKeyVal :: EventM EKey KeyVal
- eventKeyName :: EventM EKey DefaultGlibString
- eventHardwareKeycode :: EventM EKey KeyCode
- eventKeyboardGroup :: EventM EKey Word8
- data MouseButton
  - = LeftButton
  - | MiddleButton
  - | RightButton
  - | OtherButton Int
- eventButton :: EventM EButton MouseButton
- data Click
  - = SingleClick
  - | DoubleClick
  - | TripleClick
  - | ReleaseClick
- eventClick :: EventM EButton Click
- data ScrollDirection
  - = ScrollUp
  - | ScrollDown
  - | ScrollLeft
  - | ScrollRight
  - | ScrollSmooth
- eventScrollDirection :: EventM EScroll ScrollDirection
- eventIsHint :: EventM EMotion Bool
- eventRequestMotions :: EventM EMotion ()
- eventArea :: EventM EExpose Rectangle
- data VisibilityState
  - = VisibilityUnobscured
  - | VisibilityPartialObscured
  - | VisibilityFullyObscured
- eventVisibilityState :: EventM EVisibility VisibilityState
- data CrossingMode
  - = CrossingNormal
  - | CrossingGrab
  - | CrossingUngrab
  - | CrossingGtkGrab
  - | CrossingGtkUngrab
  - | CrossingStateChanged
  - | CrossingTouchBegin
  - | CrossingTouchEnd
  - | CrossingDeviceSwitch
- eventCrossingMode :: EventM ECrossing CrossingMode
- data NotifyType
  - = NotifyAncestor
  - | NotifyVirtual
  - | NotifyInferior
  - | NotifyNonlinear
  - | NotifyNonlinearVirtual
  - | NotifyUnknown
- eventNotifyType :: EventM ECrossing NotifyType
- eventCrossingFocus :: EventM ECrossing Bool
- eventFocusIn :: EventM EFocus Bool
- eventPosition :: EventM EConfigure (Int, Int)
- eventSize :: EventM EConfigure (Int, Int)
- eventProperty :: EventM EProperty Atom
- data WindowState
  - = WindowStateWithdrawn
  - | WindowStateIconified
  - | WindowStateMaximized
  - | WindowStateSticky
  - | WindowStateFullscreen
  - | WindowStateAbove
  - | WindowStateBelow
  - | WindowStateFocused
  - | WindowStateTiled
- eventWindowStateChanged :: EventM EWindowState [WindowState]
- eventWindowState :: EventM EWindowState [WindowState]
- data OwnerChange
  - = OwnerChangeNewOwner
  - | OwnerChangeDestroy
  - | OwnerChangeClose
- eventChangeReason :: EventM EOwnerChange OwnerChange
- eventSelection :: EventM EOwnerChange SelectionTag
- eventSelectionTime :: EventM EOwnerChange TimeStamp
- eventKeyboardGrab :: EventM EGrabBroken Bool
- eventImplicit :: EventM EGrabBroken Bool
- eventGrabWindow :: EventM EGrabBroken (Maybe DrawWindow)
- data Modifier
  - = Shift
  - | Lock
  - | Control
  - | Alt
  - | Alt2
  - | Alt3
  - | Alt4
  - | Alt5
  - | Button1
  - | Button2
  - | Button3
  - | Button4
  - | Button5
  - | MODIFIER_RESERVED_13_MASK
  - | MODIFIER_RESERVED_14_MASK
  - | MODIFIER_RESERVED_15_MASK
  - | MODIFIER_RESERVED_16_MASK
  - | MODIFIER_RESERVED_17_MASK
  - | MODIFIER_RESERVED_18_MASK
  - | MODIFIER_RESERVED_19_MASK
  - | MODIFIER_RESERVED_20_MASK
  - | MODIFIER_RESERVED_21_MASK
  - | MODIFIER_RESERVED_22_MASK
  - | MODIFIER_RESERVED_23_MASK
  - | MODIFIER_RESERVED_24_MASK
  - | MODIFIER_RESERVED_25_MASK
  - | Super
  - | Hyper
  - | Meta
  - | MODIFIER_RESERVED_29_MASK
  - | Release
  - | ModifierMask
- type TimeStamp = Word32
- currentTime :: TimeStamp
- tryEvent :: EventM any () -> EventM any Bool
- stopEvent :: EventM any ()

# Detail

This modules provides a monad that encapsulates the information in an event.

The events a widget can receive are defined in Graphics.UI.Gtk.Abstract.Widget. Every event carries additional information which is accessible through functions in the `EventM` monad. For instance, every event is associated with a `DrawWindow` which is accessed using the `eventWindow` accessor function. Other information is only available in one specific event. For example, the area that has to be redrawn, accessed by `eventArea` is only available in the `exposeEvent`. Indeed, you can only call `eventArea` if the first type parameter of `EventM` is the phantom type `EExpose`. (A phantom type is a type for which no values exist and which is only used to enforce certain constraints on the usage of functions such as `eventArea`.) Some information is available in several but not all events. In order to express these constraints the module defines type classes whose names start with `Has...` but which are not exported, implying that no new instance can be created. (They could be called phantom type classes.) For instance, the mouse pointer coordinates can be retrieved using the function `eventCoordinates` which requires that the first type parameter of `EventM` is in the class `HasCoordinates`. The module supplies instance of class `HasCoordinates` for the types `EButton`, `ECrossing`, `EMotion` and `EScroll`. Thus for all events that require an `EventM` action with one of the types above, the accessor function `eventCoordinates` may be used.

Note that an event handler must always returns `True` if the event was handled or `False` if the event should be dealt with by another event handler. For instance, a handler for a key press should return `False` if the pressed key is not one of those that the widget reacts to. In this case the event is passed to the parent widgets. This ensures that pressing, say, `Alt-F` opens the file menu even if the current input focus is in a text entry widget. In order to facilitate writing handlers that may abort handling an event, this module provides the function `tryEvent`. This function catches pattern match exceptions and returns `False`. If the signal successfully runs to its end, it returns `True`. A typical use is as follows:

    widget `on` keyPressEvent $ tryEvent $ do
      [Control] <- eventModifier
      "Return" <- eventKeyName
      liftIO $ putStrLn "Ctrl-Return pressed"

The rationale is that the action will throw an exception if the two event functions `eventModifier` and `eventKeyName` return something else than what is stated in the pattern. When no exception is thrown, execution continues to the last statement where the event is processed, here we merely print a message. Note that the return value of this statement must be `()` since `tryEvent` always assumes that the function handeled the event if no exception is thrown. A handler wrapped by `tryEvent` can also indicate that it cannot handle the given event by calling `stopEvent`.

Finally, not that the `EventM` monad wraps the `IO` monad. As such you can (and usually have to) use `liftIO` to execute `IO` functions.

# Event monad and type tags

type EventM t = ReaderT (Ptr t) IO #

A monad providing access to data in an event.

data EAny #

A tag for events that do not carry any event-specific information.

data EKey #

A tag for key events.

data EButton #

A tag for Button events.

data EScroll #

A tag for Scroll events.

data EMotion #

A tag for Motion events.

data EExpose #

A tag for Expose events.

data EVisibility #

A tag for Visibility events.

data ECrossing #

A tag for Crossing events.

data EFocus #

A tag for Focus events.

data EConfigure #

A tag for Configure events.

data EProperty #

A tag for Property events.

data EProximity #

A tag for Proximity events.

data EWindowState #

A tag for WindowState event.

data EOwnerChange #

A tag for OwnerChange events.

data EGrabBroken #

A tag for GrabBroken events.

# Accessor functions for event information

eventWindow :: EventM any DrawWindow #

Retrieve the `DrawWindow` that this event relates to.

eventSent :: EventM any Bool #

Query if this event was sent sent explicitly by the application (rather than being generated by human interaction).

eventCoordinates :: HasCoordinates t => EventM t (Double, Double) #

Retrieve the `(x,y)` coordinates of the mouse.

eventRootCoordinates :: HasRootCoordinates t => EventM t (Double, Double) #

Retrieve the `(x,y)` coordinates of the mouse relative to the root (origin) of the screen.

eventModifier :: HasModifier t => EventM t [Modifier] #

Query the modifier keys that were depressed when the event happened. Sticky modifiers such as CapsLock are omitted in the return value. Use `eventModifierAll` your application requires all modifiers. Use `eventModifierMouse` if you just need the mouse buttons.

eventModifierAll :: HasModifier t => EventM t [Modifier] #

Query the modifier keys that were depressed when the event happened. The result includes sticky modifiers such as CapsLock. Normally, `eventModifier` is more appropriate in applications.

eventModifierMouse :: HasModifier t => EventM t [Modifier] #

Query the mouse buttons that were depressed when the event happened.

eventTime :: HasTime t => EventM t TimeStamp #

Query the time when the event occurred.

eventKeyVal :: EventM EKey KeyVal #

The key value. See `KeyVal`.

eventKeyName :: EventM EKey DefaultGlibString #

The key value as a string. See `KeyVal`.

eventHardwareKeycode :: EventM EKey KeyCode #

The hardware key code.

eventKeyboardGroup :: EventM EKey Word8 #

The keyboard group.

data MouseButton #

Mouse buttons.

Constructors

LeftButton |
MiddleButton |
RightButton |
OtherButton Int |

Instances

Enum MouseButton # |
Methods succ :: MouseButton -> MouseButton # pred :: MouseButton -> MouseButton # toEnum :: Int -> MouseButton # fromEnum :: MouseButton -> Int # enumFrom :: MouseButton -> [MouseButton] # enumFromThen :: MouseButton -> MouseButton -> [MouseButton] # enumFromTo :: MouseButton -> MouseButton -> [MouseButton] # enumFromThenTo :: MouseButton -> MouseButton -> MouseButton -> [MouseButton] #
Eq MouseButton # |
Methods (==) :: MouseButton -> MouseButton -> Bool # (/=) :: MouseButton -> MouseButton -> Bool #
Show MouseButton # |
Methods showsPrec :: Int -> MouseButton -> ShowS # show :: MouseButton -> String # showList :: [MouseButton] -> ShowS #

eventButton :: EventM EButton MouseButton #

Query the mouse buttons.

data Click #

Type of mouse click

Constructors

SingleClick |
DoubleClick |
TripleClick |
ReleaseClick |

Instances

Enum Click # |
Methods succ :: Click -> Click # pred :: Click -> Click # toEnum :: Int -> Click # fromEnum :: Click -> Int # enumFrom :: Click -> [Click] # enumFromThen :: Click -> Click -> [Click] # enumFromTo :: Click -> Click -> [Click] # enumFromThenTo :: Click -> Click -> Click -> [Click] #
Eq Click # |
Methods (==) :: Click -> Click -> Bool # (/=) :: Click -> Click -> Bool #
Show Click # |
Methods showsPrec :: Int -> Click -> ShowS # show :: Click -> String # showList :: [Click] -> ShowS #

eventClick :: EventM EButton Click #

data ScrollDirection #

in which direction was scrolled?

Constructors

ScrollUp |
ScrollDown |
ScrollLeft |
ScrollRight |
ScrollSmooth |

Instances

Enum ScrollDirection # |
Methods succ :: ScrollDirection -> ScrollDirection # pred :: ScrollDirection -> ScrollDirection # toEnum :: Int -> ScrollDirection # fromEnum :: ScrollDirection -> Int # enumFrom :: ScrollDirection -> [ScrollDirection] # enumFromThen :: ScrollDirection -> ScrollDirection -> [ScrollDirection] # enumFromTo :: ScrollDirection -> ScrollDirection -> [ScrollDirection] # enumFromThenTo :: ScrollDirection -> ScrollDirection -> ScrollDirection -> [ScrollDirection] #
Eq ScrollDirection # |
Methods (==) :: ScrollDirection -> ScrollDirection -> Bool # (/=) :: ScrollDirection -> ScrollDirection -> Bool #
Show ScrollDirection # |
Methods showsPrec :: Int -> ScrollDirection -> ShowS # show :: ScrollDirection -> String # showList :: [ScrollDirection] -> ShowS #

eventScrollDirection :: EventM EScroll ScrollDirection #

Query the direction of scrolling.

eventIsHint :: EventM EMotion Bool #

Check if the motion event is only a hint rather than the full mouse movement information.

eventRequestMotions :: EventM EMotion () #

Request more motion notifies if this event is a motion notify hint event.

This action should be used instead of `drawWindowGetPointer` to request further motion notifies, because it also works for extension events where motion notifies are provided for devices other than the core pointer.

Coordinate extraction, processing and requesting more motion events from a `motionNotifyEvent` usually works like this:

    on widget motionNotifyEvent $ do
      (x, y) <- eventCoordinates
      -- handle the x,y motion:
      ...
      -- finally, notify that we are ready to get more motion events:
      eventRequestMotions

eventArea :: EventM EExpose Rectangle #

Query a bounding box of the region that needs to be updated.

data VisibilityState #

visibility of a window

Constructors

VisibilityUnobscured |
VisibilityPartialObscured |
VisibilityFullyObscured |

Instances

Enum VisibilityState # |
Methods succ :: VisibilityState -> VisibilityState # pred :: VisibilityState -> VisibilityState # toEnum :: Int -> VisibilityState # fromEnum :: VisibilityState -> Int # enumFrom :: VisibilityState -> [VisibilityState] # enumFromThen :: VisibilityState -> VisibilityState -> [VisibilityState] # enumFromTo :: VisibilityState -> VisibilityState -> [VisibilityState] # enumFromThenTo :: VisibilityState -> VisibilityState -> VisibilityState -> [VisibilityState] #
Eq VisibilityState # |
Methods (==) :: VisibilityState -> VisibilityState -> Bool # (/=) :: VisibilityState -> VisibilityState -> Bool #
Show VisibilityState # |
Methods showsPrec :: Int -> VisibilityState -> ShowS # show :: VisibilityState -> String # showList :: [VisibilityState] -> ShowS #

eventVisibilityState :: EventM EVisibility VisibilityState #

Get the visibility status of a window.

data CrossingMode #

How focus is crossing the widget.

Constructors

CrossingNormal |
CrossingGrab |
CrossingUngrab |
CrossingGtkGrab |
CrossingGtkUngrab |
CrossingStateChanged |
CrossingTouchBegin |
CrossingTouchEnd |
CrossingDeviceSwitch |

Instances

Enum CrossingMode # |
Methods succ :: CrossingMode -> CrossingMode # pred :: CrossingMode -> CrossingMode # toEnum :: Int -> CrossingMode # fromEnum :: CrossingMode -> Int # enumFrom :: CrossingMode -> [CrossingMode] # enumFromThen :: CrossingMode -> CrossingMode -> [CrossingMode] # enumFromTo :: CrossingMode -> CrossingMode -> [CrossingMode] # enumFromThenTo :: CrossingMode -> CrossingMode -> CrossingMode -> [CrossingMode] #
Eq CrossingMode # |
Methods (==) :: CrossingMode -> CrossingMode -> Bool # (/=) :: CrossingMode -> CrossingMode -> Bool #
Show CrossingMode # |
Methods showsPrec :: Int -> CrossingMode -> ShowS # show :: CrossingMode -> String # showList :: [CrossingMode] -> ShowS #

eventCrossingMode :: EventM ECrossing CrossingMode #

Get the mode of the mouse cursor crossing a window.

data NotifyType #

Information on from what level of the widget hierarchy the mouse cursor came.

`NotifyAncestor`
  The window is entered from an ancestor or left towards an ancestor.
`NotifyVirtual`
  The pointer moves between an ancestor and an inferior of the window.
`NotifyInferior`
  The window is entered from an inferior or left towards an inferior.
`NotifyNonlinear`
  The window is entered from or left towards a window which is neither an ancestor nor an inferior.
`NotifyNonlinearVirtual`
  The pointer moves between two windows which are not ancestors of each other and the window is part of the ancestor chain between one of these windows and their least common ancestor.
`NotifyUnknown`
  The level change does not fit into any of the other categories or could not be determined.

Constructors

NotifyAncestor |
NotifyVirtual |
NotifyInferior |
NotifyNonlinear |
NotifyNonlinearVirtual |
NotifyUnknown |

Instances

Enum NotifyType # |
Methods succ :: NotifyType -> NotifyType # pred :: NotifyType -> NotifyType # toEnum :: Int -> NotifyType # fromEnum :: NotifyType -> Int # enumFrom :: NotifyType -> [NotifyType] # enumFromThen :: NotifyType -> NotifyType -> [NotifyType] # enumFromTo :: NotifyType -> NotifyType -> [NotifyType] # enumFromThenTo :: NotifyType -> NotifyType -> NotifyType -> [NotifyType] #
Eq NotifyType # |
Methods (==) :: NotifyType -> NotifyType -> Bool # (/=) :: NotifyType -> NotifyType -> Bool #
Show NotifyType # |
Methods showsPrec :: Int -> NotifyType -> ShowS # show :: NotifyType -> String # showList :: [NotifyType] -> ShowS #

eventNotifyType :: EventM ECrossing NotifyType #

Get the notify type of the mouse cursor crossing a window.

eventCrossingFocus :: EventM ECrossing Bool #

Query if the window has the focus or is an inferior window.

eventFocusIn :: EventM EFocus Bool #

Query if a window gained focus (`True`) or lost the focus (`False`).

eventPosition :: EventM EConfigure (Int, Int) #

Get the `(x,y)` position of the window within the parent window.

eventSize :: EventM EConfigure (Int, Int) #

Get the new size of the window as `(width,height)`.

eventProperty :: EventM EProperty Atom #

data WindowState #

The state a `DrawWindow` is in.

Constructors

WindowStateWithdrawn |
WindowStateIconified |
WindowStateMaximized |
WindowStateSticky |
WindowStateFullscreen |
WindowStateAbove |
WindowStateBelow |
WindowStateFocused |
WindowStateTiled |

Instances

Bounded WindowState # |
Methods minBound :: WindowState # maxBound :: WindowState #
Enum WindowState # | These are hints for the window manager that indicate what type of function the window has. The window manager can use this when determining decoration and behaviour of the window. The hint must be set before mapping the window. See the extended window manager hints specification for more details about window types.
Methods succ :: WindowState -> WindowState # pred :: WindowState -> WindowState # toEnum :: Int -> WindowState # fromEnum :: WindowState -> Int # enumFrom :: WindowState -> [WindowState] # enumFromThen :: WindowState -> WindowState -> [WindowState] # enumFromTo :: WindowState -> WindowState -> [WindowState] # enumFromThenTo :: WindowState -> WindowState -> WindowState -> [WindowState] #
Eq WindowState # |
Methods (==) :: WindowState -> WindowState -> Bool # (/=) :: WindowState -> WindowState -> Bool #
Show WindowState # |
Methods showsPrec :: Int -> WindowState -> ShowS # show :: WindowState -> String # showList :: [WindowState] -> ShowS #
Flags WindowState # |

eventWindowStateChanged :: EventM EWindowState [WindowState] #

Query which window state bits have changed.

eventWindowState :: EventM EWindowState [WindowState] #

Query the new window state.

data OwnerChange #

Constructors

OwnerChangeNewOwner |
OwnerChangeDestroy |
OwnerChangeClose |

Instances

Enum OwnerChange # |
Methods succ :: OwnerChange -> OwnerChange # pred :: OwnerChange -> OwnerChange # toEnum :: Int -> OwnerChange # fromEnum :: OwnerChange -> Int # enumFrom :: OwnerChange -> [OwnerChange] # enumFromThen :: OwnerChange -> OwnerChange -> [OwnerChange] # enumFromTo :: OwnerChange -> OwnerChange -> [OwnerChange] # enumFromThenTo :: OwnerChange -> OwnerChange -> OwnerChange -> [OwnerChange] #
Eq OwnerChange # |
Methods (==) :: OwnerChange -> OwnerChange -> Bool # (/=) :: OwnerChange -> OwnerChange -> Bool #
Show OwnerChange # |
Methods showsPrec :: Int -> OwnerChange -> ShowS # show :: OwnerChange -> String # showList :: [OwnerChange] -> ShowS #

eventChangeReason :: EventM EOwnerChange OwnerChange #

Query why a seleciton changed its owner.

eventSelection :: EventM EOwnerChange SelectionTag #

Query what selection changed its owner.

eventSelectionTime :: EventM EOwnerChange TimeStamp #

Query the time when the selection was taken over.

eventKeyboardGrab :: EventM EGrabBroken Bool #

Check if a keyboard (`True`) or a mouse pointer grap (`False`) was broken.

eventImplicit :: EventM EGrabBroken Bool #

Check if a grab was broken implicitly.

eventGrabWindow :: EventM EGrabBroken (Maybe DrawWindow) #

Get the new window that owns the grab or `Nothing` if the window is not part of this application.

# Auxilliary Definitions

data Modifier #

Keyboard modifiers that are depressed when the user presses a key or a mouse button.

- This data type is used to build lists of modifers that were active during an event.
- The Apple key on Macintoshs is mapped to `Alt2` and the `Meta` key (if available).
- Since Gtk 2.10, there are also `Super`, `Hyper` and `Meta` modifiers which are simply generated from `Alt` .. `Compose` modifier keys, depending on the mapping used by the windowing system. Due to one key being mapped to e.g. `Alt2` and `Meta`, you shouldn't pattern match directly against a certain key but check whether a key is in the list using the `elem` function, say.

Constructors

Shift |
Lock |
Control |
Alt |
Alt2 |
Alt3 |
Alt4 |
Alt5 |
Button1 |
Button2 |
Button3 |
Button4 |
Button5 |
MODIFIER_RESERVED_13_MASK |
MODIFIER_RESERVED_14_MASK |
MODIFIER_RESERVED_15_MASK |
MODIFIER_RESERVED_16_MASK |
MODIFIER_RESERVED_17_MASK |
MODIFIER_RESERVED_18_MASK |
MODIFIER_RESERVED_19_MASK |
MODIFIER_RESERVED_20_MASK |
MODIFIER_RESERVED_21_MASK |
MODIFIER_RESERVED_22_MASK |
MODIFIER_RESERVED_23_MASK |
MODIFIER_RESERVED_24_MASK |
MODIFIER_RESERVED_25_MASK |
Super |
Hyper |
Meta |
MODIFIER_RESERVED_29_MASK |
Release |
ModifierMask |

Instances

Bounded Modifier # |
Methods minBound :: Modifier # maxBound :: Modifier #
Enum Modifier # |
Methods succ :: Modifier -> Modifier # pred :: Modifier -> Modifier # toEnum :: Int -> Modifier # fromEnum :: Modifier -> Int # enumFrom :: Modifier -> [Modifier] # enumFromThen :: Modifier -> Modifier -> [Modifier] # enumFromTo :: Modifier -> Modifier -> [Modifier] # enumFromThenTo :: Modifier -> Modifier -> Modifier -> [Modifier] #
Eq Modifier # |
Methods (==) :: Modifier -> Modifier -> Bool # (/=) :: Modifier -> Modifier -> Bool #
Show Modifier # |
Methods showsPrec :: Int -> Modifier -> ShowS # show :: Modifier -> String # showList :: [Modifier] -> ShowS #
Flags Modifier # |

type TimeStamp = Word32 #

The time (in milliseconds) when an event happened. This is used mostly for ordering events and responses to events.

currentTime :: TimeStamp #

Represents the current time, and can be used anywhere a time is expected.

tryEvent :: EventM any () -> EventM any Bool #

Execute an event handler and assume it handled the event unless it threw a pattern match exception or calls mzero (e.g. via guard).

stopEvent :: EventM any () #

Explicitly stop the handling of an event. This function should only be called inside a handler that is wrapped with `tryEvent`. (It merely throws a bogus pattern matching error which `tryEvent` interprets as if the handler does not handle the event.)