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# System Messages

Declared in: app/AppDefs.h

### B_MOUSE_WHEEL_CHANGED

Source: The system.

Target: The BWindow of the view the mouse is pointing to, and that of any other view that sets `B_POINTER_EVENTS` in its event mask (see `BView::SetEventMask()`)

Hook: none

`B_MOUSE_WHEEL_CHANGED` is sent when the mouse wheel is distrubed. The message contains these fields:

Field | Type code | Description
"when" | `B_INT64_TYPE` | Event time, in microseconds since 01/01/70
"be:wheel_delta_x" | `B_FLOAT_TYPE` | The change in the wheel's horizontal position as a value between -1.0 (left) and 1.0 (right).
"be:wheel_delta_x" | `B_FLOAT_TYPE` | The change in the wheel's vertical position as a value between -1.0 (down) and 1.0 (up).

### B_OBSERVER_NOTICE_CHANGE

Source: `BHandler::SendNotices()`

Target: Other BHandlers that are watching the notice-sender

Hook: none

`B_OBSERVER_NOTICE_CHANGE` is sent when the source object (a BHandler) invokes `SendNotices()`, which broadcasts a notice to all of its "watchers" (other BHandlers). A BHandler can register to be a watcher by calling `BHandler::StartWatching()`.

Field | Type code | Description
`B_OBSERVE_WHICH_CHANGE` | `B_INT32_TYPE` | The change that caused the notification to be sent, as taken from an argument to `SendNotices()`.
"be:old_what" | `B_INT32_TYPE` | The `what` field from the BMessage that's passed to `SendNotices()`.
Other fields | `...` | The rest of the fields are copied from the message that's passed to `SendNotices()`.

## Channel Control Message

Source: A BChannelControl object

Target: Set through `SetTarget()` et al.

Hook: none

An n-channeled BChannelControl adds the following field to the BControl-defined value-changed message that it sends to its target.

Field | Type code | Description
"be:channel_value" [n] | `B_INT32_TYPE` | The value of each of the object's channels.

This field usurps (but doesn't remove) the "be:value" field that's added by BControl.

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