class ViewTreeWatcher

A method of obtaining view tree snapshots for a particular view, the "context

view", and its child views, if any. The returned data is a sequence of

snapshots during the period of observation, which starts at the client's

prior Watch() call's [`epoch_end`] (or zx.Time 0), and ending at the

current [`epoch_end`]. The timebase is ZX_CLOCK_MONOTONIC.

Clients typically obtain a `ViewTreeWatcher` capability from within a test,

and it is not generally possible to obtain outside of a test environment.

For more information see `fuchsia.ui.observation.test.Registry` and

`fuchsia.ui.test.scene.Controller`.

Usage note. With this protocol, a client can watch for changes to the view

tree over which it has authority. For example, if a client owns view A, then

A serves as the context view for A's subtree (i.e., a "root view"), where A

is a parent of view B, and B is a parent of view C. The client can then

observe key lifecycle events in all of A, B, and C, such as newly connected

views, changes to view position and size, etc. In doing so, a client can

gate its actions on changes to the view tree, in a reliable and ergonomic

manner. For example, a client can wait for a descendant view C to become

connected before requesting a focus transfer to C.

Configuration: The context view is determined outside of this protocol.

Frequency: A client can receive one or more snapshots per frame. Clients

should not "count snapshots", as the per-frame snapshot count can be

non-deterministic. Instead, clients should look for specific conditions on

the snapshot state.

Issuance: If the context view is disconnected from a display, no

frames are issued on behalf of the context view, and a Watch() call will

sit quietly.

Lifecycle: The server endpoint is closed when the context view dies.