Struct TestBackend 

pub struct TestBackend<C: Component> { /* private fields */ }

Headless runtime for unit-testing components.

TestBackend runs the same reconciliation, nested-component expansion, and message processing as AppRunner, but without entering terminal raw mode or rendering via a backend.

This is intended for:

• verifying state transitions in response to messages
• validating view() output (as an Element)
• exercising Command scheduling and message routing
• testing keyboard-driven flows via send_key

Implementations

impl<C> TestBackend<C>

where C: Component,

pub fn new(component: C) -> Self

where C::Properties: Default,

Mount a root component with default properties.

pub fn new_transcript(component: C) -> Self

where C::Properties: Default,

pub fn new_with_props(component: C, props: C::Properties) -> Self

Mount a root component with explicit properties.

pub fn new_transcript_with_props(component: C, props: C::Properties) -> Self

pub fn viewport(&self) -> Rect

Returns the current viewport used for layout.

pub fn transcript_history_len(&self) -> usize

pub fn transcript_replay_summary( &self, include_live_viewport: bool, ) -> Vec<String>

pub fn set_viewport(&mut self, viewport: Rect)

Set the viewport used for layout.

pub fn link(&self) -> Link<C::Message>

Returns a cloneable link to the root component.

pub fn component(&self) -> &C

Borrow the mounted component instance.

pub fn component_mut(&mut self) -> &mut C

Mutably borrow the mounted component instance.

pub fn state(&self) -> &C::State

Borrow the component state.

pub fn state_mut(&mut self) -> &mut C::State

Mutably borrow the component state.

pub fn focused(&self) -> Option<NodeId>

Returns the currently focused node, if any.

pub fn set_focused(&mut self, id: NodeId)

Manually set which node has focus.

pub fn element(&self) -> &Element

Borrow the last rendered Element tree.

pub fn content_min_size(&self) -> (u16, u16)

Returns the minimum content size of the last rendered Element tree.

Call Self::render first so this measures the current element, just like Self::element. Returned dimensions are clamped to at least 1 so they are safe to use as capture viewport dimensions.

pub fn enqueue(&self, msg: C::Message)

Enqueue a message for the root component without processing it.

pub fn dispatch(&mut self, msg: C::Message) -> Result<bool>

Enqueue a message and process the runtime until idle.

pub fn send_key(&mut self, key: KeyEvent) -> Result<bool>

Inject a key event through the same dispatch pipeline as the real runner.

1. Dispatches to the focused widget via keyboard::dispatch_key
2. If unhandled, bubbles up through component scopes via on_key
3. If still unhandled, PageUp/PageDown may target one ambient ScrollView
4. Processes any queued messages and re-renders if dirty

Returns true if the focused widget, a bubbling on_key scope, or an ambient page-scroll target consumed the key.

Draining the message queue in Self::pump can re-render without this key having been handled; that work is not counted here so browser embeddings can use the return value for preventDefault without stealing shortcuts when unrelated updates flush.

pub fn send_paste(&mut self, text: &str) -> Result<bool>

Inject a text paste event through the same focused-widget pipeline as the real runner.

pub fn send_mouse(&mut self, ev: MouseEvent) -> Result<bool>

Dispatch a mouse event through the same pipeline as the real runner.

Returns true if the event was handled by any widget or component.

pub fn focus_next(&mut self)

Move focus to the next focusable node (Tab behavior).

pub fn focus_prev(&mut self)

Move focus to the previous focusable node (Shift+Tab behavior).

pub fn pump(&mut self) -> Result<bool>

Process all queued messages and any messages produced by background commands.

Returns true if any update requested a re-render.

pub fn render(&mut self)

Recompute the current Element tree and layout.

pub fn set_screen_background(&mut self, style: Option<Style>)

Set an opt-in root viewport background fill for captures.

Mirrors App::screen_background for headless rendering: pass a resolved style (typically Style::new().bg(color)), or None to leave the background transparent. Useful for previewing a theme’s filled backdrop.

pub fn capture_frame(&self) -> CapturedFrame

Render and capture the current frame output.

pub fn capture_frame_with_margin( &mut self, margin_w: u16, margin_h: u16, ) -> CapturedFrame

Capture a frame using a temporary fit-to-content viewport plus margins.

margin_w and margin_h are columns and rows added to the measured content minimum size. The backend temporarily lays out at (content_min_size + margin), captures via Self::capture_frame, then restores the original viewport and layout before returning.

pub fn capture_ui_snapshot(&self) -> UiSnapshot

Capture a combined visual + semantic UI snapshot.

Call Self::render first so the node tree and layout reflect the latest state.

pub fn capture_ui_snapshot_with_options( &self, options: &UiSnapshotOptions, ) -> UiSnapshot

Capture a UI snapshot with custom describe options.

pub fn capture_ui_snapshot_with_margin( &mut self, margin_w: u16, margin_h: u16, options: &UiSnapshotOptions, ) -> UiSnapshot

Capture a UI snapshot using a temporary fit-to-content viewport plus margins.

margin_w and margin_h are columns and rows added to the measured content minimum size. The backend temporarily lays out at (content_min_size + margin), captures via Self::capture_ui_snapshot_with_options, then restores the original viewport and layout before returning.

pub fn focused_key(&self) -> Option<&Key>

Returns the reconciliation key of the currently focused node, if any.

pub fn hovered(&self) -> Option<NodeId>

Returns the node id currently under the mouse, if any.