script_traits/

lib.rs

/* This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at https://mozilla.org/MPL/2.0/. */

//! This module contains traits in script used generically in the rest of Servo.
//! The traits are here instead of in script so that these modules won't have
//! to depend on script.

#![deny(missing_docs)]
#![deny(unsafe_code)]

use std::cell::Cell;
use std::fmt;

use accesskit::ActionRequest;
use bitflags::bitflags;
use crossbeam_channel::RecvTimeoutError;
#[cfg(feature = "devtools")]
use devtools_traits::ScriptToDevtoolsControlMsg;
use embedder_traits::user_contents::{UserContentManagerId, UserContents};
use embedder_traits::{
    EmbedderControlId, EmbedderControlResponse, FocusSequenceNumber, InputEvent, InputEventAndId,
    JavaScriptEvaluationId, MediaSessionActionType, MouseButton, PaintHitTestResult,
    ScriptToEmbedderChan, Theme, ViewportDetails, WebDriverScriptCommand,
};
use euclid::{Scale, Size2D};
use fonts_traits::{SystemFontServiceProxySender, WebFontLoadEvent};
use keyboard_types::Modifiers;
use malloc_size_of::malloc_size_of_is_0;
use malloc_size_of_derive::MallocSizeOf;
use media::WindowGLContext;
use net_traits::ResourceThreads;
use paint_api::{CrossProcessPaintApi, PinchZoomInfos};
use pixels::PixelFormat;
use profile_traits::mem;
use rustc_hash::FxHashMap;
use serde::{Deserialize, Serialize};
use servo_base::Epoch;
#[cfg(feature = "devtools")]
use servo_base::generic_channel::GenericCallback;
use servo_base::generic_channel::{GenericReceiver, GenericSender};
use servo_base::id::{
    BrowsingContextId, HistoryStateId, LCPCandidateID, PipelineId, PipelineNamespaceId,
    PipelineNamespaceRequest, ScriptEventLoopId, ServiceWorkerId, WebViewId,
};
#[cfg(feature = "bluetooth")]
use servo_bluetooth_traits::BluetoothRequest;
#[cfg(feature = "webgl")]
use servo_canvas_traits::webgl::WebGLPipeline;
use servo_config::prefs::PrefValue;
use servo_constellation_traits::{
    KeyboardScroll, LoadData, NavigationHistoryBehavior, PaintMetricEvent, RemoteFocusOperation,
    ScriptToConstellationSender, ScrollStateUpdate, ServiceWorkerState, StructuredSerializedData,
    TargetSnapshotParams, WindowSizeType,
};
use servo_url::{ImmutableOrigin, OriginSnapshot, ServoUrl};
use storage_traits::StorageThreads;
use storage_traits::webstorage_thread::WebStorageType;
use strum::IntoStaticStr;
use style_traits::{CSSPixel, SpeculativePainter};
use stylo_atoms::Atom;
#[cfg(feature = "webgpu")]
use webgpu_traits::WebGPUMsg;
use webrender_api::ImageKey;
use webrender_api::units::DevicePixel;

/// A representation of the `WebView` state that is kept in each `EventLoop`
/// and in the `Constellation`.
#[derive(Clone, Debug, Deserialize, MallocSizeOf, Serialize)]
pub struct WebViewState {
    /// The [`WebViewId`] of this `WebView`.
    pub id: WebViewId,
    /// The platform [`Theme`] to use for this `WebView`.
    pub theme: Cell<Theme>,
    /// Whether or not the `WebView` has system focus. More than one `WebView` may have
    /// system focus at one time.
    pub has_system_focus: Cell<bool>,
}

/// The initial data required to create a new `Pipeline` attached to an existing `ScriptThread`.
#[derive(Clone, Debug, Deserialize, Serialize)]
pub struct NewPipelineInfo {
    /// The [`WebViewState`] of the `WebView` that this pipeline belongs to.
    pub webview_state: WebViewState,
    /// The ID of the parent pipeline and frame type, if any.
    /// If `None`, this is a root pipeline.
    pub parent_info: Option<PipelineId>,
    /// Id of the newly-created pipeline.
    pub new_pipeline_id: PipelineId,
    /// Id of the browsing context associated with this pipeline.
    pub browsing_context_id: BrowsingContextId,
    /// Id of the opener, if any
    pub opener: Option<BrowsingContextId>,
    /// Network request data which will be initiated by the script thread.
    pub load_data: LoadData,
    /// Initial [`ViewportDetails`] for this layout.
    pub viewport_details: ViewportDetails,
    /// The ID of the `UserContentManager` associated with this new pipeline's `WebView`.
    pub user_content_manager_id: Option<UserContentManagerId>,
    /// A snapshot of the navigation parameters of the target of this navigation.
    pub target_snapshot_params: TargetSnapshotParams,
    /// Name of this iframe, if any
    pub frame_name: Option<String>,
}

/// When a pipeline is closed, should its browsing context be discarded too?
#[derive(Clone, Copy, Debug, Deserialize, Eq, Hash, PartialEq, Serialize)]
pub enum DiscardBrowsingContext {
    /// Discard the browsing context
    Yes,
    /// Don't discard the browsing context
    No,
}

/// Is a document fully active, active or inactive?
/// A document is active if it is the current active document in its session history,
/// it is fuly active if it is active and all of its ancestors are active,
/// and it is inactive otherwise.
///
/// * <https://html.spec.whatwg.org/multipage/#active-document>
/// * <https://html.spec.whatwg.org/multipage/#fully-active>
#[derive(Clone, Copy, Debug, Deserialize, Eq, Hash, MallocSizeOf, PartialEq, Serialize)]
pub enum DocumentActivity {
    /// An inactive document
    Inactive,
    /// An active but not fully active document
    Active,
    /// A fully active document
    FullyActive,
}

/// Type of recorded progressive web metric
#[derive(Clone, Debug, Deserialize, Serialize)]
pub enum ProgressiveWebMetricType {
    /// Time to first Paint
    FirstPaint,
    /// Time to first contentful paint
    FirstContentfulPaint,
    /// Time for the largest contentful paint
    LargestContentfulPaint {
        /// The identity of the element, if any.
        id: LCPCandidateID,
    },
    /// Time to interactive
    TimeToInteractive,
}

/// The reason why the pipeline id of an iframe is being updated.
#[derive(Clone, Copy, Debug, Deserialize, Eq, Hash, MallocSizeOf, PartialEq, Serialize)]
pub enum UpdatePipelineIdReason {
    /// The pipeline id is being updated due to a navigation.
    Navigation,
    /// The pipeline id is being updated due to a history traversal.
    Traversal,
}

/// Messages sent to the `ScriptThread` event loop from the `Constellation`, `Paint`, and (for
/// now) `Layout`.
#[derive(Deserialize, IntoStaticStr, Serialize)]
pub enum ScriptThreadMessage {
    /// Update the state of a service worker.
    UpdateServiceWorkerState {
        /// The identity of the service worker.
        worker_id: ServiceWorkerId,
        /// The new state of the service worker.
        state: ServiceWorkerState,
        /// The pipeline.
        pipeline_id: PipelineId,
    },
    /// Span a new `Pipeline` in this `ScriptThread` and start fetching the contents
    /// according to the provided `LoadData`. This will ultimately create a `Window`
    /// and all associated data structures such as `Layout` in the `ScriptThread`.
    SpawnPipeline(NewPipelineInfo),
    /// Takes the associated window proxy out of "delaying-load-events-mode",
    /// used if a scheduled navigated was refused by the embedder.
    /// <https://html.spec.whatwg.org/multipage/#delaying-load-events-mode>
    StopDelayingLoadEventsMode(PipelineId),
    /// Window resized.  Sends a DOM event eventually, but first we combine events.
    Resize(PipelineId, ViewportDetails, WindowSizeType),
    /// Inform the ScriptThread that some aspect of the WebViewState has changed.
    UpdateWebViewState(WebViewState),
    /// Notifies script that window has been resized but to not take immediate action.
    ResizeInactive(PipelineId, ViewportDetails),
    /// Window switched from fullscreen mode.
    ExitFullScreen(PipelineId),
    /// Notifies the script that the document associated with this pipeline should 'unload'.
    UnloadDocument(PipelineId),
    /// Notifies the script that a pipeline should be closed.
    ExitPipeline(WebViewId, PipelineId, DiscardBrowsingContext),
    /// Notifies the script that the whole thread should be closed.
    ExitScriptThread,
    /// Sends a DOM event.
    SendInputEvent(WebViewId, PipelineId, ConstellationInputEvent),
    /// Request that the given pipeline refresh the cursor by doing a hit test at the most
    /// recently hovered cursor position and resetting the cursor. This happens after a
    /// display list update is rendered.
    RefreshCursor(PipelineId),
    /// Requests that the script thread immediately send the constellation the title of a pipeline.
    GetTitle(PipelineId),
    /// Retrieve the origin of a document for a pipeline, in case a child needs to retrieve the
    /// origin of a parent in a different script thread.
    GetDocumentOrigin(PipelineId, GenericSender<Option<OriginSnapshot>>),
    /// Retrieve the origin and internal ancestor origin objects list of a
    /// `Document` for a given `PipelineId`, in case a child needs to retrieve
    /// the origin of a parent in a different event loop.
    GetDocumentOriginDetails(
        PipelineId,
        GenericSender<Option<(OriginSnapshot, Vec<ImmutableOrigin>)>>,
    ),
    /// Notifies script thread of a change to one of its document's activity
    SetDocumentActivity(PipelineId, DocumentActivity),
    /// Set whether to use less resources by running timers at a heavily limited rate.
    SetThrottled(PipelineId, bool),
    /// Notifies script thread that a url should be loaded in this iframe.
    /// PipelineId is for the parent, BrowsingContextId is for the nested browsing context
    NavigateIframe(
        PipelineId,
        BrowsingContextId,
        LoadData,
        NavigationHistoryBehavior,
        TargetSnapshotParams,
    ),
    /// Post a message to a given window.
    PostMessage {
        /// The target of the message.
        target: PipelineId,
        /// The webview associated with the source pipeline.
        source_webview: WebViewId,
        /// The ancestry of browsing context associated with the source,
        /// starting with the source itself.
        source_with_ancestry: Vec<BrowsingContextId>,
        /// The expected origin of the target.
        target_origin: Option<ImmutableOrigin>,
        /// The source origin of the message.
        /// <https://html.spec.whatwg.org/multipage/#dom-messageevent-origin>
        source_origin: ImmutableOrigin,
        /// The data to be posted.
        data: Box<StructuredSerializedData>,
    },
    /// Updates the current pipeline ID of a given iframe.
    /// First PipelineId is for the parent, second is the new PipelineId for the frame.
    UpdatePipelineId(
        PipelineId,
        BrowsingContextId,
        WebViewId,
        PipelineId,
        UpdatePipelineIdReason,
    ),
    /// Updates the history state and url of a given pipeline.
    UpdateHistoryState(PipelineId, Option<HistoryStateId>, ServoUrl),
    /// Removes inaccesible history states.
    RemoveHistoryStates(PipelineId, Vec<HistoryStateId>),
    /// Focus a `Document` as part of the focusing steps which focuses all parent `Document`s of a
    /// newly focused `<iframe>`. Note that this is not used for the `Document` and `Element` that
    /// is gaining focus as that is handled locally in the originating `ScriptThread`.
    FocusDocumentAsPartOfFocusingSteps(PipelineId, FocusSequenceNumber, Option<BrowsingContextId>),
    /// Unfocus a `Document` as part of the focusing steps which unfocuses all parent `Document`s of an
    /// `<iframe>` losing focus. This does not do anything for a top-level `Document`, which can never
    /// lose focus (apart from losing system focus, which is a separate concept).
    UnfocusDocumentAsPartOfFocusingSteps(PipelineId, FocusSequenceNumber),
    /// Focus a `Document` and run the focusing steps. This is used in two situations:
    /// - When calling the DOM `focus()` API on a remote `Window` as well as from
    ///   WebDriver. The difference between this and `FocusDocumentAsPartOfFocusingSteps` is that this
    ///   version actually does run the focusing steps and may result in blur and focus events firing
    ///   up the frame tree.
    /// - When doing sequential focus navigation into and out of frames.
    FocusDocument(PipelineId, RemoteFocusOperation),
    /// Passes a webdriver command to the script thread for execution
    WebDriverScriptCommand(PipelineId, WebDriverScriptCommand),
    /// Notifies script thread that all animations are done
    TickAllAnimations(Vec<WebViewId>),
    /// Notifies the script thread that a web font has finished loading.
    ///
    /// This is sent if either the web font loaded successfully, or to notify the script thread
    /// that it should try to resolve `document.fonts.ready` because the font was the last one
    /// loading.
    WebFontLoadFinished(PipelineId, WebFontLoadEvent),
    /// Notify a parent pipeline that one of its `<iframe>` children has finished loading,
    /// either by completing or by being aborted. When completing this causes the `load` event
    /// to be dispatched at the appropriate iframe element.
    IFrameLoadFinished {
        /// The frame that has been marked as loaded.
        target: BrowsingContextId,
        /// The pipeline that contains a frame loading the target pipeline.
        parent: PipelineId,
        /// The pipeline that has completed loading.
        child: PipelineId,
        /// Whether or not the load was finished by aborting.
        aborted: bool,
    },
    /// Cause a `storage` event to be dispatched at the appropriate window.
    /// The strings are key, old value and new value.
    DispatchStorageEvent(
        PipelineId,
        WebStorageType,
        ServoUrl,
        Option<String>,
        Option<String>,
        Option<String>,
    ),
    /// Report an error from a CSS parser for the given pipeline
    #[cfg(feature = "devtools")]
    ReportCSSError(PipelineId, String, u32, u32, String),
    /// Reload the given page.
    Reload(PipelineId),
    /// Notifies the script thread about a new recorded paint metric.
    PaintMetric(PipelineId, PaintMetricEvent),
    /// Notifies the media session about a user requested media session action.
    MediaSessionAction(PipelineId, MediaSessionActionType),
    /// Notifies script thread that WebGPU server has started
    #[cfg(feature = "webgpu")]
    SetWebGPUPort(GenericReceiver<WebGPUMsg>),
    /// `Paint` scrolled and is updating the scroll states of the nodes in the given
    /// pipeline via the Constellation.
    SetScrollStates(PipelineId, ScrollStateUpdate),
    /// Evaluate the given JavaScript and return a result via a corresponding message
    /// to the Constellation.
    EvaluateJavaScript(WebViewId, PipelineId, JavaScriptEvaluationId, String),
    /// A new batch of keys for the image cache for the specific pipeline.
    SendImageKeysBatch(PipelineId, Vec<ImageKey>),
    /// Preferences were updated in the parent process.
    PreferencesUpdated(Vec<(String, PrefValue)>),
    /// Notify the `ScriptThread` that the Servo renderer is no longer waiting on
    /// asynchronous image uploads for the given `Pipeline`. These are mainly used
    /// by canvas to perform uploads while the display list is being built.
    NoLongerWaitingOnAsychronousImageUpdates(PipelineId),
    /// Forward a keyboard scroll operation from an `<iframe>` to a parent pipeline.
    ForwardKeyboardScroll(PipelineId, KeyboardScroll),
    /// Request readiness for a screenshot from the given pipeline. The pipeline will
    /// respond when it is ready to take the screenshot or will not be able to take it
    /// in the future.
    RequestScreenshotReadiness(WebViewId, PipelineId),
    /// A response to a request to show an embedder user interface control.
    EmbedderControlResponse(EmbedderControlId, EmbedderControlResponse),
    /// Set the `UserContents` for the given `UserContentManagerId`. A `ScriptThread` can host many
    /// `WebView`s which share the same `UserContentManager`. Only documents loaded after
    /// the processing of this message will observe the new `UserContents` of the specified
    /// `UserContentManagerId`.
    SetUserContents(UserContentManagerId, UserContents),
    /// Release all data for the given `UserContentManagerId` from the `ScriptThread`'s
    /// `user_contents_for_manager_id` map.
    DestroyUserContentManager(UserContentManagerId),
    /// Update the pinch zoom details of a pipeline. Each `Window` stores a `VisualViewport` DOM
    /// instance that gets updated according to the changes from the `Compositor``.
    UpdatePinchZoomInfos(PipelineId, PinchZoomInfos),
    /// Activate or deactivate accessibility features for the given pipeline, assuming it represents
    /// a document.
    ///
    /// Why only one pipeline? In the Servo API, accessibility is activated on a per-webview basis,
    /// and webviews have a simple one-to-many mapping to pipelines that represent documents. But
    /// those pipelines run in script threads, which complicates things: the pipelines in a webview
    /// may be split across multiple script threads, and the pipelines in a script thread may belong
    /// to multiple webviews. So the simplest approach is to activate it for one pipeline at a time.
    SetAccessibilityActive(PipelineId, bool, Epoch),
    /// Forward the given [`accesskit::ActionRequest`] to the given pipeline.
    ForwardAccessibilityAction(PipelineId, ActionRequest),
    /// Force a garbage collection in this script thread.
    TriggerGarbageCollection,
    /// Inform the `ScriptThread` that the embedder has indicated that the network has gone
    /// online or offline.
    SetNetworkOnlineState(bool),
}

impl fmt::Debug for ScriptThreadMessage {
    fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
        let variant_string: &'static str = self.into();
        write!(formatter, "ConstellationControlMsg::{variant_string}")
    }
}

/// Used to determine if a script has any pending asynchronous activity.
#[derive(Clone, Copy, Debug, Deserialize, PartialEq, Serialize)]
pub enum DocumentState {
    /// The document has been loaded and is idle.
    Idle,
    /// The document is either loading or waiting on an event.
    Pending,
}

bitflags! {
    #[derive(Clone, Copy, Default, Debug, Deserialize, Eq, PartialEq, Serialize)]
    /// <https://w3c.github.io/pointerevents/#dom-mouseevent-buttons>
    pub struct MouseButtons: u16 {
        /// > 1 MUST indicate the primary button of the device (in general, the left
        /// > button or the only button on single-button devices, used to activate a user
        /// > interface control or select text).
        const Primary = 1;
        /// > 2 MUST indicate the secondary button (in general, the right button, often
        /// > used to display a context menu), if present.
        const Secondary = 2;
        /// > 4 MUST indicate the auxiliary button (in general, the middle button, often
        /// > combined with a mouse wheel).
        const Auxiliary = 4;
        /// The 'back' button:
        ///
        /// > Some pointing devices provide or simulate more buttons. To represent such
        /// > buttons, the value MUST be doubled for each successive button (in the binary
        /// > series 8, 16, 32, ... ).
        const Back = 8;
        /// The 'forward' button:
        ///
        /// > Some pointing devices provide or simulate more buttons. To represent such
        /// > buttons, the value MUST be doubled for each successive button (in the binary
        /// > series 8, 16, 32, ... ).
        const Forward = 16;
    }
}

impl MouseButtons {
    /// Returns whether exactly one button is pressed.
    pub fn exactly_one_button_pressed(&self) -> bool {
        // Exactly one button is pressed iff mouse_button_state is a power of 2
        !self.is_empty() && (self.bits() & (self.bits() - 1)) == 0
    }
}

malloc_size_of_is_0!(MouseButtons);

impl TryFrom<MouseButton> for MouseButtons {
    type Error = ();

    fn try_from(button: MouseButton) -> Result<Self, Self::Error> {
        match button {
            MouseButton::Primary => Ok(Self::Primary),
            MouseButton::Secondary => Ok(Self::Secondary),
            MouseButton::Auxiliary => Ok(Self::Auxiliary),
            MouseButton::Back => Ok(Self::Back),
            MouseButton::Forward => Ok(Self::Forward),
            MouseButton::None | MouseButton::Other(_) => Err(()),
        }
    }
}

/// Input events from the embedder that are sent via the `Constellation`` to the `ScriptThread`.
#[derive(Clone, Debug, Deserialize, MallocSizeOf, Serialize)]
pub struct ConstellationInputEvent {
    /// The hit test result of this input event, if any.
    pub hit_test_result: Option<PaintHitTestResult>,
    /// The pressed mouse button state of the constellation when this input
    /// event was triggered.
    pub pressed_mouse_buttons: MouseButtons,
    /// The currently active keyboard modifiers.
    pub active_keyboard_modifiers: Modifiers,
    /// The [`InputEventAndId`] itself.
    pub event: InputEventAndId,
}

impl ConstellationInputEvent {
    /// Returns whether `pressed_mouse_buttons` includes the primary button
    pub fn primary_button_is_pressed(&self) -> bool {
        self.pressed_mouse_buttons.contains(MouseButtons::Primary)
    }

    /// Returns whether `pressed_mouse_buttons` includes the auxiliary (middle) button
    pub fn auxiliary_button_is_pressed(&self) -> bool {
        self.pressed_mouse_buttons.contains(MouseButtons::Auxiliary)
    }

    /// Returns whether this is a compatibility mouse event synthesized for a touch.
    pub fn is_compatibility_event_for_touch(&self) -> bool {
        match self.event.event {
            InputEvent::MouseMove(ref event) => event.is_compatibility_event_for_touch,
            InputEvent::MouseButton(ref event) => event.is_compatibility_event_for_touch,
            _ => false,
        }
    }
}

/// All of the information necessary to create a new [`ScriptThread`] for a new [`EventLoop`].
///
/// NB: *DO NOT* add any Senders or Receivers here! pcwalton will have to rewrite your code if you
/// do! Use IPC senders and receivers instead.
#[derive(Deserialize, Serialize)]
pub struct InitialScriptState {
    /// The id of the script event loop that this state will start. This is used to uniquely
    /// identify an event loop.
    pub id: ScriptEventLoopId,
    /// The sender to use to install the `Pipeline` namespace into this process (if necessary).
    pub namespace_request_sender: GenericSender<PipelineNamespaceRequest>,
    /// A channel with which messages can be sent to us (the script thread).
    pub constellation_to_script_sender: GenericSender<ScriptThreadMessage>,
    /// A port on which messages sent by the constellation to script can be received.
    pub constellation_to_script_receiver: GenericReceiver<ScriptThreadMessage>,
    /// A channel on which messages can be sent to the constellation from script.
    pub script_to_constellation_sender: ScriptToConstellationSender,
    /// A channel which allows script to send messages directly to the Embedder
    /// This will pump the embedder event loop.
    pub script_to_embedder_sender: ScriptToEmbedderChan,
    /// An IpcSender to the `SystemFontService` used to create a `SystemFontServiceProxy`.
    pub system_font_service: SystemFontServiceProxySender,
    /// A channel to the resource manager thread.
    pub resource_threads: ResourceThreads,
    /// A channel to the storage manager thread.
    pub storage_threads: StorageThreads,
    /// A channel to the bluetooth thread.
    #[cfg(feature = "bluetooth")]
    pub bluetooth_sender: GenericSender<BluetoothRequest>,
    /// A channel to the time profiler thread.
    pub time_profiler_sender: profile_traits::time::ProfilerChan,
    /// A channel to the memory profiler thread.
    pub memory_profiler_sender: mem::ProfilerChan,
    /// A channel to the developer tools, if applicable.
    #[cfg(feature = "devtools")]
    pub devtools_server_sender: Option<GenericCallback<ScriptToDevtoolsControlMsg>>,
    /// The ID of the pipeline namespace for this script thread.
    pub pipeline_namespace_id: PipelineNamespaceId,
    /// A channel to the WebGL thread used in this pipeline.
    #[cfg(feature = "webgl")]
    pub webgl_chan: Option<WebGLPipeline>,
    /// The XR device registry
    pub webxr_registry: Option<webxr_api::Registry>,
    /// Access to `Paint` across a process boundary.
    pub cross_process_paint_api: CrossProcessPaintApi,
    /// Application window's GL Context for Media player
    pub player_context: WindowGLContext,
    /// A list of URLs that can access privileged internal APIs.
    pub privileged_urls: Vec<ServoUrl>,
    /// A copy of constellation's `UserContentManagerId` to `UserContents` map.
    pub user_contents_for_manager_id: FxHashMap<UserContentManagerId, UserContents>,
    /// Whether or not the embedder has indicated that networking is online.
    pub online: bool,
}

/// Errors from executing a paint worklet
#[derive(Clone, Debug, Deserialize, Serialize)]
pub enum PaintWorkletError {
    /// Execution timed out.
    Timeout,
    /// No such worklet.
    WorkletNotFound,
}

impl From<RecvTimeoutError> for PaintWorkletError {
    fn from(_: RecvTimeoutError) -> PaintWorkletError {
        PaintWorkletError::Timeout
    }
}

/// Execute paint code in the worklet thread pool.
pub trait Painter: SpeculativePainter {
    /// <https://drafts.css-houdini.org/css-paint-api/#draw-a-paint-image>
    fn draw_a_paint_image(
        &self,
        size: Size2D<f32, CSSPixel>,
        zoom: Scale<f32, CSSPixel, DevicePixel>,
        properties: Vec<(Atom, String)>,
        arguments: Vec<String>,
    ) -> Result<DrawAPaintImageResult, PaintWorkletError>;
}

impl fmt::Debug for dyn Painter {
    fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
        fmt.debug_tuple("Painter")
            .field(&format_args!(".."))
            .finish()
    }
}

/// The result of executing paint code: the image together with any image URLs that need to be loaded.
///
/// TODO: this should return a WR display list. <https://github.com/servo/servo/issues/17497>
#[derive(Clone, Debug, Deserialize, MallocSizeOf, Serialize)]
pub struct DrawAPaintImageResult {
    /// The image height
    pub width: u32,
    /// The image width
    pub height: u32,
    /// The image format
    pub format: PixelFormat,
    /// The image drawn, or None if an invalid paint image was drawn
    pub image_key: Option<ImageKey>,
    /// Drawing the image might have requested loading some image URLs.
    pub missing_image_urls: Vec<ServoUrl>,
}