RenderRepaintBoundary class

A render object that owns a CellBuffer cache for its subtree's paint.

On the first frame (and any frame after something inside it changed), the boundary repaints its subtree into the cache and clears needsPaint. On subsequent frames it skips the subtree walk entirely and composites the cache into the destination (CellBuffer.compositeRectFrom) — one pass over the cached cells instead of a recursive paint chain.

This is a CPU paint-memoization, NOT Flutter's GPU compositing layer — there is no layer tree here, and it does not isolate the subtree from repaint the way a GPU layer does. Two limits to keep in mind: it caches paint, not layout (a render object that isn't marked for layout and gets the same constraints returns its cached size from RenderObject.layout, with or without a boundary), and every rendered frame is still compared cell by cell with the previous one (CellBuffer.diffAgainst), so the blit just repopulates cells that comparison re-examines.

The win is the skipped paint walk: on a localized update, a boundary'd subtree blits its cache instead of re-running its paint chain. Measured on the paint-walk probe this is ~3x even for trivially cheap rows and 6-16x for styled ones — the walk over N siblings costs more than blitting N-1 caches regardless of per-row cost, so the historical "neutral for cheap subtrees" guidance held only for a SINGLE boundary in isolation, not for the one-of-many-changes shape ListView auto-wraps. The cost is one reused cache buffer per boundary (bounded by what's on screen). Reach for a direct boundary when a subtree's neighbour churns and it doesn't; the list case is handled for you.

The boundary is opaque to its caller: parents call paint(buffer, offset) as usual; the cache discipline is internal. Use the RepaintBoundary widget to wrap subtrees that are expensive to paint and change rarely.

Inheritance
Implemented types

Constructors

RenderRepaintBoundary({bool cachingEnabled = true})

Properties

cachingEnabled ↔ bool
Whether this boundary currently caches its subtree's paint.
getter/setter pair
child ↔ RenderObject?
The single child render object, if any.
getter/setter pairoverride
constraints → CellConstraints
The constraints from the most recent layout pass.
no setterinherited
hashCode → int
The hash code for this object.
no setterinherited
hasLayout → bool
Whether this render object has been laid out at least once.
no setterinherited
hitTestsBeyondBounds → bool
Whether a point outside this render object's box can hit something in its subtree. False by default: hit-testing prunes a subtree by its box, as Flutter does, so a pointer event walks one chain of the tree rather than every region. A container that places children outside its box and wants them to stay interactive there — a Stack with an overflowing Positioned — answers true.
no setterinherited
isRepaintBoundary → bool
Whether this render object owns its own paint cache (a CellBuffer it can blit instead of re-walking its subtree's paint). Override to true in subclasses that implement the cache discipline.
no setteroverride
needsLayout → bool
Whether this render object must run performLayout the next time it is reached with the same constraints. Constraints changes always force a new layout even when this flag is false.
no setterinherited
needsPaint ↔ bool
Whether the enclosing boundary's cache has been invalidated. Set true by markNeedsLayout and markNeedsPaintOnly; subclasses that implement a paint cache clear it once they have re-painted into their cache.
getter/setter pairinherited
paintOverflow → CellRect?
Conservative local bounds when this subtree can paint outside size. Null means its layout box contains all paint. Containers derive this from presented children, their offsets and clips. Results are cached; layout or paint invalidation invalidates the enclosing bounds as well. Custom painters that draw beyond their box override computePaintOverflow.
no setterinherited
parent → RenderObject?
no setterinherited
parentData ↔ ParentData?
getter/setter pairinherited
rootFrameDamage → RenderDamageTracker?
The frame damage tracker attached at this tree's root — the per-owner object a render object publishes frame-scoped facts into — or null while detached or before the first frame.
no setterinherited
runtimeType → Type
A representation of the runtime type of the object.
no setterinherited
size → CellSize
The size computed by the most recent layout pass.
no setterinherited

Methods

adoptChild(RenderObject child) → void
Attaches child to this render object as its parent. Subclasses that hold children call this whenever they accept one; it also gives the parent a chance to ensure RenderObject.parentData is the right type via setupParentData.
inherited
attachFrameDamageTracker(RenderDamageTracker tracker) → bool
Attaches the per-runtime damage tracker at this (root) render object.
inherited
childClipOf(RenderObject child) → CellRect?
The clip this render object imposes on child, in its own coordinates, or null when it does not clip. Usually the same rectangle for every child (a viewport); a container with several scrolling regions — a table with a pinned header — answers per child.
override
childOffsetOf(RenderObject child) → CellOffset
Where this render object paints child, relative to its own origin.
inherited
computeMaxIntrinsicHeight(int? width) → int
The tallest this render object would want to be at the given width.
inherited
computeMaxIntrinsicWidth(int? height) → int
The widest this render object would want to be at the given height. Most relevant override: text returns its unwrapped width.
inherited
computeMinIntrinsicHeight(int? width) → int
The shortest this render object can be at the given width.
inherited
computeMinIntrinsicWidth(int? height) → int
The narrowest width below which content would clip. Text-like leaves can return a longest-token width if they word-wrap.
inherited
computePaintOverflow() → CellRect?
inherited
dropChild(RenderObject child) → void
Detaches child from this render object.
inherited
layout(CellConstraints constraints) → CellSize
Lays out this render object against constraints and returns the chosen size. Subclasses implement performLayout rather than overriding this; the framework needs the bookkeeping around it.
inherited
markAncestorRepaintBoundariesDirty() → void
Marks every repaint boundary STRICTLY ABOVE this node as needing paint.
inherited
markNeedsLayout() → void
Marks this render object and its ancestors as needing layout, and marks the nearest enclosing repaint boundary as dirty. Use this for changes that can affect size, child constraints, child offsets, or layout-derived paint state.
inherited
markNeedsPaintOnly() → void
Marks this render object's painted output as stale without touching layout.
inherited
noSuchMethod(Invocation invocation) → dynamic
Invoked when a nonexistent method or property is accessed.
inherited
paint(CellBuffer buffer, CellOffset offset) → void
Paints this render object into buffer at offset: its origin in the buffer's coordinates. The buffer's bounds are the clip.
inherited
performLayout(CellConstraints constraints) → CellSize
Override to compute the chosen size and lay out children. Must return a size that satisfies constraints.
override
performPaint(CellBuffer buffer, CellOffset offset) → void
Paints this render object's cells into buffer at offset.
override
presentsChild(RenderObject child) → bool
Whether child is presented this frame: painted and interactive. False for a child hidden by policy — an inactive IndexedStack child, a route under an opaque route, an overlay entry under an opaque one, a list row outside the mounted window, a contained-error subtree.
inherited
screenGeometry() → RenderGeometry?
This render object's screen geometry, derived from layout state: where it is, and how much of it is visible.
inherited
setupParentData(RenderObject child) → void
Override to install a subclass of ParentData on the child. The default is a no-op; multi-child render objects override this so a child's parentData is always the type that parent expects.
inherited
toString() → String
A string representation of this object.
inherited
visitRenderChildren(void visitor(RenderObject child)) → void
Visits this render object's children in paint order. The default reads the single-child / multi-child interfaces; a container whose RenderObjectWithChildren.children accessor copies its list overrides this to iterate in place, since hit-testing and geometry tooling walk the tree on every event.
inherited

Operators

operator ==(Object other) → bool
The equality operator.
inherited