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fix(streamdown): render code blocks incrementally during streaming #474

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fix(streamdown): render code blocks incrementally during streaming #474

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4 changes: 4 additions & 0 deletions packages/streamdown/index.tsx
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Original file line number Diff line number Diff line change
Expand Up @@ -347,6 +347,10 @@ export const Block = memo(
if (animatePluginProp) {
const prevCount = animatePluginProp.getLastRenderCharCount();
animatePluginProp.setPrevContentLength(prevCount);
// When the block has an incomplete code fence, animate code block
// content incrementally instead of skipping it. This gives visual
// feedback that code is streaming in token-by-token.
animatePluginProp.setAnimateCodeBlocks(isIncomplete);
}

// Note: remend is already applied to the entire markdown before parsing into blocks
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25 changes: 21 additions & 4 deletions packages/streamdown/lib/animate.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -11,6 +11,12 @@ export interface AnimatePlugin {
getLastRenderCharCount: () => number;
name: "animate";
rehypePlugin: Pluggable;
/**
* Enable or disable animation inside code/pre blocks.
* When true, code block content will be animated incrementally
* during streaming (useful for incomplete/unclosed code fences).
*/
setAnimateCodeBlocks: (enabled: boolean) => void;
/**
* Set the number of HAST text characters from a previous render.
* Characters up to this count will get duration=0ms, preventing
Expand All @@ -31,17 +37,23 @@ export interface AnimateOptions {
const WHITESPACE_RE = /\s/;
const WHITESPACE_ONLY_RE = /^\s+$/;
const SKIP_TAGS = new Set(["code", "pre", "svg", "math", "annotation"]);
const SKIP_TAGS_WITHOUT_CODE = new Set(["svg", "math", "annotation"]);

const isElement = (node: unknown): node is Element =>
typeof node === "object" &&
node !== null &&
"type" in node &&
(node as Element).type === "element";

const hasSkipAncestor = (ancestors: Node[]): boolean =>
ancestors.some(
(ancestor) => isElement(ancestor) && SKIP_TAGS.has(ancestor.tagName)
const hasSkipAncestor = (
ancestors: Node[],
animateCodeBlocks: boolean
): boolean => {
const tags = animateCodeBlocks ? SKIP_TAGS_WITHOUT_CODE : SKIP_TAGS;
return ancestors.some(
(ancestor) => isElement(ancestor) && tags.has(ancestor.tagName)
);
};

const splitByWord = (text: string): string[] => {
const parts: string[] = [];
Expand Down Expand Up @@ -126,6 +138,7 @@ interface AnimateConfig {
* object that setPrevContentLength / getLastRenderCharCount mutate.
*/
interface AnimateRenderState {
animateCodeBlocks: boolean;
lastRenderCharCount: number;
prevContentLength: number;
}
Expand All @@ -143,7 +156,7 @@ const processTextNode = (
return;
}

if (hasSkipAncestor(ancestors)) {
if (hasSkipAncestor(ancestors, renderState.animateCodeBlocks)) {
return SKIP;
}

Expand Down Expand Up @@ -203,6 +216,7 @@ export function createAnimatePlugin(options?: AnimateOptions): AnimatePlugin {
// Mutable render state — the rehype closure and the plugin API methods
// both reference this same object.
const renderState: AnimateRenderState = {
animateCodeBlocks: false,
prevContentLength: 0,
lastRenderCharCount: 0,
};
Expand Down Expand Up @@ -230,6 +244,9 @@ export function createAnimatePlugin(options?: AnimateOptions): AnimatePlugin {
name: "animate",
type: "animate",
rehypePlugin: rehypeAnimate,
setAnimateCodeBlocks(enabled: boolean) {
renderState.animateCodeBlocks = enabled;
},
setPrevContentLength(length: number) {
renderState.prevContentLength = length;
},
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