Added centerOn function

Author: niosus

animation/projects/src/dummy/scenes/dummy.tsx

@@ -3,6 +3,9 @@ import { makeScene2D, Code, LezerHighlighter } from '@motion-canvas/2d';
 import { all, createRef, waitFor } from '@motion-canvas/core';
 import { MyStyle } from '../../styles';
 import { parser as parser_cpp } from '@lezer/cpp';
+import { DEFAULT } from '@motion-canvas/core/lib/signals';
+import { centerOn } from '../../utils';
+import { lines } from '@motion-canvas/2d';
 
 import dummyCode from '@lectures/dummy.md?snippet=dummy_snippet/main.cpp';
 
@@ -21,7 +24,10 @@ export default makeScene2D(function* (view) {
         />
     );
 
-    yield* waitFor(0.5);
-    yield* codeRef().code(dummyCode, 1);
+    yield* codeRef().code(dummyCode, 0);
+    yield* centerOn(codeRef(), DEFAULT, 1, 36);
     yield* waitFor(1);
+
+    yield* centerOn(codeRef(), lines(3, 3), 1, 40);
+    yield* waitFor(3);
 });

animation/projects/src/utils.ts

@@ -0,0 +1,63 @@
+import { Code } from '@motion-canvas/2d';
+import { BBox, all, DEFAULT, ThreadGenerator } from '@motion-canvas/core';
+
+export function* centerOn(
+    codeRef: Code,
+    selectionInput: any,
+    duration: number,
+    targetFontSize?: number,
+): ThreadGenerator {
+    // We only care about the bounding box of the new selection
+    let bboxes: BBox[] = [];
+    if (selectionInput !== DEFAULT) {
+        // getSelectionBBox computes the bounds for the given selection using the CURRENT layout state.
+        // We do NOT need to set the selection signal to measure it!
+        bboxes = codeRef.getSelectionBBox(selectionInput);
+    }
+
+    if (bboxes.length === 0) {
+        const fallbackAnimations: ThreadGenerator[] = [
+            codeRef.selection(selectionInput, duration),
+            codeRef.y(0, duration)
+        ];
+        if (targetFontSize !== undefined) {
+            fallbackAnimations.push(codeRef.fontSize(targetFontSize, duration));
+        }
+        yield* all(...fallbackAnimations);
+        return;
+    }
+
+    let minY = Infinity;
+    let maxY = -Infinity;
+
+    for (const bbox of bboxes) {
+        if (bbox.top !== undefined && !isNaN(bbox.top)) minY = Math.min(minY, bbox.top);
+        if (bbox.bottom !== undefined && !isNaN(bbox.bottom)) maxY = Math.max(maxY, bbox.bottom);
+    }
+
+    let centerY = (minY + maxY) / 2;
+    if (!isFinite(centerY) || isNaN(centerY)) {
+        centerY = 0;
+    }
+
+    // Scale the calculated target offset based on how the font size will change.
+    // This perfectly centers it without needing to pollute the animation timeline
+    // with synchronous layout mutations.
+    if (targetFontSize !== undefined) {
+        const currentFontSize = codeRef.fontSize();
+        if (currentFontSize > 0) {
+            centerY = centerY * (targetFontSize / currentFontSize);
+        }
+    }
+
+    const animations: ThreadGenerator[] = [
+        codeRef.selection(selectionInput, duration),
+        codeRef.y(-centerY, duration)
+    ];
+
+    if (targetFontSize !== undefined) {
+        animations.push(codeRef.fontSize(targetFontSize, duration));
+    }
+
+    yield* all(...animations);
+}