Mermix/src/lib/components/Preview.svelte

<script lang="ts">
  import { tick } from 'svelte';
  import { store } from '../store.svelte';
  import { renderMermaid } from '../mermaid';

  let svg = $state('');
  let error = $state('');
  let rendering = $state(false);
  let zoom = $state(1);
  let pan = $state({ x: 0, y: 0 });
  let canvasEl = $state<HTMLDivElement>();
  let timer: ReturnType<typeof setTimeout> | undefined;
  let fittedFor = '';

  // Debounced re-render whenever the source or theme changes.
  $effect(() => {
    const code = store.content;
    const theme = store.theme;
    clearTimeout(timer);
    timer = setTimeout(() => void run(code, theme), 220);
    return () => clearTimeout(timer);
  });

  // Re-fit when the available space changes (layout / panel toggles).
  $effect(() => {
    void store.layout;
    void store.showGit;
    tick().then(fit);
  });

  async function run(code: string, theme: string) {
    rendering = true;
    const result = await renderMermaid(code, theme);
    rendering = false;
    if ('svg' in result) {
      svg = result.svg;
      store.lastSvg = result.svg;
      error = '';
      // Auto-fit the first time each diagram renders.
      if (store.activeId !== fittedFor) {
        fittedFor = store.activeId ?? '';
        await tick();
        fit();
      }
    } else {
      error = result.error;
    }
  }

  function svgDims(s: string): { w: number; h: number } {
    const vb = s.match(/viewBox="([\d.\s-]+)"/);
    if (vb) {
      const p = vb[1].trim().split(/\s+/).map(Number);
      if (p.length === 4 && p.every((n) => !Number.isNaN(n))) return { w: p[2], h: p[3] };
    }
    return { w: 0, h: 0 };
  }

  function zoomBy(factor: number) {
    zoom = Math.min(6, Math.max(0.1, zoom * factor));
  }

  /** Scale the diagram so it fits within the visible canvas. */
  function fit() {
    if (!canvasEl || !svg) return;
    const { w, h } = svgDims(svg);
    if (!w || !h) return;
    const margin = store.theme !== 'dark' ? 72 : 32;
    const cw = canvasEl.clientWidth - margin;
    const ch = canvasEl.clientHeight - margin;
    if (cw <= 0 || ch <= 0) return;
    zoom = Math.max(0.1, Math.min(cw / w, ch / h, 3));
    pan = { x: 0, y: 0 };
  }

  function reset() {
    zoom = 1;
    pan = { x: 0, y: 0 };
  }

  // Drag-to-pan.
  let dragging = $state(false);
  let last = { x: 0, y: 0 };
  function onDown(e: MouseEvent) {
    dragging = true;
    last = { x: e.clientX, y: e.clientY };
  }
  function onMove(e: MouseEvent) {
    if (!dragging) return;
    pan = { x: pan.x + (e.clientX - last.x), y: pan.y + (e.clientY - last.y) };
    last = { x: e.clientX, y: e.clientY };
  }
  function onUp() {
    dragging = false;
  }
  function onWheel(e: WheelEvent) {
    if (e.ctrlKey || e.metaKey) {
      e.preventDefault();
      zoomBy(e.deltaY < 0 ? 1.1 : 0.9);
    }
  }
</script>

<div class="preview">
  <div class="ctrls">
    {#if rendering}<span class="dot" title="Rendering…"></span>{/if}
    <button class="ghost icon" title="Zoom out" onclick={() => zoomBy(0.83)}>−</button>
    <button class="ghost zoom" title="Reset to 100%" onclick={reset}
      >{Math.round(zoom * 100)}%</button
    >
    <button class="ghost icon" title="Zoom in" onclick={() => zoomBy(1.2)}>+</button>
    <button class="ghost icon" title="Fit to view" onclick={fit}>⤢</button>
  </div>

  <div
    bind:this={canvasEl}
    class="canvas"
    class:grab={!dragging}
    class:grabbing={dragging}
    onmousedown={onDown}
    onmousemove={onMove}
    onmouseup={onUp}
    onmouseleave={onUp}
    onwheel={onWheel}
    role="presentation"
  >
    {#if error}
      <div class="error">
        <div class="error-title">Diagram error</div>
        <pre>{error}</pre>
      </div>
    {:else if svg}
      <div
        class="stage"
        class:paper={store.theme !== 'dark'}
        style="transform: translate({pan.x}px, {pan.y}px) scale({zoom});"
      >
        {@html svg}
      </div>
    {:else}
      <div class="empty faint">Start typing to see your diagram.</div>
    {/if}
  </div>
</div>

<style>
  .preview {
    position: relative;
    height: 100%;
    background:
      radial-gradient(circle at 1px 1px, rgba(255, 255, 255, 0.05) 1px, transparent 0) 0 0 /
      22px 22px,
      var(--bg);
    overflow: hidden;
  }
  .ctrls {
    position: absolute;
    top: 10px;
    right: 12px;
    display: flex;
    align-items: center;
    gap: 4px;
    background: var(--bg-elev);
    border: 1px solid var(--border);
    border-radius: var(--radius-sm);
    padding: 3px 5px;
    z-index: 5;
  }
  .zoom {
    font-size: 12px;
    color: var(--text-dim);
    min-width: 38px;
    text-align: center;
  }
  .dot {
    width: 8px;
    height: 8px;
    border-radius: 50%;
    background: var(--accent);
    margin-right: 4px;
    animation: pulse 1s infinite;
  }
  @keyframes pulse {
    50% {
      opacity: 0.3;
    }
  }
  .canvas {
    height: 100%;
    display: grid;
    place-items: center;
    overflow: hidden;
  }
  .canvas.grab {
    cursor: grab;
  }
  .canvas.grabbing {
    cursor: grabbing;
  }
  .stage {
    transform-origin: center center;
    transition: transform 0.05s linear;
  }
  /* Light Mermaid themes are designed for a white page, so give them a
     "paper" sheet; the dark theme renders directly on the dark canvas. */
  .stage.paper {
    background: #ffffff;
    padding: 28px;
    border-radius: 10px;
    box-shadow: 0 12px 48px rgba(0, 0, 0, 0.45);
  }
  .stage :global(svg) {
    display: block;
    max-width: none;
    height: auto;
  }
  .empty {
    font-size: 14px;
  }
  .error {
    max-width: 80%;
    background: rgba(248, 81, 73, 0.08);
    border: 1px solid rgba(248, 81, 73, 0.4);
    border-radius: var(--radius);
    padding: 16px 18px;
    color: var(--red);
  }
  .error-title {
    font-weight: 600;
    margin-bottom: 8px;
  }
  .error pre {
    margin: 0;
    white-space: pre-wrap;
    font-family: var(--mono);
    font-size: 12px;
    color: #ffb4ae;
  }
</style>