feat(android): add HTML-to-Bitmap renderer#470
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Adds HtmlToBitmapRenderer, an off-screen WebView wrapper that loads an HTML string and snapshots the laid-out content to a Bitmap. Mirrors the iOS HTMLWebViewRenderer MVP: no caching, no pooling — each call creates and destroys its own WebView. A follow-up will layer caching and WebView reuse on top. All WebView interaction is marshalled onto the main thread internally, so callers can invoke the suspending API from any dispatcher.
After onPageFinished, poll Array.from(document.images).every(i => i.complete) at 50ms intervals up to a 4s soft timeout. Catches the common case where <img> tags with srcset, lazy decoding, or async DOM insertion are still in flight when window.load fires. On timeout, proceed with whatever has loaded rather than fail the render — a partial thumbnail is better than none. Same race exists on iOS; identical JS would close it there.
Adds JUnit instrumented tests that exercise the renderer against a real Android WebView on an emulator or device — the unit-test layer can't stand in for this because Robolectric's WebView shadow doesn't raster. Runs in CI via the `:android: Test Android Library Instrumented` step on the `mac-metal` queue. Each test writes its rendered PNG under the app's external cache dir and logs the absolute path under the `GBKRendererTest` tag so failures can be inspected with `adb pull` + an image viewer.
Without a parent window, Chromium-backed WebViews render into a GPU texture that WebView.draw(canvas) can't reach, producing blank bitmaps for CSS-only content. Force LAYER_TYPE_SOFTWARE and add a short post-layout settle so the software layer actually paints before pixels are read.
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Summary
Groundwork for rendering block patterns as thumbnails in the native Android inserter. Introduces
HtmlToBitmapRenderer: an off-screenWebViewthat loads an HTML string and snapshots the laid-out content to aBitmapviaCanvas.draw(). Lays out at the viewport width in device pixels, awaitsonPageFinished, pollsdocument.images.every(i => i.complete)so async-loaded images aren't missing from the snapshot, queriesdocument.documentElement.scrollHeight, re-lays out to content height, then draws into a scaled bitmap. Never upscales.Mirrors the iOS
HTMLWebViewRendererMVP underios/Sources/GutenbergKit/Sources/Views/HTMLPreview/. No consumer yet — theBlockInserterDialogwiring that turns patterns into previews lands separately.Design notes
WebViewin afinally. Pooling is a deliberate follow-up — iOS pools becauseWKWebViewallocation is expensive, but we want to see the simpler model exercised before adding a pool.setLayerType(LAYER_TYPE_SOFTWARE)forces the WebView to raster onto a software canvas rather than a GPU texture (whichWebView.draw()can't reach off-screen). After re-layout wedelay(POST_LAYOUT_SETTLE_MS)because there's no parent-window invalidation cycle to drive a repaint.Known gaps
WebViewdoesn't actually render. Coverage comes from theandroidTest/instrumented tests, which run in CI via the:android: Test Android Library Instrumentedstep (ci(android): run Gutenberg-module instrumented tests #472).Test plan
./gradlew :Gutenberg:detekt :Gutenberg:lintDebug :Gutenberg:compileDebugKotlin :Gutenberg:test—BUILD SUCCESSFUL./gradlew :Gutenberg:connectedAndroidTest):android: Test Android Library Instrumentedstep passes on this PRFollow-ups
CachingHtmlToBitmapRenderer+BlobCache/FileBlobCache) on top of the renderer.BlockInserterDialogso the renderer is exercised end-to-end.