# How data is passed from renderer to browser for page load metrics observers
1. Blink's core rendering code hooks into a "detector" that implements the
semantics of the metric. Example:
[`PaintTimingDetector`](https://source.chromium.org/chromium/chromium/src/+/main:third_party/blink/renderer/core/paint/timing/paint_timing_detector.cc).
(This is more of a convention and not a specific shared interface.)
2. The detector notifies the
[LocalFrameClient](https://source.chromium.org/chromium/chromium/src/+/main:third_party/blink/renderer/core/frame/local_frame_client.h)
that the metric's value has changed. For some metrics, this uses the generic
`DidChangePerformanceTiming` method, but other metrics have dedicated methods
on this interface.
3. The `LocalFrameClientImpl` notifies the
[`RenderFrameImpl`](https://source.chromium.org/chromium/chromium/src/+/main:content/renderer/render_frame_impl.h)
that the metric's value has changed.
4. The `RenderFrameImpl` notifies `PageLoadMetrics`'
[`MetricsRenderFrameObserver`](https://source.chromium.org/chromium/chromium/src/+/main:components/page_load_metrics/renderer/metrics_render_frame_observer.h)
(through a generic observer interface) that the metric's value has changed.
5. Sometimes, new data is passed in to `MetricsRenderFrameObserver` in the same
method that notifies it of the change.
But for metrics that rely on `DidChangePerformanceTiming`,
`MetricsRenderFrameObserver` has to go back to Blink to ask for the actual
data, by querying the [`WebPerformanceMetricsForReporting`](https://source.chromium.org/chromium/chromium/src/+/main:third_party/blink/public/web/web_performance_metrics_for_reporting.h)
object, which in turn queries the detector from step 1.
6. `MetricsRenderFrameObserver` passes the new data into `PageLoadMetrics`'s
[`PageTimingMetricsSender`](https://source.chromium.org/chromium/chromium/src/+/main:components/page_load_metrics/renderer/page_timing_metrics_sender.h).
7. `PageTimingMetricsSender` buffers the data for up
to 100 ms, then issues an IPC to the browser which is handled by
[`MetricsWebContentsObserver`](https://source.chromium.org/chromium/chromium/src/+/main:components/page_load_metrics/browser/metrics_web_contents_observer.h).
8. `MetricsWebContentsObserver` passes the data to the
[`PageLoadMetricsUpdateDispatcher`](https://source.chromium.org/chromium/chromium/src/+/main:components/page_load_metrics/browser/page_load_metrics_update_dispatcher.h)
for the currently tracked page load.
9. For some metrics, like FCP, `PageLoadMetricsUpdateDispatcher` "merges"
updates from all frames (using `PageLoadTimingMerger`) into a single
page-wide value, which it then passes up to its owner, the
[`PageLoadTracker`](https://source.chromium.org/chromium/chromium/src/+/main:components/page_load_metrics/browser/page_load_tracker.h).
But for other metrics, `PageLoadMetricsUpdateDispatcher` passes per-frame
values up to `PageLoadTracker`. This is the case for LCP, which is merged by
a `PageLoadTracker`-owned object
([`LargestContentfulPaintHandler`](https://source.chromium.org/chromium/chromium/src/+/main:components/page_load_metrics/browser/observers/core/largest_contentful_paint_handler.h))
and not by `PageLoadMetricsUpdateDispatcher`.
10. `PageLoadTracker` broadcasts metric values (some page-wide, some
frame-specific) to various
[`PageLoadMetricsObserver`](https://source.chromium.org/chromium/chromium/src/+/main:components/page_load_metrics/browser/page_load_metrics_observer.h)
implementers. Some metrics pass through the generic `OnTimingUpdate`
method, while others have dedicated methods on the observer interface.
11. `PageLoadMetricsObserver` implementations like
[`UmaPageLoadMetricsObserver`](https://source.chromium.org/chromium/chromium/src/+/main:components/page_load_metrics/browser/observers/core/uma_page_load_metrics_observer.h)
and [`UkmPageLoadMetricsObserver`](https://source.chromium.org/chromium/chromium/src/+/main:chrome/browser/page_load_metrics/observers/core/ukm_page_load_metrics_observer.h)
perform the final step of recording to UMA/UKM, calling into generic
components like
[`base::Histogram`](https://source.chromium.org/chromium/chromium/src/+/main:base/metrics/histogram.h)
and
[`UkmRecorder`](https://source.chromium.org/chromium/chromium/src/+/main:services/metrics/public/cpp/ukm_recorder.h).
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