Add a document to explain how to add a feature flag (chrome://flags)
TBR=asvitkine@chromium.org Bug: N/A Change-Id: I53bda41c047ed970417df5a5ca308855cbccbde5 Reviewed-on: https://chromium-review.googlesource.com/594754 Commit-Queue: Makoto Shimazu <shimazu@chromium.org> Reviewed-by: Nico Weber <thakis@chromium.org> Cr-Commit-Position: refs/heads/master@{#492501}
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more performant
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more performant
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* [ES6 Support in Chromium](es6_chromium.md) - Implementation of ECMAScript6
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* [ES6 Support in Chromium](es6_chromium.md) - Implementation of ECMAScript6
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features in Chromium
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features in Chromium
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* [Adding a new feature flag in chrome://flags](how_to_add_your_feature_flag.md) - Quick
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guide to add a new feature flag to experiment your feature.
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### Testing
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### Testing
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* [Running and Debugging Layout Tests](testing/layout_tests.md)
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* [Running and Debugging Layout Tests](testing/layout_tests.md)
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docs/how_to_add_your_feature_flag.md
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docs/how_to_add_your_feature_flag.md
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# Adding a new feature flag in chrome://flags
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This document describes how to add your new feature behind a flag.
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## Step 1: Adding a new `base::Feature`
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This step would be different between where you want to use the flag.
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For example, if you want to use the flag in src/content, you should add a base::Feature to the following files:
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* [content/public/common/content_features.cc](https://cs.chromium.org/chromium/src/content/public/common/content_features.cc)
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* [content/public/common/content_features.h](https://cs.chromium.org/chromium/src/content/public/common/content_features.h)
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If you want to use the flag in blink, you should also read
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[Runtime Enable Features](https://www.chromium.org/blink/runtime-enabled-features).
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You can refer to [this CL](https://chromium-review.googlesource.com/c/554510/)
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to see
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1. where to add the base::Feature
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[[1](https://chromium-review.googlesource.com/c/554510/8/content/public/common/content_features.cc#253)]
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[[2](https://chromium-review.googlesource.com/c/554510/8/content/public/common/content_features.h)]
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2. how to use it
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[[1](https://chromium-review.googlesource.com/c/554510/8/content/common/service_worker/service_worker_utils.cc#153)]
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3. how to wire the base::Feature to WebRuntimeFeatures
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[[1](https://chromium-review.googlesource.com/c/554510/8/content/child/runtime_features.cc)]
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[[2](https://chromium-review.googlesource.com/c/554510/8/third_party/WebKit/public/platform/WebRuntimeFeatures.h)]
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[[3](https://chromium-review.googlesource.com/c/554510/third_party/WebKit/Source/platform/exported/WebRuntimeFeatures.cpp)]
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[[4](https://chromium-review.googlesource.com/c/554510/8/third_party/WebKit/Source/platform/RuntimeEnabledFeatures.json5)]
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4. how to use it in blink
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[[1](https://chromium-review.googlesource.com/c/554510/8/third_party/WebKit/Source/core/workers/WorkerThread.cpp)]
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Also, this patch added a virtual test for running layout tests with the flag.
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When you add a flag, you can consider to use that.
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## Step 2: Adding the feature flag to the chrome://flags UI.
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You have to modify these four files in total.
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* [chrome/browser/about_flags.cc](https://cs.chromium.org/chromium/src/chrome/browser/about_flags.cc)
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* [chrome/browser/flag_descriptions.cc](https://cs.chromium.org/chromium/src/chrome/browser/flag_descriptions.cc)
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* [chrome/browser/flag_descriptions.h](https://cs.chromium.org/chromium/src/chrome/browser/flag_descriptions.h)
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* [tools/metrics/histograms/enums.xml](https://cs.chromium.org/chromium/src/tools/metrics/histograms/enums.xml)
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At first you need to add an entry to __about_flags.cc__,
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__flag_descriptions.cc__ and __flag_descriptions.h__. After that, try running the following test.
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```bash
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# Build unit_tests
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ninja -C out/Default unit_tests
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# Run AboutFlagsHistogramTest.CheckHistograms
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./out/Default/unit_tests --gtest_filter=AboutFlagsHistogramTest.CheckHistograms
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```
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That test will ask you to add several entries to enums.xml.
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You can refer to [this CL](https://chromium-review.googlesource.com/c/593707) as an example.
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