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接前一篇文章：Chromium源码由浅入深（二）

上一回说到了关键的“钥匙”：browserBridge.gpuInfo，本文就针对其进行深入探究。

先来看前半部分，browserBridge。

在content/browser/resources/gpu/gpu_internals.js中有以下代码：

import './info_view.js';import {BrowserBridge} from './browser_bridge.js';// Injected script from C++ or test environments may reference `browserBridge`
// as a property of the global object.
window.browserBridge = new BrowserBridge();/*** Main entry point. called once the page has loaded.*/
function onLoad() {// Create the views.document.querySelector('info-view').addBrowserBridgeListeners(window.browserBridge);// Because of inherent raciness (between the deprecated DevTools API which// telemtry uses to drive the relevant tests, and the asynchronous loading of// JS modules like this one) it's possible for telemetry tests to inject code// *before* `browserBridge` is set and the DOM is populated. This flag is used// to synchronize script injection by tests to prevent such races.window.gpuPagePopulated = true;
}document.addEventListener('DOMContentLoaded', onLoad);

代码注释说得清楚：从C++或测试环境注入的脚本可能引用“browserBridge”作为全局对象的属性。

BrowserBridge类的定义在Cromium源码中一共有两处，一处在chrome/browser/resources/net_internals/browser_bridge.js中，另一处在content/browser/resources/gpu/browser_bridge.js中。由于上边代码中是“import {BrowserBridge} from './browser_bridge.js';”，也就是引用的是当前路径下的browser_bridge.js，即content/browser/resources/gpu/下的browser_bridge.js，也就是第二处定义。

代码如下：

/*** This class provides a 'bridge' for communicating between javascript and the* browser. When run outside of WebUI, e.g. as a regular webpage, it provides* synthetic data to assist in testing.*/
export class BrowserBridge extends EventTarget {constructor() {super();this.nextRequestId_ = 0;this.pendingCallbacks_ = [];this.logMessages_ = [];// Tell c++ code that we are ready to receive GPU Info.chrome.send('browserBridgeInitialized');this.beginRequestClientInfo_();this.beginRequestLogMessages_();}dispatchEvent_(eventName) {this.dispatchEvent(new CustomEvent(eventName, {bubbles: true, composed: true}));}applySimulatedData_(data) {// set up things according to the simulated datathis.gpuInfo_ = data.gpuInfo;this.clientInfo_ = data.clientInfo;this.logMessages_ = data.logMessages;this.dispatchEvent_('gpuInfoUpdate');this.dispatchEvent_('clientInfoChange');this.dispatchEvent_('logMessagesChange');}/*** Sends a message to the browser with specified args. The* browser will reply asynchronously via the provided callback.*/callAsync(submessage, args, callback) {const requestId = this.nextRequestId_;this.nextRequestId_ += 1;this.pendingCallbacks_[requestId] = callback;if (!args) {chrome.send('callAsync', [requestId.toString(), submessage]);} else {const allArgs = [requestId.toString(), submessage].concat(args);chrome.send('callAsync', allArgs);}}/*** Called by gpu c++ code when client info is ready.*/onCallAsyncReply(requestId, args) {if (this.pendingCallbacks_[requestId] === undefined) {throw new Error('requestId ' + requestId + ' is not pending');}const callback = this.pendingCallbacks_[requestId];callback(args);delete this.pendingCallbacks_[requestId];}/*** Get gpuInfo data.*/get gpuInfo() {return this.gpuInfo_;}/*** Called from gpu c++ code when GPU Info is updated.*/onGpuInfoUpdate(gpuInfo) {this.gpuInfo_ = gpuInfo;this.dispatchEvent_('gpuInfoUpdate');}/*** This function begins a request for the ClientInfo. If it comes back* as undefined, then we will issue the request again in 250ms.*/beginRequestClientInfo_() {this.callAsync('requestClientInfo', undefined,(function(data) {if (data === undefined) {  // try again in 250 mswindow.setTimeout(this.beginRequestClientInfo_.bind(this), 250);} else {this.clientInfo_ = data;this.dispatchEvent_('clientInfoChange');}}).bind(this));}/*** Returns information about the currently running Chrome build.*/get clientInfo() {return this.clientInfo_;}/*** This function checks for new GPU_LOG messages.* If any are found, a refresh is triggered.*/beginRequestLogMessages_() {this.callAsync('requestLogMessages', undefined,(function(messages) {if (messages.length !== this.logMessages_.length) {this.logMessages_ = messages;this.dispatchEvent_('logMessagesChange');}// check again in 250 mswindow.setTimeout(this.beginRequestLogMessages_.bind(this), 250);}).bind(this));}/*** Returns an array of log messages issued by the GPU process, if any.*/get logMessages() {return this.logMessages_;}/*** Returns the value of the "Sandboxed" row.*/isSandboxedForTesting() {for (const info of this.gpuInfo_.basicInfo) {if (info.description === 'Sandboxed') {return info.value;}}return false;}
}

再来看后半部分，gpuInfo。

在上边content/browser/resources/gpu/gpu_internals.js的代码中有以下一段：

/*** Main entry point. called once the page has loaded.*/
function onLoad() {// Create the views.document.querySelector('info-view').addBrowserBridgeListeners(window.browserBridge);

addBrowserBridgeListeners函数在content/browser/resources/gpu/info_view.js中，代码如下：

addBrowserBridgeListeners(browserBridge) {browserBridge.addEventListener('gpuInfoUpdate', this.refresh.bind(this, browserBridge));browserBridge.addEventListener('logMessagesChange', this.refresh.bind(this, browserBridge));browserBridge.addEventListener('clientInfoChange', this.refresh.bind(this, browserBridge));this.refresh(browserBridge);}

而'gpuInfoUpdate'这个关键字则有两处相关调用。一处在content/browser/resources/gpu/browser_bridge.js中，代码如下：

applySimulatedData_(data) {// set up things according to the simulated datathis.gpuInfo_ = data.gpuInfo;this.clientInfo_ = data.clientInfo;this.logMessages_ = data.logMessages;this.dispatchEvent_('gpuInfoUpdate');this.dispatchEvent_('clientInfoChange');this.dispatchEvent_('logMessagesChange');}

另一处也是在同文件中，代码如下：

 /*** Called from gpu c++ code when GPU Info is updated.*/onGpuInfoUpdate(gpuInfo) {this.gpuInfo_ = gpuInfo;this.dispatchEvent_('gpuInfoUpdate');}

显然后者更为关键和常用。再次查找调用onGpuInfoUpdate函数的地方，只有一处，在content/browser/gpu/gpu_internals_ui.cc中，代码如下：

void GpuMessageHandler::OnGpuInfoUpdate() {// Get GPU Info.const gpu::GPUInfo gpu_info = GpuDataManagerImpl::GetInstance()->GetGPUInfo();const gfx::GpuExtraInfo gpu_extra_info =GpuDataManagerImpl::GetInstance()->GetGpuExtraInfo();base::Value::Dict gpu_info_val = GetGpuInfo();// Add in blocklisting featuresbase::Value::Dict feature_status;feature_status.Set("featureStatus", GetFeatureStatus());feature_status.Set("problems", GetProblems());base::Value::List workarounds;for (const auto& workaround : GetDriverBugWorkarounds())workarounds.Append(workaround);feature_status.Set("workarounds", std::move(workarounds));gpu_info_val.Set("featureStatus", std::move(feature_status));if (!GpuDataManagerImpl::GetInstance()->IsGpuProcessUsingHardwareGpu()) {const gpu::GPUInfo gpu_info_for_hardware_gpu =GpuDataManagerImpl::GetInstance()->GetGPUInfoForHardwareGpu();if (gpu_info_for_hardware_gpu.IsInitialized()) {base::Value::Dict feature_status_for_hardware_gpu;feature_status_for_hardware_gpu.Set("featureStatus",GetFeatureStatusForHardwareGpu());feature_status_for_hardware_gpu.Set("problems",GetProblemsForHardwareGpu());base::Value::List workarounds_for_hardware_gpu;for (const auto& workaround : GetDriverBugWorkaroundsForHardwareGpu())workarounds_for_hardware_gpu.Append(workaround);feature_status_for_hardware_gpu.Set("workarounds", std::move(workarounds_for_hardware_gpu));gpu_info_val.Set("featureStatusForHardwareGpu",std::move(feature_status_for_hardware_gpu));const gpu::GpuFeatureInfo gpu_feature_info_for_hardware_gpu =GpuDataManagerImpl::GetInstance()->GetGpuFeatureInfoForHardwareGpu();base::Value::List gpu_info_for_hardware_gpu_val = GetBasicGpuInfo(gpu_info_for_hardware_gpu, gpu_feature_info_for_hardware_gpu,gfx::GpuExtraInfo{});gpu_info_val.Set("basicInfoForHardwareGpu",std::move(gpu_info_for_hardware_gpu_val));}}gpu_info_val.Set("compositorInfo", CompositorInfo());gpu_info_val.Set("gpuMemoryBufferInfo", GpuMemoryBufferInfo(gpu_extra_info));gpu_info_val.Set("displayInfo", GetDisplayInfo());gpu_info_val.Set("videoAcceleratorsInfo", GetVideoAcceleratorsInfo());gpu_info_val.Set("ANGLEFeatures", GetANGLEFeatures());gpu_info_val.Set("devicePerfInfo", GetDevicePerfInfo());gpu_info_val.Set("dawnInfo", GetDawnInfo());// Send GPU Info to javascript.web_ui()->CallJavascriptFunctionUnsafe("browserBridge.onGpuInfoUpdate",std::move(gpu_info_val));
}

这个函数就是接下来要花大力气探究的核心函数。

欲知后事如何，且看下回分解。