android webview loadUrl实现原理浅析

先简单介绍一下android里面navive和js互相调用

Android与JS通过WebView互相调用方法(二者沟通的桥梁是WebView)，实际上是：

• Android去调用JS的代码
• JS去调用Android的代码

对于Android调用JS代码的方法有2种：

• 通过 WebView.loadUrl()
• 通过 WebView.evaluateJavascript()

对于JS调用Android代码的方法有3种：

• 通过 WebView.addJavascriptInterface() 进行对象映射
• 通过 WebViewClient.shouldOverrideUrlLoading()方法回调拦截 url
• 通过 WebChromeClient 的onJsAlert()、onJsConfirm()、onJsPrompt()方法回调拦截JS对话框alert()、confirm()、prompt（） 消息

使用的示例可以参考：https://developer.android.google.cn/guide/webapps/webview?hl=zh_cn

跟踪WebView的loadUrl实现的时候，发现在AOSP的代码里面找不到，在网上也基本上找不到相关的说明，大部分是Android与JS互相调用的实例，以及chromium源代码下载、编译，原理实现等，感觉这中间还缺点什么，下方的目标是把这一块给串联起来

chromium源代码下载、编译

chromium源代码下载

代码下载推荐2种方式：

• 从gitee上下载 https://github.com/chromium/chromium
• 从github上下载 https://github.com/chromium

我是通过gitee直接下载zip包的，通过git clone命令下载，网速太感人了

当然有条件的，还是使用官方的提供的depot_tools，可以少踩一些坑

git clone https://chromium.googlesource.com/chromium/tools/depot_tools.git
export PATH="$PATH:/path/to/depot_tools"
mkdir ~/chromium && cd ~/chromium
fetch --nohooks android
// 同步对应的平台的依赖编译
gclient sync
// 安装额外的编译依赖
build/install-build-deps-android.sh
           

chromium源代码编译

参考 https://github.com/chromium/chromium/blob/master/docs/android_build_instructions.md

// 设置编译平台
target_os = "android"
target_cpu = "arm64"  # See "Figuring out target_cpu" below
// 启动编译
autoninja -C out/Default chrome_public_apk
           

不同的平台的编译方式不一样，具体的可以看

https://github.com/chromium/chromium/blob/master/docs/get_the_code.md

Webview

源代码路径：

frameworks/base/core/java/android/webkit/WebView.java

loadUrl和evaluateJavascript方法的实现比较简单，通过调用WebViewProvider来实现的

/**
     * Loads the given URL.
     * <p>
     * Also see compatibility note on {@link #evaluateJavascript}.
     *
     * @param url the URL of the resource to load
     */
    public void loadUrl(String url) {
        checkThread();
        mProvider.loadUrl(url);
    }
    
    /**
     * Asynchronously evaluates JavaScript in the context of the currently displayed page.
     * If non-null, {@code resultCallback} will be invoked with any result returned from that
     * execution. This method must be called on the UI thread and the callback will
     * be made on the UI thread.
     * <p>
     * Compatibility note. Applications targeting {@link android.os.Build.VERSION_CODES#N} or
     * later, JavaScript state from an empty WebView is no longer persisted across navigations like
     * {@link #loadUrl(String)}. For example, global variables and functions defined before calling
     * {@link #loadUrl(String)} will not exist in the loaded page. Applications should use
     * {@link #addJavascriptInterface} instead to persist JavaScript objects across navigations.
     *
     * @param script the JavaScript to execute.
     * @param resultCallback A callback to be invoked when the script execution
     *                       completes with the result of the execution (if any).
     *                       May be {@code null} if no notification of the result is required.
     */
    public void evaluateJavascript(String script, @Nullable ValueCallback<String> resultCallback) {
        checkThread();
        mProvider.evaluateJavaScript(script, resultCallback);
    }
           

WebViewProvider是一个接口，提供了WebView的能力，每个WebView绑定到一个具体的WebViewProvider对象上，这个对象实现了运行时WebView的行为能力

/**
 * WebView backend provider interface: this interface is the abstract backend to a WebView
 * instance; each WebView object is bound to exactly one WebViewProvider object which implements
 * the runtime behavior of that WebView.
 *
 * All methods must behave as per their namesake in {@link WebView}, unless otherwise noted.
 *
 * @hide Not part of the public API; only required by system implementors.
 */
@SystemApi
public interface WebViewProvider
           

WebViewProvider是一个接口，具体的实现不在framework里面。最开始的时候，WebView是android framework的一部分，从Android 5.0 (Lollipop)开始，WebView的实现由一个单独的apk来提供，apk 预置在设备里面，可以和普通应用一样更新

WebView的实现apk的源代码保存在

https://github.com/chromium/chromium/tree/master/android_webview

作为Chromium项目代码的一部分，当前AOSP已经不支持从源代码里面编译出WebView

在AOSP里面，针对不同平台预置了不同版本的WebView APK，适合的apk会默认包含在system image里面。apk文件可以在external/chromium-webview下找到

具体的说明可以参考

aosp-system-integration.md

chromium/android_webview

https://github.com/chromium/chromium/tree/master/android_webview

Android WebView是一个android系统组件，用来展示web内容，WebView以及相关的android类都是实现在chromium/android_webview目录下的

chromium/android_webview目录包含了android WebView的实现，也包含了AndroidX Webkit库的实现

其中WebViewProvider的实现类是WebViewChromium

class WebViewChromium implements WebViewProvider, WebViewProvider.ScrollDelegate,
                                 WebViewProvider.ViewDelegate, SmartClipProvider
           

类文件路径：

android_webview/glue/java/src/com/android/webview/chromium/WebViewChromium.java

其中loadUrl的实现为

@Override
    public void loadUrl(final String url) {
        mFactory.startYourEngines(true);
        if (checkNeedsPost()) {
            // Disallowed in WebView API for apps targeting a new SDK
            assert mAppTargetSdkVersion < Build.VERSION_CODES.JELLY_BEAN_MR2;
            mFactory.addTask(new Runnable() {
                @Override
                public void run() {
                    mAwContents.loadUrl(url);
                }
            });
            return;
        }
        mAwContents.loadUrl(url);
    }
           

在这段代码里面，有对android版本号做一个判断，从android 4.4开始，android上的WebView是基于Chromium内核来实现的。最终的实现是调用AwContents.loadUrl

在AwContents里面，通过逐层调用，最终调用此方法

/**
     * Load url without fixing up the url string. Consumers of ContentView are responsible for
     * ensuring the URL passed in is properly formatted (i.e. the scheme has been added if left
     * off during user input).
     *
     * @param params Parameters for this load.
     */
    @VisibleForTesting
    public void loadUrl(LoadUrlParams params) {
        if (params.getBaseUrl() == null) {
            // Don't record the URL if this was loaded via loadDataWithBaseURL(). That API is
            // tracked separately under Android.WebView.LoadDataWithBaseUrl.BaseUrl.
            recordLoadUrlScheme(schemeForUrl(params.getUrl()));
        }

        if (params.getLoadUrlType() == LoadURLType.DATA && !params.isBaseUrlDataScheme()) {
            // This allows data URLs with a non-data base URL access to file:///android_asset/ and
            // file:///android_res/ URLs. If AwSettings.getAllowFileAccess permits, it will also
            // allow access to file:// URLs (subject to OS level permission checks).
            params.setCanLoadLocalResources(true);
            AwContentsJni.get().grantFileSchemeAccesstoChildProcess(
                    mNativeAwContents, AwContents.this);
        }

        // If we are reloading the same url, then set transition type as reload.
        if (params.getUrl() != null && params.getUrl().equals(mWebContents.getLastCommittedUrl())
                && params.getTransitionType() == PageTransition.TYPED) {
            params.setTransitionType(PageTransition.RELOAD);
        }
        params.setTransitionType(
                params.getTransitionType() | PageTransition.FROM_API);

        // For WebView, always use the user agent override, which is set
        // every time the user agent in AwSettings is modified.
        params.setOverrideUserAgent(UserAgentOverrideOption.TRUE);


        // We don't pass extra headers to the content layer, as WebViewClassic
        // was adding them in a very narrow set of conditions. See http://crbug.com/306873
        // However, if the embedder is attempting to inject a Referer header for their
        // loadUrl call, then we set that separately and remove it from the extra headers map/
        final String referer = "referer";
        Map<String, String> extraHeaders = params.getExtraHeaders();
        if (extraHeaders != null) {
            for (String header : extraHeaders.keySet()) {
                if (referer.equals(header.toLowerCase(Locale.US))) {
                    params.setReferrer(
                            new Referrer(extraHeaders.remove(header), ReferrerPolicy.DEFAULT));
                    params.setExtraHeaders(extraHeaders);
                    break;
                }
            }
        }

        AwContentsJni.get().setExtraHeadersForUrl(mNativeAwContents, AwContents.this,
                params.getUrl(), params.getExtraHttpRequestHeadersString());
        params.setExtraHeaders(new HashMap<String, String>());

        // Ideally, the URL would only be "fixed" for user input (e.g. for URLs
        // entered into the Omnibox), but some WebView API consumers rely on
        // the legacy behavior where all navigations were subject to the
        // "fixing".  See also https://crbug.com/1145717.
        params.setUrl(UrlFormatter.fixupUrl(params.getUrl()).getPossiblyInvalidSpec());

        mNavigationController.loadUrl(params);

        // The behavior of WebViewClassic uses the populateVisitedLinks callback in WebKit.
        // Chromium does not use this use code path and the best emulation of this behavior to call
        // request visited links once on the first URL load of the WebView.
        if (!mHasRequestedVisitedHistoryFromClient) {
            mHasRequestedVisitedHistoryFromClient = true;
            requestVisitedHistoryFromClient();
        }
    }
           

TBD chromium还是很复杂，目前只有把WebView的启动以及怎么加载到Webview.apk里面的流程大概梳理出来，到具体的绘制的过程，还得好好研究研究

【一些参考资料】

WebView的启动过程

设计的一些关键概念

最后来一个chromium官方的视频

Android WebView 101 (Chrome University 2019)

WebView的层次结构

WebView的整体框架，android O版本及以上(renderer是一个单独的进程)

WebView的整体框架，android L到N版本