前言：在android HTML5 开发中有不少人遇到过 audio 标签 autoplay在某些设备上无效的问题，网上大多是讲怎么在js中操作，即在特定的时刻调用audio的play()方法，在android上还是无效。
一、解决方案
在android 4.2添加了允许用户手势触发音视频播放接口，该接口默认为 true ，即默认不允许自动播放音视频，只能是用户交互的方式由用户自己促发播放。
WebView webView = this.finishActivity(R.id.main_act_webview); // ... ... // 其他配置 // ... ... // 设置4.2以后版本支持autoPlay，非用户手势促发 if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.JELLY_BEAN_MR1) { webView.getSettings().setMediaPlaybackRequiresUserGesture(false); }
通过以上配置就可以加载带有自动播放的音视频啦！
二、 源码分析
下面我们沿着该问题来窥探下WebView的系统源码：
1、 通过getSettings()获取到的WebView的配置
/** * Gets the WebSettings object used to control the settings for this * WebView. * * @return a WebSettings object that can be used to control this WebView's * settings */ public WebSettings getSettings() { checkThread(); return mProvider.getSettings(); }
这里通过一个 mProvider来获取的配置信息，通过看WebView的源码，我们可以看到，WebView的所有操作都是交给 mProvider来进行的。
2、 mPeovider是在哪初始化的？
/** * @hide */ @SuppressWarnings("deprecation") // for super() call into deprecated base class constructor. protected WebView(Context context, AttributeSet attrs, int defStyleAttr, int defStyleRes, Map<String, Object> javaScriptInterfaces, boolean privateBrowsing) { super(context, attrs, defStyleAttr, defStyleRes); if (context == null) { throw new IllegalArgumentException("Invalid context argument"); } sEnforceThreadChecking = context.getApplicationInfo().targetSdkVersion >= Build.VERSION_CODES.JELLY_BEAN_MR2; checkThread(); ensureProviderCreated(); mProvider.init(javaScriptInterfaces, privateBrowsing); // Post condition of creating a webview is the CookieSyncManager.getInstance() is allowed. CookieSyncManager.setGetInstanceIsAllowed(); }
可以看到有个ensureProviderCreated()方法，就是在这里创建的mProvider:
private void ensureProviderCreated() { checkThread(); if (mProvider == null) { // As this can get called during the base class constructor chain, pass the minimum // number of dependencies here; the rest are deferred to init(). mProvider = getFactory().createWebView(this, new PrivateAccess()); } }
OK,到此知道了mProvider是在WebView的构造函数中创建的，并且WebView的所有操作都是交给mProvider进行的。
3、 但是这个mPeovider到底是谁派来的呢？
看下WebViewFactory#getFactory()做了什么操作：
static WebViewFactoryProvider getProvider() { synchronized (sProviderLock) { // For now the main purpose of this function (and the factory abstraction) is to keep // us honest and minimize usage of WebView internals when binding the proxy. if (sProviderInstance != null) return sProviderInstance; final int uid = android.os.Process.myUid(); if (uid == android.os.Process.ROOT_UID || uid == android.os.Process.SYSTEM_UID) { throw new UnsupportedOperationException( "For security reasons, WebView is not allowed in privileged processes"); } Trace.traceBegin(Trace.TRACE_TAG_WEBVIEW, "WebViewFactory.getProvider()"); try { Class<WebViewFactoryProvider> providerClass = getProviderClass(); StrictMode.ThreadPolicy oldPolicy = StrictMode.allowThreadDiskReads(); Trace.traceBegin(Trace.TRACE_TAG_WEBVIEW, "providerClass.newInstance()"); try { sProviderInstance = providerClass.getConstructor(WebViewDelegate.class) .newInstance(new WebViewDelegate()); if (DEBUG) Log.v(LOGTAG, "Loaded provider: " + sProviderInstance); return sProviderInstance; } catch (Exception e) { Log.e(LOGTAG, "error instantiating provider", e); throw new AndroidRuntimeException(e); } finally { Trace.traceEnd(Trace.TRACE_TAG_WEBVIEW); StrictMode.setThreadPolicy(oldPolicy); } } finally { Trace.traceEnd(Trace.TRACE_TAG_WEBVIEW); } } }
可见在23行返回了sProviderInstance， 是由 providerClass 通过反射创建的，15行中通过getProviderClass() 得到了providerClass.
private static Class<WebViewFactoryProvider> getProviderClass() { try { // First fetch the package info so we can log the webview package version. sPackageInfo = fetchPackageInfo(); Log.i(LOGTAG, "Loading " + sPackageInfo.packageName + " version " + sPackageInfo.versionName + " (code " + sPackageInfo.versionCode + ")"); Trace.traceBegin(Trace.TRACE_TAG_WEBVIEW, "WebViewFactory.loadNativeLibrary()"); loadNativeLibrary(); Trace.traceEnd(Trace.TRACE_TAG_WEBVIEW); Trace.traceBegin(Trace.TRACE_TAG_WEBVIEW, "WebViewFactory.getChromiumProviderClass()"); try { return getChromiumProviderClass(); } catch (ClassNotFoundException e) { Log.e(LOGTAG, "error loading provider", e); throw new AndroidRuntimeException(e); } finally { Trace.traceEnd(Trace.TRACE_TAG_WEBVIEW); } } catch (MissingWebViewPackageException e) { // If the package doesn't exist, then try loading the null WebView instead. // If that succeeds, then this is a device without WebView support; if it fails then // swallow the failure, complain that the real WebView is missing and rethrow the // original exception. try { return (Class<WebViewFactoryProvider>) Class.forName(NULL_WEBVIEW_FACTORY); } catch (ClassNotFoundException e2) { // Ignore. } Log.e(LOGTAG, "Chromium WebView package does not exist", e); throw new AndroidRuntimeException(e); } }
主要的 14行 返回了一个 getChromiumProviderClass(); 是不是有点熟悉，没错Android在4.4开始使用强大的Chromium替换掉了原来的WebKit。来看下这个getChromiumProviderClass()。
// throws MissingWebViewPackageException private static Class<WebViewFactoryProvider> getChromiumProviderClass() throws ClassNotFoundException { Application initialApplication = AppGlobals.getInitialApplication(); try { // Construct a package context to load the Java code into the current app. Context webViewContext = initialApplication.createPackageContext( sPackageInfo.packageName, Context.CONTEXT_INCLUDE_CODE | Context.CONTEXT_IGNORE_SECURITY); initialApplication.getAssets().addAssetPath( webViewContext.getApplicationInfo().sourceDir); ClassLoader clazzLoader = webViewContext.getClassLoader(); Trace.traceBegin(Trace.TRACE_TAG_WEBVIEW, "Class.forName()"); try { return (Class<WebViewFactoryProvider>) Class.forName(CHROMIUM_WEBVIEW_FACTORY, true, clazzLoader); } finally { Trace.traceEnd(Trace.TRACE_TAG_WEBVIEW); } } catch (PackageManager.NameNotFoundException e) { throw new MissingWebViewPackageException(e); } }
最后找到了这个 CHROMIUM_WEBVIEW_FACTORY， 可以看到在 WebViewFactory 中的定义：
private static final String CHROMIUM_WEBVIEW_FACTORY = "com.android.webview.chromium.WebViewChromiumFactoryProvider";
回答2小节的mProvider的初始化，在WebViewChromiumFactoryProvider 的 createWebView(…) 中进行了mProvider的初始化：
@Override public WebViewProvider createWebView(WebView webView, WebView.PrivateAccess privateAccess) { WebViewChromium wvc = new WebViewChromium(this, webView, privateAccess); synchronized (mLock) { if (mWebViewsToStart != null) { mWebViewsToStart.add(new WeakReference<WebViewChromium>(wvc)); } } ResourceProvider.registerResources(webView.getContext()); return wvc; }
OK，到这里就真正找到了mProvider 的真正初始化位置，其实它就是一个WebViewChromium，不要忘了我们为什么费这么大劲找mProvider，其实是为了分析 webView.getSettings()，这样就回到了第一小节，通过getSettings()获取到的WebView的配置。
4、 Settings的初始化
通过第一小节，我们知道Settings是mProvider的一个变量，要想找到Settings就要到 WebViewChromium 来看下：
@Override public WebSettings getSettings() { return mWebSettings; }
接下来就是Settings初始化的地方啦
@Override // BUG=6790250 |javaScriptInterfaces| was only ever used by the obsolete DumpRenderTree // so is ignored. TODO: remove it from WebViewProvider. public void init(final Map<String, Object> javaScriptInterfaces, final boolean privateBrowsing) { if (privateBrowsing) { mFactory.startYourEngines(true); final String msg = "Private browsing is not supported in WebView."; if (mAppTargetSdkVersion >= Build.VERSION_CODES.KITKAT) { throw new IllegalArgumentException(msg); } else { Log.w(TAG, msg); TextView warningLabel = new TextView(mWebView.getContext()); warningLabel.setText(mWebView.getContext().getString( com.android.internal.R.string.webviewchromium_private_browsing_warning)); mWebView.addView(warningLabel); } } // We will defer real initialization until we know which thread to do it on, unless: // - we are on the main thread already (common case), // - the app is targeting >= JB MR2, in which case checkThread enforces that all usage // comes from a single thread. (Note in JB MR2 this exception was in WebView.java). if (mAppTargetSdkVersion >= Build.VERSION_CODES.JELLY_BEAN_MR2) { mFactory.startYourEngines(false); checkThread(); } else if (!mFactory.hasStarted()) { if (Looper.myLooper() == Looper.getMainLooper()) { mFactory.startYourEngines(true); } } final boolean isAccessFromFileURLsGrantedByDefault = mAppTargetSdkVersion < Build.VERSION_CODES.JELLY_BEAN; final boolean areLegacyQuirksEnabled = mAppTargetSdkVersion < Build.VERSION_CODES.KITKAT; mContentsClientAdapter = new WebViewContentsClientAdapter(mWebView); mWebSettings = new ContentSettingsAdapter(new AwSettings( mWebView.getContext(), isAccessFromFileURLsGrantedByDefault, areLegacyQuirksEnabled)); mRunQueue.addTask(new Runnable() { @Override public void run() { initForReal(); if (privateBrowsing) { // Intentionally irreversibly disable the webview instance, so that private // user data cannot leak through misuse of a non-privateBrowing WebView // instance. Can't just null out mAwContents as we never null-check it // before use. destroy(); } } }); }
在第39行进行了 mWebSettings 的初始化，原来是 ContentSettingsAdapter。
5、 setMediaPlaybackRequiresUserGesture() 分析
经过以上我们队Google大神的膜拜，我们找到了mWebSettings，下面来看下 setMediaPlaybackRequiresUserGesture方法：
@Override public void setMediaPlaybackRequiresUserGesture(boolean require) { mAwSettings.setMediaPlaybackRequiresUserGesture(require); }
好吧，又是调用的 mAwSettings 的 setMediaPlaybackRequiresUserGesture 方法，那 mAwSettings 是什么呢？
public ContentSettingsAdapter(AwSettings awSettings) { mAwSettings = awSettings; }
原来是在构造函数中注入的，回到第4小节的最后，这里 new 了一个AwSettings。
mWebSettings = new ContentSettingsAdapter(new AwSettings( mWebView.getContext(), isAccessFromFileURLsGrantedByDefault, areLegacyQuirksEnabled));
那么久来 AwSettings 中看下 setMediaPlaybackRequiresUserGesture 吧：
该类位于系统源码 external/chromium_org/android_webview/java/src/org/chromium/android_webview/AwSettings.java
/** * See {@link android.webkit.WebSettings#setMediaPlaybackRequiresUserGesture}. */ public void setMediaPlaybackRequiresUserGesture(boolean require) { synchronized (mAwSettingsLock) { if (mMediaPlaybackRequiresUserGesture != require) { mMediaPlaybackRequiresUserGesture = require; mEventHandler.updateWebkitPreferencesLocked(); } } }
可以看到这里只是给一个变量 mMediaPlaybackRequiresUserGesture 设置了值，然后看到下面一个方法，豁然开朗：
@CalledByNative private boolean getMediaPlaybackRequiresUserGestureLocked() { return mMediaPlaybackRequiresUserGesture; }
该方法是由JNI层调用的，external/chromium_org/android_webview/native/aw_settings.cc 中我们看到了：
web_prefs->user_gesture_required_for_media_playback = Java_AwSettings_getMediaPlaybackRequiresUserGestureLocked(env, obj);
可见在内核中去调用该接口，判断是否允许音视频的自动播放。
以上所述是小编给大家介绍的关于Android HTML5 audio autoplay无效问题的解决方案，希望对大家有所帮助，如果大家有任何疑问请给我留言，小编会及时回复大家的。在此也非常感谢大家对脚本之家网站的支持！