編輯：關於Android編程
前言：在android HTML5 開發中有不少人遇到過 audio 標簽 autoplay在某些設備上無效的問題，網上大多是講怎麼在js中操作，即在特定的時刻調用audio的play()方法，在android上還是無效。
在android 4.2添加了允許用戶手勢觸發音視頻播放接口，該接口默認為 true ，即默認不允許自動播放音視頻，只能是用戶交互的方式由用戶自己促發播放。
WebView webView = this.finishActivity(R.id.main_act_webview);
// ... ...
// 其他配置
// ... ...
// 設置4.2以後版本支持autoPlay，非用戶手勢促發
if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.JELLY_BEAN_MR1) {
webView.getSettings().setMediaPlaybackRequiresUserGesture(false);
}
通過以上配置就可以加載帶有自動播放的音視頻啦！
下面我們沿著該問題來窺探下WebView的系統源碼：
/**
* 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();
}
這裡通過一個 mProvider來獲取的配置信息，通過看WebView的源碼，我們可以看到，WebView的所有操作都是交給 mProvider來進行的。
/**
* @hide
*/
@SuppressWarnings("deprecation") // for super() call into deprecated base class constructor.
protected WebView(Context context, AttributeSet attrs, int defStyleAttr, int defStyleRes,
Map 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();
}
可以看到有個ensureProviderCreated()方法，就是在這裡創建的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的構造函數中創建的，並且WebView的所有操作都是交給mProvider進行的。
看下WebViewFactory#getFactory()做了什麼操作：
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 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);
}
}
}
可見在23行返回了sProviderInstance， 是由 providerClass 通過反射創建的，15行中通過getProviderClass() 得到了providerClass.
private static ClassgetProviderClass() { 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 ) 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 返回了一個 getChromiumProviderClass(); 是不是有點熟悉，沒錯Android在4.4開始使用強大的Chromium替換掉了原來的WebKit。來看下這個getChromiumProviderClass()。
// throws MissingWebViewPackageException private static ClassgetChromiumProviderClass() 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 ) Class.forName(CHROMIUM_WEBVIEW_FACTORY, true, clazzLoader); } finally { Trace.traceEnd(Trace.TRACE_TAG_WEBVIEW); } } catch (PackageManager.NameNotFoundException e) { throw new MissingWebViewPackageException(e); } }
最後找到了這個 CHROMIUM_WEBVIEW_FACTORY， 可以看到在 WebViewFactory 中的定義：
private static final String CHROMIUM_WEBVIEW_FACTORY = "com.android.webview.chromium.WebViewChromiumFactoryProvider";
回答2小節的mProvider的初始化，在WebViewChromiumFactoryProvider 的 createWebView(…) 中進行了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(wvc));
}
}
ResourceProvider.registerResources(webView.getContext());
return wvc;
}
OK，到這裡就真正找到了mProvider 的真正初始化位置，其實它就是一個WebViewChromium，不要忘了我們為什麼費這麼大勁找mProvider，其實是為了分析 webView.getSettings()，這樣就回到了第一小節，通過getSettings()獲取到的WebView的配置。
通過第一小節，我們知道Settings是mProvider的一個變量，要想找到Settings就要到 WebViewChromium 來看下：
@Override
public WebSettings getSettings() {
return mWebSettings;
}
接下來就是Settings初始化的地方啦
