WebView getContentHeight()的内部实现是

探究WebView getContentHeight的内部实现看看是否需要进行额外计算

先从WebView入手

WebView.java

    /**
     * Gets the height of the HTML content.
     *
     * @return the height of the HTML content
     */
    @ViewDebug.ExportedProperty(category = "webview")
    public int getContentHeight() {
        checkThread();
        return mProvider.getContentHeight();
    }

再找mProvider的实现

WebView.java

    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());
        }
    }

getFactory()是关键

WebView.java

    private static WebViewFactoryProvider getFactory() {
        return WebViewFactory.getProvider();
    }

查看WebViewFactory.getProvider(),直接看return

WebViewFactory.java

static WebViewFactoryProvider getProvider() {
    //...
    if (sProviderInstance != null) return sProviderInstance;
    //...
    
    try {
        Class<WebViewFactoryProvider> providerClass = getProviderClass();
        Method staticFactory = null;
        try {
            staticFactory = providerClass.getMethod(
                        CHROMIUM_WEBVIEW_FACTORY_METHOD, WebViewDelegate.class);
            } catch (Exception e) {
               if (DEBUG) {
                Log.w(LOGTAG, "error instantiating provider with static factory method", e);
            }
       }
    
    //...
     try {
        sProviderInstance = (WebViewFactoryProvider)
                            staticFactory.invoke(null, new WebViewDelegate());
        if (DEBUG) Log.v(LOGTAG, "Loaded provider: " + sProviderInstance);
        return sProviderInstance;
        } catch (Exception e) {
    
    //...
}

用到了反射，关键的getProviderClass()，也是直接看return

WebViewFactory.java

private static Class<WebViewFactoryProvider> getProviderClass() {
        //...
    try {
                return getWebViewProviderClass(clazzLoader);
                } finally {
                    Trace.traceEnd(Trace.TRACE_TAG_WEBVIEW);
                }
        //...
                
     try {
                return (Class<WebViewFactoryProvider>) Class.forName(NULL_WEBVIEW_FACTORY);
            } catch (ClassNotFoundException e2) {
                // Ignore.
            }
       //...

}

第二个return可以不用关心，因为有null,关注第一个return

WebViewFactory.java

    /**
     * @hide
     */
    public static Class<WebViewFactoryProvider> getWebViewProviderClass(ClassLoader clazzLoader)
            throws ClassNotFoundException {
        return (Class<WebViewFactoryProvider>) Class.forName(CHROMIUM_WEBVIEW_FACTORY,
                true, clazzLoader);
    }

隐藏方法，看看CHROMIUM_WEBVIEW_FACTORY是什么东东

WebViewFactory.java

    /** @hide */
    private static final String CHROMIUM_WEBVIEW_FACTORY =
            "com.android.webview.chromium.WebViewChromiumFactoryProviderForO";

OK,反射了这个类，去sdk source里找找

...
找啊找
...
发现在android-19,android-20,android-22里找到，高版本的都是null实现

随便找个android-22吧，路径如下：

android-sdk\sources\android-22\com\android\webview\chromium\WebViewChromiumFactoryProvider.java

回到上面
getProviderClass()完毕，反射出WebViewChromiumFactoryProvider实例，就可以执行其createWebView()方法了

WebViewChromiumFactoryProvider.java

        @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));
            }
        }

        return wvc;
    }

OK 真正的实现找到了--->WebViewChromium.java，并且在同目录下

至此，WebView的傀儡 WebViewProvider mProvider已揭开真面目

下一步肯定是不忘初心，看看getContentHeight()的实现啦

WebViewChromiumFactoryProvider.java

    @Override
    public int getContentHeight() {
        if (mAwContents == null) return 0;
        // No checkThread() as it is mostly thread safe (workaround for b/10594869).
        return mAwContents.getContentHeightCss();
    }

又是桥接其他类来实现，AwContents.java，在 org.chromium.android_webview.AwContents;

AwContents提供的不是WebView的API，所以，需要一层桥接部分，将AwContents桥接到WebView，这就是图中的桥接模块，该模块位于Android源代码中的frameworks/webview/chromium/java/com/android/webview/chromium/目录下，WebViewChromium和WebViewChromiumFactory类作为WebView的具体实现，依赖于Chromium项目的AwContents模块。

github clone 一个Chromium项目看看
https://android.googlesource.com/platform/external/chromium_org

找啊找
在\chromium_org\android_webview\java\src\org\chromium\android_webview目录下找到AwContents.java

ok 查看其内部实现即可

AwContents.java

    public int getContentHeightCss() {
        return (int) Math.ceil(mContentHeightDip);
    }

这里可以看出直接返回成员变量，未进行计算

那这个成员变量赋值在哪

AwContents.java

    @CalledByNative
    private void updateScrollState(int maxContainerViewScrollOffsetX,
            int maxContainerViewScrollOffsetY, int contentWidthDip, int contentHeightDip,
            float pageScaleFactor, float minPageScaleFactor, float maxPageScaleFactor) {
        mContentWidthDip = contentWidthDip;
        mContentHeightDip = contentHeightDip;
        mScrollOffsetManager.setMaxScrollOffset(maxContainerViewScrollOffsetX,
                maxContainerViewScrollOffsetY);
        setPageScaleFactorAndLimits(pageScaleFactor, minPageScaleFactor, maxPageScaleFactor);
    }
    

在这里赋值，可以根据注解@CalledByNative看出是Native层主动回调出来进行设置。

（也可在该实现中查看其它webview操作的实现。）

所以得到了想要的答案。

参考：
http://www.jcodecraeer.com/a/anzhuokaifa/androidkaifa/2014/1106/1921.html

https://android.googlesource.com/platform/external/chromium_org