Cordova—Android源码分析二：JS调用Native

        在CordovaWebView初始化时，会根据Android版本的不同，初始化不同的JS调用Native的方法，当Android版本小于4.2(API 17)时，会采用prompt的方式处理JS的调用，当Android版本大于4.2时，会采用JavaScriptInterface的方式调用，初始化的方法在CordovaWebViewImpl的init(CordovaInterface cordova, List pluginEntries, CordovaPreferences preferences)方法中的：

engine.init(this, cordova, engineClient, resourceApi, pluginManager, nativeToJsMessageQueue);

        engine的类型为CordovaWebViewEngine接口，它的实现类为SystemWebViewEngine，在init()方法的最后一行调用了exposeJsInterface()方法：

exposeJsInterface(webView, bridge);

        在exposeJsInterface()方法中，根据Android版本的不同，初始化了不同的JS调用Android的方式：

private static void exposeJsInterface(WebView webView, CordovaBridge bridge) {
    if ((Build.VERSION.SDK_INT < Build.VERSION_CODES.JELLY_BEAN_MR1)) {
        Log.i(TAG, "Disabled addJavascriptInterface() bridge since Android version is old.");
        // Bug being that Java Strings do not get converted to JS strings automatically.
        // This isn't hard to work-around on the JS side, but it's easier to just
        // use the prompt bridge instead.
        return;            
    }
    SystemExposedJsApi exposedJsApi = new SystemExposedJsApi(bridge);
    webView.addJavascriptInterface(exposedJsApi, "_cordovaNative");
}

        在JS调用Native时，会根据_cordovaNative对象是否存在来判断使用哪种方式调用Native，SystemExposedJsApi类实现了ExposedJsApi，有以下三个方法：

public interface ExposedJsApi {
    public String exec(int bridgeSecret, String service, String action, String callbackId, String arguments) throws JSONException, IllegalAccessException;
    public void setNativeToJsBridgeMode(int bridgeSecret, int value) throws IllegalAccessException;
    public String retrieveJsMessages(int bridgeSecret, boolean fromOnlineEvent) throws IllegalAccessException;
}

SystemExposedJsApi如下：

class SystemExposedJsApi implements ExposedJsApi {
    private final CordovaBridge bridge;
    SystemExposedJsApi(CordovaBridge bridge) {
        this.bridge = bridge;
    }
    @JavascriptInterface
    public String exec(int bridgeSecret, String service, String action, String callbackId, String arguments) throws JSONException, IllegalAccessException {
        return bridge.jsExec(bridgeSecret, service, action, callbackId, arguments);
    }
    @JavascriptInterface
    public void setNativeToJsBridgeMode(int bridgeSecret, int value) throws IllegalAccessException {
        bridge.jsSetNativeToJsBridgeMode(bridgeSecret, value);
    }
    @JavascriptInterface
    public String retrieveJsMessages(int bridgeSecret, boolean fromOnlineEvent) throws IllegalAccessException {
        return bridge.jsRetrieveJsMessages(bridgeSecret, fromOnlineEvent);
    }
}

        exec()方法是JS调用Native的方法。当Android版本低于4.2时，会采用prompt的方式，此方式在继承WebChromeClient的SystemWebChromeClient类中的onJsPrompt()方法中处理JS。
下面来分析Android 4.2以上版本的情况，当JS调用Native的exec方法时，会调用SystemExposedJsApi对象(“_cordovaNative”)的exec方法，从而调用CordovaBridge的jsExec方法，如下：

 public String jsExec(int bridgeSecret, String service, String action, String callbackId, String arguments) throws JSONException, IllegalAccessException {
    if (!verifySecret("exec()", bridgeSecret)) {
        return null;
    }
    // If the arguments weren't received, send a message back to JS.  It will switch bridge modes and try again.  See CB-2666.
    // We send a message meant specifically for this case.  It starts with "@" so no other message can be encoded into the same string.
    if (arguments == null) {
        return "@Null arguments.";
    }
    jsMessageQueue.setPaused(true);
    try {
        // Tell the resourceApi what thread the JS is running on.
        CordovaResourceApi.jsThread = Thread.currentThread();
        pluginManager.exec(service, action, callbackId, arguments);
        String ret = null;
        if (!NativeToJsMessageQueue.DISABLE_EXEC_CHAINING) {
            ret = jsMessageQueue.popAndEncode(false);
        }
        return ret;
    } catch (Throwable e) {
        e.printStackTrace();
        return "";
    } finally {
        jsMessageQueue.setPaused(false);
    }
}

        其中，又执行了PluginManager的exec(service, action, callbackId, arguments)方法，如下：

public void exec(final String service, final String action, final String callbackId, final String rawArgs) {
    CordovaPlugin plugin = getPlugin(service);
    if (plugin == null) {
        Log.d(TAG, "exec() call to unknown plugin: " + service);
        PluginResult cr = new PluginResult(PluginResult.Status.CLASS_NOT_FOUND_EXCEPTION);
        app.sendPluginResult(cr, callbackId);
        return;
    }
    CallbackContext callbackContext = new CallbackContext(callbackId, app);
    try {
        long pluginStartTime = System.currentTimeMillis();
        boolean wasValidAction = plugin.execute(action, rawArgs, callbackContext);
        long duration = System.currentTimeMillis() - pluginStartTime;
        if (duration > SLOW_EXEC_WARNING_THRESHOLD) {
            Log.w(TAG, "THREAD WARNING: exec() call to " + service + "." + action + " blocked the main thread for " + duration + "ms. Plugin should use CordovaInterface.getThreadPool().");
        }
        if (!wasValidAction) {
            PluginResult cr = new PluginResult(PluginResult.Status.INVALID_ACTION);
            callbackContext.sendPluginResult(cr);
        }
    } catch (JSONException e) {
        PluginResult cr = new PluginResult(PluginResult.Status.JSON_EXCEPTION);
        callbackContext.sendPluginResult(cr);
    } catch (Exception e) {
        Log.e(TAG, "Uncaught exception from plugin", e);
        callbackContext.error(e.getMessage());
    }
}

        在PluginManager的exec方法中，首先根据CordovaPlugin的名称得到插件，如果找不到插件，则设置错误回调信息并return。如果找到该插件，则调用：

boolean wasValidAction = plugin.execute(action, rawArgs, callbackContext);

        并监听返回值，返回值标识是否正确执行，如果返回false，则设置错误信息并回调：

if (!wasValidAction) {
            PluginResult cr = new PluginResult(PluginResult.Status.INVALID_ACTION);
            callbackContext.sendPluginResult(cr);
        }

        同时，还会监控执行时间，如果时间过长，则警告用户插件执行阻塞了主线程，为了防止阻塞主线程，需要在子线程中进行耗时操作，Cordova提供了一个线程池供插件调用，在CordovaPlugin中使用以下方法获取线程时：

cordova.getThreadPool();

        以上为Cordova中JS调用本地插件的全部过程。