上一篇文章中介紹了cordova外掛初始化流程，本文開始介紹，任何呼叫到java程式碼

上文已經介紹到呼叫

		navigator.camera.getPicture(onSuccess, onFail, {
			quality: 50,
			sourceType: Camera.PictureSourceType.CAMERA,
			destinationType: Camera.DestinationType.FILE_URI
		});

Camera.js

cameraExport.getPicture = function (successCallback, errorCallback, options) {
	console.log('takePicture getPicture  ')

    argscheck.checkArgs('fFO', 'Camera.getPicture', arguments);
    options = options || {};
    var getValue = argscheck.getValue;

    var quality = getValue(options.quality, 50);
    var destinationType = getValue(options.destinationType, Camera.DestinationType.FILE_URI);
    var sourceType = getValue(options.sourceType, Camera.PictureSourceType.CAMERA);
    var targetWidth = getValue(options.targetWidth, -1);
    var targetHeight = getValue(options.targetHeight, -1);
    var encodingType = getValue(options.encodingType, Camera.EncodingType.JPEG);
    var mediaType = getValue(options.mediaType, Camera.MediaType.PICTURE);
    var allowEdit = !!options.allowEdit;
    var correctOrientation = !!options.correctOrientation;
    var saveToPhotoAlbum = !!options.saveToPhotoAlbum;
    var popoverOptions = getValue(options.popoverOptions, null);
    var cameraDirection = getValue(options.cameraDirection, Camera.Direction.BACK);

    var args = [quality, destinationType, sourceType, targetWidth, targetHeight, encodingType,
        mediaType, allowEdit, correctOrientation, saveToPhotoAlbum, popoverOptions, cameraDirection];
	console.log('takePicture click 3')

    exec(successCallback, errorCallback, 'Camera', 'takePicture', args);//關鍵程式碼
    // XXX: commented out
    // return new CameraPopoverHandle();
};
 

上面引入了exec方法

var exec = require('cordova/exec');

定義實在cordova.js中定義的

function androidExec(success, fail, service, action, args) {

    if (bridgeSecret < 0) {
        // If we ever catch this firing, we'll need to queue up exec()s
        // and fire them once we get a secret. For now, I don't think
        // it's possible for exec() to be called since plugins are parsed but
        // not run until until after onNativeReady.
        throw new Error('exec() called without bridgeSecret');
    }
    // Set default bridge modes if they have not already been set.
    // By default, we use the failsafe, since addJavascriptInterface breaks too often
    if (jsToNativeBridgeMode === undefined) {
        androidExec.setJsToNativeBridgeMode(jsToNativeModes.JS_OBJECT);
    }

    // If args is not provided, default to an empty array
    args = args || [];

    // Process any ArrayBuffers in the args into a string.
    for (var i = 0; i < args.length; i++) {
        if (utils.typeName(args[i]) == 'ArrayBuffer') {
            args[i] = base64.fromArrayBuffer(args[i]);
        }
    }

    var callbackId = service + cordova.callbackId++,
        argsJson = JSON.stringify(args);
    if (success || fail) {
        cordova.callbacks[callbackId] = {success:success, fail:fail};//這個後面有用
    }

    var msgs = nativeApiProvider.get().exec(bridgeSecret, service, action, callbackId, argsJson);//繼續深入程式碼分析
    // If argsJson was received by Java as null, try again with the PROMPT bridge mode.
    // This happens in rare circumstances, such as when certain Unicode characters are passed over the bridge on a Galaxy S2.  See CB-2666.
    if (jsToNativeBridgeMode == jsToNativeModes.JS_OBJECT && msgs === "@Null arguments.") {
        androidExec.setJsToNativeBridgeMode(jsToNativeModes.PROMPT);
        androidExec(success, fail, service, action, args);
        androidExec.setJsToNativeBridgeMode(jsToNativeModes.JS_OBJECT);
    } else if (msgs) {
        messagesFromNative.push(msgs);
        // Always process async to avoid exceptions messing up stack.
        nextTick(processMessages);
    }
}
 

下面我們看看nativeApiProvider是從哪裡來的

define("cordova/android/nativeapiprovider", function(require, exports, module) {

/**
 * Exports the ExposedJsApi.java object if available, otherwise exports the PromptBasedNativeApi.
 */

var nativeApi = this._cordovaNative || require('cordova/android/promptbasednativeapi');//這裡是關鍵
var currentApi = nativeApi;

module.exports = {
    get: function() { return currentApi; },
    setPreferPrompt: function(value) {
        currentApi = value ? require('cordova/android/promptbasednativeapi') : nativeApi;
    },
    // Used only by tests.
    set: function(value) {
        currentApi = value;
    }
};

});
 

看到_cordovaNative這個是和native通訊的橋

在SystemWebViewEngine.java中（這裡在介紹java端程式碼的時候，會詳細介紹，目前只有一個印象即可）

    @SuppressLint("AddJavascriptInterface")
    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");//與js建立通訊
    }

    @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);
    }

接著呼叫到CordovaBridge.java中

    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);//到外掛管理中心去進行分發
            String ret = null;
            if (!NativeToJsMessageQueue.DISABLE_EXEC_CHAINING) {
                ret = jsMessageQueue.popAndEncode(false);
            }
            return ret;
        } catch (Throwable e) {
            e.printStackTrace();
            return "";
        } finally {
            jsMessageQueue.setPaused(false);
        }
    }

PluginManager.java中

    public void exec(final String service, final String action, final String callbackId, final String rawArgs) {
        LOG.d(TAG, "exec() call stack: " + Log.getStackTraceString(new Throwable()));
        CordovaPlugin plugin = getPlugin(service);//獲取外掛
        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);//執行外掛程式碼
            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());
        }
    }

下面以相機外掛為例:CameraLauncher.java,後面就是吊起照相機的邏輯

    public boolean execute(String action, JSONArray args, CallbackContext callbackContext) throws JSONException {
        this.callbackContext = callbackContext;
        //Adding an API to CoreAndroid to get the BuildConfigValue
        //This allows us to not make this a breaking change to embedding
        this.applicationId = (String) BuildHelper.getBuildConfigValue(cordova.getActivity(), "APPLICATION_ID");
        this.applicationId = preferences.getString("applicationId", this.applicationId);


        if (action.equals("takePicture")) {
            this.srcType = CAMERA;
            this.destType = FILE_URI;
            this.saveToPhotoAlbum = false;
            this.targetHeight = 0;
            this.targetWidth = 0;
            this.encodingType = JPEG;
            this.mediaType = PICTURE;
            this.mQuality = 50;

            //Take the values from the arguments if they're not already defined (this is tricky)
            this.destType = args.getInt(1);
            this.srcType = args.getInt(2);
            this.mQuality = args.getInt(0);
            this.targetWidth = args.getInt(3);
            this.targetHeight = args.getInt(4);
            this.encodingType = args.getInt(5);
            this.mediaType = args.getInt(6);
            this.allowEdit = args.getBoolean(7);
            this.correctOrientation = args.getBoolean(8);
            this.saveToPhotoAlbum = args.getBoolean(9);

            // If the user specifies a 0 or smaller width/height
            // make it -1 so later comparisons succeed
            if (this.targetWidth < 1) {
                this.targetWidth = -1;
            }
            if (this.targetHeight < 1) {
                this.targetHeight = -1;
            }

            // We don't return full-quality PNG files. The camera outputs a JPEG
            // so requesting it as a PNG provides no actual benefit
            if (this.targetHeight == -1 && this.targetWidth == -1 && this.mQuality == 100 &&
                    !this.correctOrientation && this.encodingType == PNG && this.srcType == CAMERA) {
                this.encodingType = JPEG;
            }

            try {
                if (this.srcType == CAMERA) {
                    this.callTakePicture(destType, encodingType);
                }
                else if ((this.srcType == PHOTOLIBRARY) || (this.srcType == SAVEDPHOTOALBUM)) {
                    // FIXME: Stop always requesting the permission
                    if(!PermissionHelper.hasPermission(this, Manifest.permission.READ_EXTERNAL_STORAGE)) {
                        PermissionHelper.requestPermission(this, SAVE_TO_ALBUM_SEC, Manifest.permission.READ_EXTERNAL_STORAGE);
                    } else {
                        this.getImage(this.srcType, destType, encodingType);
                    }
                }
            }
            catch (IllegalArgumentException e)
            {
                callbackContext.error("Illegal Argument Exception");
                PluginResult r = new PluginResult(PluginResult.Status.ERROR);
                callbackContext.sendPluginResult(r);
                return true;
            }

            PluginResult r = new PluginResult(PluginResult.Status.NO_RESULT);
            r.setKeepCallback(true);
            callbackContext.sendPluginResult(r);

            return true;
        }
        return false;
    }