本文主要是介绍Cordova源码深入分析-第三讲，希望对大家解决编程问题提供一定的参考价值，需要的开发者们随着小编来一起学习吧！
上一讲讲解了js端，调用到java端的代码逻辑
这一讲，主要介绍，js->native带有callback的形式，是如何调用回去的
上一篇介绍到了调用照相机代码的地方，在最后调用了:sendPluginResult
这里调用一下：
@Overridepublic void sendPluginResult(PluginResult cr, String callbackId) {nativeToJsMessageQueue.addPluginResult(cr, callbackId);}NativeToJsMessageQueue.java
public void addPluginResult(PluginResult result, String callbackId) {if (callbackId == null) {LOG.e(LOG_TAG, "Got plugin result with no callbackId", new Throwable());return;}// Don't send anything if there is no result and there is no need to// clear the callbacks.boolean noResult = result.getStatus() == PluginResult.Status.NO_RESULT.ordinal();boolean keepCallback = result.getKeepCallback();if (noResult && keepCallback) {return;}JsMessage message = new JsMessage(result, callbackId);if (FORCE_ENCODE_USING_EVAL) {StringBuilder sb = new StringBuilder(message.calculateEncodedLength() + 50);message.encodeAsJsMessage(sb);message = new JsMessage(sb.toString());}enqueueMessage(message);//将消息放到队列中，里面含有} private void enqueueMessage(JsMessage message) {synchronized (this) {if (activeBridgeMode == null) {LOG.d(LOG_TAG, "Dropping Native->JS message due to disabled bridge");return;}queue.add(message);//将消息加入队列中if (!paused) { //如果没有pause则直接执行，有pause的话，下次触发执行activeBridgeMode.onNativeToJsMessageAvailable(this);//开始执行}}}根据不同的bridge走不同代码，这里先列举一种情况
@Overridepublic void onNativeToJsMessageAvailable(final NativeToJsMessageQueue queue) {cordova.getActivity().runOnUiThread(new Runnable() {public void run() {String js = queue.popAndEncodeAsJs();//取出刚才加入进去的消息，并且转换为jsif (js != null) {engine.loadUrl("javascript:" + js, false);}}});}这里是组装js的代码：
public String popAndEncodeAsJs() {synchronized (this) {int length = queue.size();if (length == 0) {return null;}int totalPayloadLen = 0;int numMessagesToSend = 0;for (JsMessage message : queue) {int messageSize = message.calculateEncodedLength() + 50; // overestimate.if (numMessagesToSend > 0 && totalPayloadLen + messageSize > MAX_PAYLOAD_SIZE && MAX_PAYLOAD_SIZE > 0) {break;}totalPayloadLen += messageSize;numMessagesToSend += 1;}boolean willSendAllMessages = numMessagesToSend == queue.size();StringBuilder sb = new StringBuilder(totalPayloadLen + (willSendAllMessages ? 0 : 100));// Wrap each statement in a try/finally so that if one throws it does// not affect the next.for (int i = 0; i < numMessagesToSend; ++i) {JsMessage message = queue.removeFirst();if (willSendAllMessages && (i + 1 == numMessagesToSend)) {message.encodeAsJsMessage(sb);} else {sb.append("try{");message.encodeAsJsMessage(sb);sb.append("}finally{");}}if (!willSendAllMessages) {sb.append("window.setTimeout(function(){cordova.require('cordova/plugin/android/polling').pollOnce();},0);");}for (int i = willSendAllMessages ? 1 : 0; i < numMessagesToSend; ++i) {sb.append('}');}String ret = sb.toString();Log.v("XPC","ret="+ret);//ret=cordova.callbackFromNative('Camera1024838304',false,9,["No Image Selected"],false);return ret;}}看一下encodeAsJsMessage
void encodeAsJsMessage(StringBuilder sb) {if (pluginResult == null) {sb.append(jsPayloadOrCallbackId);} else {int status = pluginResult.getStatus();boolean success = (status == PluginResult.Status.OK.ordinal()) || (status == PluginResult.Status.NO_RESULT.ordinal());sb.append("cordova.callbackFromNative('").append(jsPayloadOrCallbackId).append("',").append(success).append(",").append(status).append(",[");buildJsMessage(sb);sb.append("],").append(pluginResult.getKeepCallback()).append(");");}}这样就调回了js端的cordova.js端
callbackFromNative: function(callbackId, isSuccess, status, args, keepCallback) {try {var callback = cordova.callbacks[callbackId];//根据callbackId找到我们之前的函数，这个在上一讲中，写到过，if (callback) {if (isSuccess && status == cordova.callbackStatus.OK) {callback.success && callback.success.apply(null, args);//这里会直接调用camera.js的成功和失败方法中} else if (!isSuccess) {callback.fail && callback.fail.apply(null, args);}/*elseNote, this case is intentionally not caught.this can happen if isSuccess is true, but callbackStatus is NO_RESULTwhich is used to remove a callback from the list without calling the callbackstypically keepCallback is false in this case*/// Clear callback if not expecting any more resultsif (!keepCallback) {delete cordova.callbacks[callbackId];}}}catch (err) {var msg = "Error in " + (isSuccess ? "Success" : "Error") + " callbackId: " + callbackId + " : " + err;console && console.log && console.log(msg);cordova.fireWindowEvent("cordovacallbackerror", { 'message': msg });throw err;}},这篇关于Cordova源码深入分析-第三讲的文章就介绍到这儿，希望我们推荐的文章对编程师们有所帮助！
