Send notifications to Android devices with Google Cloud Messaging without additional web server

I wanted to use Google Cloud Messaging on my ESP8266 to push notifications to Android devices. Most tutorials found on the web use a external webserver, PHP scripts and MySQL databases as an interface to send notifications via the Google Cloud Messaging (GCM) service. As the ESP8266 has a webserver capability, but no PHP or MySQL support I wrote this code to replace the external webserver, PHP scripts and MySQL database.

Instead of PHP scripts and a MySQL database I use below explained functions and SPIFFS filesystem for the GCM connection functions and saving the Android registration ids.

To send requests to the GCM servers JSON objects are used. The easiest way to implement JSON decoding and encoding is with the ArduinoJson library written by Benoit Blanchon.
All credits for the ArduinoJson library go to Benoit Blanchon. You can get it from:
Blog on Good Code Smell
Github sources
Install the library as explained in the ArduinoJson Wiki: Using the library with Arduino

Blog
Wiki

###boolean gcmSendMsg() #####Description prepares the JSON object holding the registration IDs and data and calls gcmSendOut to forward the request to the GCM server #####Signatures boolean gcmSendMsg(JsonArray& pushMessageIds, JsonArray& pushMessages); boolean gcmSendMsg(JsonObject& pushMessages); #####Arguments pushMessageIds Json array with the key(s) for the message(s)
pushMessages Json array with the message(s) or Json object with key:message fields #####Used global variables Global string array regAndroidIds[] contains the ids
Global int regDevNum contains number of devices #####Return value true if the request was successful send to the GCM server
false if sending the request to the GCM server failed #####Example // Create messages & keys as JSON arrays DynamicJsonBuffer jsonBufferKeys; DynamicJsonBuffer jsonBufferMsgs; JsonArray& msgKeysJSON = jsonBufferKeys.createArray(); JsonArray& msgsJSON = jsonBufferMsgs.createArray(); msgKeysJSON.add("sensortype"); msgKeysJSON.add("value"); msgsJSON.add("humitity"); msgsJSON.add(95); gcmSendMsg(msgKeysJSON, msgsJSON);

###boolean writeRegIds() #####Description reads Android registration ids from the array regAndroidIds[] and saves them to the file gcm.txt as JSON object #####Arguments none #####Used global variables Global string array regAndroidIds[] contains the ids
Global int regDevNum contains number of devices #####Return value true if the registration ids were successfully saved
false if a file error occured

###boolean getRegisteredDevices() #####Description reads Android registration ids from the file gcm.txt and stores them in the array regAndroidIds[] #####Arguments none #####Used global variables Global string array regAndroidIds[] contains the ids
Global int regDevNum contains number of devices #####Return value true if the registration ids were successfully read
false if a file error occured or if the content of the file was corrupted #####Example if (!getRegisteredDevice()) { Serial.println("Failed to read IDs"); } else { if (regDevNum != 0) { // Any devices already registered? for (int i=0; i<regDevNum; i++) { Serial.println("Device #"+String(i)":"); Serial.println(regAndroidIds[i]); } } }

###boolean addRegisteredDevice() #####Description adds a new Android registration id to the file gcm.txt #####Arguments newDeviceID String containing the registration id #####Used global variables Global string array regAndroidIds[] contains the ids
Global int regDevNum contains number of devices #####Return value true if the registration id was successfully added
false if the registration id was invalid, if the max number of ids was reached or if a file error occured #####Example String newID = "XXX91bFZIZMVJPeWjfEfaqMOWctyfAOifSl6Tz52BpCVHIsGmJnq7dr8XIAueSV2SsjkTTW_vlhDGOS8t-uuITk3jAe-d8NnYuuzhGdS3jGiXpgJYFAfz1gqndx_yz0zo3cWcLsJ0Usx"; if (!addRegisteredDevice(newID)) { Serial.println("Failed to save ID"); } else { Serial.println("Successful saved ID"); }

###boolean delRegisteredDevice() #####Description deletes one or all Android registration id(s) from the file gcm.txt #####Signatures boolean delRegisterDevice(); boolean delRegisterDevice(String delRegId); boolean delRegisterDevice(int delRegIndex); #####Arguments delRegId String with the registration id to be deleted
delRegIndex Index of the registration id to be deleted #####Used global variables Global string array regAndroidIds[] contains the ids
Global int regDevNum contains number of devices #####Return value true if the registration id(s) was/were successfully deleted
false if the registration id was not found, if the indwx was invalid or if a file error occured #####Example // Delete registration id by the id itself String delID = "XXX91bFZIZMVJPeWjfEfaqMOWctyfAOifSl6Tz52BpCVHIsGmJnq7dr8XIAueSV2SsjkTTW_vlhDGOS8t-uuITk3jAe-d8NnYuuzhGdS3jGiXpgJYFAfz1gqndx_yz0zo3cWcLsJ0Usx"; if (!delRegisteredDevice(delID)) { Serial.println("Failed to delete ID"); } else { Serial.println("Successful deleted ID"); }

// Delete registration id at index
int delIndex = 1;
if (!delRegisteredDevice(delIndex)) {
    Serial.println("Failed to delete ID");
} else {
    Serial.println("Successful deleted ID");
}

// Delete all saved registration ids
if (!delRegisteredDevice()) {
    Serial.println("Failed to delete IDs");
} else {
    Serial.println("Successful deleted all IDs");
}

###boolean gcmSendOut() #####Description sends message to https://android.googleapis.com/gcm/send to request a push notification to all registered Android devices used internal only #####Arguments data String with the Json object containing the reg IDs and data #####Used global variables none #####Return value true if the request was successful send to the GCM server
false if sending the request to the GCM server failed #####Example used internal only