A NEW LIBRARY FOR GOOGLE CALENDAR API, OAUTH 2, ETC. (OBSOLETE GUIDE)

The figures below show the main customizations to the source code, which do not change the behavior of the sketch.

The example sketch, as written, provides that an administrator:

• create a calendar in own Google account with the same title set in the "algorithm.h" file ("calendar_summary" variable), relating to the development boards;

• use one or more development boards, each with a different "esp32_id" value ("algorithm.h" file).

The setup() function of the "sketch.ino" file:

• connects to the WiFi network, logs in to the Google server and initializes the calendar of development boards;

• if present, reads the event with the title "From: Administrator     To: All".

The loop () function of "sketch.ino" cycles through the following algorithm:

1. restart the development board if it has lost the WiFi connection;

2. the event containing in the title the string "To: " + esp32_id (for example "To: ESP32-1"), with older start date and time is searched; if nothing is found, the loop returns to point 1;

3. if the start date and time have already passed, the event is cancelled, otherwise the cycle resumes from step 1;

4. an event identical to the previous one removed is created, but with the start date and time shifted forward by about 45 seconds.

The events of point 2 must have as title"From: Administrator     To: " + esp32_id (recipient) or "From: " + esp32_id (sender) + "     To: " + esp32_id (recipient); those of the first type are created by the administrator, seconds from development boards. There can be at most one event with title"From: Administrator     To: All". If errors occur during the execution of the sketch, the device is restarted. 

In this way, by appropriately modifying the functions present in the files "sketch.ino", "algorithm.h", "algorithm.cpp" and the descriptions of the events present in the calendar, it is possible to customize the behavior of the development cards.

Some clarifications:

• the date and time of the end of events are not considered;

• in the event descriptions you can't insert the strings"From: Administrator     To: All" and "From: " + esp32_id;

• when creating an event it is recommended to modify only the title, description, start and end dates and times fields to avoid problems during the execution of the sketch;

• there may be unmanaged errors in the source code.

The sketch supports oled displays as the one shown on the page startingelectronics.org/tutorials...n5qQgRisBcSrohFCA, with the only difference working on 3.3 V; it must connected to the development board as follows.

At the first start each board requires access to own Google account; this is summarized below.

The login procedure won't be repeated until the credentials stored in the ESP32 are valid. 

The following image depicts a calendar with an event "From: Administrator     To: All"  and one "From: ESP32-1     To: ESP32-1"; the latter is deleted and recreated cyclically from the development board.

The video below shows the behavior of the ESP32 with the display.

For this project I helped myself with the examples contained in Arduino IDE, the libraries  OakOLED (display support, by netguy204) and Time (time management, by PaulStoffregen). Therefore I thank their authors.

Notes:

1) the project does not implement all modern security systems for data protection, so it is better to avoid entering sensitive data in the information exchanged between the development boards and the Internet;

2) my programming skills are not very high and aim to improve them over time; therefore pay attention, because you could find programming errors and wrong algorithms in the source code;

3) the software is provided "as is", without warranty of any kind; I take no responsibility for the consequences deriving from the use of the information contained on this page.