# Implementing a Blynk client (library)

{% hint style="warning" %}
This documentation is for the LEGACY version of Blynk platform which is no longer supported and will be shut down. 

You can sign up for the current version of Blynk platform [here](http://blynk.cloud/dashboard/register).

The new mobile apps can be downloaded from [App Store](https://apps.apple.com/us/app/blynk-iot/id1559317868) and [Google Play](https://play.google.com/store/apps/details?id=cloud.blynk\&hl=en\&gl=US).

The actual Blynk documentation is [here](https://docs.blynk.io/).
{% endhint %}

We provide a reference C++/Arduino implementation of the library. It is very extensible and modular, just look at [the list of supported hardware](http://community.blynk.cc/t/hardware-supported-by-blynk/16). Adding new connection types and Arduino-compatible boards is easy.

But some devices are programmed in other languages and environments, like:

* Vendor-specific SDK and IDE
* JavaScript, Node.JS, Espruino 
*  Python, MicroPython 
* NodeMCU, eLua

This document hints how to write a custom library.

#### Blynk library main functions <a href="#blynk-library-main-functions" id="blynk-library-main-functions"></a>

* Provide easy-to use API
* Virtual pin handlers registration
* Provide comfortable wrappers for some widgets
* Manage connection
* Should support different connection type/hardware, if applicable
* Serialize/deserialize Blynk protocol
* Should be portable across similar devices (or same technology/programming language), if possible
* Should detect and notify the user about [troubles](https://github.com/blynkkk/blynk-library/blob/master/extras/docs/Troubleshooting.md) where possible (especially Flood)
* Handle direct pin operations \[optional]<br>

#### Adding new HW board to the Blynk mobile app <a href="#adding-new-hw-board-to-the-blynk-mobile-app" id="adding-new-hw-board-to-the-blynk-mobile-app"></a>

Different boards can be added by creating JSON board description file - look at the examples [here](https://github.com/blynkkk/boards). You can send us your own board description files **for review and App integration**.

There may be a problem that you want to start testing your implementation, but your board is not listed int the Blynk App. We have a **"Generic Board"** specially for such purposes. Or you can just select UNO board and check how it works using just virtual pins. Most digital pins should also work. Analog IO/PWM will not work in general, until we add your board to the App.

#### Blynk protocol <a href="#blynk-protocol" id="blynk-protocol"></a>

Blynk transfers binary messages with the following structure:

**Command  Message Id   Length/Status      Body**\
1 byte            2 bytes               2 bytes              Variable&#x20;

**Message Id** and **Length** are [big endian](http://en.wikipedia.org/wiki/Endianness#Big-endian). Body has a command-specific format.

Command and Status definitions: [BlynkProtocolDefs.h](https://github.com/blynkkk/blynk-library/blob/master/src/Blynk/BlynkProtocolDefs.h)

Another protocol description can be found [here](https://github.com/blynkkk/blynk-server/blob/master/docs/README_FOR_APP_DEVS.md).

Typical Blynk library knows how to **send**(S) and **process**(P):

```
S   BLYNK_CMD_LOGIN + auth token
SP  BLYNK_CMD_PING
SP  BLYNK_CMD_RESPONSE
SP  BLYNK_CMD_BRIDGE
SP  BLYNK_CMD_HARDWARE
S   BLYNK_CMD_TWEET
S   BLYNK_CMD_EMAIL
S   BLYNK_CMD_PUSH_NOTIFICATION
```

#### HARDWARE/BRIDGE command body <a href="#hardwarebridge-command-body" id="hardwarebridge-command-body"></a>

The body of these commands are encoded as a sequence of strings, separated by **'\0'** ([Null character](http://en.wikipedia.org/wiki/Null_character)). Please note that the last value may be not [Null-terminated.In](http://null-terminated.in/) the following command examples **'\0'** chars are replaced with spaces.

#### Pin mode <a href="#pin-mode" id="pin-mode"></a>

PinMode command is received by library after connection, or when a mobile application starts.

```
pm <pin> <mode>
pm <pin> <mode> <pin> <mode> <pin> <mode> ...
```

Mode:

* in - INPUT
* out - OUTPUT
* pu - INPUT\_PULLUP
* pd - INPUT\_PULLDOWN

#### Digital pin operations <a href="#digital-pin-operations" id="digital-pin-operations"></a>

Digital write:

```
dw <pin> <val>
```

Digital read:

```
dr <pin>
```

#### Analog pin operations <a href="#analog-pin-operations" id="analog-pin-operations"></a>

```
aw <pin> <val>

ar <pin>
```

#### Virtual pin operations <a href="#virtual-pin-operations" id="virtual-pin-operations"></a>

```
vw <pin> <param0> <param1> <param2> <param3> ...

vr <pin>
```

#### Developer notes <a href="#developer-notes" id="developer-notes"></a>

* Values in HW commands are plain text.
* In response to dr/ar command, library should send dw/aw command on the same pin and with the same message id.
* These situations should cause a connection drop or reconnection attempt: Message with ID=0 is received, Message with unknown type is received

#### Example implementations <a href="#example-implementations" id="example-implementations"></a>

Use these to play with the protocol and understand the basics:

* [Python + MicroPython ](https://github.com/vshymanskyy/blynk-library-python)
* [Node.js + Espruino](https://github.com/vshymanskyy/blynk-library-js)
* [Arduino](https://github.com/blynkkk/blynk-library)
* [Particle Core/Photon/Electron](https://github.com/vshymanskyy/blynk-library-spark)

#### Adding network interface <a href="#adding-network-interface" id="adding-network-interface"></a>

4 entities should be created to add a new network interface to Arduino library:

1 - Select connection interface that will be used for Blynk operation. This should be something like <http://www.arduino.cc/en/Tutorial/WebClient>

Based on the API of the connection, create the Transport. Some examples may be found in the Adapters folder:

* BlynkTransportSerial
* BlynkTransportCC3000
* BlynkArduinoClient **- should be reused, if possible**

2 - Create Blynk representative class, which contains connection-specific helper functions (like begin).Examples:

* BlynkEthernet
* BlynkStream
* BlynkCC3000
* BlynkWildFire
* BlynkYun

3 - Create `BlynkSimple*`  header for your connection. This constructs main Blynk instance, so the user (mostly) doesn't need to get into such details.Examples:

* BlynkSimpleEthernet.h
* BlynkSimpleCC3000.h
* BlynkSimpleWifi.h
* BlynkSimpleUIPEthernet.h

4 - Create a simple example for your platform ;)\ <br>


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