Send key strokes
Send text
Press/release individual keys
Media keys are supported
Read Numlock/Capslock/Scrolllock state
Set battery level (basically works, but doesn't show up in Android's status bar)
Compatible with Android
Compatible with Windows
Compatible with Linux
Compatible with MacOS X (not stable, some people have issues, doesn't work with old devices)
Compatible with iOS (not stable, some people have issues, doesn't work with old devices)

Installation

(Make sure you can use the ESP32 with the Arduino IDE. Instructions can be found here.)
Download the latest release of this library from the release page.
In the Arduino IDE go to "Sketch" -> "Include Library" -> "Add .ZIP Library..." and select the file you just downloaded.
You can now go to "File" -> "Examples" -> "ESP32 BLE Keyboard" and select any of the examples to get started.

Example

/**
 * This example turns the ESP32 into a Bluetooth LE keyboard that writes the words, presses Enter, presses a media key and then Ctrl+Alt+Delete
 */
#include <BleKeyboard.h>

BleKeyboard bleKeyboard;

void setup() {
  Serial.begin(115200);
  Serial.println("Starting BLE work!");
  bleKeyboard.begin();
}

void loop() {
  if(bleKeyboard.isConnected()) {
    Serial.println("Sending 'Hello world'...");
    bleKeyboard.print("Hello world");

    delay(1000);

    Serial.println("Sending Enter key...");
    bleKeyboard.write(KEY_RETURN);

    delay(1000);

    Serial.println("Sending Play/Pause media key...");
    bleKeyboard.write(KEY_MEDIA_PLAY_PAUSE);

    delay(1000);

    Serial.println("Sending Ctrl+Alt+Delete...");
    bleKeyboard.press(KEY_LEFT_CTRL);
    bleKeyboard.press(KEY_LEFT_ALT);
    bleKeyboard.press(KEY_DELETE);
    delay(100);
    bleKeyboard.releaseAll();

  }
  Serial.println("Waiting 5 seconds...");
  delay(5000);
}

API docs

The BleKeyboard interface is almost identical to the Keyboard Interface, so you can use documentation right here: https://www.arduino.cc/reference/en/language/functions/usb/keyboard/

Just remember that you have to use bleKeyboard instead of just Keyboard and you need these two lines at the top of your script:

#include <BleKeyboard.h>
BleKeyboard bleKeyboard;

In addition to that you can send media keys (which is not possible with the USB keyboard library). Supported are the following:

KEY_MEDIA_NEXT_TRACK
KEY_MEDIA_PREVIOUS_TRACK
KEY_MEDIA_STOP
KEY_MEDIA_PLAY_PAUSE
KEY_MEDIA_MUTE
KEY_MEDIA_VOLUME_UP
KEY_MEDIA_VOLUME_DOWN
KEY_MEDIA_WWW_HOME
KEY_MEDIA_LOCAL_MACHINE_BROWSER // Opens "My Computer" on Windows
KEY_MEDIA_CALCULATOR
KEY_MEDIA_WWW_BOOKMARKS
KEY_MEDIA_WWW_SEARCH
KEY_MEDIA_WWW_STOP
KEY_MEDIA_WWW_BACK
KEY_MEDIA_CONSUMER_CONTROL_CONFIGURATION // Media Selection
KEY_MEDIA_EMAIL_READER

There is also Bluetooth specific information that you can set (optional): Instead of BleKeyboard bleKeyboard; you can do BleKeyboard bleKeyboard("Bluetooth Device Name", "Bluetooth Device Manufacturer", 100);. The third parameter is the initial battery level of your device. To adjust the battery level later on you can simply call e.g. bleKeyboard.setBatteryLevel(50) (set battery level to 50%). By default the battery level will be set to 100%, the device name will be ESP32 Bluetooth Keyboard and the manufacturer will be Espressif.

NimBLE-Mode

The NimBLE mode enables a significant saving of RAM and FLASH memory.

Comparison (SendKeyStrokes.ino at compile-time)

Standard

RAM:   [=         ]   9.3% (used 30548 bytes from 327680 bytes)
Flash: [========  ]  75.8% (used 994120 bytes from 1310720 bytes)

NimBLE mode

RAM:   [=         ]   8.3% (used 27180 bytes from 327680 bytes)
Flash: [====      ]  44.2% (used 579158 bytes from 1310720 bytes)

Comparison (SendKeyStrokes.ino at run-time)

	Standard	NimBLE mode	difference
`ESP.getHeapSize()`	296.804	321.252	+ 24.448
`ESP.getFreeHeap()`	143.572	260.764	+ 117.192
`ESP.getSketchSize()`	994.224	579.264	- 414.960

How to activate NimBLE mode?

ArduinoIDE: Before including the library, insert the line #define USE_NIMBLE

#define USE_NIMBLE
#include <BleKeyboard.h>

PlatformIO: Change your platformio.ini to the following settings

lib_deps = 
  NimBLE-Arduino

build-flags = 
  -D USE_NIMBLE

Credits

Credits to chegewara and the authors of the USB keyboard library as this project is heavily based on their work! Also, credits to duke2421 who helped a lot with testing, debugging and fixing the device descriptor!