BLEServer Class

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An easy-to-use Bluetooth Low Energy (BLE) GATT Server class.

Syntax

ipworksble.BLEServer

Remarks

The BLEServer class provides a simple but flexible BLE GATT server (peripheral) implementation, making it easy to advertise your own services and characteristics, and to respond to the requests of remote BLE GATT clients that connect to them.

Prerequisites

Windows Notes:

Please note that the BLE classes are only supported by Windows 10 Creator's Update (10.0.15063.0) and later.

Java Notes:

The Java edition uses Java Native Interface (JNI) and relies on included native C++ libraries that perform the actual BLE functions. The BLE classes will attempt to load the platform-specific binary at runtime, so they need to be included in the project as follows:

• jniipworksble24.dll for Windows
• libjniipworksble.24.0.so for Linux
• libjniipworksble.24.0.dylib for macOS

Android Notes:

The BLE classes require an Android API level of 21 or higher.

Note that on Android, even when pairing is not explicitly enabled in the component, the operating system may automatically perform pairing as it deems appropriate.

iOS/macOS Notes:

The BLE classes have the following operating system requirements:

• iOS 5.0+
• iPadOS 5.0+
• Mac Catalyst 13.0+
• macOS 10.10+

Note that for iOS and macOS, the operating system handles all pairing, and this is not controllable from the component.

Linux Notes:

On Linux, the BLE classes rely on BlueZ and DBus. These packages must be installed in order to use the components. Many Linux distributions come with BlueZ and DBus pre-installed.

Adding Services and Characteristics

Before you can advertise a GATT service, you must define the characteristics it exposes. Call the AddCharacteristic method, passing the UUID of the service the characteristic belongs to, the UUID of the characteristic itself, and a bitmask of flags describing what remote clients are permitted to do with it. If the service doesn't already exist, it's created automatically the first time a characteristic is added to it.

As with BLEClient, UUIDs may be given as 16-, 32-, or 128-bit hexadecimal strings (a mixture of formats is fine; they're all converted to 128-bit internally).

Props is a bitmask made up of one or more of the following flags:

• 0x0002 (Read): The characteristic's value may be read by clients. Handle the ReadRequested event to supply the value.
• 0x0004 (WriteWithoutResponse): Clients may write the characteristic's value without expecting an acknowledgement.
• 0x0008 (Write): Clients may write the characteristic's value and receive an acknowledgement indicating success or failure.
• 0x0010 (Notify): Clients may subscribe to receive unacknowledged notifications when the characteristic's value changes.
• 0x0020 (Indicate): Clients may subscribe to receive acknowledged indications when the characteristic's value changes.

Handle the WriteRequested event to be notified of, and respond to, write and write-without-response requests. Handle the SubscriptionChanged event to be notified when a client subscribes to, or unsubscribes from, a Notify or Indicate characteristic.

// Add a simple read-only "Battery Level" characteristic to a "Battery Service". bleserver1.AddCharacteristic("180F", "2A19", 0x0002);

Refer to AddCharacteristic for more detailed information.

Advertising

Once you've added the characteristics you need for a service, call StartListening (passing the service's UUID) to begin advertising it, allowing nearby GATT clients to discover and connect to it. The AdvertisementStatusChanged event fires once advertising has actually begun (or if it fails to start, or is unexpectedly aborted).

You can check whether a particular service is currently being advertised by setting the ServiceId property to its UUID, and then inspecting the Listening property. Call StopListening to stop advertising a service.

// AdvertisementStatusChanged event handler. bleserver1.OnAdvertisementStatusChanged += (s, e) => Console.WriteLine("Advertising status: " + e.Status); bleserver1.StartListening("180F"); // ... bleserver1.StopListening("180F");

Handling Client Requests

The class is not explicitly notified when a remote device forms a low-level BLE connection to it. Instead, the Connected event fires the first time a client interacts with one of your characteristics, by reading it, writing it, or subscribing to it. The ClientId it provides is also passed to the events below, so that you can associate related requests with a particular client.

Handle the ReadRequested event to respond to read requests for characteristics that have the Read flag set. Set the event's ResponseValue parameter to a hex-encoded string holding the bytes to return, and leave its Accept parameter set to its default value of True. To reject the request instead, set Accept to False.

bleserver1.OnReadRequested += (s, e) => { if (e.CharacteristicId.Equals("00002A19-0000-1000-8000-00805F9B34FB", StringComparison.OrdinalIgnoreCase)) { e.ResponseValue = "5A"; // 90 (%), as a single hex-encoded byte. } else { e.Accept = false; } };

Similarly, handle the WriteRequested event to respond to write requests for characteristics that have the Write or WriteWithoutResponse flag set. Its Value parameter holds the hex-encoded string of bytes the client wants to write, and WithResponse indicates whether the client is expecting an acknowledgement. Set Accept to False to reject a write made with acknowledgement; Accept has no effect on writes made without acknowledgement, since the client isn't told the outcome either way.

bleserver1.OnWriteRequested += (s, e) => { Console.WriteLine("Client wrote: " + e.Value); };

Handle the SubscriptionChanged event to know when a Notify or Indicate characteristic gains or loses subscribed clients, and the SessionStatusChanged and Disconnected events to track a client's connection over time.

Once a client has subscribed to a Notify or Indicate characteristic, call UpdateValue whenever its value changes to push the new value to every currently subscribed client. If no clients are currently subscribed, calling UpdateValue has no effect.

// Push a new value to all clients subscribed to notifications/indications on this characteristic. bleserver1.UpdateValue("180F", "2A19", new byte[] { 0x5A });

Usage Notes

It is very important that you do not block or perform any long-running operations within the class's event handlers, particularly ReadRequested and WriteRequested. The platform APIs that the class uses rely on timely delivery of these events in order to function properly.

Most platforms will in some way limit the use of Bluetooth APIs when your application is in the background. Be aware of the limitations your platform places on the usage of BLE APIs while in the background, and be ready to act accordingly.

Be sure to call StopListening for each of your services when you're finished advertising them.

Property List

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The following is the full list of the properties of the class with short descriptions. Click on the links for further details.

Method List

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The following is the full list of the methods of the class with short descriptions. Click on the links for further details.

Event List

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The following is the full list of the events fired by the class with short descriptions. Click on the links for further details.

Config Settings

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The following is a list of config settings for the class with short descriptions. Click on the links for further details.

Characteristics Property (BLEServer Class)

The GATT characteristics which have been added to the server.

Syntax

public ServerCharacteristicList getCharacteristics();

Remarks

The GATT characteristics which have been added to the server through calls to the AddCharacteristic method. This collection is read-only.

This property is read-only and not available at design time.

Please refer to the ServerCharacteristic type for a complete list of fields.

Listening Property (BLEServer Class)

Whether the service identified by ServiceId is currently being advertised.

Syntax

public boolean isListening();

Default Value

False

Remarks

This property indicates whether the service identified by the ServiceId property is currently being advertised, and can therefore be discovered by remote clients.

Use the StartListening and StopListening methods to start and stop advertising a service.

This property is read-only and not available at design time.

ServiceId Property (BLEServer Class)

The Id of the service to check the listening status of.

Syntax

public String getServiceId();
public void setServiceId(String serviceId);

Default Value

""

Remarks

Set ServiceId to the Id of a service you've added characteristics to (via AddCharacteristic) in order to inspect its advertising status using the Listening property.

Services are identified by the same UUID string passed to AddCharacteristic, StartListening, and StopListening.

This property is not available at design time.

Services Property (BLEServer Class)

The GATT services which have been added to the server.

Syntax

public ServerServiceList getServices();

Remarks

The GATT services which have been added to the server through calls to the AddCharacteristic method. This collection is read-only.

This property is read-only and not available at design time.

Please refer to the ServerService type for a complete list of fields.

AddCharacteristic Method (BLEServer Class)

Adds a characteristic to a service, creating the service if necessary.

Syntax

public void addCharacteristic(String serviceId, String characteristicId, int props);

Remarks

This method adds a characteristic, identified by CharacteristicId, to the service identified by ServiceId. If no service with that Id has been added yet, it is created automatically.

ServiceId and CharacteristicId are UUID strings, and may be specified using 16-, 32-, or 128-bit hexadecimal notation (dashes are optional); they are converted to full 128-bit UUIDs internally.

Props is a bitmask describing the operations remote clients are permitted to perform on the characteristic once it starts being advertised. It should be one or more of the following flags, combined with a bitwise OR:

Handle the WriteRequested event to respond to write requests, and use UpdateValue to push value changes to subscribed clients.

A characteristic cannot be added to a service that is already being advertised (see StartListening); doing so will cause the class to throws an exception. Once a service has started listening, its set of characteristics is fixed until you call StopListening.

// Add a read-only "Battery Level" characteristic to a "Battery Service". bleserver1.AddCharacteristic("180F", "2A19", 0x0002); // Add a write-only "Alert Level" characteristic to an "Immediate Alert" service. bleserver1.AddCharacteristic("1802", "2A06", 0x0008);

Config Method (BLEServer Class)

Sets or retrieves a configuration setting.

Syntax

public String config(String configurationString);

Remarks

Config is a generic method available in every class. It is used to set and retrieve configuration settings for the class.

These settings are similar in functionality to properties, but they are rarely used. In order to avoid "polluting" the property namespace of the class, access to these internal properties is provided through the Config method.

To set a configuration setting named PROPERTY, you must call Config("PROPERTY=VALUE"), where VALUE is the value of the setting expressed as a string. For boolean values, use the strings "True", "False", "0", "1", "Yes", or "No" (case does not matter).

To read (query) the value of a configuration setting, you must call Config("PROPERTY"). The value will be returned as a string.

DoEvents Method (BLEServer Class)

This method processes events from the internal message queue.

Syntax

public void doEvents();

Remarks

When DoEvents is called, the class processes any available events. If no events are available, it waits for a preset period of time, and then returns.

StartListening Method (BLEServer Class)

Begins advertising a service so that remote clients can discover and connect to it.

Syntax

public void startListening(String serviceId);

Remarks

This method begins advertising the service identified by ServiceId, making it discoverable by nearby GATT clients. ServiceId must identify a service that characteristics have already been added to via AddCharacteristic.

The AdvertisementStatusChanged event fires once advertising actually starts (and again if it later stops or is aborted). You can check whether a service is currently being advertised at any time by setting the ServiceId property to its Id and inspecting the Listening property.

Calling this method for a service that doesn't exist, or that is already being advertised, will cause the class to throws an exception.

bleserver1.AddCharacteristic("180F", "2A19", 0x0002); bleserver1.StartListening("180F");

StopListening Method (BLEServer Class)

Stops advertising a service.

Syntax

public void stopListening(String serviceId);

Remarks

This method stops advertising the service identified by ServiceId.

The AdvertisementStatusChanged event fires once advertising has actually stopped.

Calling this method for a service that doesn't exist, or that isn't currently being advertised, will cause the class to throws an exception.

bleserver1.StopListening("180F");

UpdateValue Method (BLEServer Class)

Updates the value of a characteristic and sends a notification or indication to all currently subscribed clients.

Syntax

public void updateValue(String serviceId, String characteristicId, byte[] value);

Remarks

This method broadcasts a value to all subscribed clients. It does not target a specific client.

The value is specified as a byte array. The characteristic must support notifications or indications. If no clients are subscribed, no notification is sent.

Calling this method for a service that is not currently being advertised will cause the class to throws an exception.

AdvertisementStatusChanged Event (BLEServer Class)

Fired when the advertising status of a service changes.

Syntax

public class DefaultBLEServerEventListener implements BLEServerEventListener {
  ...
  public void advertisementStatusChanged(BLEServerAdvertisementStatusChangedEvent e) {}
  ...
}

public class BLEServerAdvertisementStatusChangedEvent {
  public int status;
  public int error;
}

Remarks

This event fires in response to StartListening and StopListening, and also if advertising is unexpectedly aborted by the underlying Bluetooth stack (for example, because the adapter was disabled).

Status indicates whether advertising has started, stopped, or was aborted; Error holds the underlying Bluetooth-stack error code if Status indicates an abort, or 0 otherwise. The exact numeric values used for Status are platform-dependent.

Note that this event does not identify which service's advertising status has changed. If you are advertising more than one service at a time, set the ServiceId property to each of your service Ids in turn and check the Listening property to determine which one this event pertains to.

Connected Event (BLEServer Class)

Fired when a remote client connects.

Syntax

public class DefaultBLEServerEventListener implements BLEServerEventListener {
  ...
  public void connected(BLEServerConnectedEvent e) {}
  ...
}

public class BLEServerConnectedEvent {
  public String clientId;
  public int statusCode;
  public String description;
}

Remarks

The underlying Bluetooth APIs do not notify the class when a remote device forms a low-level BLE connection to it. Instead, this event fires the first time a client interacts with one of your characteristics, by reading it (ReadRequested), writing it (WriteRequested), or subscribing to it (SubscriptionChanged).

ClientId is a platform-specific string that identifies the client, and is also provided as a parameter to the events above, as well as SessionStatusChanged and Disconnected, so that you can associate requests with a particular client.

StatusCode is always 0 and Description is always "OK", since a connection has, by definition, already been established by the time this event can fire.

Disconnected Event (BLEServer Class)

Fired when a connected client disconnects.

Syntax

public class DefaultBLEServerEventListener implements BLEServerEventListener {
  ...
  public void disconnected(BLEServerDisconnectedEvent e) {}
  ...
}

public class BLEServerDisconnectedEvent {
  public String clientId;
  public int statusCode;
  public String description;
}

Remarks

This event fires when the GATT session for a previously-connected client (see Connected) is closed; refer to SessionStatusChanged for more information about session state changes.

ClientId identifies which client disconnected. StatusCode is always 0 and Description is always "OK".

If the disconnecting client had any active characteristic subscriptions, the SubscriptionChanged event will fire separately to reflect their removal.

Error Event (BLEServer Class)

Fired when information is available about errors during data delivery.

Syntax

public class DefaultBLEServerEventListener implements BLEServerEventListener {
  ...
  public void error(BLEServerErrorEvent e) {}
  ...
}

public class BLEServerErrorEvent {
  public int errorCode;
  public String description;
}

Remarks

The Error event is fired in case of exceptional conditions during message processing. Normally the class throws an exception.

The ErrorCode parameter contains an error code, and the Description parameter contains a textual description of the error. For a list of valid error codes and their descriptions, please refer to the Error Codes section.

Log Event (BLEServer Class)

Fires once for each log message.

Syntax

public class DefaultBLEServerEventListener implements BLEServerEventListener {
  ...
  public void log(BLEServerLogEvent e) {}
  ...
}

public class BLEServerLogEvent {
  public int logLevel;
  public String message;
  public String logType;
}

Remarks

This events fires once for each log message generated by the class. The verbosity is controlled by the LogLevel setting.

LogLevel indicates the level of the Message. Possible values are:

The default log level, Info, provides information when most BLE operations begin and end, and should be sufficient to get a good idea of what the class is doing at a high level.

The Verbose log level adds a few extra operation timing messages, and adds more detail to some messages logged at the Info level.

The Debug log level causes the class to output as much information as possible about what it is doing at all times. All BLE communications, even those which are typically abstracted away by the API, are logged in full detail. Typically the additional information added by this log level is not helpful to the end user, and would only be necessary to capture if you wish to report an issue to the support team.

LogType identifies the type of log entry. Possible values are:

• Connection: Messages relating to connecting, disconnecting, and connection failures.
• Read: Messages relating to reading values from the remote device.
• Write: Messages relating to writing values to the remote device.
• Subscriptions: Messages relating to subscribing to and unsubscribing from characteristics.
• Info, Verbose, and Debug: Miscellaneous messages fired at the applicable log level concerning internal class operations.

ReadRequested Event (BLEServer Class)

Fired when a client requests to read a characteristic's value.

Syntax

public class DefaultBLEServerEventListener implements BLEServerEventListener {
  ...
  public void readRequested(BLEServerReadRequestedEvent e) {}
  ...
}

public class BLEServerReadRequestedEvent {
  public String clientId;
  public String serviceId;
  public String characteristicId;
  public String responseValue; //read-write
  public boolean accept; //read-write
}

Remarks

This event fires when a connected client sends a request to read the current value of a characteristic that was added (via AddCharacteristic) with the Read flag set.

ClientId identifies the requesting client, and ServiceId and CharacteristicId identify the characteristic being read.

To answer the request, set ResponseValue to a hex-encoded string representing the bytes to return to the client, and leave Accept set to its default value of True. If you do not set ResponseValue, an empty value is returned. To reject the request instead (for example, if the value isn't currently available), set Accept to False.

bleserver1.OnReadRequested += (s, e) => { if (e.CharacteristicId.Equals("00002A19-0000-1000-8000-00805F9B34FB", StringComparison.OrdinalIgnoreCase)) { // Battery Level characteristic: respond with 90%, encoded as a single hex byte. e.ResponseValue = "5A"; } else { e.Accept = false; } };

SessionStatusChanged Event (BLEServer Class)

Fired when a connected client's GATT session status changes.

Syntax

public class DefaultBLEServerEventListener implements BLEServerEventListener {
  ...
  public void sessionStatusChanged(BLEServerSessionStatusChangedEvent e) {}
  ...
}

public class BLEServerSessionStatusChangedEvent {
  public String clientId;
  public int status;
  public int error;
}

Remarks

This event fires whenever the underlying GATT session for a connected client (see Connected) becomes active or is closed. It may fire more than once for the same client over the lifetime of a connection, for instance if the client temporarily goes out of range.

ClientId identifies the client. Status indicates whether the session is now active or closed (the exact numeric values used are platform-dependent). Error holds the underlying Bluetooth-stack error code associated with the change, or 0 if there isn't one.

When Status indicates that a client's session has closed, the Disconnected event fires immediately afterward for the same client.

SubscriptionChanged Event (BLEServer Class)

Fired when a characteristic gains or loses a subscribed client.

Syntax

public class DefaultBLEServerEventListener implements BLEServerEventListener {
  ...
  public void subscriptionChanged(BLEServerSubscriptionChangedEvent e) {}
  ...
}

public class BLEServerSubscriptionChangedEvent {
  public String serviceId;
  public String characteristicId;
  public boolean isSubscribed;
}

Remarks

This event fires when a client subscribes to, or unsubscribes from, notifications or indications on a characteristic that was added (via AddCharacteristic) with the Notify or Indicate flag set.

ServiceId and CharacteristicId identify the characteristic. IsSubscribed reflects the characteristic's overall subscription state: True if it now has one or more subscribed clients, or False if it has none. This event does not indicate which specific client subscribed or unsubscribed, nor how many clients are currently subscribed.

To push a new value to any subscribed clients, use UpdateValue.

WriteRequested Event (BLEServer Class)

Fired when a client requests to write a characteristic's value.

Syntax

public class DefaultBLEServerEventListener implements BLEServerEventListener {
  ...
  public void writeRequested(BLEServerWriteRequestedEvent e) {}
  ...
}

public class BLEServerWriteRequestedEvent {
  public String clientId;
  public String serviceId;
  public String characteristicId;
  public byte[] value;
  public boolean withResponse;
  public boolean accept; //read-write
}

Remarks

This event fires when a connected client sends a request to write a new value to a characteristic that was added (via AddCharacteristic) with the Write or WriteWithoutResponse flag set.

ClientId identifies the requesting client, and ServiceId and CharacteristicId identify the characteristic being written to. Value holds the hex-encoded string of bytes the client wants to write. WithResponse is True if the client is expecting an acknowledgement (a "Write"), or False if it is not (a "Write Without Response").

If WithResponse is True, the response will use Accept to indicate if the request was accepted or rejected.

bleserver1.OnWriteRequested += (s, e) => { Console.WriteLine("Client " + e.ClientId + " wrote: " + e.Value); };

ServerCharacteristic Type

A GATT characteristic added to a BLE server.

Remarks

This type represents a GATT characteristic added to a BLE server.

• CanSubscribe
• Flags
• Id
• ServiceId
• Uuid

Fields

Constructors

public ServerCharacteristic();

ServerService Type

A GATT service added to a BLE server.

Remarks

This type represents a GATT service added to a BLE server.

• InstanceId
• Uuid

Fields

Constructors

public ServerService();

Config Settings (BLEServer Class)

The class accepts one or more of the following configuration settings. Configuration settings are similar in functionality to properties, but they are rarely used. In order to avoid "polluting" the property namespace of the class, access to these internal properties is provided through the Config method.

BLEServer Config Settings

Base Config Settings

Trappable Errors (BLEServer Class)

BLEServer Errors