BLEServer Control

Properties   Methods   Events   Config Settings   Errors  

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

Syntax

BLEServer

Remarks

The BLEServer control 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 controls are only supported by Windows 10 Creator's Update (10.0.15063.0) and later.

Android Notes:

The BLE controls 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 controls 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 controls 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 control 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 control's event handlers, particularly ReadRequested and WriteRequested. The platform APIs that the control 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 control 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 control 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 control 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 control with short descriptions. Click on the links for further details.

CharacteristicCount Property (BLEServer Control)

The number of records in the Characteristic arrays.

Syntax

bleservercontrol.CharacteristicCount

Default Value

0

Remarks

This property controls the size of the following arrays:

• CharacteristicCanSubscribe
• CharacteristicFlags
• CharacteristicId
• CharacteristicServiceId
• CharacteristicUuid

The array indices start at 0 and end at CharacteristicCount - 1.

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

Data Type

Integer

CharacteristicCanSubscribe Property (BLEServer Control)

Whether or not you can subscribe to receive value updates for this characteristic.

Syntax

bleservercontrol.CharacteristicCanSubscribe(CharacteristicIndex)

Default Value

False

Remarks

Whether or not you can subscribe to receive value updates for this characteristic.

The CharacteristicIndex parameter specifies the index of the item in the array. The size of the array is controlled by the CharacteristicCount property.

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

Data Type

Boolean

CharacteristicFlags Property (BLEServer Control)

A bitmask of this characteristic's flags.

Syntax

bleservercontrol.CharacteristicFlags(CharacteristicIndex)

Default Value

0

Remarks

A bitmask of this characteristic's flags.

This property is a bitmask which represents all of the flags (or, in BLE terms, "properties") which this characteristic has set. The following table shows what bitmask is used to represent each flag:

Note that, on some platforms, the control might have to automatically discover specific descriptors for the value this returns to be accurate. The control take care of handling this automatic discovery in the most energy-efficient manner possible.

The CharacteristicIndex parameter specifies the index of the item in the array. The size of the array is controlled by the CharacteristicCount property.

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

Data Type

Integer

CharacteristicId Property (BLEServer Control)

An identification string which uniquely identifies this instance of this characteristic.

Syntax

bleservercontrol.CharacteristicId(CharacteristicIndex)

Default Value

""

Remarks

An identification string which uniquely identifies this instance of this characteristic.

This identification string is guaranteed to be unchanged while the control remains connected to the device.

The CharacteristicIndex parameter specifies the index of the item in the array. The size of the array is controlled by the CharacteristicCount property.

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

Data Type

String

CharacteristicServiceId Property (BLEServer Control)

The identification string of the service that contains this characteristic.

Syntax

bleservercontrol.CharacteristicServiceId(CharacteristicIndex)

Default Value

""

Remarks

The identification string of the service that contains this characteristic.

The CharacteristicIndex parameter specifies the index of the item in the array. The size of the array is controlled by the CharacteristicCount property.

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

Data Type

String

CharacteristicUuid Property (BLEServer Control)

This characteristic's 128-bit UUID string.

Syntax

bleservercontrol.CharacteristicUuid(CharacteristicIndex)

Default Value

""

Remarks

This characteristic's 128-bit UUID string.

The CharacteristicIndex parameter specifies the index of the item in the array. The size of the array is controlled by the CharacteristicCount property.

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

Data Type

String

Listening Property (BLEServer Control)

Whether the service identified by ServiceId is currently being advertised.

Syntax

bleservercontrol.Listening

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.

Data Type

Boolean

ServiceId Property (BLEServer Control)

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

Syntax

bleservercontrol.ServiceId[=string]

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.

Data Type

String

ServiceCount Property (BLEServer Control)

The number of records in the Service arrays.

Syntax

bleservercontrol.ServiceCount

Default Value

0

Remarks

This property controls the size of the following arrays:

• ServiceInstanceId
• ServiceUuid

The array indices start at 0 and end at ServiceCount - 1.

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

Data Type

Integer

ServiceInstanceId Property (BLEServer Control)

An identification string which uniquely identifies this instance of this service.

Syntax

bleservercontrol.ServiceInstanceId(ServiceIndex)

Default Value

""

Remarks

An identification string which uniquely identifies this instance of this service.

This identification string is guaranteed to be unchanged while the control remains connected to the device.

The ServiceIndex parameter specifies the index of the item in the array. The size of the array is controlled by the ServiceCount property.

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

Data Type

String

ServiceUuid Property (BLEServer Control)

This service's 128-bit UUID string.

Syntax

bleservercontrol.ServiceUuid(ServiceIndex)

Default Value

""

Remarks

This service's 128-bit UUID string.

The ServiceIndex parameter specifies the index of the item in the array. The size of the array is controlled by the ServiceCount property.

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

Data Type

String

AddCharacteristic Method (BLEServer Control)

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

Syntax

bleservercontrol.AddCharacteristic ServiceId, CharacteristicId, 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 control to fails with an error. 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 Control)

Sets or retrieves a configuration setting.

Syntax

bleservercontrol.Config ConfigurationString

Remarks

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

These settings are similar in functionality to properties, but they are rarely used. In order to avoid "polluting" the property namespace of the control, 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 Control)

This method processes events from the internal message queue.

Syntax

bleservercontrol.DoEvents 

Remarks

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

StartListening Method (BLEServer Control)

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

Syntax

bleservercontrol.StartListening 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 control to fails with an error.

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

StopListening Method (BLEServer Control)

Stops advertising a service.

Syntax

bleservercontrol.StopListening 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 control to fails with an error.

bleserver1.StopListening("180F");

UpdateValue Method (BLEServer Control)

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

Syntax

bleservercontrol.UpdateValue ServiceId, CharacteristicId, 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 control to fails with an error.

AdvertisementStatusChanged Event (BLEServer Control)

Fired when the advertising status of a service changes.

Syntax

Sub bleservercontrol_AdvertisementStatusChanged(Status As Integer, Error As Integer)

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 Control)

Fired when a remote client connects.

Syntax

Sub bleservercontrol_Connected(ClientId As String, StatusCode As Integer, Description As String)

Remarks

The underlying Bluetooth APIs do not notify the control 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 Control)

Fired when a connected client disconnects.

Syntax

Sub bleservercontrol_Disconnected(ClientId As String, StatusCode As Integer, Description As String)

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 Control)

Fired when information is available about errors during data delivery.

Syntax

Sub bleservercontrol_Error(ErrorCode As Integer, Description As String)

Remarks

The Error event is fired in case of exceptional conditions during message processing. Normally the control fails with an error.

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 Control)

Fires once for each log message.

Syntax

Sub bleservercontrol_Log(LogLevel As Integer, Message As String, LogType As String)

Remarks

This events fires once for each log message generated by the control. 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 control 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 control 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 control operations.

ReadRequested Event (BLEServer Control)

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

Syntax

Sub bleservercontrol_ReadRequested(ClientId As String, ServiceId As String, CharacteristicId As String, ResponseValue As String, Accept As Boolean)

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 Control)

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

Syntax

Sub bleservercontrol_SessionStatusChanged(ClientId As String, Status As Integer, Error As Integer)

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 Control)

Fired when a characteristic gains or loses a subscribed client.

Syntax

Sub bleservercontrol_SubscriptionChanged(ServiceId As String, CharacteristicId As String, IsSubscribed As Boolean)

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 Control)

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

Syntax

Sub bleservercontrol_WriteRequested(ClientId As String, ServiceId As String, CharacteristicId As String, Value As String, WithResponse As Boolean, Accept As Boolean)

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); };

Config Settings (BLEServer Control)

The control 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 control, access to these internal properties is provided through the Config method.

BLEServer Config Settings

Base Config Settings

Trappable Errors (BLEServer Control)

BLEServer Errors