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| <h2 id="ble-peripheral-project">BLE Peripheral Project</h2> |
| <h3 id="connection-callbacks">Connection callbacks</h3> |
| <p><br></p> |
| <h4 id="overview">Overview</h4> |
| <p>Every BLE connection has a <em>connection callback</em> associated with it. A |
| connection callback is a bit of application code which NimBLE uses to inform |
| you of connection-related events. For example, if a connection is terminated, |
| NimBLE lets you know about it with a call to that connection's callback.</p> |
| <p>In the <a href="bleprph-adv/">advertising section</a> of this tutorial, we saw how the |
| application specifies a connection callback when it begins advertising. NimBLE |
| uses this callback to notify the application that a central has connected to |
| your peripheral after receiving an advertisement. Let's revisit how <em>bleprph</em> specifies its connection callback when advertising:</p> |
| <p><br></p> |
| <div class="codehilite" style="background: #ffffff"><pre style="line-height: 125%;"><span></span><code> <span style="color: #177500">/* Begin advertising. */</span> |
| <span style="color: #000000">rc</span> <span style="color: #000000">=</span> <span style="color: #000000">ble_gap_adv_start</span>(<span style="color: #000000">BLE_GAP_DISC_MODE_GEN</span>, <span style="color: #000000">BLE_GAP_CONN_MODE_UND</span>, |
| <span style="color: #A90D91">NULL</span>, <span style="color: #1C01CE">0</span>, <span style="color: #A90D91">NULL</span>, <span style="color: #000000">bleprph_on_connect</span>, <span style="color: #A90D91">NULL</span>); |
| <span style="color: #A90D91">if</span> (<span style="color: #000000">rc</span> <span style="color: #000000">!=</span> <span style="color: #1C01CE">0</span>) { |
| <span style="color: #000000">BLEPRPH_LOG</span>(<span style="color: #000000">ERROR</span>, <span style="color: #C41A16">"error enabling advertisement; rc=%d\n"</span>, <span style="color: #000000">rc</span>); |
| <span style="color: #A90D91">return</span>; |
| } |
| </code></pre></div> |
| |
| <p><br></p> |
| <h4 id="bleprph_on_connect">bleprph_on_connect()</h4> |
| <p>The <code>bleprph_on_connect()</code> function is <em>bleprph</em>'s connection callback; NimBLE |
| calls this function when the advertising operation leads to connection |
| establishment. Upon connecting, this callback becomes permanently associated |
| with the connection; all subsequent events related to this connection are |
| communicated through this callback.</p> |
| <p>Now let's look at the function that <em>bleprph</em> uses for all its connection |
| callbacks: <code>bleprph_on_connect()</code>.</p> |
| <div class="codehilite" style="background: #ffffff"><pre style="line-height: 125%;"><span></span><code><span style="color: #A90D91">static</span> <span style="color: #A90D91">int</span> |
| <span style="color: #000000">bleprph_on_connect</span>(<span style="color: #A90D91">int</span> <span style="color: #000000">event</span>, <span style="color: #A90D91">int</span> <span style="color: #000000">status</span>, <span style="color: #A90D91">struct</span> <span style="color: #3F6E75">ble_gap_conn_ctxt</span> <span style="color: #000000">*ctxt</span>, |
| <span style="color: #A90D91">void</span> <span style="color: #000000">*arg</span>) |
| { |
| <span style="color: #A90D91">switch</span> (<span style="color: #000000">event</span>) { |
| <span style="color: #A90D91">case</span> <span style="color: #000000">BLE_GAP_EVENT_CONN</span>: |
| <span style="color: #000000">BLEPRPH_LOG</span>(<span style="color: #000000">INFO</span>, <span style="color: #C41A16">"connection %s; status=%d "</span>, |
| <span style="color: #000000">status</span> <span style="color: #000000">==</span> <span style="color: #1C01CE">0</span> <span style="color: #000000">?</span> <span style="color: #C41A16">"up"</span> <span style="color: #000000">:</span> <span style="color: #C41A16">"down"</span>, <span style="color: #000000">status</span>); |
| <span style="color: #000000">bleprph_print_conn_desc</span>(<span style="color: #000000">ctxt->desc</span>); |
| <span style="color: #000000">BLEPRPH_LOG</span>(<span style="color: #000000">INFO</span>, <span style="color: #C41A16">"\n"</span>); |
| |
| <span style="color: #A90D91">if</span> (<span style="color: #000000">status</span> <span style="color: #000000">!=</span> <span style="color: #1C01CE">0</span>) { |
| <span style="color: #177500">/* Connection terminated; resume advertising. */</span> |
| <span style="color: #000000">bleprph_advertise</span>(); |
| } |
| <span style="color: #A90D91">break</span>; |
| } |
| |
| <span style="color: #A90D91">return</span> <span style="color: #1C01CE">0</span>; |
| } |
| </code></pre></div> |
| |
| <p><br></p> |
| <p>Connection callbacks are used to communicate a variety of events related to a |
| connection. An application determines the type of event that occurred by |
| inspecting the value of the <em>event</em> parameter. The full list of event codes |
| can be found in <a href="https://github.com/apache/incubator-mynewt-core/blob/master/net/nimble/host/include/host/ble_gatt.h">net/nimble/host/include/host/ble_gatt.h</a>. |
| <em>bleprph</em> only concerns itself with a single event type: <em>BLE_GAP_EVENT_CONN</em>. |
| This event indicates that a new connection has been established, or an existing |
| connection has been terminated; the <em>status</em> parameter clarifies which. As you |
| can see, <em>bleprph</em> uses the <em>status</em> parameter to determine if it should resume |
| advertising.</p> |
| <p>The <em>ctxt</em> parameter contains additional information about the connection |
| event. <em>bleprph</em> does nothing more than log some fields of this struct, but |
| some apps will likely want to perform further actions, e.g., perform service |
| discovery on the connected device. The <em>struct ble_gap_conn_ctxt</em> type is |
| defined as follows:</p> |
| <div class="codehilite" style="background: #ffffff"><pre style="line-height: 125%;"><span></span><code><span style="color: #A90D91">struct</span> <span style="color: #3F6E75">ble_gap_conn_ctxt</span> { |
| <span style="color: #A90D91">struct</span> <span style="color: #3F6E75">ble_gap_conn_desc</span> <span style="color: #000000">*desc</span>; |
| |
| <span style="color: #A90D91">union</span> { |
| <span style="color: #A90D91">struct</span> { |
| <span style="color: #A90D91">struct</span> <span style="color: #3F6E75">ble_gap_upd_params</span> <span style="color: #000000">*self_params</span>; |
| <span style="color: #A90D91">struct</span> <span style="color: #3F6E75">ble_gap_upd_params</span> <span style="color: #000000">*peer_params</span>; |
| } <span style="color: #000000">update</span>; |
| |
| <span style="color: #A90D91">struct</span> <span style="color: #3F6E75">ble_gap_sec_params</span> <span style="color: #000000">*sec_params</span>; |
| }; |
| }; |
| </code></pre></div> |
| |
| <p><br></p> |
| <p>As shown, a connection context object consists of two parts:</p> |
| <ul> |
| <li><em>desc:</em> The connection descriptor; indicates properties of the connection.</li> |
| <li><em>anonymous union:</em> The contents are event-specific; check the <em>event</em> code to determine which member field (if any) is relevant.</li> |
| </ul> |
| <p>For events of type <em>BLE_GAP_EVENT_CONN</em>, the anonymous union is not used at all, so the only information carried by the context struct is the connection descriptor. The <em>struct ble_gap_conn_desc</em> type is defined as follows:</p> |
| <div class="codehilite" style="background: #ffffff"><pre style="line-height: 125%;"><span></span><code><span style="color: #A90D91">struct</span> <span style="color: #3F6E75">ble_gap_conn_desc</span> { |
| <span style="color: #A90D91">uint8_t</span> <span style="color: #000000">peer_addr</span>[<span style="color: #1C01CE">6</span>]; |
| <span style="color: #A90D91">uint16_t</span> <span style="color: #000000">conn_handle</span>; |
| <span style="color: #A90D91">uint16_t</span> <span style="color: #000000">conn_itvl</span>; |
| <span style="color: #A90D91">uint16_t</span> <span style="color: #000000">conn_latency</span>; |
| <span style="color: #A90D91">uint16_t</span> <span style="color: #000000">supervision_timeout</span>; |
| <span style="color: #A90D91">uint8_t</span> <span style="color: #000000">peer_addr_type</span>; |
| }; |
| </code></pre></div> |
| |
| <p><br></p> |
| <p>We will examine these fields in a slightly different order from how they appear |
| in the struct definition.</p> |
| <p><br></p> |
| <table> |
| <thead> |
| <tr> |
| <th><em>Field</em></th> |
| <th><em>Purpose</em></th> |
| <th><em>Notes</em></th> |
| </tr> |
| </thead> |
| <tbody> |
| <tr> |
| <td>peer_addr</td> |
| <td>The 48-bit address of the peer device.</td> |
| <td></td> |
| </tr> |
| <tr> |
| <td>peer_addr_type</td> |
| <td>Whether the peer is using a public or random address.</td> |
| <td>The address type list is documented in <a href="https://github.com/apache/incubator-mynewt-core/blob/master/net/nimble/include/nimble/hci_common.h">net/nimble/include/nimble/hci_common.h</a>.</td> |
| </tr> |
| <tr> |
| <td>conn_handle</td> |
| <td>The 16-bit handle associated with this connection.</td> |
| <td>This number is how your app and the NimBLE stack refer to this connection.</td> |
| </tr> |
| <tr> |
| <td>conn_itvl,<br>conn_latency,<br>supervision_timeout</td> |
| <td>Low-level properties of the connection.</td> |
| <td></td> |
| </tr> |
| </tbody> |
| </table> |
| <p><br></p> |
| <h4 id="guarantees">Guarantees</h4> |
| <p>It is important to know what your application code is allowed to do from within |
| a connection callback.</p> |
| <p><strong>No restrictions on NimBLE operations</strong></p> |
| <p>Your app is free to make calls into the NimBLE stack from within a connection |
| callback. <em>bleprph</em> takes advantage of this freedom when it resumes |
| advertising upon connection termination. All other NimBLE operations are also |
| allowed (service discovery, pairing initiation, etc).</p> |
| <p><strong>All context data is transient</strong></p> |
| <p>Pointers in the context object point to data living on the stack. Your |
| callback is free to read (or write, if appropriate) through these pointers, but |
| you should not store these pointers for later use. If your application needs |
| to retain some data from a context object, it needs to make a copy.</p> |
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