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Abstract

do that:</p><div id="2e8b"><pre>eventHandler<span class="hljs-selector-class">.Handle</span>(event1, <span class="hljs-built_in">ExampleMiddlewareFoo</span>(<span class="hljs-built_in">ExampleMiddlewareBar</span>(handle)))</pre></div><h1 id="a8a5">Managing Events with Handlers and Middleware</h1><p id="f869">Most articles I read explain how to use this design pattern; this one deals with implementing the internal logic. So let's start coding.</p><p id="35c4">The full code for this article can be found <a href="https://github.com/xNok/slack-go-demo-socketmode/blob/main/examples/middleware/main.go">here</a> to help you follow along.</p><h2 id="0f88">Designing events</h2><p id="b622">First, we are going to enumerate the list of events that our system can handle. The way we create enumeration in Go is a bit different than in other programming languages. In Go, we are going to use a set of constants sharing the same <code>type</code>. Here I define a type <code>EventType</code>that represents a string with the event's name.</p><div id="b181"><pre>// <span class="hljs-keyword">type</span> <span class="hljs-type">used </span>to enumerate events <span class="hljs-keyword">type</span> <span class="hljs-type">EventType </span>string</pre></div><div id="4817"><pre>const ( event1 EventType <span class="hljs-operator">=</span> <span class="hljs-string">"event1"</span> event2 EventType <span class="hljs-operator">=</span> <span class="hljs-string">"event2"</span> )</pre></div><p id="e041">Next, we define the event itself. In our example, the<code>Event</code> as a type which can be selected among the list of <code>EventType</code> created above.</p><div id="2bc9"><pre><span class="hljs-keyword">type</span> <span class="hljs-type">Event</span> struct { <span class="hljs-type">Type</span> <span class="hljs-type">EventType</span> <span class="hljs-type">Data</span> interface{} }</pre></div><h2 id="df6c">Create an event sender (for test purpose)</h2><p id="60aa">To test our system, we will need to create a small function to send events every 2s. Each<code>Event</code> is transmitted via a <a href="https://tour.golang.org/concurrency/2">channel</a> and, the <code>eventSender</code> below sends a random <code>Event</code> of type <code>event1</code> or <code>event2</code> to a channel.</p><blockquote id="53f3"><p><i>Channels are a type which you can send and receive values, they are great for communication among goroutines. In other words, there are perfect for sending and receive event through your application.</i></p></blockquote><div id="ffd1"><pre><span class="hljs-function"><span class="hljs-keyword">func</span> <span class="hljs-title">eventSender</span><span class="hljs-params">(c <span class="hljs-keyword">chan</span> EventType)</span></span> {</pre></div><div id="7f57"><pre> for { // Send <span class="hljs-selector-tag">a</span> random event <span class="hljs-selector-tag">to</span> the channel rand<span class="hljs-selector-class">.Seed</span>(<span class="hljs-selector-tag">time</span><span class="hljs-selector-class">.Now</span>()<span class="hljs-selector-class">.Unix</span>()) events := []EventType{ event1, event2, } n := rand.<span class="hljs-built_in">Int</span>() % <span class="hljs-built_in">len</span>(events)</pre></div><div id="6101"><pre> c <- events[n] <span class="hljs-comment">// send event to channel</span></pre></div><div id="5285"><pre> // <span class="hljs-keyword">wait</span> a <span class="hljs-built_in">bit</span> <span class="hljs-built_in">time</span>.Sleep(<span class="hljs-number">2</span> * <span class="hljs-built_in">time</span>.Second) } }</pre></div><h2 id="000c">Handler and dispatcher</h2><p id="e562">We first need a struct to hold the list of events we want to listen to and which function to call whenever that event is transmitted. This struct also contains the channel used for communicating events.</p><div id="280d"><pre><span class="hljs-comment">// Create a struct to hold config</span> <span class="hljs-comment">// And simplify dependency injections</span> <span class="hljs-keyword">type</span> EventHandler <span class="hljs-keyword">struct</span> { <span class="hljs-comment">// Event channel</span> Events <span class="hljs-keyword">chan</span> Event <span class="hljs-comment">// hold the registedred event functionss</span> EventMap <span class="hljs-keyword">map</span>[EventType][]<span class="hljs-function"><span class="hljs-keyword">func</span><span class="hljs-params">(Event)</span></span> }</pre></div><p id="aa6c">Next, we need to provide an initializing constructor for our <code>EventHandler</code>.</p><div id="a4ed"><pre><span class="hljs-function"><span class="hljs-keyword">func</span> <span class="hljs-title">NewEventHandler</span><span class="hljs-params">()</span></span> *EventHandler { eventMap := <span class="hljs-built_in">make</span>(<span class="hljs-keyword">map</span>[EventType][]<span class="hljs-function"><span class="hljs-keyword">func</span><span class="hljs-params">(Event)</span></span>) event

Options

s := <span class="hljs-built_in">make</span>(<span class="hljs-keyword">chan</span> Event)</pre></div><div id="78bb"><pre> return <span class="hljs-variable">&</span>EventHandler<span class="hljs-punctuation">{</span> <span class="hljs-symbol"> Events:</span> events, <span class="hljs-symbol"> EventMap:</span> eventMap, <span class="hljs-punctuation">}</span> <span class="hljs-punctuation">}</span></pre></div><p id="98fe">Then, we can create our <code>Handle</code> function that associates the event with a callback function.</p><div id="308c"><pre><span class="hljs-comment">// register the handler function to handle an event type</span> func (h *EventHandler) <span class="hljs-built_in">Handle</span>(e EventType, f <span class="hljs-built_in">func</span>(Event)) { h<span class="hljs-selector-class">.EventMap</span><span class="hljs-selector-attr">[e]</span> = <span class="hljs-built_in">append</span>(h<span class="hljs-selector-class">.EventMap</span><span class="hljs-selector-attr">[e]</span>, f) }</pre></div><p id="e42a">Finally, we create the <code>EventDispatcher</code> function, the core of this mechanism. The <code>EventDispatcher</code>process any event sent to a channel, check its type, and if any function has been registering for that type, we call all registered functions.</p><div id="6597"><pre><span class="hljs-function"><span class="hljs-keyword">func</span> <span class="hljs-params">(h *EventHandler)</span></span> EventDispatcher() { <span class="hljs-keyword">for</span> evt := <span class="hljs-keyword">range</span> h.Events { log.Printf(<span class="hljs-string">"event recieved: %v"</span>, evt) <span class="hljs-keyword">if</span> handlers, ok := h.EventMap[evt.Type]; ok { <span class="hljs-comment">// If we registered an event</span> <span class="hljs-keyword">for</span> _, f := <span class="hljs-keyword">range</span> handlers { <span class="hljs-comment">// exacute function as goroutine</span> <span class="hljs-keyword">go</span> f(evt) } } } }</pre></div><h1 id="13e4">Using our system</h1><p id="4d4e">Everything is ready; we can start using our event handling system.</p><ol><li>Instantiate our event Handler</li><li>Register which event to listen to and what function to callback</li><li>Start the event sender</li><li>Start the event dispatcher</li></ol><div id="62ab"><pre><span class="hljs-keyword">func</span> <span class="hljs-title function_">main</span><span class="hljs-params">()</span> {</pre></div><div id="5b09"><pre> <span class="hljs-variable">eventHandler</span> := <span class="hljs-function"><span class="hljs-title">NewEventHandler</span>()</span></pre></div><div id="d244"><pre> eventHandler.Handle(event1, <span class="hljs-keyword">func</span><span class="hljs-params">()</span> { <span class="hljs-built_in">log</span>.Printf(<span class="hljs-string">"event Handled: %v"</span>, event1) })</pre></div><div id="5c56"><pre> <span class="hljs-variable">go</span> <span class="hljs-function"><span class="hljs-title">eventSender</span>(<span class="hljs-variable">eventHandler.Events</span>)</span></pre></div><div id="30f2"><pre> eventHandler<span class="hljs-selector-class">.EventDispatcher</span>()</pre></div><div id="9bf2"><pre>}</pre></div><p id="e590">The result should be along those lines:</p><figure id="5ee7"><img src="https://cdn-images-1.readmedium.com/v2/resize:fit:800/1*_D-GCRSu3K8CgTBXewJjJQ.gif"><figcaption></figcaption></figure><p id="19d6">Since we only handle an event of type <code>event1</code>, only <code>event1</code> shows as <code>Hansled</code>. All is good!</p><p id="d48a">The full code for this article can be found <a href="https://github.com/xNok/slack-go-demo-socketmode/blob/main/examples/middleware/main.go">here</a>.</p><h1 id="6f5e">Interesting Articles tackling the same topic</h1><ul><li><a href="https://drstearns.github.io/tutorials/gomiddleware/">Middleware Patterns in Go</a></li><li><a href="https://sathishvj.medium.com/web-handlers-and-middleware-in-golang-2706c2ecfb75">Web Handlers and Middleware in GoLang</a></li><li><a href="https://readmedium.com/lightweight-event-management-implemented-by-go-a654d59ac65">Lightweight event management implemented by Go</a></li><li><a href="https://readmedium.com/the-7-most-important-software-design-patterns-d60e546afb0e">The 7 Most Important Software Design Patterns</a></li></ul><p id="13e7"><i>Do you want <b>unlimited access</b> to all my content and many other writers’ content on Medium? Consider using my affiliate link to <a href="https://couedeloalexandre.medium.com/membership">become a Medium member today</a></i></p><h2 id="853a">Further reads</h2> <figure id="81d2"> <div> <div> <img class="ratio" src="http://placehold.it/16x9"> <iframe class="" src="https://couedeloalexandre.medium.com/embed/list/3f9d03f2cb8f" allowfullscreen="" frameborder="0" height="184" width="undefined"> </div> </div> </figure></iframe></div></div></figure></article></body>

9 Songs Where the Intro is Better Than the Rest of the Song

Metallica, Journey, Kebu, Marillion, W.A.S.P., Mike Oldfield, Tangerine Dream, Ozric Tentacles, Dire Straits

Image by Brigitte makes custom works from your photos, thanks a lot from Pixabay

“A great intro is like the smell of a delicious meal or seeing a sexy naked person. It whets your appetite, draws you in, and makes you want more.” — Me

But sometimes, what follows doesn’t live up to what the intro promised. Here are nine examples from music.

Metallica — Battery

The first minute of this is amazing! It starts with some haunting acoustic chords, which are quickly joined by a lovely melody, which is then perfectly harmonized, then harmonized further by yet another guitar. Soon this gives way to a heavy, distorted version of the same harmonies, played by what sounds like about 6 guitars.

It’s truly powerful stuff, and an awesome way to start an album. The only problem is, the rest of the song does nothing for me at all.

Journey — Don’t Stop Believin’

Who doesn’t love those opening piano chords? And there’s a good reason — they use the famous four chords that most human ears find pleasing.

Okay, so the rest of the song is actually pretty good. Just not as good as the intro.

Giorgio Moroder — Chase (live cover by Kebu)

Finnish electronic musician Kebu does a cover version of Giorgio Moroder’s Chase in which he adds his own intro to it. This intro is superb. The rest of the music is okay.

Marillion — Holloway Girl

I’ve always loved the first minute of this, but when the main part of the song comes crashing in, it’s okay but nothing special. At 1:34 there is a nice brief return to the opening theme, but then the last two minutes or so of the song are not much to write home about.

W.A.S.P. — Thunderhead

Don’t get me wrong, this is a reasonably good heavy metal song. But that beautifully haunting piano and keyboard intro is truly something to behold.

Mike Oldfield — Tubular Bells

Most people over a certain age know Tubular Bells. Those piano harmonies are unmistakable. But the section of music everybody knows so well is just the introduction to a larger piece, most of it not as good as this.

Just in case you’re wondering, its time signature is 15/8. Or one bar of 7/8 followed by one bar of 8/8, depending on how you look at it.

Tangerine Dream — Marakesh

The whole of this opening track from the German electronic pioneers’ 1988 album is good, but it’s those big chords in the first 28 seconds that are the best bit for me. There is a return to them at the end, nicely concluding the piece.

I tried to go for a similar effect with my own piece of music several years ago.

Ozric Tentacles — Eternal Wheel

The first track from the English space rock band’s 2nd album Erpland starts with this otherworldly synth sequence. Unfortunately, most of the rest of it is filled with crazy jazz-rock guitar improvisations which don’t really do anything for me.

Dire Straits — Love Over Gold

The title track of their 4th album from 1982 is okay, but the sweet piano and acoustic guitar at the beginning are truly beautiful. It’s the kind of music I imagine accompanying an emotional part of a movie. Maybe it has. Does anybody know?

BONUS: Jean-Michel Jarre — Second Rendez-Vous

This one is actually the opposite of the others, but I thought I’d throw it in anyway as a bonus. Rather than the intro being the best part, it’s a small section hidden away in the middle. Here we have nearly 6 minutes of extravagant-sounding but boring “music” until it eventually calms down, then at 5:53, there is a bit that’s ten times better than the rest of it. Then after 2 minutes, it fades out and there is a return to the earlier music.

What can we learn from this?

Although it’s important to have a strong beginning, it’s equally important for the standard to be kept throughout. Imagine if I started an article talking about fitness tips, but 80% of the article was actually knitting patterns. There’s nothing wrong with knitting patterns, but they’re not why you started reading the article.