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<!--replace-end-7--><!--replace-end-4--><!--replace-end-1--></head><body><div class="ui fluid container universe"><!--replace-start-2--><!--replace-start-3--><!--replace-start-6--><div class="ui text container" id="zettel-container" style="position: relative"><div class="zettel-view"><article class="ui raised attached segment zettel-content"><div class="pandoc"><h1 id="title-h1">Command pattern</h1><p>This is typically used when you want to sequence a fairly large number of actions in a series with the option to undo or reverse them. Similarly to the mediator it has a one-to-many architecture (there is a central class that sequences the commands which are handled by other classes) but in addition to sending and routing requests between colleagues, it keeps a central store of them and can reverse actions.</p><p>The typical example is a calculator. Will will generate this using constructor function syntax, rather than ES6 class syntax.</p><p>You have a bunch of arithmetic functions:</p><pre><code class="jsx language-jsx">function add(x, y) {
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return x + y;
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}
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function sub(x, y) {
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return x - y;
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}
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function mul(x, y) {
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return x * y;
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}
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function div(x, y) {
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return x / y;
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}</code></pre><p>Then a generalised <code>Command</code> class that has three parameters: undo, execute, and return a value:</p><pre><code class="jsx language-jsx">var Command = function (execute, undo, value) {
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this.execute = execute;
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this.undo = undo;
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this.value = value;
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};</code></pre><p>We then create the specific commands by combining the functions and the <code>Command</code> class:</p><pre><code class="jsx language-jsx">var AddCommand = function (value) {
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return new Command(add, sub, value);
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};
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// We would create one of these classes for each of the four operations</code></pre><p>So now the <code>add</code> and <code>subtract</code> functions slot into <code>execute</code> and <code>undo</code> on the <code>Command</code> class.</p><p>Finally we create the centralised command class that will return values based on the individual commands it sequences and issues, store them and remove them:</p><pre><code class="jsx language-jsx">var Calculator = function () {
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var current = 0;
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var commands = [];
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function action(command) {
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var name = command.execute.toString().substr(9, 3);
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return name.charAt(0).toUpperCase() + name.slice(1);
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}
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return {
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execute: function (command) {
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current = command.execute(current, command.value);
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commands.push(command);
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log.add(action(command) + ": " + command.value);
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},
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undo: function () {
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var command = commands.pop();
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current = command.undo(current, command.value);
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log.add("Undo " + action(command) + ": " + command.value);
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},
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getCurrentValue: function () {
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return current;
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}
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}</code></pre><pre><code class="jsx language-jsx">calculator.execute(new AddCommand(100));
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calculator.execute(new SubCommand(24));
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calculator.execute(new MulCommand(6));
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calculator.execute(new DivCommand(2));</code></pre><p>Or undo them with:</p><pre><code class="jsx language-jsx">calculator.undo();</code></pre></div></article><nav class="ui attached segment deemphasized bottomPane" id="neuron-tags-pane"><div><span class="ui basic label zettel-tag" title="Tag">design-patterns</span></div></nav><nav class="ui bottom attached icon compact inverted menu blue" id="neuron-nav-bar"><!--replace-start-9--><!--replace-end-9--><a class="right item" href="impulse.html" title="Open Impulse"><i class="wave square icon"></i></a></nav></div></div><!--replace-end-6--><!--replace-end-3--><!--replace-end-2--><div class="ui center aligned container footer-version"><div class="ui tiny image"><a href="https://neuron.zettel.page"><img alt="logo" src="https://raw.githubusercontent.com/srid/neuron/master/assets/neuron.svg" title="Generated by Neuron 1.9.35.3" /></a></div></div></div></body></html>
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