add VolumeEffect

src/effects/VolumeEffect.js

@@ -0,0 +1,54 @@
+const Effect = require('./Effect');
+
+/**
+ * Affect the volume of an effect chain.
+ */
+class VolumeEffect extends Effect {
+    /**
+     * Default value to set the Effect to when constructed and when clear'ed.
+     * @const {number}
+     */
+    get DEFAULT_VALUE () {
+        return 100;
+    }
+
+    /**
+     * Initialize the Effect.
+     * Effects start out uninitialized. Then initialize when they are first set
+     * with some value.
+     * @throws {Error} throws when left unimplemented
+     */
+    initialize () {
+        this.inputNode = this.audioEngine.audioContext.createGain();
+        this.outputNode = this.inputNode;
+
+        this.initialized = true;
+    }
+
+    /**
+     * Set the effects value.
+     * @private
+     * @param {number} value - new value to set effect to
+     */
+    _set (value) {
+        this.value = value;
+        // A gain of 1 is normal. Scale down scratch's volume value. Apply the
+        // change over a tiny period of time.
+        this.outputNode.gain.setTargetAtTime(value / 100, 0, this.audioEngine.DECAY_TIME);
+    }
+
+    /**
+     * Clean up and disconnect audio nodes.
+     */
+    dispose () {
+        this.outputNode.disconnect();
+
+        this.inputNode = null;
+        this.outputNode = null;
+        this.target = null;
+
+        this.initialized = false;
+    }
+}
+
+module.exports = VolumeEffect;