Dynamic property (#749)

* Add DynamicProperty class

* Add tests for DynamicProperty

1 files changed, 104 insertions, 1 deletions

diff --git a/ext/mixed/js/core.js b/ext/mixed/js/core.js
index 9142a846..5bee4670 100644
--- a/ext/mixed/js/core.js
+++ b/ext/mixed/js/core.js
@@ -260,7 +260,7 @@ function promiseTimeout(delay, resolveValue) {
 
 
 /*
- * Common events
+ * Common classes
  */
 
 class EventDispatcher {
@@ -348,3 +348,106 @@ class EventListenerCollection {
         this._eventListeners = [];
     }
 }
+
+/**
+ * Class representing a generic value with an override stack.
+ * Changes can be observed by listening to the 'change' event.
+ */
+class DynamicProperty extends EventDispatcher {
+    /**
+     * Creates a new instance with the specified value.
+     * @param value The value to assign.
+     */
+    constructor(value) {
+        super();
+        this._value = value;
+        this._defaultValue = value;
+        this._overrides = [];
+    }
+
+    /**
+     * Gets the default value for the property, which is assigned to the
+     * public value property when no overrides are present.
+     */
+    get defaultValue() {
+        return this._defaultValue;
+    }
+
+    /**
+     * Assigns the default value for the property. If no overrides are present
+     * and if the value is different than the current default value,
+     * the 'change' event will be triggered.
+     * @param value The value to assign.
+     */
+    set defaultValue(value) {
+        this._defaultValue = value;
+        if (this._overrides.length === 0) { this._updateValue(); }
+    }
+
+    /**
+     * Gets the current value for the property, taking any overrides into account.
+     */
+    get value() {
+        return this._value;
+    }
+
+    /**
+     * Gets the number of overrides added to the property.
+     */
+    get overrideCount() {
+        return this._overrides.length;
+    }
+
+    /**
+     * Adds an override value with the specified priority to the override stack.
+     * Values with higher priority will take precedence over those with lower.
+     * For tie breaks, the override value added first will take precedence.
+     * If the newly added override has the highest priority of all overrides
+     * and if the override value is different from the current value,
+     * the 'change' event will be fired.
+     * @param value The override value to assign.
+     * @param priority The priority value to use, as a number.
+     * @returns A string token which can be passed to the clearOverride function
+     *  to remove the override.
+     */
+    setOverride(value, priority=0) {
+        const overridesCount = this._overrides.length;
+        let i = 0;
+        for (; i < overridesCount; ++i) {
+            if (priority > this._overrides[i].priority) { break; }
+        }
+        const token = generateId(16);
+        this._overrides.splice(i, 0, {value, priority, token});
+        if (i === 0) { this._updateValue(); }
+        return token;
+    }
+
+    /**
+     * Removes a specific override value. If the removed override
+     * had the highest priority, and the new value is different from
+     * the previous value, the 'change' event will be fired.
+     * @param token The token for the corresponding override which is to be removed.
+     * @returns true if an override was returned, false otherwise.
+     */
+    clearOverride(token) {
+        for (let i = 0, ii = this._overrides.length; i < ii; ++i) {
+            if (this._overrides[i].token === token) {
+                this._overrides.splice(i, 1);
+                if (i === 0) { this._updateValue(); }
+                return true;
+            }
+        }
+        return false;
+    }
+
+    /**
+     * Updates the current value using the current overrides and default value.
+     * If the new value differs from the previous value, the 'change' event will be fired.
+     */
+    _updateValue() {
+        const value = this._overrides.length > 0 ? this._overrides[0].value : this._defaultValue;
+        if (this._value === value) { return; }
+        this._value = value;
+        this.trigger('change', {value});
+    }
+}