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node_modules/lit-html/development/directives/guard.js

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+/**
+ * @license
+ * Copyright 2018 Google LLC
+ * SPDX-License-Identifier: BSD-3-Clause
+ */
+import { noChange } from '../lit-html.js';
+import { directive, Directive } from '../directive.js';
+// A sentinel that indicates guard() hasn't rendered anything yet
+const initialValue = {};
+class GuardDirective extends Directive {
+    constructor() {
+        super(...arguments);
+        this._previousValue = initialValue;
+    }
+    render(_value, f) {
+        return f();
+    }
+    update(_part, [value, f]) {
+        if (Array.isArray(value)) {
+            // Dirty-check arrays by item
+            if (Array.isArray(this._previousValue) &&
+                this._previousValue.length === value.length &&
+                value.every((v, i) => v === this._previousValue[i])) {
+                return noChange;
+            }
+        }
+        else if (this._previousValue === value) {
+            // Dirty-check non-arrays by identity
+            return noChange;
+        }
+        // Copy the value if it's an array so that if it's mutated we don't forget
+        // what the previous values were.
+        this._previousValue = Array.isArray(value) ? Array.from(value) : value;
+        const r = this.render(value, f);
+        return r;
+    }
+}
+/**
+ * Prevents re-render of a template function until a single value or an array of
+ * values changes.
+ *
+ * Values are checked against previous values with strict equality (`===`), and
+ * so the check won't detect nested property changes inside objects or arrays.
+ * Arrays values have each item checked against the previous value at the same
+ * index with strict equality. Nested arrays are also checked only by strict
+ * equality.
+ *
+ * Example:
+ *
+ * ```js
+ * html`
+ *   <div>
+ *     ${guard([user.id, company.id], () => html`...`)}
+ *   </div>
+ * `
+ * ```
+ *
+ * In this case, the template only rerenders if either `user.id` or `company.id`
+ * changes.
+ *
+ * guard() is useful with immutable data patterns, by preventing expensive work
+ * until data updates.
+ *
+ * Example:
+ *
+ * ```js
+ * html`
+ *   <div>
+ *     ${guard([immutableItems], () => immutableItems.map(i => html`${i}`))}
+ *   </div>
+ * `
+ * ```
+ *
+ * In this case, items are mapped over only when the array reference changes.
+ *
+ * @param value the value to check before re-rendering
+ * @param f the template function
+ */
+export const guard = directive(GuardDirective);
+//# sourceMappingURL=guard.js.map