Keep track of used labels

src/ol/render/canvas/LabelCache.js

@@ -0,0 +1,69 @@
+import {getUid} from '../../util.js';
+import LRUCache from '../../structs/LRUCache.js';
+
+/**
+ * @module ol/render/canvas/LabelCache
+ */
+
+/**
+ * @classdesc
+ * Cache of pre-rendered labels.
+ * @fires import("../events/Event.js").Event
+ */
+class LabelCache extends LRUCache {
+
+  /**
+   * @inheritDoc
+   */
+  constructor(opt_highWaterMark) {
+    super(opt_highWaterMark);
+    this.consumers = {};
+  }
+
+  clear() {
+    super.clear();
+    this.consumers = {};
+  }
+
+  /**
+   * @override
+   * @param {string} key Label key.
+   * @param {import("./Executor.js").default} consumer Label consumer.
+   * @return {HTMLCanvasElement} Label.
+   */
+  get(key, consumer) {
+    const canvas = super.get(key);
+    const consumerId = getUid(consumer);
+    if (!(consumerId in this.consumers)) {
+      this.consumers[consumerId] = {};
+    }
+    this.consumers[consumerId][key] = true;
+    return canvas;
+  }
+
+  prune() {
+    outer:
+    while (this.canExpireCache()) {
+      const key = this.peekLastKey();
+      for (const consumerId in this.consumers) {
+        if (key in this.consumers[consumerId]) {
+          break outer;
+        }
+      }
+      const canvas = this.pop();
+      canvas.width = canvas.height = 0;
+      for (const consumerId in this.consumers) {
+        delete this.consumers[consumerId][key];
+      }
+    }
+  }
+
+  /**
+   * @param {import("./Executor.js").default} consumer Label consumer.
+   */
+  release(consumer) {
+    delete this.consumers[getUid(consumer)];
+  }
+}
+
+export default LabelCache;