map operator for observable lists

src/observable/index.js

@@ -5,6 +5,7 @@ import BaseObservableMap from "./map/BaseObservableMap.js";
 // re-export "root" (of chain) collections
 export { default as ObservableArray } from "./list/ObservableArray.js";
 export { default as SortedArray } from "./list/SortedArray.js";
+export { default as MappedList } from "./list/MappedList.js";
 export { default as ObservableMap } from "./map/ObservableMap.js";
 
 // avoid circular dependency between these classes

src/observable/list/MappedList.js

@@ -0,0 +1,115 @@
+import BaseObservableList from "./BaseObservableList.js";
+
+export default class MappedList extends BaseObservableList {
+    constructor(sourceList, mapper, updater) {
+        super();
+        this._sourceList = sourceList;
+        this._mapper = mapper;
+        this._updater = updater;
+        this._sourceUnsubscribe = null;
+        this._mappedValues = null;
+    }
+
+    onSubscribeFirst() {
+        this._sourceUnsubscribe = this._sourceList.subscribe(this);
+        this._mappedValues = [];
+        for (const item of this._sourceList) {
+            this._mappedValues.push(this._mapper(item));
+        }
+    }
+
+    onReset() {
+        this._mappedValues = [];
+        this.emitReset();
+    }
+
+    onAdd(index, value) {
+        const mappedValue = this._mapper(value);
+        this._mappedValues.splice(index, 0, mappedValue);
+        this.emitAdd(index, mappedValue);
+    }
+
+    onUpdate(index, value, params) {
+        const mappedValue = this._mappedValues[index];
+        if (this._updater) {
+            this._updater(mappedValue, value);
+        }
+        this.emitUpdate(index, mappedValue, params);
+    }
+
+    onRemove(index) {
+        const mappedValue = this._mappedValues[index];
+        this._mappedValues.splice(index, 1);
+        this.emitRemove(index, mappedValue);
+    }
+
+    onMove(fromIdx, toIdx) {
+        const mappedValue = this._mappedValues[fromIdx];
+        this._mappedValues.splice(fromIdx, 1);
+        this._mappedValues.splice(toIdx, 0, mappedValue);
+        this.emitMove(fromIdx, toIdx, mappedValue);
+    }
+
+    onUnsubscribeLast() {
+        this._sourceUnsubscribe();
+    }
+
+    get length() {
+        return this._mappedValues.length;
+    }
+
+    [Symbol.iterator]() {
+        return this._mappedValues.values();
+    }
+}
+
+export async function tests() {
+    class MockList extends BaseObservableList {
+        get length() {
+            return 0;
+        }
+        [Symbol.iterator]() {
+            return [].values();
+        }
+    }
+
+    return {
+        test_add(assert) {
+            const source = new MockList();
+            const mapped = new MappedList(source, n => {return {n: n*n};});
+            let fired = false;
+            const unsubscribe = mapped.subscribe({
+                onAdd(idx, value) {
+                    fired = true;
+                    assert.equal(idx, 0);
+                    assert.equal(value.n, 36);
+                }
+            });
+            source.emitAdd(0, 6);
+            assert(fired);
+            unsubscribe();
+        },
+        test_update(assert) {
+            const source = new MockList();
+            const mapped = new MappedList(
+                source,
+                n => {return {n: n*n};},
+                (o, n) => o.m = n*n
+            );
+            let fired = false;
+            const unsubscribe = mapped.subscribe({
+                onAdd() {},
+                onUpdate(idx, value) {
+                    fired = true;
+                    assert.equal(idx, 0);
+                    assert.equal(value.n, 36);
+                    assert.equal(value.m, 49);
+                }
+            });
+            source.emitAdd(0, 6);
+            source.emitUpdate(0, 7);
+            assert(fired);
+            unsubscribe();
+        }
+    };
+}