From fdc5f599df59286cc59268d3b82b8fa36607e502 Mon Sep 17 00:00:00 2001
From: Justin Fagnani <justinfagnani@google.com>
Date: Fri, 7 Jul 2023 17:19:26 -0700
Subject: [PATCH 1/2] Allow tasks to have an initial value

---
 .changeset/heavy-planets-obey.md         |  5 ++++
 packages/labs/task/src/task.ts           | 17 ++++++++++++++
 packages/labs/task/src/test/task_test.ts | 30 ++++++++++++++++++++++++
 3 files changed, 52 insertions(+)
 create mode 100644 .changeset/heavy-planets-obey.md

diff --git a/.changeset/heavy-planets-obey.md b/.changeset/heavy-planets-obey.md
new file mode 100644
index 0000000000..dd2ac02f72
--- /dev/null
+++ b/.changeset/heavy-planets-obey.md
@@ -0,0 +1,5 @@
+---
+'@lit-labs/task': minor
+---
+
+Allow tasks to have an initial value
diff --git a/packages/labs/task/src/task.ts b/packages/labs/task/src/task.ts
index 5e336a0da7..5e7f6f4322 100644
--- a/packages/labs/task/src/task.ts
+++ b/packages/labs/task/src/task.ts
@@ -49,6 +49,16 @@ export interface TaskConfig<T extends ReadonlyArray<unknown>, R> {
   task: TaskFunction<T, R>;
   args?: ArgsFunction<T>;
   autoRun?: boolean;
+
+  /**
+   * If initialValue is provided, the task is initialized to the COMPLETE
+   * status and the value is set to initialData.
+   *
+   * Initial args should be coherent with the initialValue, since they are
+   * assumed to be the args that would produce that value. When autoRun is
+   * `true` the task will not auto-run again until the args change.
+   */
+  initialValue?: R;
   onComplete?: (value: R) => unknown;
   onError?: (error: unknown) => unknown;
 }
@@ -185,6 +195,13 @@ export class Task<
     if (taskConfig.autoRun !== undefined) {
       this.autoRun = taskConfig.autoRun;
     }
+    // Providing initialValue puts the task in COMPLETE state and stores the
+    // args immediately so it only runs when they change again.
+    if ('initialValue' in taskConfig) {
+      this._value = taskConfig.initialValue;
+      this.status = TaskStatus.COMPLETE;
+      this._previousArgs = this._getArgs?.();
+    }
   }
 
   hostUpdated() {
diff --git a/packages/labs/task/src/test/task_test.ts b/packages/labs/task/src/test/task_test.ts
index e086af85ac..085e49166c 100644
--- a/packages/labs/task/src/test/task_test.ts
+++ b/packages/labs/task/src/test/task_test.ts
@@ -225,6 +225,36 @@ suite('Task', () => {
     assert.equal(el.taskValue, `a1,b1`);
   });
 
+  test('tasks can have initialValue', async () => {
+    // The test helpers make it hard to write proper args callbacks
+    // `args: () => [this.a, this.b]` would work, but `[el.a]` doesn't
+    // since `el` isn't initialized yet. This little hack works:
+    let initializing = true;
+    const el = getTestElement({
+      initialValue: 'initial',
+      args: () => [initializing ? 'a' : el.a, initializing ? 'b' : el.b],
+    });
+    initializing = false;
+    await renderElement(el);
+    assert.equal(el.task.status, TaskStatus.COMPLETE);
+    assert.equal(el.task.value, 'initial');
+    assert.equal(el.taskValue, 'initial');
+
+    // The task is complete, so waiting for it shouldn't change anything
+    await tasksUpdateComplete();
+    assert.equal(el.task.status, TaskStatus.COMPLETE);
+    assert.equal(el.task.value, 'initial');
+
+    // The task still reruns when arguments change
+    el.a = 'a1';
+    await tasksUpdateComplete();
+    assert.equal(el.task.status, TaskStatus.PENDING);
+    el.resolveTask();
+    await tasksUpdateComplete();
+    assert.equal(el.task.status, TaskStatus.COMPLETE);
+    assert.equal(el.taskValue, `a1,b`);
+  });
+
   test('task error is not reset on rerun', async () => {
     const el = getTestElement({args: () => [el.a, el.b]});
     await renderElement(el);

From 8487dfb5dc59e32a4e5e541db84f1a5e1182a3e2 Mon Sep 17 00:00:00 2001
From: Justin Fagnani <justinfagnani@google.com>
Date: Fri, 7 Jul 2023 19:05:07 -0700
Subject: [PATCH 2/2] Add an assertion

---
 packages/labs/task/src/test/task_test.ts | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/packages/labs/task/src/test/task_test.ts b/packages/labs/task/src/test/task_test.ts
index 085e49166c..bce98225a6 100644
--- a/packages/labs/task/src/test/task_test.ts
+++ b/packages/labs/task/src/test/task_test.ts
@@ -245,6 +245,12 @@ suite('Task', () => {
     assert.equal(el.task.status, TaskStatus.COMPLETE);
     assert.equal(el.task.value, 'initial');
 
+    // An element update (which checks args again) should not cause a rerun
+    el.requestUpdate();
+    await tasksUpdateComplete();
+    assert.equal(el.task.status, TaskStatus.COMPLETE);
+    assert.equal(el.task.value, 'initial');
+
     // The task still reruns when arguments change
     el.a = 'a1';
     await tasksUpdateComplete();