Use one-way binding for loading-state (#4186)

Also, make some code health improvements while I'm here:
- Mark private methods with an underscore
- Use console.error for error messages
- Use numerice comparison for numeric variables
- Make isLoading read-only
- Add comments

webapp/components/loading-state.js

@@ -4,46 +4,74 @@
  * found in the LICENSE file.
  */
 
-/*
-LoadingState is a behaviour component for indicating when information is
-still being loaded (generally, fetched).
-*/
-
-
+/**
+ * LoadingState is a behaviour component for indicating when information is
+ * still being loaded (generally, fetched).
+ */
 const LoadingState = (superClass) => class extends superClass {
   static get properties() {
     return {
+      /**
+       * The number of active loading operations.
+       */
       loadingCount: {
         type: Number,
         value: 0,
-        observer: 'loadingCountChanged',
+        observer: '_loadingCountChanged',
       },
+      /**
+       * Whether the component is currently loading data.
+       * Computed based on `loadingCount`.
+       */
       isLoading: {
         type: Boolean,
-        computed: 'computeIsLoading(loadingCount)',
-        notify: true,
+        value: false,
+        computed: '_computeIsLoading(loadingCount)',
+        readOnly: true,
+      },
+      /**
+       * A callback function to be executed when loading is complete.
+       */
+      onLoadingComplete: {
+        type: Function,
       },
-      onLoadingComplete: Function,
     };
   }
 
-  computeIsLoading(loadingCount) {
-    return !!loadingCount;
+  /**
+   * Computes the `isLoading` property based on `loadingCount`.
+   * @param {number} loadingCount The current loading count.
+   * @return {boolean} True if loading, false otherwise.
+   */
+  _computeIsLoading(loadingCount) {
+    return loadingCount > 0;
   }
 
-  loadingCountChanged(now, then) {
+  /**
+   * Observer for `loadingCount` changes.
+   * Calls `onLoadingComplete` when loading finishes.
+   * @param {number} now The new loading count.
+   * @param {number} then The previous loading count.
+   */
+  _loadingCountChanged(now, then) {
     if (now === 0 && then > 0 && this.onLoadingComplete) {
       this.onLoadingComplete();
     }
   }
 
+  /**
+   * Tracks a promise, incrementing `loadingCount` while it's pending.
+   * @param {Promise} promise The promise to track.
+   * @param {Function} opt_errHandler An optional error handler.
+   * @return {Promise} A promise that resolves/rejects with the original promise.
+   */
   async load(promise, opt_errHandler) {
     this.loadingCount++;
     try {
       return await promise;
     } catch (e) {
       // eslint-disable-next-line no-console
-      console.log(`Failed to load: ${e}`);
+      console.error(`Failed to load: ${e}`);
       if (opt_errHandler) {
         opt_errHandler(e);
       }
@@ -52,6 +80,14 @@ const LoadingState = (superClass) => class extends superClass {
     }
   }
 
+  /**
+   * Retries a function with exponential backoff.
+   * @param {Function} f The function to retry.
+   * @param {Function} shouldRetry A function that determines if retrying should continue.
+   * @param {number} num The maximum number of retries.
+   * @param {number} wait The initial wait time in milliseconds.
+   * @return {Promise} A promise that resolves with the result of `f` or rejects.
+   */
   retry(f, shouldRetry, num, wait) {
     let count = 0;
     const retry = () => {