Angular 5中如何直接赋值Observable变量？无需mergeMap与scan

Hey there! Let's start by breaking down what your original code does: it takes your this.groups array, prepends an empty array, then emits each element with a 1-second delay between them. Finally, scan accumulates these elements into a growing array, so your $groups observable emits a new array every second with all previous items plus the latest one.

If you want to avoid mergeMap and scan and instead have a more "direct assignment" feel, BehaviorSubject is absolutely the right tool here—you just might not have used it the right way to replicate the accumulation behavior. Here's a step-by-step solution:

Solution 1: Use BehaviorSubject with Manual Interval Updates #

A BehaviorSubject holds the current value and emits it to new subscribers, which makes it perfect for maintaining your growing array state. You can manually push updates to it on a timer:

// Initialize with an empty array as the initial value
private $groups = new BehaviorSubject<any[]>([]);

// Function to push groups one by one with 1-second delays
private startEmittingGroups(): void {
  let currentIndex = 0;
  const emitNext = () => {
    if (currentIndex >= this.groups.length) return;

    // Get the current array state, add the next group, and push the new array
    const currentArray = this.$groups.value;
    const updatedArray = [...currentArray, this.groups[currentIndex]];
    this.$groups.next(updatedArray);

    currentIndex++;
    setTimeout(emitNext, 1000);
  };

  emitNext();
}

// Call this when you want to start emitting (e.g., in ngOnInit if using Angular)
ngOnInit(): void {
  this.startEmittingGroups();
}

This approach matches your original behavior exactly:

• $groups emits a new array every second
• Each new array includes all previous groups plus the latest one
• No mergeMap or scan needed—you're directly controlling the state and emissions

Why Your Previous Subject Attempt Might Have Failed #

If you tried a regular Subject instead of BehaviorSubject, it doesn't hold a current value—so you couldn't easily access the existing array to append new items. A BehaviorSubject solves this by letting you get the latest emitted value with .value, which is key for building the updated array.

Bonus: Simplified Version (No mergeMap, but still uses scan) #

If you're open to keeping scan but ditching mergeMap, you can use delayWhen to handle the timing instead:

this.$groups = from(this.groups)
  .pipe(
    delayWhen((_, index) => timer(index * 1000)),
    scan((acc, curr) => [...acc, curr], [])
  );

This is more concise, but since you asked to avoid scan, the BehaviorSubject approach is your best bet for that "direct assignment" control you're looking for.

内容的提问来源于stack exchange，提问作者Sudhanshu sharma