RxJS: Use Higher Order Observables in RxJS Effectively

## 项目纵向拆分

### ⓵ 定义目标和原则

### ⓶ 展望结果（OKRs）

### ⓷ 头脑风暴（发散）

### ⓸ 组织整理（收敛）

### ⓹ 明确「下一步行动」

总计 58:34

- [ ] Get started with higher order observables in RxJS 4:35
- [ ] Flatten a higher order observable with RxJS switch 4:52
- [ ] Flatten a higher order observable with mergeAll in RxJS 3:19
- [ ] Flatten a higher order observable with concatAll in RxJS 3:22
- [ ] Use RxJS switchMap to map and flatten higher order observables 6:04
- [ ] Use RxJS mergeMap to map and merge high order observables 4:00
- [ ] Use RxJS concatMap to map and concat high order observables 2:11
- [ ] Use RxJS mergeMap for fine-grain custom behavior 5:39
- [ ] Split an RxJS observable with window 6:58
- [ ] Split an RxJS observable conditionally with windowToggle 4:32
- [ ] Split an RxJS Observable into groups with groupBy 4:02
- [ ] Use groupBy in real RxJS applications 5:21
- [ ] Use switchMap as a safe default to flatten observables in RxJS 3:39