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Nanaloveyuki
76 lines
2.0 KiB
Markdown
76 lines
2.0 KiB
Markdown
---
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name: queued-sink-drain
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group: api
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category: sink
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update-time: 20260613
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description: Drain queued records from a QueuedSink with an optional item limit.
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key-word:
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- sink
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- queue
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- drain
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- public
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---
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## Queued-sink-drain
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Drain queued records from a `QueuedSink[S]`. This is the direct sink-level queue delivery API when code owns a queued sink and wants controlled progress into the wrapped sink.
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### Interface
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```moonbit
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pub fn[S : Sink] QueuedSink::drain(self : QueuedSink[S], max_items~ : Int = -1) -> Int {
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```
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#### input
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- `self : QueuedSink[S]` - Queued sink whose pending records should be drained to the wrapped sink.
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- `max_items : Int` - Optional upper bound on drained queued records. Negative values mean no explicit bound.
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#### output
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- `Int` - Number of drained records.
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### Explanation
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Detailed rules explaining key parameters and behaviors
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- `max_items == 0` returns `0` immediately.
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- Negative `max_items` means drain up to the current `pending_count()`.
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- The method repeatedly pops from the queue and writes each record to the wrapped sink in order until the limit is reached or the queue becomes empty.
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- The return value is the number of records actually drained.
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### How to Use
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Here are some specific examples provided.
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#### When Need Bounded Queue Progress
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When queued work should be advanced in chunks:
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```moonbit
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let drained = sink.drain(max_items=16)
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```
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In this example, callers cap how much queue backlog is forwarded in one step.
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#### When Need Full Manual Drain
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When a queued sink should be emptied explicitly:
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```moonbit
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ignore(sink.drain())
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```
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In this example, the queue is drained without an explicit item cap.
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### Error Case
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e.g.:
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- If the queue is already empty, the method returns `0`.
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- If callers only need the shorthand that drains the whole queue, `flush()` is the simpler API.
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### Notes
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1. Prefer this helper when queue progress should be bounded or directly observed.
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2. Pair it with `pending_count()` to inspect remaining backlog after a drain step.
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