Transforms don't execute until the consumer pulls. There's no eager evaluation, no hidden buffering. Data flows on-demand from source, through transforms, to the consumer. If you stop iterating, processing stops.
40 是视野角度（FOV），就像你睁大眼睛还是眯着眼睛看。
，这一点在Line官方版本下载中也有详细论述
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Harpreet Matharu