Smalltalk Daily 8/21/07: Using Promise, Part II

On today's Smalltalk Daily, we take another look at class promise - but at the kind of usage that would be more common. Say you had a set of tasks (like, say, a bunch of HTTP downloads) that had to hapen before the next action could take place. Well, that's what class Promise is for. Here's the example code from today's Screencast:

"collection of urls"
urls := #('http://www.yahoo.com' 'http://www.cincomsmalltalk.com' 
'http://www.cincomsmalltalk.com/blog/blogView' 
'http://www.podcastalley.com' 'http://www.cincom.com' 
'http://www.smalltalkindustrycouncil.org' 'http://www.stic.st' 
'http://www.cincomsmalltalk.com/CincomSmalltalkWiki').


"block to download each one"
downloadBlock := [:url | | client response |
	client := HttpClient new.
	response := client get: url.
	Transcript show: 'Downloaded: ', url; cr].

"with Promise"
downloads := urls collect: [:each | [downloadBlock value: each] promise].
downloads do: [:each | each value].



"without Promise"
downloads := urls collect: [:each | | sem |
	sem := Semaphore new.
	[downloadBlock value: each.
	sem signal] fork.
	sem].

downloads do: [:each | each wait].

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