comparison demo/prose @ 697:755a71c99be5

Threads demo
author Adam Chlipala <adamc@hcoop.net>
date Sun, 05 Apr 2009 10:48:11 -0400
parents bab524996fca
children 9b29ce0babb8
comparison
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696:79a49c509007 697:755a71c99be5
232 </ol> 232 </ol>
233 233
234 <p><tt>BatchFun.Make</tt> handles the plumbing of allocating the local state, using it to create widgets, and reading the state values when the user clicks "Batch it."</p> 234 <p><tt>BatchFun.Make</tt> handles the plumbing of allocating the local state, using it to create widgets, and reading the state values when the user clicks "Batch it."</p>
235 235
236 <p><tt>batchG.ur</tt> contains an example instantiation, which is just as easy to write as in the <tt>Crud1</tt> example.</p> 236 <p><tt>batchG.ur</tt> contains an example instantiation, which is just as easy to write as in the <tt>Crud1</tt> example.</p>
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238 threads.urp
239
240 <p>Ur/Web makes it easy to write multi-threaded client-side code. This example demonstrates two threads writing to a page at once.</p>
241
242 <p>First, we define a useful component for sections of pages that can have lines of text added to them dynamically. This is the <tt>Buffer</tt> module. It contains an abstract type of writable regions, along with functions to create a region, retrieve a signal representing its HTML rendering, and add a new line to it.</p>
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244 <p>The entry point to the main module <tt>Threads</tt> begins by creating a buffer. The function <tt>loop</tt> implements writing to that buffer periodically, incrementing a counter each time. The arguments to <tt>loop</tt> specify a prefix for the messages and the number of milliseconds to wait between writes.</p>
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246 <p>We specify some client-side code to run on page load using the <tt>onload</tt> attribute of <tt>&lt;body&gt;</tt>. The <tt>onload</tt> code in this example spawns two separate threads running the <tt>loop</tt> code with different prefixes, update intervals, and starting counters.</p>
247
248 <p>Old hands at concurrent programming may be worried at the lack of synchronization in this program. Ur/Web uses <i>cooperative multi-threading</i>, not the more common <i>preemptive</i> multi-threading. Only one thread runs at a time, and only particular function calls can trigger context switches. In this example, <tt>sleep</tt> is the only such function that appears.</p>