]]><![CDATA[Be Careful with NSString's Hash]]>2012-12-08T11:24:00+08:00http://luosky.com/2012/12/08/be-careful-with-nsstring-hashMy recent iOS project, Aimeiwei, is an image-oriented app focused on food. We use lots of images, and one picture user updated can have two different aspect ratios, one is the original square food picture, and one is a cropped rectangle picutre that can possiblly be used as the restaurant’s header image. I utilized EGOImageLoading to cache and display images from our remote server. But I came across a strange problem: Sometimes the rectangle image view displays the square one instead, dispite the fact that the urls of the two images are different and the url of the rectangle one points to the correct rectangle image.
After a long time digging, I found that the problem rooted in the hash method of NSString. These two url:
Their hash is identical! (And EGOImageLoading uses the hash result as a key to cache the image. So if the square image had been cached, the rectangle view used the cached square image to display.)
NSString/CFString/NSURL
The hash is a convolution of the first and last eight bytes plus the length of the string basically the byte values are shifted and added to the string length.
I do know that the hash result is not guaranteed to be unique, but I don’t expect a “weak unique” like these. The implementation of hash must have been improved after these years because the data provided in the article no longer give the same result. But as of now 2012, on iOS 6 SDK, those two urls in my project still yield the same hash.
The fix is simple, just use MD5 as the key. Now I use this fork instead. If someday I had time, I may give SDWebImage a try.
]]><![CDATA[Create Custom Rss Feed for Octopress]]>2012-07-24T22:27:00+08:00http://luosky.com/2012/07/24/create-custom-rss-feed-for-octopressOctopress offers an atom.xml feed only. But many other blog providers’ default feed is different from this, such as /feed on wordpress. So how to move your existing blog subscribers smoothly from your old blog to Octopress?
Rss feed address can be change by modifying the subscribe_rss value in _config.yml. But this only change the url for the feed link on your page. It won’t generate a feed there. Then how do you generate a feed there?
If you have ssh access to your blog server:
just ssh to the server and do $ ln -s atom.xml $your_rss_file, then you are done.
If you don’t have ssh access:
If you host your Octopress on heroku or github, then you have no ssh access to the server.
For heroku user
You can redirect the request. Details can be found in the redirect part of this article, and code can be found here.
For github user
Simple method:
You can simply copy the atom.xml in your source(not public) directory to any location in the source directory or its sub-directory.
But this method has a little disadvantage, that due to your have two(or more) same feed source files now, Octopress will generate the rss feed more than one time. But that would not be a problem.
Better method:
To eliminate this disadvantage, I added a rake task to copy the generated atom.xml to other feed files in the Rakefile as follow:
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# Feed files other than atom.xml that needed to be compatible with previous blogfeed_files=["feed"]desc"copy atom.xml to feed_files"task:copyfeedsdofeed_files.eachdo|filename|cp("#{public_dir}/atom.xml","#{public_dir}/#{filename}")endend
And invoke the task before push to github by modify the deploy task in Rakefile:
Methods above serve well if the feed you want is somehing like rss.xml. But there is still a little problem if your old feed url is like /feed.
Since in github pages, the content-type of a file is determinate by the file extension. The response content-type of the request to /feed is application/octet-stream rather than text/xml as a normal rss feed. When user clicked on the /feed clink, it eventual downloads the feed file other than displaying the feed content or bringing up the Rss subscribe interface. And even if you add the address to google reader manually , although it can read the content first time you import this feed, you can’t get further updates.
What is worse is that github has no support for .htaccess
or other ways to redirect /feed to atom.xml.
After searching a long time and no solution was found, I almost gave up. But suddenly I came up with an idea: how about look /feed as a directory rather than a file? Then the /feed request eventually redirect to /feed/index.html, and the content-type can be specified in the html.
I tried that. And it DID WORK!
So the solution is :
copy your source/atom.xml to source/feed/index.xml if you use the simple method
or
set the feed_files = [“feed/index.html”] if you use the better method
Two roads diverged in a yellow wood,
And sorry I could not travel both
And be one traveler, long I stood
And looked down one as far as I could
To where it bent in the undergrowth;
Then took the other, as just as fair,
And having perhaps the better claim
Because it was grassy and wanted wear;
Though as for that, the passing there
Had worn them really about the same,
And both that morning equally lay
In leaves no step had trodden black.
Oh, I marked the first for another day!
Yet knowing how way leads on to way
I doubted if I should ever come back.
I shall be telling this with a sigh
Somewhere ages and ages hence:
Two roads diverged in a wood, and I,
I took the one less traveled by,
And that has made all the difference.
]]><![CDATA[First Week on Xcode ]]>2011-04-03T00:36:00+08:00http://luosky.com/2011/04/03/first-week-on-xcode-4经历了上周的one week on rails,这周又切换回iOS了.自从更新了Xcode 4之后,就跑去做支付宝连接和学习Ruby on Rails去了,一直没有碰Xcode.
