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资源说明：A last-in-first-out NSOperation queue written in Objective-C.
LIFOOperationQueue
==================
A last-in-first-out NSOperation queue written in Objective-C.
What's it for?
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A LIFO queue can help priortize new operations over old ones. One such example would be loading images in a UITableView. The LIFO approach will ensure that images on screen take higher priority than those a user has already scrolled past.
How does it work?
-----------------
LIFOOperationQueue is very much like `NSOperationQueue`. Just like the native implementation, you can configure the maximum number of concurrent operations. The only difference is that operations are added to the front of the queue and `NSOperationQueuePriority` has no effect. Initialization looks like this:
// initialize LIFOOperationQueue with a maximum thread count of 4
LIFOOperationQueue *operationQueue = [[LIFOOperationQueue alloc] initWithMaxConcurrentOperationCount:4];
Here are some quick examples of loading images in a `UITableView` with [AFNetworking](https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&ved=0CEcQFjAA&url=https%3A%2F%2Fgithub.com%2FAFNetworking%2FAFNetworking&ei=jTwxUNnPNY6NigLmuYHoAw&usg=AFQjCNE6c3SnPVzdrmQ1-UQ5mEf8Kl9JXg&sig2=WtTzATbO_YTH888N5ZEcAQ) and LIFOOperationQueue.
- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath {
// create cell
UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:@"CellIdentifier"];
if (cell == nil) {
cell = [[UITableViewCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:@"CellIdentifier"];
}
// create image request
NSURL *imageUrl = [NSURL URLWithString:@"http://www.i-love-cats.com/software/Adorable-Cats-Screensaver.jpg"];
NSURLRequest *urlRequest = [NSURLRequest requestWithURL:imageUrl];
AFImageRequestOperation *imageRequestOperation = [AFImageRequestOperation imageRequestOperationWithRequest:urlRequest success:^(UIImage *image) {
cell.imageView.image = image;
}];
// add to LIFOOperationQueue
[self.operationQueue addOperation:imageRequestOperation];
return cell;
}
The code above would prioritize the latest cell's image over thsoe that may not be on screen anymore. You can accomplish the same thing without AFNetworking by using blocks.
- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath {
// create cell
UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:@"CellIdentifier"];
if (cell == nil) {
cell = [[UITableViewCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:@"CellIdentifier"];
}
// add block to LIFOOperationQueue
[self.operationQueue addOperationWithBlock:^{
UIImage *image = [UIImage imageWithData:[NSData dataWithContentsOfURL:[NSURL URLWithString:url]]];
// be sure to display image on main thread
dispatch_sync(dispatch_get_main_queue(), ^{
cell.imageView.image = image;
});
}];
return cell;
}
`addOperationWithBlock:` executes the block asynchronously by default.
License
-------
LIFOOperationQueue is available under the MIT license. See the LICENSE file for more info.
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