PromiseKit aims to make dealing with assyncronicity in your iPhone app delightful. PromiseKit is not just a Promises implementation, it is also a collection of helper functions that make the typical asyncronous patterns we use in iOS development delightful too.
A promise is an intent to accomplish an asyncronous task. Eg. some library promises to download a gravatar:
#import "PromiseKit.h"
- (void)gravatar {
[ABCDoEverythingLibrary gravatar:myURL].then(^(UIImage *img){
self.imageView.image = img;
}).fail(^(NSError *error){
NSLog(@"%@", error);
})
}The designer changes their mind (again). They want us to overlay a kitten on the gravatar:
#import "PromiseKit.h"
- (void)gravatar {
UIImage *gravatarImage = nil;
[ABCDoEverythingLibrary gravatar:myURL].then(^(UIImage *img){
gravatarImage = img;
}).fail(^(NSError *error){
NSLog(@"%@", error);
}).then(^{
return [NSURLConnection GET:@"http://placekitten.com/100/100"];
}).then(^(UIImage *kittenImage){
self.imageView.image = [UIImage draw:kittenImage over:gravatarImage];
});
}Whoops we didn't handle the error for the NSURLConnection:
#import "PromiseKit.h"
- (void)gravatar {
UIImage *gravatarImage = nil;
[ABCDoEverythingLibrary gravatar:myURL].then(^(UIImage *img){
gravatarImage = img;
}).then(^{
return [NSURLConnection GET:@"http://placekitten.com/100/100"];
}).then(^(UIImage *kittenImage){
self.imageView.image = [UIImage draw:kittenImage over:gravatarImage];
}).fail(^(NSError *error){
// moved to the end: any errors or thrown exceptions will bubble up
NSLog(@"%@", error);
});
}We need to refactor this code so our imageView is set elsewhere:
#import "PromiseKit.h"
- (Promise *)gravatar {
UIImage *gravatarImage = nil;
[ABCDoEverythingLibrary gravatar:myURL].then(^(UIImage *img){
gravatarImage = img;
}).then(^{
return [NSURLConnection GET:@"http://placekitten.com/100/100"];
}).then(^(UIImage *kittenImage){
return [UIImage draw:kittenImage over:gravatarImage];
});
}
- (void)viewDidLoad {
self.gravatar.then(^(UIImage *img){
self.imageView.image = img;
}).fail(^(NSError *error){
[UIAlertView show:error];
});
}In production we find that ABCDoEverythingLibrary is a buggy, bloated mostrosity. So we hastily reinvent the wheel:
#import "PromiseKit.h"
- (Promise *)gravatar {
UIImage *gravatarImage = nil;
[Promise md5:self.email].then(^(NSString *md5){
// The MD5 is crunched in a background GCD queue
return [NSURLConnection GET:@"http://gravatar.com/avatar/%@", md5];
}).then(^{
return [NSURLConnection GET:@"http://placekitten.com/100/100"];
}).then(^(UIImage *kittenImage){
return [UIImage draw:kittenImage over:gravatarImage];
});
}Later we realize we’re wasting our users’ lives by downloading the two images consequentively:
#import "PromiseKit.h"
- (Promise *)gravatar {
UIImage *gravatarImage = nil;
[Promise md5:self.email].then(^(NSString *md5){
id a = [NSURLConnection GET:@"http://gravatar.com/avatar/%@", md5];
id b = [NSURLConnection GET:@"http://placekitten.com/100/100"];
return [Promise when:@[a, b]];
}).then(^(NSArray *images){
return [UIImage draw:images[0] over:images[1]];
});
}- UIViewControllers
- NSURLConnections
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