ソースコードがヘッダやコメントを入れても１００行にも満たない無駄の無いコード。それでいて十分な機能。
ここでいうシーケンサとは、あらかじめ登録した複数の処理を順番に実行する仕組みのこと。ネットワークアクセスやユーザインタラクションは基本非同期な処理になるが、特定の順番で同期的に処理を行いたい場合がある。シーケンサはそんな用途で利用できる。
単純な例
Sequencer *sequencer = [[Sequencer alloc] init];
[sequencer enqueueStep:^(id result, SequencerCompletion completion) {
NSLog(@"This is the first step");
completion(nil);
}];
[sequencer enqueueStep:^(id result, SequencerCompletion completion) {
NSLog(@"This is another step");
completion(nil);
}];
[sequencer enqueueStep:^(id result, SequencerCompletion completion) {
NSLog(@"This step is going to do some async work...");
int64_t delayInSeconds = 2.0;
dispatch_time_t popTime = dispatch_time(DISPATCH_TIME_NOW, delayInSeconds * NSEC_PER_SEC);
dispatch_after(popTime, dispatch_get_main_queue(), ^(void){
NSLog(@"finished the async work.");
completion(nil);
});
}];
[sequencer enqueueStep:^(id result, SequencerCompletion completion) {
NSLog(@"This is the last step");
completion(nil);
}];
[sequencer run];completionの引数が次のステップの引数 result として渡っていく。ネットワークアクセスの例
Sequencer *sequencer = [[Sequencer alloc] init];
[sequencer enqueueStep:^(id result, SequencerCompletion completion) {
NSURL *url = [NSURL URLWithString:@"https://alpha-api.app.net/stream/0/posts/stream/global"];
NSURLRequest *request = [NSURLRequest requestWithURL:url];
AFJSONRequestOperation *operation = [AFJSONRequestOperation JSONRequestOperationWithRequest:request success:^(NSURLRequest *request, NSHTTPURLResponse *response, id JSON) {
completion(JSON);
} failure:nil];
[operation start];
}];
[sequencer enqueueStep:^(NSDictionary *feed, SequencerCompletion completion) {
NSArray *data = [feed objectForKey:@"data"];
NSDictionary *lastFeedItem = [data lastObject];
NSString *cononicalURL = [lastFeedItem objectForKey:@"canonical_url"];
NSURLRequest *request = [NSURLRequest requestWithURL:[NSURL URLWithString:cononicalURL]];
AFHTTPRequestOperation *operation = [[AFHTTPRequestOperation alloc] initWithRequest:request];
[operation setCompletionBlockWithSuccess:^(AFHTTPRequestOperation *operation, id responseObject) {
completion(responseObject);
} failure:nil];
[operation start];
}];
[sequencer enqueueStep:^(NSData *htmlData, SequencerCompletion completion) {
NSString *html = [[NSString alloc] initWithData:htmlData encoding:NSUTF8StringEncoding];
NSLog(@"HTML Page: %@", html);
completion(nil);
}];
[sequencer run];最初のステップでJSONを取得し、次にURL情報から HTMLを取得。最後にそれをログに表示。シーケンサを使うとデリゲートチェーンやblocksの多段ネストの必要が無くなり見通し良いコードが書ける。なかなかいい感じ。