Pick up the right collections

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代码1：

NSArray *sessionsFromServer = ...;
NSManagedObjectContext *context = ...;

for (WWDCSession *aSession in sessionsFromServer) {
    NSFetchRequest *req = [NSFetchRequest fetchRequestWithEntityName:@"WWDCSession"];
    req.predicate = [NSPredicate predicateWithFormat:@"sessionID == %@", aSession.sessionID];
    NSArray *results = [context executeFetchRequest:req error:nil];

    WWDCSession *existSesion = [results firstObject];
    if (!existSesion) {
        //Insert a New One
    }else {
        // Do something with the existSession
    }
}

代码2：

NSArray *sessionsFromServer = ...;
NSManagedObjectContext *context = ...;

NSArray *sessionIDs = [sessionsFromServer valueForKey:@"SessionID"];
NSFetchRequest *req = [NSFetchRequest fetchRequestWithEntityName:@"WWDCSession"];
req.predicate = [NSPredicate predicateWithFormat:@"sessionID in %@", sessionIDs];
NSArray *results = [context executeFetchRequest:req error:nil];

for (WWDCSession *aSession in sessionsFromServer) {
    NSInteger existingSessionIndex = [results indexOfObject:aSession];
    if (existingSessionIndex == NSNotFound) {
        //Insert a New One
    }else {
        // Do something with the existSession
    }
}

代码3：

NSArray *sessionsFromServer = ...;
NSManagedObjectContext *context = ...;

NSArray *sessionIDs = [sessionsFromServer valueForKey:@"SessionID"];
NSFetchRequest *req = [NSFetchRequest fetchRequestWithEntityName:@"WWDCSession"];
req.predicate = [NSPredicate predicateWithFormat:@"sessionID in %@", sessionIDs];
NSArray *results = [context executeFetchRequest:req error:nil];
NSDictionary *mapDic = [NSDictionary dictionaryWithObjects:results forKeys:[results valueForkey:@"sessionID"]];

for (WWDCSession *aSession in sessionsFromServer) {
    WWDCSession *existingSession = mapDic[aSession.sessionID];
    if (!existingSession) {
        //Insert a New One
    }else {
        // Do something with the existSession
    }
}

代码1循环里每次都要进行次数据库查询。

代码2改进数据查询为一次，但是for循环里面还是for了一遍results。

代码3继承代码2的优点，但是for循环里面根据Dictionary的优势只进行了一次查询，可谓高手风范。

有些法则需要遵循：

  1. 特别要注意循环里面干什么！
  2. 可能的话，尽量使用NSDictionary和NSSet来查询。（这两个都是Hash table store）
  3. 能使用mutable collections(或者strings)的时候就用他们来addObject吧。不要重新来初始化一个新的inmutable的了。
  4. 精简数据库查询代码，比如代码1中的这种，能一次搞定就一次搞定。