iOS slow SQL query within the viewDidLoad method - ios

Question

I've got a chat app that is based on a navigation controller.
Within the view that displays the conversation i load all the previous history from SQLite within the viewDidLoad method.
When a conversation grows it takes a few seconds to open up the view and therefore feels "laggy".
Below is my code that i use to load the history from the database.
Any ideas what could be improved to make the UI feel more responsive?
NSString *sql = [NSString stringWithFormat:#"SELECT * FROM chatHistory WHERE channelID = '%#' ORDER BY time ASC", box.activeChannel];
sqlite3_stmt *statement;
if(sqlite3_prepare_v2([box db], [sql UTF8String], -1, &statement, nil) == SQLITE_OK) {
while(sqlite3_step(statement) == SQLITE_ROW) {
char *field1 = (char *) sqlite3_column_text(statement, 0);
NSString *channelID = [[NSString alloc] initWithUTF8String:field1];
char *field2 = (char *) sqlite3_column_text(statement, 1);
NSString *sender = [[NSString alloc] initWithUTF8String:field2];
char *field3 = (char *) sqlite3_column_text(statement, 2);
NSString *message = [[NSString alloc] initWithUTF8String:field3];
char *field4 = (char *) sqlite3_column_text(statement, 3);
NSString *recipient = [[NSString alloc] initWithUTF8String:field4];
char *field5 = (char *) sqlite3_column_text(statement, 4);
NSString *time = [[NSString alloc] initWithUTF8String:field5];
NSString *str = [[NSString alloc] initWithFormat:#"%#", message];
NSString *str1 = [[NSString alloc] initWithFormat:#"%#", channelID];
NSString *str2 = [[NSString alloc] initWithFormat:#"%#", sender];
NSString *str3 = [[NSString alloc] initWithFormat:#"%#", time];
NSString *trimmedStr2 = [str2 stringByTrimmingCharactersInSet:
[NSCharacterSet whitespaceCharacterSet]];
NSString *trimmedAgentName = [box.agentName stringByTrimmingCharactersInSet:
[NSCharacterSet whitespaceCharacterSet]];
NSString *trimmedTime = [str3 stringByTrimmingCharactersInSet:
[NSCharacterSet whitespaceCharacterSet]];
NSDateFormatter *df = [[NSDateFormatter alloc] init];
[df setDateFormat:#"yyyy-MM-dd HH:mm:ss ZZZ"];
NSDate *myDate = [df dateFromString:trimmedTime];
if ([trimmedStr2 isEqualToString:trimmedAgentName]) {
[self.messages addObject:[[JSMessage alloc] initWithText:str sender:trimmedAgentName date:myDate]];
[self finishSend];
[self scrollToBottomAnimated:YES];
} else {
[self.messages addObject:[[JSMessage alloc] initWithText:str sender:kSubtitleVisitor date:myDate]];
[self finishSend];
[self scrollToBottomAnimated:YES];
}
}
}
Edit
I've also tried to put the code inside a dispatch_queue, that made the switch to the view go a lot faster but the messages are still lagging behind when being added.

Answer 1

There are some places you can optimise in your code:
No need to create str str1 str2 str3, just use the message channelID sender time.
NSDateFormatter creation is expansive, you should create a instance outside the while loop.
viewDidLoad is called in UI thread, the query will take a long time to execute and block UI, you might want to execute SQL query in background. See Apple's Concurrency Programming Guide for more.