I have some JSON data that I need to display in a table view. What is the best approach for fetching and loading this data in the background using dispatch methods or child threads?
I have some JSON data that I need to display in a table view. What is the best approach for fetching and loading this data in the background using dispatch methods or child threads?
One effective technique is to load data in a background thread using the dispatch_async method and ensuring that a queue other than the main one is used to prevent UI blocking. Once the data is ready, simply call reload table view on the main thread... Below is an example from a real project showcasing a class-level method for loading transactions.
+ (void)getAllTransactionsWithHandler:(void(^)(NSArray *transactions, NSError *error))handler{
dispatch_queue_t q = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_async(q, ^{
NSURL *url = [[NSBundle mainBundle] URLForResource:trnasactionFileName withExtension:@"json"];
NSData *data = [NSData dataWithContentsOfURL:url];
if (!data) {
if (handler) {
handler(nil, [NSError errorWithDomain:@"bank" code:900 userInfo:nil]);
}
return ;
}
NSError *error = nil;
NSArray *jsonArray = [NSJSONSerialization JSONObjectWithData:data options:0 error:&error];
if (error) {
if (handler) {
handler(nil, error);
}
}
else{
Transaction *transaction;
NSString *dateString;
NSMutableArray *objects = [NSMutableArray new];
for (NSDictionary *dic in jsonArray) {
transaction = [Transaction new];
dateString = dic[kOccured];
NSDateFormatter *dateFormater = [NSDateFormatter new];
[dateFormater setDateFormat:@"yyyy-MM-dd"];
transaction.occurred = [dateFormater dateFromString:dateString];
transaction.processed = [dateFormater dateFromString:dic[kProcessed]];
transaction.desc = dic[kDescription];
transaction.amount = dic[kAmount];
[objects addObject:transaction];
}
if (handler) {
handler([NSArray arrayWithArray:objects], nil);
}
}
});
}
You can implement this as follows:
[Transaction getAllTransactionsWithHandler:^(NSArray *transactions, NSError *error) {
if (error) {
}else{
if ([transactions count] > 0) {
weakSelf.objects = transactions;
runOnMainThread(^{
[weakSelf.tableView reloadData];
});
}
}
}];
The runOnMainthread function provided below is a utility method that ensures a block of code runs on the main thread.
void runOnMainThread(void(^block)(void)){
if ([[NSThread currentThread] isMainThread])
block();
else{
dispatch_sync(dispatch_get_main_queue(), ^{
block();
});
}
}
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