I have a block of code which loops through an array and performs block code on it. Currently it looks like this:

```
for (NSString *myString in myArray) {

    [self doSomethingToString:myString WithCompletion:^(BOOL completion) {
        string = [NSString stringByAppendingString:@"Test"];
    }];

}

```

I want to wait for the previous iteration to finish before I start on the next one. How can I loop through some block code like this?