Editor’s Note: This is from a blog I started back in 2014, when I aimed to teach Objective-C to people who were totally unfamiliar with programming. It’s verbose, and I’m sorry, but also I am not sorry.
Welcome! I’ll explain more about my overall goal in another post, but I’d like to hit the ground running.
I’ll be showing you a chunk of code every week, and taking the time to break it down and explain exactly what it does. The goal is to have someone who has never written code understand what’s going on. I don’t ever want to confuse you, so I promise to never:
show you code blocks that are longer than 50 lines give you some long tutorial about how to build entire applications
This week, I’ll be showing you what happens when you want a label to show a countdown that updates every second.
-(void) updateCountdown {
NSTimeInterval timeToClosing = [_deleteDate timeIntervalSinceDate:[NSDate date]];
div_t h = div(timeToClosing, 3600);
int hours = h.quot;
div_t m = div(h.rem, 60);
int minutes = m.quot;
int seconds = m.rem;
NSString *hoursStr, *minutesStr, *secondsStr;
if (hours < 10) {
hoursStr = [NSString stringWithFormat:@"0%d", hours];
} else {
hoursStr = [NSString stringWithFormat:@"%d", hours];
}
if (minutes < 10) {
minutesStr = [NSString stringWithFormat:@"0%d", minutes];
} else {
minutesStr = [NSString stringWithFormat:@"%d", minutes];
}
if (seconds < 10) {
secondsStr = [NSString stringWithFormat:@"0%d", seconds];
} else {
secondsStr = [NSString stringWithFormat:@"%d", seconds];
}
if (seconds < 0) {
[self dismissButtonTapped];
} else {
self.countdownLabel.text = [NSString stringWithFormat:@"%@:%@:%@", hoursStr, minutesStr, secondsStr];
}
if (timeToClosing <= 30) {
[_switchTimer invalidate];
_switchTimer = nil;
[UIView animateWithDuration:0.7
animations:^{
self.eventEndsLabel.alpha = 0.0f;
self.countdownLabel.alpha = 1.0f;
}];
}
}
So! This is pretty intimidating at first. I’m going to make it as easy to understand as spoken word. Strap in.
Let’s start with the first line…
-(void)updateCountdown {
…wait - what did I even show you? A rule of thumb is that any block of code I ever show you will be a ‘method’. Think of this as an action that the program can do. I can make up things that exist in the program, but what good are they if they can’t do anything? They have many names (pointers, vars, ivars, etc.) but we’ll call them ‘things’ for now. In a program, ‘things’ have to ‘do’ stuff, and when you tell a ‘thing’ to ‘do’ something, you are ‘invoking a method’. A method is simply a directive for a ‘thing’ to do.
Let’s look at the ‘void’ part of this method. Think about this - when you ask someone to cook you a cheeseburger, you would expect a cheeseburger in return, right? But if you ask someone to fix something they already own, you would expect nothing in return. If this method was asking for a cheeseburger in return, then you would put a cheeseburger in those parentheses. However, we are asking something that already owns the label to simply update its own content. We aren’t asking for anything in return, so we say that by signifying (void) in the method signature.
The open bracket signifies the beginning of the method. You can tell when a method ends by searching for the ending bracket. Brackets are used commonly in code, so you have to keep track of how many brackets you use in code.
So, a ‘thing’ wants to update the countdown. Line 2 says…
NSTimeInterval timeToClosing = [_deleteDate timeIntervalSinceDate:[NSDate date]];
timeToClosing is the name of the variable I have created. Everything to the left of the equal sign is for saying that there is a ‘thing’ of a ‘type’. The ‘thing’ is called timeToClosing, and the ‘type’ is NSTimeInterval. NSTimeInterval is a type of ‘thing’ that is measured in seconds. I will need this later!
Everything to the right of the equal sign is how I figure out what the ‘thing’ will be defined by. _deleteDate is a ‘thing’ that already exists in my program. timeIntervalSinceDate:[xxxxxxx xxxxx] simply tells me how to calculate it based on _deleteDate. This is another example of a method, that returns a ‘thing’ of type NSTimeInterval. So when this line of code runs, I get back a NSTimeInterval to work with. When you see [NSDate date], just assume that it invokes a method to get the current date in a format that’s easy for the computer to understand. So we can say that timeToClosing is equal to a value that is in seconds from the current date to the value _deleteDate, which has already been set as a type of NSDate, which is a type of date.
Now we have THIS mess:
div_t h = div(timeToClosing, 3600);
int hours = h.quot;
div_t m = div(h.rem, 60);
int minutes = m.quot;
int seconds = m.rem;
As you might have guessed, div_t and int are types of ‘things’. To get super technical, they are basic C-level structs, which is a step down from the typical Objective-C code that I write in…
int is just an integer. You only give it a number, nothing more.
div_t is slightly more complicated. You give a div_t ‘thing’ two values: a dividend, and a divisor. A div_t will give you a quotient and a remainder, if you ask nicely.
…when you see something like div(timeToClosing, 3600);, that is a way to show you what values you are assigning to a div_t ‘thing’. The first line is making a div_t ‘thing’ that is called h. h is created by giving the values of timeToClosing and 3600 to a div_t, and you can get the quotient and the remainder from that value. Here’s some simplification:
h.quot = the quotient of timeToClosing and 3600
h.rem = the remainder of timeToClosing and 3600
This is how we are able to discern the int values of hours, minutes, and seconds.
Here comes the fun part…
NSString *hoursStr, *minutesStr, *secondsStr;
if (hours < 10) {
hoursStr = [NSString stringWithFormat:@"0%d", hours];
} else {
hoursStr = [NSString stringWithFormat:@"%d", hours];
}
if (minutes < 10) {
minutesStr = [NSString stringWithFormat:@"0%d", minutes];
} else {
minutesStr = [NSString stringWithFormat:@"%d", minutes];
}
if (seconds < 10) {
secondsStr = [NSString stringWithFormat:@"0%d", seconds];
} else {
secondsStr = [NSString stringWithFormat:@"%d", seconds];
}
Let’s look at NSString first. If you’ve been reading diligently, you recognize the NS. NS stands for NextStep, which is attributed to the computing company NeXt Computing, which Steve Jobs originally started after leaving Apple, and subsequently got acquired by Apple. NS is a tribute to that.
NSString is a type of ‘thing’ that simply refers to a string of text. For example, you could say…
NSString *example = @"This is an example of a string.";
…you can probably gather what it does at a high level, so you can ignore the other stuff going on there for now, but you should just understand that, when you create a NSString, you are creating a string of text that you want to use elsewhere.
In our code example, you see some if-else stuff going on. When you see…
if (something == true)
…then that means that, whatever exists past that line of code will happen ONLY IF something is true. In our example, we’ve already got a number for hours assigned, and we are looking to see if it is less than 10 or not. If it is, infact, less than 10, as our code describes, then we assign the following value to hoursStr, which we have declared as a NSString…
[NSString stringWithFormat:@"0%d", hours];
Don’t be scared! Let’s look at everything inside the @" " marks. For right now, think of a string of text that the computer understands is @"string of text"…
I say tomato…
…the computer says @"tomato"
When you tell the program to do: [NSString stringWithFormat:@" "];, that simply means you are making a string of text look exactly the way you want it to. Inside that @" ", you will often see a % sign followed by either the @ sign or a letter (usually f or d). Our example of 0%d means that you want to make a string that starts with the text “0” and then substitutes an int thing that we already created. You can see that we use hours, something we created earlier, to substitute. This method CAN get more complex, but this lays the basic foundation for how it works. So, if, in our example, hours = 9, then hoursStr = @"09".
Ok, cool story bro, but what if hours = 13?
else {
hoursStr = [NSString stringWithFormat:@"%d", hours];
}
In this case, per our previous example, we would just create a string formatted according to the rules we created. Since there is no 0 in front of the %d, and the value for hours did not satisfy the if statement earlier, we fall into the else statement. Since hours = 13, hoursStr = @"13" in our situation. This applies for the next few lines of our example as well.
We’re close! Two more code examples from our block…
if (seconds < 0) {
[self dismissButtonTapped];
} else {
self.countdownLabel.text = [NSString stringWithFormat:@"%@:%@:%@", hoursStr, minutesStr, secondsStr];
}
If the number of seconds we have is less than 0, we want to ‘call’ a method called dismissButtonTapped. For posterity, this countdown timer determines how long you have to view a screen, and when the timer is up, you can no longer view the screen. Therefore, the time being up forces the user to dismiss the screen and no longer be able to view it. However, if the value for seconds is greater than zero, which it usually will be, as I am updating my timer every second, then I update the value of a thing called a UILabel I created earlier called countdownLabel. A UILabel contains a NSString, and I can access that by writing the .text property of it. Don’t worry about the self part for now - just assume that we are talking about a label contained in the screen we are looking at.
When seconds is not less than zero, we update the label to show the proper kind of string with our hours, minutes, and seconds value, as calculated above. We do the math ahead of time so creating our string is nice and easy.
Last bit!
if (timeToClosing <= 30) {
[_switchTimer invalidate];
_switchTimer = nil;
[UIView animateWithDuration:0.7
animations:^{
self.eventEndsLabel.alpha = 0.0f;
self.countdownLabel.alpha = 1.0f;
}];
}
In terms of necessity, this is purely optional, and only valuable to the code I’ve written in the rest of the app, but I’ll explain a little. I make sure that the countdown timer and a message explaining the countdown timer alternate in visibility, and when we get to less than 30 seconds, I only want the user to see the countdown timer. I use an NSTimer called _switchTimer to display the two in succession, and at < 30 seconds, I no longer want to switch.
Finally, the .alpha property is how visible something is. For instance, if a UILabel has an alpha of 1.0f, it is 100% visible. If a UILabel has an alpha of 0.0f, it is 100% invisible. Don’t stress too much over the UIView gobble-de-gook for right now, but assume that it is a fancy way for me to make eventEndsLabel invisible, and countdownLabel totally visible.
Whew! You made it! I hope I was able to make sense of this for you. I promise to try and keep it simple for you at all times. Feel free to correct me with a comment, or email me a suggestion here with a code block to explain. Thanks for reading!