SoFunction
Updated on 2025-04-03

Summary of implementation of several timers in iOS

In the software development process, we often need to execute a certain method after a certain time, or execute a certain method throughout a certain period of time. At this time, we need to use a timer.

However, there are many ways to complete the above tasks in iOS. How many methods are there? After reviewing the information, there are roughly three methods: NSTimer, CADisplayLink, and GCD. Next, I will introduce their usage one by one.

1. NSTimer

1. Create method

NSTimer *timer = [NSTimer scheduledTimerWithTimeInterval:1.0 target:self selector:@selector(action:) userInfo:nil repeats:NO];
  • TimerInterval : The time to wait before execution. For example, setting it to 1.0 means that the method is executed after 1 second
  • target : The object that needs to execute the method.
  • selector : Methods that need to be executed
  • repeats  : Is a loop required

2. Release method

[timer invalidate];

Notice :
After calling the create method, the counter of the target object will be incremented by 1 until the execution is completed and will be automatically reduced by 1. If it is a loop execution, it must be closed manually, otherwise the release method can be not executed.

3. Features

There is a delay
Regardless of whether it is a one-time or periodic timer's actual triggering event, it will be related to the RunLoop and RunLoop Mode added. If this RunLoop is performing a continuous operation, the timer will be delayed. Repeatable timer encounters this situation. If the delay exceeds one cycle, it will be executed immediately after the delay is over and continue to execute according to the previously specified cycle.

Must join Runloop
Using the above creation method, the timer will be automatically added to the NSDefaultRunLoopMode of MainRunloop. If you create a timer using the following method, you must manually join the Runloop:

NSTimer *timer = [NSTimer timerWithTimeInterval:5 target:self selector:@selector(timerAction) userInfo:nil repeats:YES];
[[NSRunLoop mainRunLoop] addTimer:timer forMode:NSDefaultRunLoopMode];

2. CADisplayLink

1. Create method

 = [CADisplayLink displayLinkWithTarget:self selector:@selector(handleDisplayLink:)];  
[ addToRunLoop:[NSRunLoop currentRunLoop] forMode:NSDefaultRunLoopMode];

2. Stop method

[ invalidate]; 
 = nil;

**When the CADisplayLink object is added to the runloop, the selector can be called periodically, similar to the repeated NSTimer is started; when the invalidate operation is performed, the CADisplayLink object will be removed from the runloop, and the selector call will be stopped immediately, similar to the invalidate method of the NSTimer. **

3. Features

Calling CADisplayLink when screen refresh is a timer class that allows us to draw specific content on the screen at a frequency synchronized with the screen refresh rate. After CADisplayLink registers to runloop in a specific mode, whenever the screen displays content refresh, runloop will send a specified selector message to the target specified by CADisplayLink, and the selector corresponding to the CADisplayLink class will be called once. So usually, according to the refresh rate of iOS device screen 60 times/second

Delay

  • The screen refresh frequency of iOS devices is fixed. Under normal circumstances, CADisplayLink will be called at the end of each refresh, which has a very high accuracy. However, if the method called is time-consuming and exceeds the screen refresh cycle, several callback calls will be skipped.
  • If the CPU is too busy and cannot guarantee the refresh rate of 60 times per second on the screen, it will lead to several chances of calling the callback method, and the number of skipping depends on the busyness of the CPU.

Usage scenarios From the principle, it can be seen that CADisplayLink is suitable for continuous repainting of the interface. For example, when playing video, you need to constantly obtain the next frame for interface rendering.

4. Important attributes

  • frameInterval The value of NSInteger type is used to set how many frames to call the selector method once. The default value is 1, that is, it is called once every frame.
  • duration readOnly The CFTimeInterval value indicates the time interval between two screen refreshes. It should be noted that this property will not be assigned after the target selector is called for the first time. The call interval time of the selector is calculated as: call interval time = duration × frameInterval.

3. GCD method

Execute once

double delayInSeconds = 2.0;
dispatch_time_t popTime = dispatch_time(DISPATCH_TIME_NOW, delayInSeconds * NSEC_PER_SEC); 
dispatch_after(popTime, dispatch_get_main_queue(), ^(void){ 
  //Execute event});

Repeat execution

NSTimeInterval period = 1.0; //Set the time intervaldispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_source_t _timer = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0, queue);
dispatch_source_set_timer(_timer, dispatch_walltime(NULL, 0), period * NSEC_PER_SEC, 0); //Execute every seconddispatch_source_set_event_handler(_timer, ^{
  //Execute events here});
dispatch_resume(_timer);

GCD method, I can only find these materials online. I am still studying and will update it in the future.

This is the end of this article about the implementation of several timers in iOS. For more related iOS timer content, please search for my previous articles or continue browsing the related articles below. I hope everyone will support me in the future!