GCD (Grand Central Dispatch) is a low-level concurrency framework provided by Apple to help you manage multithreading and execute tasks asynchronously or concurrently in iOS\/macOS apps.<\/p>\n\n\n\n
It lets you run time-consuming tasks in the background (like API calls, file reading, or image processing) while keeping the UI smooth and responsive.<\/p>\n\n\n\n
A dispatch queue is an object that manages the execution of tasks serially or concurrently.<\/p>\n\n\n\n Specifies the priority level of tasks:<\/strong><\/p>\n\n\n\nQueue Type<\/strong><\/td> Description<\/strong><\/td><\/tr> Main Queue<\/strong><\/td> Executes tasks on the main thread (UI updates must run here).<\/td><\/tr> Global Queues<\/strong><\/td> Concurrent background queues managed by the system.<\/td><\/tr> Custom Queues<\/strong><\/td> You create your own serial or concurrent queues.<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n 2. Sync vs Async<\/h3>\n\n\n\n
Method<\/strong><\/td> Blocks Current Thread?<\/strong><\/td> Waits for Task to Finish?<\/strong><\/td><\/tr> sync<\/strong><\/td> Yes<\/td> Yes<\/td><\/tr> async<\/strong><\/td> No<\/td> No<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n Common GCD Examples in Swift<\/h2>\n\n\n\n
1. Perform Background Task, Then Update UI<\/h3>\n\n\n\n
DispatchQueue.global(qos: .background).async {\n \/\/ Background task (e.g., download, compute)\n \n DispatchQueue.main.async {\n \/\/ Update UI on main thread\n }\n}<\/code><\/pre>\n\n\n\n2. Delay Execution<\/h3>\n\n\n\n
DispatchQueue.main.asyncAfter(deadline: .now() + 2.0) {\n print(\"Executed after 2 seconds\")\n}<\/code><\/pre>\n\n\n\n3. Create a Custom Serial Queue<\/h3>\n\n\n\n
let serialQueue = DispatchQueue(label: \"com.myapp.serial\")\nserialQueue.async {\n print(\"Task 1\")\n}\nserialQueue.async {\n print(\"Task 2\") \/\/ Runs after Task 1\n}<\/code><\/pre>\n\n\n\n4. Create a Concurrent Queue<\/h3>\n\n\n\n
let concurrentQueue = DispatchQueue(label: \"com.myapp.concurrent\", attributes: .concurrent)\nconcurrentQueue.async {\n print(\"Task A\")\n}\nconcurrentQueue.async {\n print(\"Task B\") \/\/ Can run in parallel with Task A\n}<\/code><\/pre>\n\n\n\nQuality of Service (QoS)<\/h2>\n\n\n\n
QoS<\/strong><\/td> Use Case<\/strong><\/td><\/tr> .userInteractive<\/strong><\/td> For UI-critical tasks<\/td><\/tr> .userInitiated<\/strong><\/td> High-priority tasks initiated by user<\/td><\/tr> .utility<\/strong><\/td> Long-running tasks (e.g., downloads)<\/td><\/tr> .background<\/strong><\/td> Lowest priority (e.g., syncing data)<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n When to Use GCD<\/h2>\n\n\n\n
\n
Conclusion<\/h2>\n\n\n\n