Dispatch Queues execute work items in FIFO order，including two kind of execution
Asynchronous ：可以先加入队列，将来再执行
Synchronous：必须执行完才到下一个
let queue = DispatchQueue(label: "com.example.imagetransform")
queue.async {
let smallImage = image.resize(to: rect)
DispatchQueue.main.async {
imageView.image = smallImage
}
}
Getting Work Off Your Main Thread
Create a Dispatch Queue to which you submit work
Dispatch Queues execute work items in FIFO order
Use .async to execute your work on the queue
Dispatch main queue executes all items on the main thread
Simple to chain work between queues
let queue = DispatchQueue(label: "com.example.imagetransform")
queue.async {
let smallImage = image.resize(to: rect)
DispatchQueue.main.async {
imageView.image = smallImage
}
}
Grouping Work Together
Choosing a Quality of Service
Using Quality of Service Classes
Use .async to submit work with a specific QoS class
Dispatch helps resolve priority inversions
Create single-purpose queues with a specific QoS class
queue.async(qos: .background) {
print("Maintenance work")
}
queue.async(qos: .userInitiated) {
print(“Button tapped”)
}