Tell us about the global queue in multithreaded programming on iOS.

The Global Queue in Grand Central Dispatch (GCD) refers to predefined system queues accessible to all applications. They are Concurrent Queues, meaning tasks within them can be executed in parallel on different threads.

Main characteristics:

• Types: Differ based on Quality of Service (QoS) levels.
• QoS: Determines the priority of task execution, influencing which tasks the system should perform faster.
• Concurrency: Allows multiple tasks to be executed simultaneously.
• System: Created and managed by the operating system.

Types of global queues (in order of decreasing priority):

QoS Level	Description
userInteractive	Tasks that directly affect the user interface (animations, events).
userInitiated	Tasks initiated by the user but not blocking the UI (loading content).
default	Default level without explicit QoS specification.
utility	Tasks that require time to execute but do not immediately affect the UI (loading large files).
background	Tasks performed in the background, not visible to the user (data synchronization).
unspecified	Rarely used when QoS is not important. Equivalent to default.

Accessing global queues:

// Accessing a global queue with a specified QoS level
let userInitiatedQueue = DispatchQueue.global(qos: .userInitiated)

// Accessing the default global queue
let defaultQueue = DispatchQueue.global()

Using global queues for task execution:

// Asynchronous execution of a task in a background queue
DispatchQueue.global(qos: .background).async {
    // Long-running code that does not block the UI
}

// Synchronous execution of a task in a queue (less common for global queues)
DispatchQueue.global(qos: .userInitiated).sync {
    // Code that must be completed before the current thread continues
}

Important notes:

• Tasks that directly affect the user interface should not be executed in global queues (except userInteractive for very fast operations). Use the main queue (DispatchQueue.main) for UI updates.
• Global queues are intended for parallel execution. The order of task execution is not guaranteed.
• Using sync in a global queue will block the current thread until the task completes. This can lead to UI blocking if called from the main thread.