iOS event response chain and event delivery principle

In iOS applications, one of the core mechanisms for users to interact with the interface is event response. When users touch the screen, click buttons, or perform other operations, the application needs to accurately capture, process, and respond to these events. To achieve this goal, iOS introduces the event response chain and event delivery mechanism. This article will explore the event response chain and event delivery principles of iOS in depth, including its basic concepts, implementation process, code examples, and practical application scenarios.

Step 1: What is the event response chain and event delivery?

The event response chain is a mechanism that passes events from the event source (such as touching the screen) to the object that ultimately handles the event. In iOS, the event source can be user operations such as touch, gesture, button click, etc. Event delivery refers to the process of passing events from one object to another, which involves event capture, delivery, and response.

The event delivery mechanism of iOS is based on the responder chain. Each UI object can become a responder, and they form a responder chain in a certain order. When an event occurs, the system will look for a suitable responder in the responder chain to handle the event. This process is the event delivery.

Step 2: The Principle of Event Response Chain

The principles of the event response chain can be divided into the following key steps:

• An event occurs: The user triggers an event, such as touching the screen or clicking a button.
• Event delivery starts: Events are first sent to the application's main window (UIWindow).
• Find the first responder: The main window starts from its own root view (Root View) and traverses the view hierarchy level by level, looking for the first view that can respond to the event (that is, the view with the userInteractionEnabled property set to YES).
• Event delivery: Once the first responder is found, the event will be passed from the first responder to other objects in the responder chain until the actual event handler is found.
• Event response: When an event reaches the event handler, the event handler will perform corresponding operations, such as executing a method or changing the interface state.
• Event End: Once the event is processed, event delivery ends.

Step 3: Code example of event delivery

Let's use a simple code example to demonstrate the process of event delivery:

 import UIKit class MyView: UIView { override func touchesBegan(_ touches: Set<UITouch>, with event: UIEvent?) { super.touchesBegan(touches, with: event) print("MyView - touchesBegan") } } class ViewController: UIViewController { override func viewDidLoad() { super.viewDidLoad() let myView = MyView(frame: CGRect(x: 100, y: 100, width: 200, height: 200)) myView.backgroundColor = .blue view.addSubview(myView) } override func touchesBegan(_ touches: Set<UITouch>, with event: UIEvent?) { super.touchesBegan(touches, with: event) print("ViewController - touchesBegan") } override func touchesEnded(_ touches: Set<UITouch>, with event: UIEvent?) { super.touchesEnded(touches, with: event) print("ViewController - touchesEnded") } }

In this example, we create a ViewController that contains a blue view. When the user touches the screen, both ViewController and MyView implement the touchesBegan method, so they can both become responders to the event.

When the user touches the screen, the event is first delivered by the main window. Then, because MyView is at the top of the view hierarchy, it becomes the first responder and receives the event. Then, the event is delivered to ViewController, but only the touchesBegan method implemented by MyView is called, so the touchesBegan method of ViewController is not executed.

Step 4: Practical application scenarios of the event response chain

The event response chain is one of the core mechanisms for handling user interactions in iOS applications. The following are some common application scenarios:

• Button click event: When the user clicks a button, the button becomes the first responder and executes the button's click event processing method.
• Gesture recognition: iOS provides gesture recognizers, which can convert various gestures (such as clicks, slides, pinches, etc.) into events, which are then processed by corresponding gesture processors.
• Text input: When the user enters text in a text field, the text field becomes the first responder and handles the text input events.
• Interface control: View controllers are usually used to manage the life cycle and interaction events of the interface. They can handle interface switching, navigation, and user input.
• Custom views: Custom views can implement customized event handling logic such as drawing, animation, and interaction as needed.
• Multi-touch: iOS supports multi-touch, so multiple touch events can be processed simultaneously. The event response chain ensures that each touch point is handled correctly.

Step 5: Notes

When dealing with the iOS event response chain, there are a few things to keep in mind:

• Event bubbling and capture: Event transmission is the process of event bubbling from parent view to child view, while event response is the process of event capture from child view to parent view. Understanding these two directions helps to understand how events are transmitted and responded to.
• Interruption of event delivery: If a responder handles an event and marks it as handled, the event will not be passed on to other responders. This can be used to prevent an event from being handled multiple times.
• Appropriate responder: Ensure that events are delivered to the most appropriate responder to ensure that user actions receive the correct response.
• Multi-touch processing: When processing multi-touch events, you need to consider the position and state of multiple touch points to ensure that multi-touch is handled correctly.
• Performance optimization: The event response chain may involve multiple responders, so attention should be paid to performance optimization to avoid unnecessary event processing.

Step 6: Summary

iOS's event response chain and event delivery mechanism are key components for building user interface interactions. By understanding the event delivery process and the principles of the response chain, developers can better handle user operations and achieve rich user interfaces and interactive experiences.

The event response chain mechanism allows iOS applications to pass events from event sources to final event handlers when users interact with the interface. The flexibility and scalability of this mechanism enable developers to create a variety of complex user interfaces and implement rich interactive behaviors.

A deep understanding of the iOS event response chain and event delivery principle is the key to iOS development. It helps developers better master the implementation of interface interaction and improve the quality of applications and user experience. Whether developing basic button click functions or implementing complex gesture recognition and interface control, understanding the event response chain and event delivery mechanism is crucial.