Event Delivery: The Responder Chain

When you design your app, it’s likely that you want to respond to events dynamically. For example, a touch can occur in many different objects onscreen, and you have to decide which object you want to respond to a given event and understand how that object receives the event.

When a user-generated event occurs, UIKit creates an event object containing the information needed to process the event. Then it places the event object in the active app’s event queue. For touch events, that object is a set of touches packaged in a UIEvent object. For motion events, the event object varies depending on which framework you use and what type of motion event you are interested in.

An event travels along a specific path until it is delivered to an object that can handle it. First, the singleton UIApplication object takes an event from the top of the queue and dispatches it for handling. Typically, it sends the event to the app’s key window object, which passes the event to an initial object for handling. The initial object depends on the type of event.

The ultimate goal of these event paths is to find an object that can handle and respond to an event. Therefore, UIKit first sends the event to the object that is best suited to handle the event. For touch events, that object is the hit-test view, and for other events, that object is the first responder. The following sections explain in more detail how the hit-test view and first responder objects are determined.

Hit-Testing Returns the View Where a Touch Occurred

iOS uses hit-testing to find the view that is under a touch. Hit-testing involves checking whether a touch is within the bounds of any relevant view objects. If it is, it recursively checks all of that view’s subviews. The lowest view in the view hierarchy that contains the touch point becomes the hit-test view. After iOS determines the hit-test view, it passes the touch event to that view for handling.

To illustrate, suppose that the user touches view E in Figure 2-1. iOS finds the hit-test view by checking the subviews in this order:

  1. The touch is within the bounds of view A, so it checks subviews B and C.

  2. The touch is not within the bounds of view B, but it’s within the bounds of view C, so it checks subviews D and E.

  3. The touch is not within the bounds of view D, but it’s within the bounds of view E.

    View E is the lowest view in the view hierarchy that contains the touch, so it becomes the hit-test view.

Figure 2-1  Hit-testing returns the subview that was touched

The hitTest:withEvent: method returns the hit test view for a given CGPoint and UIEvent. The hitTest:withEvent: method begins by calling the pointInside:withEvent: method on itself. If the point passed into hitTest:withEvent: is inside the bounds of the view, pointInside:withEvent: returns YES. Then, the method recursively calls hitTest:withEvent: on every subview that returns YES.

If the point passed into hitTest:withEvent: is not inside the bounds of the view, the first call to the pointInside:withEvent: method returns NO, the point is ignored, and hitTest:withEvent: returns nil. If a subview returns NO, that whole branch of the view hierarchy is ignored, because if the touch did not occur in that subview, it also did not occur in any of that subview’s subviews. This means that any point in a subview that is outside of its superview can’t receive touch events because the touch point has to be within the bounds of the superview and the subview. This can occur if the subview’s clipsToBounds property is set to NO.

The hit-test view is given the first opportunity to handle a touch event. If the hit-test view cannot handle an event, the event travels up that view’s chain of responders as described in “The Responder Chain Is Made Up of Responder Objects” until the system finds an object that can handle it.

The Responder Chain Is Made Up of Responder Objects

Many types of events rely on a responder chain for event delivery. The responder chain is a series of linked responder objects. It starts with the first responder and ends with the application object. If the first responder cannot handle an event, it forwards the event to the next responder in the responder chain.

A responder object is an object that can respond to and handle events. The UIResponder class is the base class for all responder objects, and it defines the programmatic interface not only for event handling but also for common responder behavior. Instances of the UIApplication, UIViewController, and UIView classes are responders, which means that all views and most key controller objects are responders. Note that Core Animation layers are not responders.

The first responder is designated to receive events first. Typically, the first responder is a view object. An object becomes the first responder by doing two things:

  1. Overriding the canBecomeFirstResponder method to return YES.

  2. Receiving a becomeFirstResponder message. If necessary, an object can send itself this message.

Events are not the only objects that rely on the responder chain. The responder chain is used in all of the following:

UIKit automatically sets the text field or text view that a user taps to be the first responder; Apps must explicitly set all other first responder objects with the becomeFirstResponder method.

The Responder Chain Follows a Specific Delivery Path

If the initial object—either the hit-test view or the first responder—doesn’t handle an event, UIKit passes the event to the next responder in the chain. Each responder decides whether it wants to handle the event or pass it along to its own next responder by calling the nextResponder method.This process continues until a responder object either handles the event or there are no more responders.

The responder chain sequence begins when iOS detects an event and passes it to an initial object, which is typically a view. The initial view has the first opportunity to handle an event. Figure 2-2 shows two different event delivery paths for two app configurations. An app’s event delivery path depends on its specific construction, but all event delivery paths adhere to the same heuristics.

Figure 2-2  The responder chain on iOS

For the app on the left, the event follows this path:

  1. The initial view attempts to handle the event or message. If it can’t handle the event, it passes the event to its superview, because the initial view is not the top most view in its view controller’s view hierarchy.

  2. The superview attempts to handle the event. If the superview can’t handle the event, it passes the event to its superview, because it is still not the top most view in the view hierarchy.

  3. The topmost view in the view controller’s view hierarchy attempts to handle the event. If the topmost view can’t handle the event, it passes the event to its view controller.

  4. The view controller attempts to handle the event, and if it can’t, passes the event to the window.

  5. If the window object can’t handle the event, it passes the event to the singleton app object.

  6. If the app object can’t handle the event, it discards the event.

The app on the right follows a slightly different path, but all event delivery paths follow these heuristics:

  1. A view passes an event up its view controller’s view hierarchy until it reaches the topmost view.

  2. The topmost view passes the event to its view controller.

  3. The view controller passes the event to its topmost view’s superview.

    Steps 1-3 repeat until the event reaches the root view controller.

  4. The root view controller passes the event to the window object.

  5. The window passes the event to the app object.