Use of first responders and first responder vs. target-action

Question

I understand the use and need of target-actions. But I encountered this concept of "First Responder". Can someone explain why is it needed? What can it do that can't be done using target-actions?

Answer 1

In an app, the responder object that first receives many kinds of events is known as the first responder. It receives key events, motion events, and action messages, among others. (Mouse events and multitouch events first go to the view that is under the mouse pointer or finger; that view might or might not be the first responder.) The first responder is typically the view in a window that an app deems best suited for handling an event. To receive an event, the responder must also indicate its willingness to become first responder; it does this in different ways for each platform

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.

Touch events. For touch events, the window object first tries to deliver the event to the view where the touch occurred. That view is known as the hit-test view. The process of finding the hit-test view is called hit-testing, which is described in the “Hit-Testing Returns the View Where a Touch Occurred.” doc.

For Motion and remote control events. With these events, the window object sends the shaking-motion or remote control event to the first responder for handling. The first responder is described in “The Responder Chain Is Made Up of Responder Objects.”

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.

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