The U.S. Patent and Trademark Office granted Apple U.S. Patent No. 8,291,344 for a "Device, method, and graphical user interface for managing concurrently open software applications," otherwise known as multitasking.
While switching between apps running in the background has become part of the iOS experience, and possibly taken for granted for those new to the operating system, the feature wasn't available until version 4.0 of what was then called the iPhone OS.
Apple's first steps into multitasking actually came with app switching on the iPad in early 2010, though at the time, the system still limited third party apps to run in the background. It was only until iPhone OS 4.0 that true multitasking was allowed for apps made by outside developers.
Apple was well aware of the constraints seen in iPhone OS, especially as titles in the App Store grew exponentially, and in September of 2010 filed a patent application outlining its solution to bring multitasking to its mobile platform.
From the '344 patent's background:
For portable electronic devices, existing methods for managing concurrently open applications are cumbersome and inefficient. For example, portable devices with small screens (e.g., smart phones and other pocket-sized devices) typically display a single application at a time, even though multiple applications may be running on the device. With such devices, a user may have difficulty seeing and managing the currently open applications. This situation creates a significant cognitive burden on a user. In addition, existing methods for managing currently open applications take longer than necessary, thereby wasting energy. This latter consideration is particularly important in battery-operated devices.
The comprehensive patent goes on to describe a number of embodiments, each focusing on a device first displaying an open application, which is then replaced by a second open app that takes the place of the first. A slew of user interface options are described, including a number of solutions resembling Apple's Time Machine "three-dimensional stack" GUI, except with representations of open apps.
Illustration of "three-dimensional stack" UI.
In another embodiment, the app windows are downsized and represented as a linear strip that can be scrolled through, much like mobile Safari's multi-pane GUI.
Illustration of scrollable linear strip embodiment.
Ultimately, the patent describes the system iOS users have come to know, which is the double tapping of an iDevice's home button, which brings up a row of icons on the bottom of the screen. Each icon in the multitasking bar represents a recently opened app, and can be scrolled through and selected as wanted by the user.
The '344 patent also accommodates gesture-driven multitasking, as is seen on the iPad when a four-finger swipe up reveals the multitasking bar. Basically every instance and embodiment seen in modern iOS iterations, meaning iOS 4 or later, are covered in the invention, including the multitasking bar's orientation rotation lock and music player controls.
Illustration of multitasking bar device controls.
It should be noted that the Exposé-like iOS multitasking bar does not technically represent a list of running apps, but instead shows those programs that were recently opened by the user. Background tasks are still managed by the OS itself, and usually have a max time limit in which to complete a task. For example, a podcast downloader may be run in the background while a user is browsing the web, but if the selected media does not download in the specified amount of time, iOS will suspend the app's CPU access. There are instances when this time limit does not apply, such as when music player apps or first-party software are functioning.