HB Blog 124: Android Multi-Window Mode.

Android 7.0 allows several apps to share the screen at once. For example, a user could split the screen, viewing a web page on the left side while composing an email on the right side. The user experience depends on the device:

• Handheld devices running Android 7.0 offer split-screen mode. In this mode, the system fills the screen with two apps, showing them either side-by-side or one-above-the-other. The user can drag the dividing line separating the two to make one app larger and the other smaller.
• On TV devices, apps can put themselves in picture-in-picture mode, allowing them to continue showing content while the user browses or interacts with other apps.
•  Manufacturers of larger devices can choose to enable freeform mode, in which the user can freely resize each activity. If the manufacturer enables this feature, the device offers freeform mode in addition to split-screen mode.

The user can switch into multi-window mode in the following ways:

• If the user opens the Overview screen and performs a long press on an activity title, they can drag that activity to a highlighted portion of the screen to put the activity in multi-window mode.
• If the user performs a long press on the Overview button, the device puts the current activity in multi-window mode, and opens the Overview screen to let the user choose another activity to share the screen.

Users can drag and drop data from one activity to another while the activities are sharing the screen. 

Multi-window mode does not change the activity lifecycle. In multi-window mode, only the activity the user has most recently interacted with is active at a given time. This activity is considered topmost. All other activities are in the paused state, even if they are visible. However, the system gives these paused-but-visible activities higher priority than activities that are not visible. If the user interacts with one of the paused activities, that activity is resumed, and the previously topmost activity is paused. When the user puts an app into multi-window mode, the system notifies the activity of a configuration change, as specified in Handling Runtime Changes. This also happens when the user resizes the app, or puts the app back into full-screen mode. Essentially, this change has the same activity-lifecycle implications as when the system notifies the app that the device has switched from portrait to landscape mode, except that the device dimensions are changed instead of just being swapped. As discussed in Handling Runtime Changes, your activity can handle the configuration change itself, or it can allow the system to destroy the activity and recreate it with the new dimensions. If the user is resizing a window and makes it larger in either dimension, the system resizes the activity to match the user action and issues runtime changes as needed. If the app lags behind in drawing in newly-exposed areas, the system temporarily fills those areas with the color specified by the windowBackground attribute or by the default windowBackgroundFallback style attribute.