As a user navigates through, out of, and back to your app, the Show
Within the lifecycle callback methods, you can declare how your activity behaves when the user leaves and re-enters the activity. For example, if you're building a streaming video player, you might pause the video and terminate the network connection when the user switches to another app. When the user returns, you can reconnect to the network and allow the user to resume the video from the same spot. In other words, each callback allows you to perform specific work that's appropriate to a given change of state. Doing the right work at the right time and handling transitions properly make your app more robust and performant. For example, good implementation of the lifecycle callbacks can help ensure that your app avoids:
This document explains the activity lifecycle in detail. The document begins by describing the lifecycle paradigm. Next, it explains each of the callbacks: what happens internally while they execute, and what you should implement during them. It then briefly introduces the relationship between activity state and a process’s vulnerability to being killed by the system. Last, it discusses several topics related to transitions between activity states. For information about handling lifecycles, including guidance about best practices, see Handling Lifecycles with Lifecycle-Aware Components and Saving UI States. To learn how to architect a robust, production-quality app using activities in combination with architecture components, see Guide to App Architecture. Activity-lifecycle concepts To navigate transitions between stages of the activity lifecycle, the Activity class provides a core set of six callbacks: Figure 1 presents a visual representation of this paradigm. Figure 1. A simplified illustration of the activity lifecycle. As the user begins to leave the activity, the system calls methods to dismantle the activity. In some cases, this dismantlement is only partial; the activity still resides in memory (such as when the user switches to another app), and can still come back to the foreground. If the user returns to that activity, the activity resumes from where the user left off. With a few exceptions, apps are restricted from starting activities when running in the background. The system’s likelihood of killing a given process—along with the activities in it—depends on the state of the activity at the time. Activity state and ejection from memory provides more information on the relationship between state and vulnerability to ejection. Depending on the complexity of your activity, you probably don't need to implement all the lifecycle methods. However, it's important that you understand each one and implement those that ensure your app behaves the way users expect. The next section of this document provides detail on the callbacks that you use to handle transitions between states. Lifecycle callbacksThis section provides conceptual and implementation information about the callback methods used during the activity lifecycle. Some actions, such as calling onCreate() You
must implement this callback, which fires when the system first creates the activity. On activity creation, the activity enters the Created state. In the If you have a lifecycle-aware component that is hooked up to the lifecycle of your activity it will receive the
The following example of the
As an alternative to defining the XML file and passing it to Your activity does not reside in the Created state. After the onStart() When the activity enters the Started state, the system invokes this callback. The When the activity moves to the started state, any lifecycle-aware component tied to the activity's lifecycle will receive the
The onResume() When the activity enters the Resumed state,
it comes to the foreground, and then the system invokes the When the activity moves to the resumed state, any lifecycle-aware component tied to the activity's lifecycle will receive the
When an interruptive event occurs, the activity enters the Paused state, and the system invokes the If the activity returns to the Resumed state from
the Paused state, the system once again calls Here is an example of a lifecycle-aware component that accesses the camera when the component receives the
The code above initializes the camera once the Regardless of which build-up event you choose to perform an initialization operation in, make sure to use the corresponding lifecycle event to release the resource. If you initialize something after the ON_START event, release or terminate it after the ON_STOP event. If you initialize after the ON_RESUME event, release after the ON_PAUSE event. Note, the code snippet above places camera initialization code in a lifecycle aware component. You can instead put this code directly into the activity lifecycle callbacks such as onPause() The system calls this method as the first indication that
the user is leaving your activity (though it does not always mean the activity is being destroyed); it indicates that the activity is no longer in the foreground (though it may still be visible if the user is in multi-window mode). Use the
When the activity moves to the paused state, any lifecycle-aware component tied to the activity's lifecycle will receive the
You can also use the The following example of a
Note, the code snippet above places camera release code after the ON_PAUSE event is received by the LifecycleObserver. As previously mentioned, see Handling Lifecycles with Lifecycle-Aware Components to learn how to create a lifecycle-aware component. Completion of the onStop() When your activity is no longer
visible to the user, it has entered the Stopped state, and the system invokes the When the activity moves to the stopped state, any lifecycle-aware component tied to the activity's lifecycle will receive the
In the You should also use
Note, the code sample above uses SQLite directly. You should instead use Room, a persistence library that provides an abstraction layer over SQLite. To learn more about the benefits of using Room, and how to implement Room in your app, see the Room Persistence Library guide. When your activity enters the Stopped state, the Note: Once your activity is stopped, the system might destroy the process that contains the activity if the system needs to recover memory. Even if the system destroys the process while the activity is stopped, the system still retains the state of the From the Stopped state, the activity either comes back to interact with the user, or the activity is finished running and goes away. If the activity comes back, the system invokes onDestroy()
When the activity moves to the destroyed state, any lifecycle-aware component tied to the activity's lifecycle will receive the
Instead of putting logic in your Activity to determine why it is being destroyed you should use a You can
distinguish between these two scenarios with the If the activity is finishing, onDestroy() is the final lifecycle callback the activity receives. If onDestroy() is called as the result of a configuration change, the system immediately creates a new activity instance and then calls
The Activity state and ejection from memoryThe system kills processes when it needs to free up RAM; the likelihood of the system killing a given process depends on the state of the process at the time. Process state, in turn, depends on the state of the activity running in the process. Table 1 shows the correlation among process state, activity state, and likelihood of the system’s killing the process. Note: This table only applies if a process is not running other types of application components.
Table 1. Relationship between process lifecycle and activity state The system never kills an activity directly to free up memory. Instead, it kills the process in which the activity runs, destroying not only the activity but everything else running in the process, as well. To learn how to preserve and restore your activity's UI state when system-initiated process death occurs, see Saving and restoring activity state. A user can also kill a process by using the Application Manager under Settings to kill the corresponding app. For more information about processes in general, see Processes and Threads. For more information about how the lifecycle of a process is tied to the states of the activities in it, see the Process Lifecycle section of that page. Saving and restoring transient UI stateA user expects an activity’s UI state to remain the same throughout a configuration change, such as rotation or switching into multi-window mode. However, the system destroys the activity by default when such a configuration change occurs, wiping away any UI state stored in the activity instance. Similarly, a user expects UI state to remain the same if they temporarily switch away from your app to a different app and then come back to your app later. However, the system may destroy your application’s process while the user is away and your activity is stopped. When the activity is destroyed due to system constraints, you should preserve the user’s
transient UI state using a combination of This section outlines what instance state is and how to implement the onSaveInstance() method, which is a callback on the activity itself. If your UI data is simple and lightweight, such as a primitive data type or a simple object (like String), you can use onSaveInstanceState() alone to persist the UI state across both configuration changes and system-initiated process death. In most cases, though, you should use both ViewModel and onSaveInstanceState() (as outlined in Saving UI State) since onSaveInstanceState() incurs serialization/deserialization costs. Instance state There are a few scenarios in which your activity is destroyed due to normal app behavior, such as when the user presses
the Back button or your activity signals its own destruction by calling the However, if the system destroys the activity due to system constraints (such as a configuration change or memory pressure), then although the actual The saved data that the system uses to restore the previous state is called the instance state and is a collection of key-value pairs stored in a Note: In order for the Android system to restore the state of the views in your activity, each view must have a unique ID, supplied by the A Save simple, lightweight UI state using onSaveInstanceState() As your activity begins to stop, the system calls the
To save additional instance state information for your activity, you must override
Note:
To save persistent data, such as user preferences or data for a database, you should take appropriate opportunities when your activity is in the foreground. If no such opportunity arises, you should save such data during the
Restore activity UI state using saved instance state When your activity is recreated after it was previously destroyed, you can recover your saved instance state from the
Because the For example, the following code snippet shows how you can restore some state data in
Instead of
restoring the state during
Caution:
Always call the superclass implementation of Navigating between activitiesAn app is likely to enter and exit an activity, perhaps many times, during the app’s lifetime. For example, the user may tap the device’s Back button, or the activity may need to launch a different activity. This section covers topics you need to know to implement successful activity transitions. These topics include starting an activity from another activity, saving activity state, and restoring activity state. Starting one activity from anotherAn activity often needs to start another activity at some point. This need arises, for instance, when an app needs to move from the current screen to a new one. Depending on whether your activity wants a result back from the new activity it’s about to start, you start the new activity using either the The startActivity() If the newly started activity does not need to return a result, the current activity can start it by calling the When working within your own application, you often need to simply launch a known activity. For example, the following code snippet shows how to launch an activity called
Your application might also want to perform some action, such as send an email, text message, or status update, using data from your activity. In this case, your application might not have its own activities to perform such actions, so you can instead leverage the activities provided by other applications on the device, which can perform the actions for you. This is where intents are really valuable: You can create an intent that describes an action you want to perform and the system launches the appropriate activity from another application. If there are multiple activities that can handle the intent, then the user can select which one to use. For example, if you want to allow the user to send an email message, you can create the following intent:
The startActivityForResult() Sometimes you want to get a result back from an activity when it ends. For example, you may start an activity that lets the user pick a person in a list of contacts; when it ends, it returns the person that was selected. To do this, you
call the When a child activity exits, it can call If a child activity fails for any reason, such as crashing, the parent activity receives a result with the code
Coordinating activitiesWhen one activity starts another, they both experience lifecycle transitions. The first activity stops operating and enters the Paused or Stopped state, while the other activity is created. In case these activities share data saved to disc or elsewhere, it's important to understand that the first activity is not completely stopped before the second one is created. Rather, the process of starting the second one overlaps with the process of stopping the first one. The order of lifecycle callbacks is well defined, particularly when the two activities are in the same process (app) and one is starting the other. Here's the order of operations that occur when Activity A starts Activity B:
This predictable sequence of lifecycle callbacks allows you to manage the transition of information from one activity to another. Which method of production describes the process of one item manufactured at a time?Batch Production: Custom homes Makes one product at a time.
Which method of production describes the process where groups of items go through stages of production together until finished?Batch production is a manufacturing method where sets of identical goods go through different production stages together, i.e. the production process creates one batch of finished goods at a time.
What are CAD CAM and JIT?4 CAD – Computer Aided Design drawing with computers CAM – Computer Aided Machining machines making things from computer drawings CIM –Computer Integrated Manufacturing both of the above working together JIT – just in time Materials arriving at the work place just in time EPOS – Electronic point of ...
Is a form of computer aided production that builds a three dimensional object by adding layers of material one upon the other until an object is created?3D printing, also known as additive manufacturing, is a method of creating a three dimensional object layer-by-layer using a computer created design. 3D printing is an additive process whereby layers of material are built up to create a 3D part.
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