The Activity class is a crucial component of an Android app, and the way activities are launched and put together is a fundamental part of the platform's application model. Unlike programming paradigms in which apps are launched with a main method, the Android system initiates code in an Activity instance by invoking specific callback methods that correspond to specific stages of its lifecycle.
This document introduces the concept of activities, and then provides some lightweight guidance about how to work with them. For additional information about best practices in architecting your app, see Guide to App Architecture. The mobile-app experience differs from its desktop counterpart in that a user's interaction with the app doesn't always begin in the same place.
How to reload activity in Android?
Instead, the user journey often begins non-deterministically. For instance, if you open an email app from your home screen, you might see a list of emails. By contrast, if you are using a social media app that then launches your email app, you might go directly to the email app's screen for composing an email.
The Activity class is designed to facilitate this paradigm. When one app invokes another, the calling app invokes an activity in the other app, rather than the app as an atomic whole. In this way, the activity serves as the entry point for an app's interaction with the user. You implement an activity as a subclass of the Activity class. An activity provides the window in which the app draws its UI. This window typically fills the screen, but may be smaller than the screen and float on top of other windows.
Generally, one activity implements one screen in an app. Most apps contain multiple screens, which means they comprise multiple activities. Typically, one activity in an app is specified as the main activitywhich is the first screen to appear when the user launches the app.
Each activity can then start another activity in order to perform different actions. For example, the main activity in a simple e-mail app may provide the screen that shows an e-mail inbox. From there, the main activity might launch other activities that provide screens for tasks like writing e-mails and opening individual e-mails. Although activities work together to form a cohesive user experience in an app, each activity is only loosely bound to the other activities; there are usually minimal dependencies among the activities in an app.
Stack Overflow for Teams is a private, secure spot for you and your coworkers to find and share information. How do I call the method startChronometer in another class when the method is declared inside the main activity? All Activities in Android must go through the Activity lifecycle so that they have a valid context attached to them.
By treating an Activity as a normal Java class, you end up with a null context.
As most methods in an Activity are called on its Context, you will get a null pointer exception, which is why your app crashes. Instead, move all such methods which need to be called from other classes into a Utility class which accepts a valid context in its constructor, and then use that context in the methods to do the work.
Thats because, you never initialized your MainActivity. You have to pass instance of MainActivity into another class, then you can call everything public in MainActivity from everywhere. What i have done and it works is create an instance in the MainActivity and getter for that instance:. What I have done with no memory leaks or lint warnings is to use f. Learn more. How to call a method in MainActivity from another class? Ask Question. Asked 7 years ago. Active 3 months ago. Viewed 91k times.
May I know what more I need to add to the code? Vasilis G. What you're trying to do reeks of bad design and tight coupling. You should have activity classes only for activity related purposes.Here is some code from an AndroidManifest. With minor tweaks, this same code could accompany lots of examples.
In addition, the device executes several other methods. On most Android devices, the user sees the Home screen. When the user taps the icon, the app starts running. So there you have it. With the proper secret sauce namely, the action and category elements in the AndroidManifest. Often, the words extends and Override tell an important story — a story that applies to all Java programs, not only to Android apps.
AppCompatActivity class, you create a new kind of Android activity. The folks at Google have already written thousands of lines of Java code to describe what an Android AppCompatActivity can do. So a MainActivity is automatically a screenful of components with lots and lots of handy, prewritten code.
Of course, in some apps, you might not want all that prewritten code. Instead, the MainActivity contains a declaration for its own onCreate method. In particular, the onCreate method calls setContentView R. Every Android activity has a lifecycle — a set of stages that the activity undergoes from birth to death to rebirth, and so on. You can declare your own onCreate method without declaring your own onStart and onResume methods.
So, one complete sweep of your activity, from birth to death, involves the run of at least six methods: onCreatethen onStartand then onResumeand later onPaus e, and then onStopand, finally, onDestroy. For an Android activity, reincarnation is a common phenomenon.
In this case, Android can kill some running activities. A call to super. He hosts podcasts and videos about software and other technology topics. He is the author of many articles and books, including Java For Dummies.Very similar way, Android system initiates its program with in an Activity starting with a call on onCreate callback method.
There is a sequence of callback methods that start up an activity and a sequence of callback methods that tear down an activity as shown in the below Activity life cycle diagram: image courtesy : android. The Activity class defines the following call backs i. You don't need to implement all the callbacks methods.
However, it's important that you understand each one and implement those that ensure your app behaves the way users expect. The paused activity does not receive user input and cannot execute any code and called when the current activity is being paused and the previous activity is being resumed. This example will take you through simple steps to show Android application activity life cycle.
This file includes each of the fundamental life cycle methods. The Log. An application can have one or more activities without any restrictions. Every activity you define for your application must be declared in your AndroidManifest.
Let's try to run our modified Hello World! I assume you had created your AVD while doing environment setup. To run the app from Android studio, open one of your project's activity files and click Run icon from the toolbar. Let us try to click lock screen button on the Android emulator and it will generate following events messages in LogCat window in android studio:. Let us again try to unlock your screen on the Android emulator and it will generate following events messages in LogCat window in Android studio:.
Next, let us again try to click Back button on the Android emulator and it will generate following events messages in LogCat window in Android studio and this completes the Activity Life Cycle for an Android Application. Android - Activities Advertisements. Previous Page. Next Page. Previous Page Print Page. Dashboard Logout. You will use Android studio to create an Android application and name it as HelloWorld under a package com.
Modify main activity file MainActivity. Keep rest of the files unchanged. Run the application to launch Android emulator and verify the result of the changes done in the application.I am using DependencyService to perform a Facebook login in each platform. In my android implementation class I use the I use the Xamarin.
So the login is performed through:. Where this is an argument of class Android. AdamP thank you for response. It is definitely useful. But for my solution it was enough to use: Xamarin. Context as MainActivity.
Yes, there is. The plugin mentioned above does exactly this but I don't think you should need to use a Nuget plugin where a single class would do. Add a MainApplication to your Android project:. There is no need to refer to it, just put it among your source files where your main activity is, too. You can refer to the static MainApplication.
An Activity represents a single screen in your app with which your user can perform a single, focused task such as taking a photo, sending an email, or viewing a map. An activity is usually presented to the user as a full-screen window.
An app usually consists of multiple screens that are loosely bound to each other. Each screen is an activity. Typically, one activity in an app is specified as the "main" activity MainActivity. The main activity can then start other activities to perform different actions. Each time a new activity starts, the previous activity is stopped, but the system preserves the activity in a stack the "back stack".
When a new activity starts, that new activity is pushed onto the back stack and takes user focus. The back stack follows basic "last in, first out" stack logic.
When the user is done with the current activity and presses the Back button, that activity is popped from the stack and destroyed, and the previous activity resumes. An activity is started or activated with an intent. An Intent is an asynchronous message that you can use in your activity to request an action from another activity, or from some other app component.
You use an intent to start one activity from another activity, and to pass data between activities. In this practical you create explicit intents. You find out how to use implicit intents in a later practical. In this chapter you create and build an app called Two Activities that, unsurprisingly, contains two Activity implementations. You build the app in three stages. In the first stage, you create an app whose main activity contains one button, Send.
When the user clicks this button, your main activity uses an intent to start the second activity. In the second stage, you add an EditText view to the main activity. The user enters a message and clicks Send.
The main activity uses an intent to start the second activity and send the user's message to the second activity. The second activity displays the message it received. In the final stage of creating the Two Activities app, you add an EditText and a Reply button to the second activity. The user can now type a reply message and tap Replyand the reply is displayed on the main activity.
At this point, you use an intent to pass the reply back from the second activity to the main activity. In this task you set up the initial project with a main Activitydefine the layout, and define a skeleton method for the onClick button event. Name your app Two Activities and choose the same Phone and Tablet settings that you used in previous practicals.
The project folder is automatically named TwoActivitiesand the app name that appears in the app bar will be "Two Activities". The attribute value is underlined in red because the launchSecondActivity method has not yet been created. Ignore this error for now; you fix it in the next task. In this task you implement the launchSecondActivity method you referred to in the layout for the android:onClick attribute. Each new activity you add to your project has its own layout and Java files, separate from those of the main activity.
As with the main activity, new activity implementations that you create in Android Studio also extend from the AppCompatActivity class. Each activity in your app is only loosely connected with other activities. However, you can define an activity as a parent of another activity in the AndroidManifest.
This parent-child relationship enables Android to add navigation hints such as left-facing arrows in the title bar for each activity.You will get the most value out of this course if you work through the codelabs in sequence:. Apps often include settings that allow users to modify app features and behaviors. For example, some apps allow the user to set their home location, default units for measurements, and other settings that apply to the entire app.
Users don't access settings frequently, because once a user changes a setting, such as a home location, they rarely need to go back and change it again. Users expect to navigate to app settings by tapping Settings in side navigation, such as a navigation drawer as shown on the left side of the figure below, or in the options menu in the app bar, shown on the right side of the figure below. In this practical you add a settings activity to an app.
Users will be able to navigate to the app settings by tapping Settingswhich will be located in the options menu in the app bar. Android Studio provides a shortcut for setting up an options menu with Settings.
Android App Development with Java: All About Android Activities
If you start an Android Studio project for a phone or tablet using the Basic Activity template, the new app includes Settings as shown below:. The template also includes a floating action button in the lower right corner of the screen with an envelope icon. You can ignore this button for this practical, as you won't be using it. You'll start by creating an app named AppWithSettings using the Basic Activity template, and you'll add a settings Activity that provides one toggle switch setting that the user can turn on or off:.
You will add code to read the setting and perform an action based on its value. For the sake of simplicity, the action will be to display a Toast message with the value of the setting.
In the second task, you will add the standard Settings Activity template provided by Android Studio to the DroidCafeOptionsUp app you created in a previous lesson. The Settings Activity template is pre-populated with settings you can customize for an app, and provides a different layout for phones and tablets:. To customize the template, you'll change the headers, setting titles, setting descriptions, and values for the settings.
The DroidCafeOptionsUp app was created in a previous lesson from the Basic Activity template, which provides an options menu in the app bar for placing the Settings option.
You will customize the supplied Settings Activity template by changing a single setting's title, description, values, and default values. You will add code to read the setting's value after the user changes it, and display that value. In order to use the PreferenceFragmentCompat version of PreferenceFragmentyou must also use the android.
The statement shown above adds the android. The SwitchPreferenceCompat line above may show a yellow light bulb icon with a warning, but you can ignore it for now. In order to use PreferenceFragmentCompatyou must also declare preferenceTheme with the PreferenceThemeOverlay style to the app theme. In order to create a settings Activity that provides a UI for settings, add an Empty Activity to the app. Follow these steps:.
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