Layout widgets in android

Layout

Что такое Layout

При разработке первых приложений мы уже встречали элемент LinearLayout, который позволяет группировать дочерние элементы в одну линию в автоматическом режиме. Существуют и другие типы контейнеров, позволяющие располагать элементы разными способами. Пришло время познакомиться с ними поближе.

Компоновка (также используются термины разметка или макет) хранится в виде XML-файла в папке /res/layout. Это сделано для того, чтобы отделить код от дизайна, как это принято во многих технологиях (HTML и CSS). Кроме основной компоновки для всего экрана, существуют дочерние элементы компоновки для группы элементов. По сути, компоновка – это некий визуальный шаблон для пользовательского интерфейса вашего приложения, который позволяет управлять элементами управления, их свойствами и расположением. В своей практике вам придется познакомиться со всеми способами размещения. Поэтому здесь мы рассмотрим только базовую часть теории, чтобы вы поняли саму сущность разметки. Кроме того, разметку можно создавать программным способом, который будет описан в конце статьи. Если вы будет обращаться к элементам управления через Java-код, то необходимо присваивать элементам уникальный идентификатор через атрибут android:id. Сам идентификатор назначается через выражение @+id/your_value. После этого вы можете обращаться к элементу через код при помощи метода findViewById(R.id.your_value).

Android Studio включает в себя специальный редактор для создания разметки двумя способами. Редактор имеет две вкладки: одна позволяет увидеть, как будут отображаться элементы управления, а вторая – создавать XML-разметку вручную.

Создавая пользовательский интерфейс в XML-файле, вы можете отделить представление приложения от программного кода. Вы можете изменять пользовательский интерфейс в файле разметки без необходимости изменения вашего программного кода. Например, вы можете создавать XML-разметки для различных ориентаций экрана мобильного устройства (portrait, landscape), размеров экрана и языков интерфейса.

Каждый файл разметки должен содержать только один корневой элемент компоновки, который должен быть объектом View или ViewGroup. Внутри корневого элемента вы можете добавлять дополнительные объекты разметки или виджеты как дочерние элементы, чтобы постепенно формировать иерархию элементов, которую определяет создаваемая разметка.

Виды разметок

Существует несколько стандартных типов разметок:

Все описываемые разметки являются подклассами ViewGroup и наследуют свойства, определённые в классе View.

Комбинирование

Компоновка ведёт себя как элемент управления и их можно группировать. Расположение элементов управления может быть вложенным. Например, вы можете использовать RelativeLayout в LinearLayout и так далее. Но будьте осторожны: слишком большая вложенность элементов управления вызывает проблемы с производительностью.

Можно внедрить готовый файл компоновки в существующую разметку при помощи тега :

Подробнее в отдельной статье Include Other Layout

Программный способ создания разметки

Для подключения созданной разметки используется код в методе onCreate():

Естественно, вы можете придумать и свое имя для файла, а также в приложениях с несколькими экранами у вас будет несколько файлов разметки: game.xml, activity_settings.xml, fragment_about.xml и т.д.

В большинстве случаев вы будете использовать XML-способ задания разметки и подключать его способом, указанным выше. Но, иногда бывают ситуации, когда вам понадобится программный способ (или придётся разбираться с чужим кодом). Вам доступны для работы классы android.widget.LinearLayout, LinearLayout.LayoutParams, а также Android.view.ViewGroup.LayoutParams, ViewGroup.MarginLayoutParams. Вместо стандартного подключения ресурса разметки через метод setContentView(), вы строите содержимое разметки в Java, а затем уже в самом конце передаёте методу setContentView() родительский объект макета:

Число макетов постоянно меняется. Например, недавно появились новые виды CoordinatorLayout и ConstraintLayout. Кроме стандартных элементов разметки существуют и сторонние разработки.

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Android (Home screen) Widgets — Tutorial

Developing Android Widgets. This article describes how to create home screen widgets in Android.

1. Prerequisites

The following description assume that you already have experience in building standard Android application. Please see https://www.vogella.com/tutorials/Android/article.html — Android Tutorial. It also partly uses Android services. You find an introduction into Android Services in https://www.vogella.com/tutorials/AndroidServices/article.html — Android Service Tutorial.

2. Android Widgets

2.1. Overview about AppWidgets

2.2. Home screen widgets

Home screen widgets are broadcast receivers which provide interactive components. They are primarily used on the Android home screen. They typically display some kind of data and allow the user to perform actions with them. For example, a widget can display a short summary of new emails and if the user selects an email, it could start the email application with the selected email.

To avoid confusion with views (which are also called widgets), this text uses the term home screen widgets, if it speaks about widgets.

A widget runs as part of the process of its host. This requires that the widget preserves the permissions of their application.

Widget use RemoteViews to create their user interface. A RemoteView can be executed by another process with the same permissions as the original application. This way the widget runs with the permissions of its defining application.

The user interface for a Widget is defined by a broadcast receiver. This receiver inflates its layout into an object of type RemoteViews . This object is delivered to Android, which hands it over the home screen application.

2.3. Steps to create a Widget

To create a widget, you:

Define a layout file

Create an XML file ( AppWidgetProviderInfo ) which describes the properties of the widget, e.g. size or the fixed update frequency.

Create a BroadcastReceiver which is used to build the user interface of the widget.

Enter the Widget configuration in the AndroidManifest.xml file.

Optional you can specify a configuration activity which is called once a new instance of the widget is added to the widget host.

2.4. Widget size

Before Android 3.1 a widget always took a fixed amount of cells on the home screen. A cell is usually used to display the icon of one application. As a calculation rule you should define the size of the widget with the formula: ((Number of columns / rows) * 74) — 2 . These are device independent pixels and the -2 is used to avoid rounding errors.

As of Android 3.1 a widget can be flexible in size, e.g., the user can make it larger or smaller. To enable this for widget, you can use the android:resizeMode=»horizontal|vertical» attribute in the XML configuration file for the widget.

3. Creating the Broadcast receiver for the widget

3.1. Create and configure widget

To register a widget, you create a broadcast receiver with an intent filter for the android.appwidget.action.APPWIDGET_UPDATE action.

The receiver can get a label and icon assigned. These are used in the list of available widgets in the Android launcher.

You also specify the meta-data for the widget via the android:name=»android.appwidget.provider attribute. The configuration file referred by this metadata contains the configuration settings for the widget. It contains, for example, the update interface, the size and the initial layout of the widget.

3.2. Available views and layouts

A widget is restricted in the View classes it can use. As layouts you can use the FrameLayout , LinearLayout and RelativeLayout classes. As views you can use AnalogClock , Button , Chromometer , ImageButton , ImageView , ProgressBar and TextView .

As of Android 3.0 more views are available: GridView , ListView , StackView , ViewFlipper and AdapterViewFlipper . These adapter views require that you define a collection view widget which is described later in this tutorial.

The only interaction that is possible with the views of a widget is via an OnClickListener event. This OnClickListener can be registered on a widget and is triggered by the user.

3.3. AppWidgetProvider

Your BroadcastReceiver implementation typically extends the AppWidgetProvider class.

The AppWidgetProvider class implements the onReceive() method, extracts the required information and calls the following widget life cycle methods.

As you can add several instances of a widget to the home screen, you have life cycle methods which are called only for the first instance added / removed to the home screen and others which are called for every instance of your widget.

Table 1. Life cycle method

Called the first time an instance of your widget is added to the home screen.

Called once the last instance of your widget is removed from the home screen.

Called for every update of the widget. Contains the ids of appWidgetIds for which an update is needed. Note that this may be all of the AppWidget instances for this provider, or just a subset of them, as stated in the method’s JavaDoc. For example, if more than one widget is added to the home screen, only the last one changes (until reinstall).

Widget instance is removed from the home screen.

3.4. Receiver and asynchronous processing

A widget has the same runtime restrictions as a normal broadcast receiver, i.e., it has only 5 seconds to finish its processing.

A receive (widget) should therefore perform time consuming operations in a service and perform the update of the widgets from the service.

4. Widget updates

A widget gets its data on a periodic timetable. There are two methods to update a widget, one is based on an XML configuration file and the other is based on the Android AlarmManager service.

In the widget configuration file you can specify a fixed update interval. The system will wake up after this time interval and call your broadcast receiver to update the widget. The smallest update interval is 1800000 milliseconds (30 minutes).

The AlarmManager allows you to be more resource efficient and to have a higher frequency of updates. To use this approach, you define a service and schedule this service via the AlarmManager regularly. This service updates the widget.

Please note that a higher update frequency will wake up the phone from the energy safe mode. As a result your widget consumes more energy.

5. Exercise: widget with fixed update interval

5.1. Target

In the following tutorial you create a widget which displays a random number. This random number is updated every 30 minutes. You also register an OnClickListener so that the widgets updates once the user clicks on it.

The resulting widget will look like the following.

5.2. Create project and widget implementation

Create a new Android project called de.vogella.android.widget.example with an activity in the package de.vogella.android.widget.example .

Create a new file myshape.xml in the /res/drawable_ folder. This file defines the drawable used as background in the widget.

Define the following widget_layout.xml file under the res/layout_ folder.

Create a new resource file called widget_info.xml via right click on the res folder and by selecting New Android resource file .

Create the following receiver class which is called during updates.

Open the AndroidManifest.xml and register your widget similar to the following listing.

This attribute specifies that the AppWidgetProvider accepts the ACTION_APPWIDGET_UPDATE broadcast and specifies the metadata for the widget.

5.3. Validate

Deploy your application on your Android device. Once your application has been deployed use the Android launcher to install your new widget on the home screen and test it.

6. Collection View Widgets

Collection view widgets add support for the usage of the ListView , StackView and GridView classes in widgets.

For collection view widgets you need two layout files, one for the widget and one for each item in the widget collection.

The widget items are filled by an instance of the RemoteViewsFactory factory class.

This factory class is provided by an Android service which extends the RemoteViewsService class. This service requires the android.permission.BIND_REMOTEVIEWS permission.

To connect your views with the service, you use your onUpdate() method in your widget implementation.

You define an intent pointing to the service and use the setRemoteAdapter method on the RemoteViews class.

7. Enabling a app widget for the lock Screen

Since Android 4.2, it is possible to add home screen app widgets to the lock screen of an Android device. To enable your widget for the look screen you need to add keyguard category in the android:widgetCategory attribute in the AppWidgetProviderInfo XML file. The following code shows an example.

In this example you declare a widget to support both — the home and the lock screens. If you recompile and launch your application now, you will be able to add the widget to the lock screen already.

You can also detect a widget category at runtime. For this, in the AppWidgetProvider.onUpdate() method, you can check for the category option of a widget with the following code.

Using this technique you can decide at runtime whether the widgets your application provides, will look differently, when they are hosted on the lock screen.

Similarly to how you used the android:initialLayout attribute for defining an initial layout for home screen widgets, you can use a new android:initialKeyguardLayout attribute for the lock screen in the AppWidgetProviderInfo XML file. This layout will appear immediately after a widget is added and will be replaced by the real layout once the widget is initialized.

8. Exercise: Update widget via a service

The following will demonstrate the usage of a service to update the widget.

Create the following UpdateWidgetService class in your project.

Add this class as a Service to your AndroidManifest.xml file.

Change MyWidgetProvider to the following. It will now only construct the service and start it.

Once called, this service will update all widgets. You can click on one of the widgets to update all widgets.

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Method	Description