Determining the Size of an Android View or Screen at Run Time
For efficient bitmap handling or dynamic View creation in an App the area that a widget or layout is using needs to be known. If no fixed sizes are allocated at design time the size of a View may not be known until an App is executed. This is because of the wide range of display sizes that Android supports. The example code snippets in this articles shows how to read the screen size and the size of Views as the App runs. To run the example code you will need to create a new Android project (those new to Android programming can view the article Your First Android Hello World Java Program to see how), we called our App View Size.
In the layout designer for activity_main.xml (or whatever you called your layout) add another TextView, called textXY, next to the existing Hello world! widget. Change the Text on the first TextView to X,Y. Add this code to the oncreate method in MainActivity.java (or whatever class you are using), you will need an imports for TextView and DisplayMetrics. :
//object to store display information
DisplayMetrics metrics = new DisplayMetrics();
//get display information
//show display width and height
This is the code running on an AVD with a 320×480 screen:
He is the layout used for this screen: Continue reading
In Android Get View Defined in XML in Activity Code
The UI screens for an App can be designed outside of the code. They are stored in XML files. This eases support for multiple screen sizes and types and helps with software maintenance. Classes in the Android SDK have methods used to access the UI components. Screens are composed of various implementations of the Android View class. The major sub-classes for Views are widgets and ViewGroups. The widget class is not to be confused with Widgets that can be added to the Android home screen. Instead View widgets are the normal components with which the Android device users interact, including the Button, Checkbox, EditText (a text box), ImageView, RadioButton, ProgressBar, TextView (label) and many more. Several widgets sit in a ViewGroup which provides a container for laying out components. Different ViewGroups provide different types of layouts, including RelativeLayout, LinearLayout, ScrollView, WebView and others. ViewGroups can contain other ViewGroups as well as widgets therefore building complex displays by nesting different Views is possible.
Create a Basic Android Screen
For this tutorial create a new, simple Android App project and call it Button Demo (if you don’t know how see Your First Android Hello World Java Program). A simple screen that just has a button on it was created using the starting layout, we kept the default layout name of activity_main.xml. In Eclipse use the Package Explorer to open the activity_main.xml file (in the res/layout folder). Using the Graphical Layout view (selected via the tabs at the bottom of the editor window) delete the TextView, showing Hello world!, and drag and drop a Button from the Form Widgets folder onto the screen. This screen’s code is stored in the activity_main.xml file:
To show how this view is accessed from the App’s code the text on the button will be changed and it displays a message when it is clicked. Continue reading
Understand How Layouts Center Views
Setting the correct XML attributes to move a ViewGroup or widget (View) to the middle of the App’s display is easier if you know how Android screens are laid out. Everything on the screen is built up in rectangles, starting with the screen itself. A LinearLayout (which is a ViewGroup) sitting on the screen is a rectangle, a TextView in the LinearLayout is a rectangle. Each rectangle has two sets of layout attributes associated with it, attributes that refer to its contents and attributes that refer to its parents layout requirements. The latter is easy to determine because they all start with layout_. The number of layout_ attributes that a widget, such as a TextView, can access will vary depending upon the ViewGroup it is inside:
To help remember that the layout_ attributes refer to the items parent ViewGroup think of it as “attributes that lay outside” of the rectangle.
The two layout_ attributes that are always required are android:layout_width and android:layout_height. These are set to tell the parent ViewGroup how big the rectangle should be to contain the child widget or ViewGroup. These are usually set to either match_parent (previously known as fill_parent) which lets the ViewGroup decide how big the rectangle should be (usually as big as possible), or wrap_content which makes the rectangles just big enough to hold what is inside of them. However, they can also be set to physical dimensions, such as setting layout_height to 40px to make the View’s height 40 pixels high.
When a new Android project is started in Eclipse with a blank Activity some default Hello world! text is displayed in a TextView. What if we wanted to center this in the middle of the screen (note we use the American spelling instead of the UK spelling of centre to match the Android SDK API settings). Looking at the various layout attributes for the TextView it would appear that android:layout_gravity=”center” would work. This though has no effect because the default RelativeLayout arranges child Views based on each others position. Instead to center the TextView set android:gravity=”center” for the RelativeLayout. Because this attribute does not start with layout_ it refers to it contents, in this case the TextView. This time centering works, provided the TextView’s android:layout_width and android:layout_height are set to wrap_content, making it small enough to be moved to the central location.
See also the Layouts article in the developer documentation.
How to Display A Smaller Window on Top of an Activity
The Activity screen is not the only way to interact with the user of an App. A brief message or dialog can be displayed in appropriate circumstances to ask a specific question or get specific input. Android has build in support for such small focused interactions that require immediate attention.
- The Toast class – to display brief informational only message.
- The Dialog class, managed by a DialogFragment – to support a flexible input that can display information and can ask for inputs or choices. There are several built in sub-classes including:
- AlertDialog – supports buttons (from zero to three), list selections, check boxes and radio buttons.
- ProgressDialog – supports a progress bar or wheel and supports buttons because it extends AlertDialog.
- DatePickerDialog – supports selecting a date.
- TimePickerDialog – supports a time selection.
- The PopupWindow class – allows a View to float on top of an existing activity. Suitable for custom informational messages.
These classes build their user interfaces through class methods with support from custom layouts when required. This article looks at using the PopupWindow class. What is the PopupWindow? The Android developer documention has the following class overview:
“A popup window that can be used to display an arbitrary view. The popup window is a floating container that appears on top of the current activity.”
It can be used to display extra information without needing to code another Activity. An example would be to show results at the end of a game (or end of a level), which we will be doing in the following tutorial. The steps for this example code are:
- Create a new project.
- Design the pop-up window.
- Add the code to load and populate the pop-up window.
- Call the code.
Open a Project to Add the Pop-Up Window
In this tutorial we are assuming that you have a working project in Eclipse to use as a base, either open an existing project or create a new one (if you need help see Your First Android Hello World Java Program).
Design the Layout for the Pop-Up
For this example we are going to have a pop-up to display a game’s result as gold, silver and bronze medal positions. The pop-up will have a graphic (ImageView) for each of the medals, a TextView for the winner’s name and one for the score. The images for the medals came from Open Clip Art Library user momoko. They have been resized for an Android project and you can download medals_png.zip ready for importing into a project.