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
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.
Understand Why Changing View Text Does Not Work or is Delayed
When changing the text for a TextView using the setText method new Android developers sometimes fail to understand why it does not appear to work. The text not updating also applies to other Views as well, such as the EditView and Button, why this happens is explained. This article is also useful in helping to understand why the User Interface (UI) in your Android App might not be as responsive as expected. The underlying problem is the same. So if you are trying to find out why your setText() on EditText, TextView, Button, etc. is not working as intended, or you UI is sluggish read on, it includes a work around to help you out.
Not All Android Code Executes Immediately
When changing the TextView text this code is second nature to all Android developers:
((TextView) findViewById(R.id.textView1)).setText("New Text");
For those new to programming the above is equivalent to these two lines but saves having to declare the TextView object:
TextView tv = (TextView) findViewById(R.id.textView1);
When setText(“New Text”) is run the text is not updated immediately. Android is an event based system. Something happens on the device (the screen is touched, a key is pressed, a call comes in, etc.) and Android raises an event. An App is notified of an event and when one occurs that it needs to respond to it does so, often running the code that you have written. Your App runs its code in a loop under the control of the Android Operating Systems (OS). This code loop is referred to as the App’s thread of execution. There is only one thread and it is responsible for both running the App code and updating the display. The setText call posts a message to update the display, so the update does not happen immediately. Once remaining App code has run the UI messages are processed, then the text changes. A running App’s execution thread can be viewed as shown in this simplified diagram. Continue reading