Transcript Lecture 10 Android Slides

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• First lecture: general introduction
• Touches many aspects of Android programming
• Quite a few times several concepts pop up at the same
time
• Can’t explain everything that comes up in this lecture
but…
• After next lectures everything that comes up today should
have been explained
• “recipes”
• snags
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General purpose OS for handheld devices
Handhelds: resource constrained
• Processing power
• Memory
• Persistent storage
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What is Android?
Linux-based OS for managing processes, memory and devices
Support for functionality common on handhelds: telephony, video,
graphics, GPS, speech etc. through Java libraries
Android SDK supports most of Java Standard Edition (SE) except the
standard java UI frameworks Abstract Windowing Toolkit (AWT) and
Swing. Instead: own UI framework that looks a lot like the java framework
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Bottom layer: Linux core and device drivers
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Second layer: C/C++ (“native”) libraries for functionality like
database access (SQLite) and access to display system (Surface
Manager) 2D/3D
System C library (libc) based on Berkeley Software Distribution,
tuned to half the original size
Android runtime: Dalvik Virtual Machine, a version of the Java VM
optimized for space, performance and battery life
Consequence: NO COFFEEDREGS!
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Top layers: application framework and the actual applications
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Fundamental Android concepts (1): the Nodes of the app
• Activity
– Usually represents single screen in the app and can contain one or more
Views (UI components) – can be seen as a component of a program
– Apps usually consist of several activities – can be seen as a Java
program
– Activities are a standard Java classes that extend the Activity class
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Content provider (not treated – too much additional information needed)
– Standard mechanism to share data among apps
– By declaring a class to extend the abstract class ContentProvider and
declaring a “provider” intent filter, you can expose data to other apps
– Simple example app: TelephoneList
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Service (not treated)
– Process running in the background, comparable to Windows services
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Fundamental Android concepts (2)
• View
– User interface element: basic building blocks of the user interface:
label, textbox, buttons, layout managers, …
– A View is a Java class with a well-defined interface of which we don’t
know its implementation
•
Documentation of all Android-specific classes:
http://developer.android.com/reference/packages.html
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AndroidManifest.xml
– Defines contents and behavior of the app
– Package, version, (write) permissions
– Lists the nodes (activities, services, providers)
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Fundamental Android concepts (3)
• Intent
– Class that is used to give a generic definition of an intention to start
execution of other tasks, e.g.
launch another activity in the same or another app
launch an app that displays a web page
– Android uses the intent to select an activity to start
– If you specify that you want to, e.g., display a web page
Intent i = new Intent(Intent.ACTION_VIEW);
i.setData(Uri.parse("http://www.tue.nl");
this.startActivity(i); // this: calling activity
Android selects an app that will
Start a browser to display TU/e site
If more browsers are installed Android will ask which one to start
– You can also specify explicitly which app should handle the request
– Demo follows later (week 6)
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Fundamental Android concepts (4)
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Android Virtual Devices (AVD) (exercises)
– Allow to execute the code on your PC without attaching a real handheld to
your PC
– Can be created in many different configurations: screen size, memory etc.
– Exam: only AVD may be used, no real devices!
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For those who have no Android device: tablets can be borrowed from BCF
(Asus, if necessary Cherry tablets)
• AVD emulates Advanced RISC Machine (ARM)
• RISC: Reduced Instruction Set Computing:
– simplicity of processor chip design
– speed
achieved by reduced set of instructions, results in “small” chip
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Development: Eclipse with Android plug-in
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In place of DrJava
Execute apps on real device or on AVD
Extensive debugging facilities both for AVD as for real device
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Breakpoints
Step over
Step into
Run to line (go to line where cursor is placed)
Values of attributes, local variables
Call stack
Context sensitive help
Opening Android/Java reference (browser)
Inspect file system of AVD/real device with DDMS perspective
Make sure that your AVD works – ask help if needed!!!
Next slides: “tour” of an Android project and how to accomplish things in
Eclipse until Activities are discussed
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Create an Android project
File->New->Android Application project - starts a wizard
The wizard creates Hello World
– A framework for your project
– a basic framework for your Activity class
– icons for your app
– an XML file that specifies that the screen consists of a tittle bar, a layout
and a label:
the layout is a ViewGroup, a container that can contain other GUI
elements
all GUI elements inherit from the View class
the outermost View is a ViewGroup, unless there is only one View
– string and other resources
– references to libraries needed
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Hello World
Create project with File->New->Android Application project
Later more details about Activities
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“Hello World”
Structure of Eclipse project made with
File->new->Android Application Project
Important parts for us:
src: source code
Android convention: package name reflects
organization (code is organized in packages)
I mostly use nl.tue.win as prefix (following
Google’s recommendation)
gen: generated code
assets: arbitrary folders/files
bin: compiled code
res: resources – icons, strings, colors, screen layouts,…
AndroidManifest.xml: application structure
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Hello.java source code
package nl.tue.win.hello;
import android.os.Bundle;
import android.app.Activity;
import android.view.Menu;
public class Hello extends Activity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.hello);
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
// Inflate the menu; this adds items to the action bar if it is present.
getMenuInflater().inflate(R.menu.hello, menu);
return true;
}
}
No main()! Instead: onCreate(Bundle
Observations?
…)
- Bundle discussed later
R.layout.hello  Layout file for the screen: res/layout/hello.xml
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Hello.java (continued)
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setContentView(R.layout.hello):
– creates the objects defined by the XML file
– sets the screen contents to the outermost
View object in the XML file
– displays the screen
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onCreateOptionsmenu: is called when GUI is
created
the MenuInflater creates the classes for the
menu from the xml file res/menu/hello.xml
menus are not treated in the lectures – not really
complicated to work out how to use
Result of pressing the menu button or pressing
the “menu” symbol (3 squares):
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Creating the Graphical User
Interface (later more detail)
The GUI is built with elements called Views
A simple control is already on the “canvas”: a
label
A label (TextView) is used to display a text to the
user
• Open hello.xml in de res/layout project
folder in Graphical Layout tab
• Righ-click the label in the canvas or in the
outline window
• Select “Edit Text”
• The “Resource Chooser” shows that the
string has name hello_world and has a
value like “Hello world!”
• The string is a “resource” that is referred
to by an identifier
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Creating the Graphical User Interface (continued)
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Click “New String”
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Enter a new value for the string, e.g., “Hello Class”
Enter a new name for the string, e.g., “helloclass”
(don’t enter the quotes in either case!)
From these data, a string is added to R, a class that represents resources
R.string are string resources
Each resource is identified by an integer
The new string is available in your program by
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Resources r = this.getResources();
String s = r.getString(R.string.helloclass);
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Resources
Key role in Android architecture –
Resource
– file (e.g. icon or file that describes layout), e.g., hello.xml
– Value (string, color), e.g. the string named hello or the color white: #ffffffff
– Identified by an ID in an XML file in the res directory:
strings in the directory res/values
pictures in the directories res/drawable, res/drawable/hdpi etc.
layouts in the directory res/layout
– When a file is added/changed, the Eclipse plugin automatically adds to or updates the
file R.java after the file is saved (but sometimes it takes a while).
– R connects the ID in the XML file with an integer that identifies it to Android
– Access to resources e.g. by this.findViewById(R.id.editText1) or
r.getString(R.string.hello) from an activity as shown before
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R.Java (incomplete - comments added):
package tue.win.hello;
public final class R {
.
public static final class drawable {
public static final int ic_launcher=0x7f020000;
}
public static final class id {
public static final int editText1=0x7f050001;
public static final int rl_id=0x7f050000;
}
public static final class layout {
public static final int main=0x7f030000;
}
public static final class string {
public static final int app_name=0x7f040001;
public static final int hello=0x7f040002;
}
}
//outer class R
//inner class R.drawable.
// application icon: R.drawable.ic_launcher
//R.id.editText1
//R.id.rl_id
// main.xml
R.layout.main
//strings.xml
//R.string.hello
: "Hello World! "
In R.java, each resource is thus identified by an integer R.<inner class>.<attribute>
Access to resources in Java: get an instance r of the class Resources from getResources()
Access a resouce with ID id with, e.g., r.getString(int id) or r.getColor(int id)
Alternatively: this.findViewById(R.id.<ID>)
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Properties of Views
All controls in the palette inherit from the View class
Right-click the TextView, shows many built-in properties like Layout width/height and text
color
“Other Properties” has a sub menu
“Defined by TextView” (properties specific for a TextView)
“Inherited from View” (properties that all classes inheriting from View have)
For a label, usually, we only need properties like
text color
font
background color
text direction
…
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Help
Documentation of the Android classes:
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http://developer.android.com/reference/packages.html
Quality varies; sometimes incomplete
Search function:
– if chosen from dropdown list: API reference
– else: general search results page, often from
http://developer.android.com/guide/components/index.html
Context sensitive help in Eclipse: after “.”: list of functions of class (F2 to fix
the pop-up screen) – very useful!
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XML
• Important role in Android applications
• Allows to define multiple layouts with less (Java) code:
– define layouts for both screen orientations
– handle different screen resolutions
– handle different screen sizes
e.g. prepare layouts for different screen sizes and resolutions
select one in the code that fits best to the actual screen
• Manifest
• Support for multi-language (example in week 6)
In general: DO NOT EDIT XML directly, use
the IDE GUI tools instead!!!
Please, no Groundhog day experience for me!
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XML
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We avoid direct use of XML (use graphical facilities of IDE or code instead);
nevertheless a few examples:
– From the file res/values/strings.xml
<string name="hello_world">Hello World!</string>
the string “Hello World!” is identified by the name hello_world
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From the file res/layout/hello.xml the xml for the label
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="@string/hello_world" />
– XML is converted to code on the device itself; can lead to errors that could have
been picked up in the IDE by, e.g., syntax checks if we had coded this in Java
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“Resources” view of the string we created ( in file res/values/strings.xml;
open file to see default “Resources” view):
Basically, this is an editor with which we can avoid
using XML directly
“Graphical layout” view of the string:
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XML view of the layout
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:paddingBottom="@dimen/activity_vertical_margin"
android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
tools:context=".Hello" >
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="@string/hello_world" />
</RelativeLayout>>
RelativeLayout: will place controls relative to parent (the layout) and to each other
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The Manifest Describes properties of the app to Android: application
structure, organizational properties, (write) permissions,…
Tab page 1 – general – usually no need to edit, except Uses Sdk
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• Sometimes needed to set minimum SDK version, in particular for
Gridlayout. New version of SDK puts Min SDK version to 8 and
Gridlayout needs 14
• Current SDK also sets Target SDK version to 18 (for your real device,
you may have to change this and install extra Android packages – will
be explained in Monday’s exercises)
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tab page 2:
Application Structure
Android creates an object of a class
derived from the abstract class
Application
All activities in the app share this
object
Notice:
app_name (banner)
Icon (launcher screen icon)
Intent Filter says activity Hello
can be started as main
is added to launcher screen
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Extend the app with a text box (class EditText) – allows user to
enter text (demo)
Two ways:
1. With the Graphical Designer:
Add the EditText by dragging it (“plain text”) from the palette (tab Text
Fields) to the “canvas” – notice the relation of position with respect to
parent/TextView
Notice: no code is added!
EditText shows up in the res/layout/main.xml file
Using the Graphical Designer in general saves many lines of code
Note:
The name “canvas” is somewhat confusing as there is also a class
Canvas; the canvas here is not a canvas on which one can draw! It is
the canvas of the Graphical Designer only.
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2.
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Add the text box with code
Assign an id to the RelativeLayout (right-click in Outline and choose “Assign ID”,
e.g. rl_id; save layout – we need to get hold of the instance via the id
The RelativeLayout defines how its Views are laid out
In the code: add attributes and add import statements (QuickFix or ctrl-shift-M)
RelativeLayout rl;
EditText et;
In onCreate:
rl= (RelativeLayout)this.findViewById(R.id.rl_id); // notice cast
et = new EditText(this);
rl.addView(et);
Notice: this must be done after the call to setContentView, as the Views are
instantiated during that call! Before the call, findViewById will return a
RelativeLayout, but its value is null…
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Activities
• One screen (usually) and some functionality
• When a new activity is started, Android puts the previously active one on a
stack
• Android builds up a stack of activities that are still “alive” but that don’t have
the focus
• Demo (Icons): what happens if we turn the screen?
• When the screen is turned, Android kills all activities and restarts them
• Likewise, when Android runs low on resources, it will also kill activities
to free resources (harder to demonstrate)
• Next slide: activity lifecycle.
New in Android: Fragments, which have more complicated life
cycle. We decided to only discuss activities.
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Activity life cycle
States:
Created
Started
Activity Running
Paused (partially visible)
Stopped (hidden)
When in the stopped state,
Android may kill the app
App process killed
Activity shut down
Methods shown are called by Android
when transitions occur
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Activity life cycle
When the activity starts for the first time, it comes to the foreground and receives
user focus
Android calls various methods depending on the state changes of the activity. When
the Activity does not override these, the corresponding methods of super are called.
The most important ones:
• onCreate
– Called when activity is first created.
– Also called when activity is restarted after the Stopped state
– Argument of the onCreate method: used to restore critical state information
When called first time argument is null
– When the activity was stopped before, information that was saved in the
Bundle is available (state of Views is saved by super!):
when stopped, not only the method onStop() but also the method
onSaveInstanceState(Bundle state) is called to allow saving state in a
Bundle
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Activity life cycle (continued)
• onPause
– Called when other activity comes in foreground.
– If your app is, e.g., running a video you may want to pause the video.
– One should also stop any action that consumes CPU cycles or uses other
resources (e.g. the camera).
– This is also the point to save unsaved changes (to a database or file)
e.g. when the user is editing a document or an email (although the onStop
method may be more appropriate).
• onResume
Called when activity comes to the foreground again
so here you can, e.g., restart the video, reinitialize the camera, etc.
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Activity life cycle (continued)
• onStop
– Called when the activity may be killed
e.g., when the user receives a phone call or starts another app that hides the
current one.
– Activity should release most of the resources it is using.
– CPU intensive work should be done here instead of in the onPause
method, like saving data in a database.
– Notice: the state of View objects (e.g. EditText) is retained in a Bundle
and handled by the super class – hence the call super.onCreate
– Wild guess: activities that use most memory (or other resources) are killed
first…
Demo (Icons): save and restore state
Other ways to solve problem?