Programming with Android: Layouts, Widgets and Events. Dipartimento di Scienze dell Informazione Università di Bologna

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1 Programming with Android: Layouts, Widgets and Events Luca Bedogni Marco Di Felice Dipartimento di Scienze dell Informazione Università di Bologna

Event Listeners Event Management: Event Handlers Luca Bedogni,

2 Outline Components of an Activity ViewGroup: definition ViewGroup: examples Widget: definition Widget: examples Event Management: Event Listeners Event Management: Event Handlers Luca Bedogni, Marco Di Felice - Programming with Android ViewGroups, Views and Events 2

3 Android Applications Design Ø Developing an Android Application means using in a proper way the Android basic components Activity Fragment Intent Service Layout Views Content Providers Broadcast Receiver Luca Bedogni, Marco Di Felice - Programming with Android ViewGroups, Views and Events 3

4 Android: Views and Layouts Components of the User Interface (UI) of an Activity ViewGroup View ² View container ² Responsible for placing other Views on the display ² Every layout must extend a ViewGroup ² Basic component of the UI ² Can handle/produce events ² Every new UI component must extend a View Luca Bedogni, Marco Di Felice - Programming with Android ViewGroups, Views and Events 4

5 Android: ViewGroups ViewGroup à define the placement of the Views. Ø Defined in Java code (within the Activity file) Ø Defined in XML (layout file) ² Use XML tags to place a ViewGroup ² Place a View within a ViewGroup using these XML attributes android:layout_width android:layout_height } match_parent wrap_content Luca Bedogni, Marco Di Felice - Programming with Android ViewGroups, Views and Events 5

6 Android: ViewGroups <?xml version="1.0" encoding="utf- 8"?> ACTIVITY_MAIN.XML <LinearLayout xmlns:android=" android:layout_width="fill_parent" android:layout_height="fill_parent" android:orientation= vertical" > <Button android:layout_width= match_parent" android:layout_height="wrap_content" /> <Button android:layout_width="wrap_content" android:layout_height="wrap_content" /> </LinearLayout> Luca Bedogni, Marco Di Felice - Programming with Android ViewGroups, Views and Events 6

7 Android: ViewGroups <?xml version="1.0" encoding="utf- 8"?> <LinearLayout xmlns:android=" android:layout_width="fill_parent" android:layout_height="fill_parent" android:orientation= vertical" > <Button android:layout_width= match_parent" android:layout_height="wrap_content" /> <Button android:layout_width="wrap_content" android:layout_height= match_parent" /> </LinearLayout> Luca Bedogni, Marco Di Felice - Programming with Android ViewGroups, Views and Events 7 ACTIVITY_MAIN.XML

8 Android: ViewGroups List of most common Layouts provided by Android Name XML Tag Description LinearLayout RelativeLayout TableLayout FrameLayout AbsoluteLayout <LinearLayout> </LinearLayout> <RelativeLayout> </RelativeLayout> <TableLayout> </TableLayout> <FrameLayout> </FrameLayout> <AbsoluteLayout> </AbsoluteLayout> arrange Views by aligning them on a single row or column arrange Views through relative positions arrange Views into rows and columns arrange a single View within a Layout arrange Views through absolute positions ² A Layout can be declared within another Layout Luca Bedogni, Marco Di Felice - Programming with Android ViewGroups, Views and Events 8

9 Android: ViewGroups List of most common Layouts provided by Android Name XML Tag Description LinearLayout RelativeLayout TableLayout FrameLayout AbsoluteLayout <LinearLayout> </LinearLayout> <RelativeLayout> </RelativeLayout> <TableLayout> </TableLayout> <FrameLayout> </FrameLayout> <AbsoluteLayout> </AbsoluteLayout> arrange Views by aligning them on a single row or column arrange Views through relative positions arrange Views into rows and columns arrange a single View within a Layout arrange Views through absolute positions ² A Layout can be declared within another Layout Luca Bedogni, Marco Di Felice - Programming with Android ViewGroups, Views and Events 9

10 Android: ViewGroups LinearLayout à view group that aligns all children in a single direction, vertically or horizontally. ² Orientation can be declared through XML tag android:orientation= HORIZONTAL VERTICAL ² Orientation can also be declared in Java through setorientation(int orientation) ² Views has also other two attributes: gravity weigth Align the View with its parent How much space is assigned to the View Luca Bedogni, Marco Di Felice - Programming with Android ViewGroups, Views and Events 10

11 Android: ViewGroups ORIENTATION VERTICAL ORIENTATION HORIZONTAL Luca Bedogni, Marco Di Felice - Programming with Android ViewGroups, Views and Events 11

12 Android: ViewGroups <?xml version="1.0" encoding="utf- 8"?> <LinearLayout xmlns:android=" android:layout_width="fill_parent" android:layout_height="fill_parent" android:orientation="vertical" > <!- - Also horizontal à <Button android:id="@+id/button1" android:layout_width="wrap_content" android:layout_height="wrap_content" android:text="@string/buttonstring1" /> <Button android:id="@+id/button2" android:layout_width="wrap_content" android:layout_height="wrap_content" android:text="@string/buttonstring2" /> </LinearLayout> Luca Bedogni, Marco Di Felice - Programming with Android ViewGroups, Views and Events 12

13 Android: ViewGroups ² For each View in a LinearLayout, we can set the android:weight XML property (integer value) Importance of a View, how much it can expand <LinearLayout> <Button android:layout_width=match_parent android:weight=1/> <Button android:layout_width=match_parent android:weight=1/> </LinearLayout> Luca Bedogni, Marco Di Felice - Programming with Android ViewGroups, Views and Events 13

14 Android: ViewGroups ² For each View in a LinearLayout, we can set the android:weight XML property (integer value) Importance of a View, how much it can expand <LinearLayout> <Button android:layout_width=match_parent android:weight=1/> <Button android:layout_width=match_parent android:weight=2/> </LinearLayout> Luca Bedogni, Marco Di Felice - Programming with Android ViewGroups, Views and Events 14

15 Android: ViewGroups ² For each View in a LinearLayout, we can set the layout_gravity XML property Align a View with its parent (LinearLayout) <LinearLayout> <Button android:layout_width=match_parent android:weigth=1/> <Button android:layout_width=match_parent android:layout_gravity= center_vertical android:weight=2/> </LinearLayout> Luca Bedogni, Marco Di Felice - Programming with Android ViewGroups, Views and Events 15

16 Android: ViewGroups List of most common Layouts provided by Android Name XML Tag Description LinearLayout RelativeLayout TableLayout FrameLayout AbsoluteLayout <LinearLayout> </LinearLayout> <RelativeLayout> </RelativeLayout> <TableLayout> </TableLayout> <FrameLayout> </FrameLayout> <AbsoluteLayout> </AbsoluteLayout> arrange Views by aligning them on a single row or column arrange Views through relative positions arrange Views into rows and columns arrange a single View within a Layout arrange Views through absolute positions ² A Layout can be declared within another Layout Luca Bedogni, Marco Di Felice - Programming with Android ViewGroups, Views and Events 16

17 Android: ViewGroups RelativeLayout à View group that displays all Views based on relative positions. RELATIVE= COMPARED to the PARENT LAYOUT android:alignparentbottom= true false android:alignparenttop= true false android:alignparentleft= true false android:alignparentright= true false android:alignparentstart= true false android:alignparentend= true false Luca Bedogni, Marco Di Felice - Programming with Android ViewGroups, Views and Events 17

RELATIVE= COMPARED to SIBLING VIEWs android:toleftof= ID android:torightof= ID

18 Android: ViewGroups RelativeLayout à View group that displays all Views based on relative positions. RELATIVE= COMPARED to SIBLING VIEWs android:toleftof= ID android:torightof= ID android:tostartof= ID android:toendof= ID XML Identifier of the View used as reference point Luca Bedogni, Marco Di Felice - Programming with Android ViewGroups, Views and Events 18

19 Android: ViewGroups <?xml version="1.0" encoding="utf- 8"?> <RelativeLayout xmlns:android=" android:layout_width="match_parent" android:layout_height="match_parent" > ACTIVITY_MAIN.XML <EditText android:text="username" android:inputtype="text" android:layout_width="wrap_content" android:layout_height="wrap_content" android:layout_alignparentright="true" > </EditText> <TextView android:layout_width="wrap_content" android:layout_height="wrap_content" android:text="username" /> CONTINUE Luca Bedogni, Marco Di Felice - Programming with Android ViewGroups, Views and Events 19

Android: ViewGroups <EditText android:id="@+id/password" android:text="password" android:inputtype="textpassword" android:layout_below="@+id/username" android:layout_width="wrap_content"

20 Android: ViewGroups <EditText android:text="password" android:inputtype="textpassword" android:layout_width="wrap_content" android:layout_height="wrap_content" android:layout_alignparentright="true" > </EditText> ACTIVITY_MAIN.XML <TextView android:layout_width="wrap_content" android:layout_height="wrap_content" android:text="password" /> </RelativeLayout> Luca Bedogni, Marco Di Felice - Programming with Android ViewGroups, Views and Events 20

21 Android: ViewGroups List of most common Layouts provided by Android Name XML Tag Description LinearLayout RelativeLayout TableLayout FrameLayout AbsoluteLayout <LinearLayout> </LinearLayout> <RelativeLayout> </RelativeLayout> <TableLayout> </TableLayout> <FrameLayout> </FrameLayout> <AbsoluteLayout> </AbsoluteLayout> arrange Views by aligning them on a single row or column arrange Views through relative positions arrange Views into rows and columns arrange a single View within a Layout arrange Views through absolute positions ² A Layout can be declared within another Layout Luca Bedogni, Marco Di Felice - Programming with Android ViewGroups, Views and Events 21

22 Android: ViewGroups TableLayout à View group that arranges all Views into rows and columns (as an HTML table). PROPERTIES 1. Consists of a list of TableRaw objects, each defyining a raw of the Table. 2. The width of a column is defined by the row with the widest cell in that column. 3. Cells can be empty, or can span multiple columns (like in HTML). 4. Border lines of the cells are not displayed. Luca Bedogni, Marco Di Felice - Programming with Android ViewGroups, Views and Events 22

23 Android: ViewGroups TableLayout à View group that arranges all Views into rows and columns (as an HTML table). ADDITIONAL XML attributes android:layout_column android:layout_span android:collapsecolumn android:shrinkcolumns android:stretchcolumns } Apply only to width layout_width is always WRAP_CONTENT layout_height is WRAP_CONTENT (default) Luca Bedogni, Marco Di Felice - Programming with Android ViewGroups, Views and Events 23

24 Android: ViewGroups <?xml version="1.0" encoding="utf- 8"?> <TableLayout android:layout_width="fill_parent" android:layout_height="fill_parent" xmlns:android=schemas.android.com/apk/res/android > <TableRow android:layout_width="wrap_content" android:layout_height="wrap_content <Button android:layout_width="wrap_content android:layout_height="wrap_content android:text="button" /> <Button android:layout_width="match_parent" android:layout_height="match_parent" android:text="button" /> <Button android:layout_width="match_parent" android:layout_height="match_parent" android:text="button" /> </TableRow> Luca Bedogni, Marco Di Felice - Programming with Android ViewGroups, Views and Events 24 ACTIVITY_MAIN.XML CONTINUE

25 Android: ViewGroups <TableRow android:layout_width="wrap_content" android:layout_height="wrap_content" ACTIVITY_MAIN.XML <Button android:layout_column="1" android:layout_span="2" android:layout_width="wrap_content" android:layout_height="wrap_content" android:text="button"> </Button> </TableRow> </TableLayout> Luca Bedogni, Marco Di Felice - Programming with Android ViewGroups, Views and Events 25

26 Android: ViewGroups List of most common Layouts provided by Android Name XML Tag Description LinearLayout RelativeLayout TableLayout FrameLayout AbsoluteLayout <LinearLayout> </LinearLayout> <RelativeLayout> </RelativeLayout> <TableLayout> </TableLayout> <FrameLayout> </FrameLayout> <AbsoluteLayout> </AbsoluteLayout> arrange Views by aligning them on a single row or column arrange Views through relative positions arrange Views into rows and columns arrange a single View within a Layout arrange Views through absolute positions ² A Layout can be declared within another Layout Luca Bedogni, Marco Di Felice - Programming with Android ViewGroups, Views and Events 26

27 Android: ViewGroups FrameLayout à Block out an area on the screen to display a single item (i.e. a single View). ² It should be used to display a single View within the Layout ² Multiple views can be controlled through android:layout_gravity AbsoluteLayout à Arrange Views on the screen by specifying absolute x-y positions of each View. ² Deprecated, since it is dependant of the screen resolution Luca Bedogni, Marco Di Felice - Programming with Android ViewGroups, Views and Events 27

28 Android: Views and Layouts Components of the User Interface (UI) of an Activity ViewGroup View ² View container. ² Responsible for placing other Views on the display ² Every layout must extend a ViewGroup ² Basic component of the UI ² Can handle/produce events ² Every new UI component must extend a View Luca Bedogni, Marco Di Felice - Programming with Android ViewGroups, Views and Events 28

Android: Views objects Views à basic building blocks for user interface components ² Rectangular area of the screen ² Responsible for drawing ² Responsible for event handling

29 Android: Views objects Views à basic building blocks for user interface components ² Rectangular area of the screen ² Responsible for drawing ² Responsible for event handling EXAMPLEs of VIEWS objects: GoogleMap WebView Widgets à topic of the day User-defined Views Luca Bedogni, Marco Di Felice - Programming with Android ViewGroups, Views and Events 29

30 Android: Views objects Widgetà Pre-defined interactive UI components (android.view.widgets) TextView EditText Button ToggleButton Spinner DataPicker Luca Bedogni, Marco Di Felice - Programming with Android ViewGroups, Views and Events 30

31 Widgets: Java and XML code Widgets can be defined in the XML layout files < TextView android:layout_width="match_parent" android:layout_height="wrap_content android:visibility="visible android:enabled="true android:scrollbars="vertical android:text= Hello World /> PROPERTIES, defined through android: attributes Luca Bedogni, Marco Di Felice - Programming with Android (c) ViewGroups, Luca Bedogni 2012 Views and Events 31

32 Widgets: Java and XML code Widgets can be defined in XML and accessed from Java < TextView /> XML public TextView text; text=(textview)findviewbyid(r.id.name1); JAVA CAST REQUIRED public TextView text; text=new TextView(); Luca Bedogni, Marco Di Felice - Programming with Android (c) ViewGroups, Luca Bedogni 2012 Views and Events 32

33 Widgets: Java and XML code ² Each Widget can have a focus and a visibility, based on the user s interaction. Ø The user can force a focus to a specific component through the requestfocus() method. Ø The user can modify the visibility of a specific component through the setvisibility(int) method. public TextView text; text=(textview) findviewbyid(r.id.name1); text.setvisibility(true) text.requestfocus(); Luca Bedogni, Marco Di Felice - Programming with Android (c) ViewGroups, Luca Bedogni 2012 Views and Events 33

34 Widgets: Hierarchy of the classes Ø Widgets are organized on a hierarchy of classes View TextView ImageView EditView Button CheckedTextView AutoCompleteTextView CompoundButton CheckBox RadioButton ToggleButton Luca Bedogni, Marco Di Felice - Programming with Android (c) ViewGroups, Luca Bedogni 2012 Views and Events 34

35 Widgets: TextView ² XML tags: <TextView> </TextView> Ø Could be filled with strings or HTML markups Ø Not directly editable by users Ø Usually used to display static informations <TextView android:layout_width="wrap_content" android:layout_height="wrap_content /> Luca Bedogni, Marco Di Felice - Programming with Android ViewGroups, Views and Events 35

Widgets: TextView methods Ø Methods to place some texts inside a TextView ² public void settext(charsequence text) ² public CharSequence gettext() ² public void setsingleline(boolean singleline) ²

36 Widgets: TextView methods Ø Methods to place some texts inside a TextView ² public void settext(charsequence text) ² public CharSequence gettext() ² public void setsingleline(boolean singleline) ² public void sethorizontallyscrolling(boolean enable) ² public void setlines(int lines) ² public void setellipsize(textutils.truncateat where) ² public void sethints(charsequence hints) ² TextUtils.TruncateAt.END ² TextUtils.TruncateAt.MARQUEE ² TextUtils.TruncateAt.MIDDLE ² TextUtils.TruncateAt.START Luca Bedogni, Marco Di Felice - Programming with Android (c) ViewGroups, Luca Bedogni 2012 Views and Events 36

37 Widgets: Linkify elements ² Simple strings could be linkified automatically. Ø How? Pick a normal string, and use Linkify.addLinks() to define the kind of links to be created. Ø Could manage: Web addresses, s, phone numbers, Maps TextView textview=(textview) findviewbyid(r.id.output); Linkify.addLinks(textView, Linkify.WEB_URLS Linkify.WEB_ADDRESSES Linkify.PHONE_NUMBERS ); Linkify.addLinks(textView, Linkify.ALL); ² It is possible to define custom Linkify objects... Luca Bedogni, Marco Di Felice - Programming with Android (c) ViewGroups, Luca Bedogni 2012 Views and Events 37

38 Widgets: EditText ² XML tags: <EditText> </EditText> Ø Similar to a TextView, but editable by the users Ø An appropriate keyboard will be displayed <EditText android:id="@+id/edittext android:inputtype= textcapsentences textcapwords textautocorrect textpassword textmultilane /> Luca Bedogni, Marco Di Felice - Programming with Android ViewGroups, Views and Events 38

39 Widgets: Button ² XML tags: <Button> </Button> Ø Subclass of a TextView, but not directly editable by users Ø Can generate events related to click, long click, etc <Button android:text="@string/textbutton" android:id="@+id/idbutton" android:background="@color/blue /> <selector> <item android:color="#ff android:state_pressed="true > </item> </selector> ² CompoundButton: Button + state (checked/unchecked) Luca Bedogni, Marco Di Felice - Programming with Android (c) ViewGroups, Luca Bedogni 2012 Views and Events 39 res/color/blue.xml

40 Widgets: Button and CompoundButton checkbox CompoundButton XML tags: <checkbox> <checkbox android:layout_width="wrap_content android:layout_height="wrap_content android:text="checkbox android:checked="true /> Luca Bedogni, Marco Di Felice - Programming with Android (c) ViewGroups, Luca Bedogni 2012 Views and Events 40

41 Widgets: Button and CompoundButton checkbox CompoundButton ² public boolean ischecked(): Returns true if the button is checked, false otherwise. ² public boolean setchecked(bool) Listener: oncheckedchangelistener Luca Bedogni, Marco Di Felice - Programming with Android (c) ViewGroups, Luca Bedogni 2012 Views and Events 41

42 Widgets: Button and CompoundButton radiobutton CompoundButton XML tags: <RadioButton> <RadioButton android:layout_width="wrap_content android:layout_height="wrap_content android:text="buttonradio android:checked="true /> Luca Bedogni, Marco Di Felice - Programming with Android (c) ViewGroups, Luca Bedogni 2012 Views and Events 42

43 Widgets: Button and CompoundButton radiobutton CompoundButton ² Define multiple (mutualexclusive) options through a <RadioGroup> tag. ² Only one button can be checked within the same RadioGroup. Listener: OnCheckedChangeListener Luca Bedogni, Marco Di Felice - Programming with Android (c) ViewGroups, Luca Bedogni 2012 Views and Events 43

44 Widgets: Button and CompoundButton <RadioGroup android:layout_width="wrap_content" android:layout_height="wrap_content" android:orientation="vertical > <RadioButton android:layout_width="wrap_content" android:layout_height="wrap_content" android:text="option 1" android:checked="true /> <RadioButton android:layout_width="wrap_content" android:layout_height="wrap_content" android:text="option 2 /> </RadioGroup> Luca Bedogni, Marco Di Felice - Programming with Android (c) ViewGroups, Luca Bedogni 2012 Views and Events 44

45 Widgets: Button and CompoundButton togglebutton CompoundButton ² It can assume only 2 states: checked/unchecked ² Different labels for the states with: android:texton and android:textoff XML attributes. Listener: OnCheckedChangeListener Luca Bedogni, Marco Di Felice - Programming with Android (c) ViewGroups, Luca Bedogni 2012 Views and Events 45

Widgets: Button and CompoundButton togglebutton CompoundButton XML tags: <ToggleButton> <ToggleButton android:layout_width="wrap_content android:layout_height="wrap_content

46 Widgets: Button and CompoundButton togglebutton CompoundButton XML tags: <ToggleButton> <ToggleButton android:layout_width="wrap_content android:layout_height="wrap_content android:texton="button ON android:textoff="button OFF android:checked="false /> Luca Bedogni, Marco Di Felice - Programming with Android (c) ViewGroups, Luca Bedogni 2012 Views and Events 46

47 Widgets: Spinners Spinner component XML tags: <Spinner> ² Provides a quick way to select values from a specific set. ² The spinner value-set can be defined in XML (through the entries tag) or through the SpinnerAdapter in Java Listener: OnItemSelectedListener Luca Bedogni, Marco Di Felice - Programming with Android (c) ViewGroups, Luca Bedogni 2012 Views and Events 47

Widgets: Spinners Spinner component XML tags: <Spinner> <resources> <string-array name="stringoptions"> <item>option 1</item> <item>option 2</item> <item>option 3</item> <item>option 4</item>

48 Widgets: Spinners Spinner component XML tags: <Spinner> <resources> <string-array name="stringoptions"> <item>option 1</item> <item>option 2</item> <item>option 3</item> <item>option 4</item> </string-array> </resources> res/values.xml <Spinner android:layout_width="wrap_content" android:layout_height="wrap_content" /> Luca Bedogni, Marco Di Felice - Programming with Android (c) ViewGroups, Luca Bedogni 2012 Views and Events 48

49 Widgets: Button and CompoundButton DataPicker component XML tags: <DataPicker> <DatePicker android:layout_width="wrap_content android:layout_height="wrap_content android:endyear="1990 android:startyear="2014 android:maxdate="10/10/2014 /> Luca Bedogni, Marco Di Felice - Programming with Android (c) ViewGroups, Luca Bedogni 2012 Views and Events 49

50 Widgets: ImageView ² ImageView: subclass of View object. ² Some methods to manipulate an image: void setscaletype(enum scaletype) void setalpha(double alpha) void setcolorfilter(colorfilter color) CENTER, CENTER_CROP, CENTER_INSIDE, FIT_CENTER, FIT_END, FIT_START, FIT_XY, MATRIX Luca Bedogni, Marco Di Felice - Programming with Android (c) ViewGroups, Luca Bedogni 2012 Views and Events 50

Widgets: ImageView ImageView component XML tags: <ImageView> <ImageView android:layout_width="wrap_content android:layout_height="wrap_content android:id="@+id/imageid

51 Widgets: ImageView ImageView component XML tags: <ImageView> <ImageView android:layout_width="wrap_content android:layout_height="wrap_content /> Source: android.jpg in drawable/ Luca Bedogni, Marco Di Felice - Programming with Android (c) ViewGroups, Luca Bedogni 2012 Views and Events 51

52 Views and Events Views/Widgets are interactive components à Upon certain action from the user, an appropriate event will be fired click, long click, focus, items selected, items checked,drag, How to handle the events generated by a View? 1. Directly from XML 2. Through Event Listeners (general, recommended) 3. Through Event Handlers (general) Luca Bedogni, Marco Di Felice - Programming with Android (c) ViewGroups, Luca Bedogni 2012 Views and Events 52

53 Views and Events ² For a limited set of components, it is possible to manage the events through callbacks indicated in the XML layout. <Button android:text="@string/textbutton android:id="@+id/idbutton android:onclick= dosomething /> XML Layout File Java code public void dosomething(view w) { // Code to manage the click event } Luca Bedogni, Marco Di Felice - Programming with Android (c) ViewGroups, Luca Bedogni 2012 Views and Events 53

54 Views and Events Views/Widgets are interactive components à Upon certain action from the user, an appropriate event will be fired click, long click, focus, items selected, items checked,drag, How to handle the events generated by a View? 1. Directly from XML 2. Through Event Listeners (general, recommended) 3. Through Event Handlers (general) Luca Bedogni, Marco Di Felice - Programming with Android (c) ViewGroups, Luca Bedogni 2012 Views and Events 54

55 Views and Events Each View contains a collection of nested interfaces (listeners). Ø Each listener handles a single event Ø Each listener contains a single callback method Ø The callback is invoked at occurrence of the event. Button interface OnClickListener public abstract void onclick(view v) {} Luca Bedogni, Marco Di Felice - Programming with Android (c) ViewGroups, Luca Bedogni 2012 Views and Events 55

56 Views and Events: ActionListener To handle events through the ActionListener mechanism: 1. Make the Activity implement the Listener Interface corrisponding to the Event generated by the View (Click Event in this example) public class ExampleActivity extends Activity implements OnClickListener { } Luca Bedogni, Marco Di Felice - Programming with Android ViewGroups, Views and Events 56

57 Views and Events: ActionListener To handle events through the ActionListener mechanism: 2. Implement the abstract method of the Listener à this will be the callback method when the Event is fired public void onclick(view w) { // Place here the code to manage the event } (Click Event in this example) Luca Bedogni, Marco Di Felice - Programming with Android ViewGroups, Views and Events 57

58 Views and Events: ActionListener To handle events through the ActionListener mechanism: 3. Associate the ActionListener to the View through the method seteventlistener(this) Button mybutton=(button) findviewbyid(r.id.button1); mybutton.setonclicklistener(this); (Click Event in this example) Luca Bedogni, Marco Di Felice - Programming with Android ViewGroups, Views and Events 58

59 Views and Events: ActionListener LIST OF ACTIONLISTENER INTERFACES Ø interface OnClickListener abstract method: onclick() Ø interface OnLongClickListener abstract method: onlongclick() Ø interface OnFocusChangeListener abstract method: onfocuschange() Ø interface OnKeyListener abstract method: onkey() Ø interface OnCheckedChangeListener abstract method: oncheckedchanged() Ø interface OnItemSelectedListener abstract method: onitemselected() Ø interface OnCheckedChangeListener abstract method: oncheckedchanged() Ø interface OnItemSelectedListener abstract method: onitemselected() Ø interface OnTouchListener abstract method: ontouch() Ø interface OnCreateContextMenuListener abstract method: oncreatecontextmenu() Luca Bedogni, Marco Di Felice - Programming with Android (c) ViewGroups, Luca Bedogni 2012 Views and Events 59

Views and Events: ActionListener It is also possible to fire an event directly from the Java code (without user s interaction) useful for debugging!

60 Views and Events: ActionListener It is also possible to fire an event directly from the Java code (without user s interaction) useful for debugging! ² Can be done through performevent() events ² The corresponding listener (if set) will be invoked Button button=(button)findviewbyid(r.id.buttonnext); button.performclick(); (c) Luca Bedogni 2012 Luca Bedogni, Marco Di Felice Events and Widgets // Callback method public void onclick(view v) {. } 60

61 Views and Events Views/Widgets are interactive components à Upon certain action from the user, an appropriate event will be fired click, long click, focus, items selected, items checked,drag, How to handle the events generated by a View? 1. Directly from XML 2. Through Event Listeners (general, recommended) 3. Through Event Handlers (general) Luca Bedogni, Marco Di Felice - Programming with Android (c) ViewGroups, Luca Bedogni 2012 Views and Events 61

Views and Events: Event Handlers Event Handlers à Some Views have callback methods to handle specific events (example: When a Button is touched à ontouchevent() called) PROBLEM: to intercept an

62 Views and Events: Event Handlers Event Handlers à Some Views have callback methods to handle specific events (example: When a Button is touched à ontouchevent() called) PROBLEM: to intercept an event, you must extend the View class and override the callback method not very practical! IN PRACTICE: ² Events Handlers are used for custom (user-defined) components ² Events Listeners are used for common View/Widget components Luca Bedogni, Marco Di Felice - Programming with Android (c) ViewGroups, Luca Bedogni 2012 Views and Events 62

Programming with Android: Widgets and Events. Luca Bedogni. Dipartimento di Scienze dell Informazione Università di Bologna

Programming with Android: Widgets and Events Luca Bedogni Dipartimento di Scienze dell Informazione Università di Bologna Outline What is a Widget? Widget: TextView and EditText Widget: Button and CompoundButton