Intents
Ilio Catallo, Eleonora Ciceri – Politecnico di Milano
ilio.catallo@polimi.it, eleonora.ciceri@polimi.it
Activities are tasks
¤ Each Activity should be associated with a specific task
2
view
activity
I can take
pictures
from t...
Applications are collection of
activities
¤ Applications are made of different activities, each of
which with its own res...
Activities should cooperate
¤ From time to time, an activity may be required to perform
a task out of its set of responsi...
Activities should cooperate
¤ Wouldn’t it be great if Activities could cooperate?
5
view
activity
view
activity
I’ll back...
Activities should cooperate
¤ Wouldn’t it be great if Activities could cooperate?
6
view
activity
view
activity
Here’s yo...
Intents
7
Intents
¤ Intents are messages that are passed between
components
8
intent
view
activity
activity
view
Intents
¤ A component may ask another component to execute
some action on its behalf
9
intent
view
activity
activity
view
Intents
¤ The message contains a description of the action to be
performed
10
intent
view
activity
activity
view
Intents
¤ As such, an intent always involves two subjects
11
intent
view
activity
activity
view
Intents
¤ The sender, which wants an action to be performed and
thus publishes a message
12
intent
activity
view
view
act...
Intents
¤ The receiver, which is any component capable of
performing the requested action
13
intent
view
activity
view
ac...
What Intents are good at
¤ Actions can be of different type:
¤ Taking pictures from the camera
¤ Starting a new web sea...
ExampleofAction
15
Intent
Action: play video
Browser application YouTube application
Types of Intents
16
Two ways of launching Activities...
¤ The calling Activity has only two ways of launching a
second Activity
17
view
activ...
...by name
¤ By name: The calling Activity knows the exact name of
the Activity to launch
18
view
activity 1
view
activit...
...or by action
¤ By action: The calling Activity wants something to be
done (e.g., taking a picture)
19
view
activity 1
...
Implicit and Explicit intents
¤ Consequently, Android provides two kinds of Intents:
¤ Explicit intents, which specify t...
Intent resolution mechanism
¤ For implicit intents, the system determines which Activity is
best to run based on the Inte...
Intent resolution mechanism
22
Activity 1
Activity 2
Intent
Resolution
Explicit Intent:
“Execute Activity3”
Implicit Inten...
Launching new Activities
23
Launching a new Activity
¤ An Activity asks for the intervention of a second Activity
by invoking on itself the startActi...
Launching a new Activity
¤ An Activity asks for the intervention of a second Activity
by invoking on itself the startActi...
Launching a new Activity
¤ Activity class:
26
public class Activity {
public Activity() {...}
public void startActivity(I...
Launching a new Activity
¤ First, the calling Activity forges an Intent object that
contains the description of the neede...
Launching a new Activity
¤ Once ready, the Intent object is passed to
startActivity()
28
public void startActivity(Intent...
Launching a new Activity
¤ Once the startActivity() method is invoked
¤ Implicit intents: the resolution mechanism ident...
30
How to create Intents?
Intent class
¤ Intent class:
31
public class Intent {
public Intent(Context context, Class<?> cls) {...}
public Intent(St...
Creating explicit Intents
¤ Explicit Intents are typically used for launching new Activities
within the same application
...
Creating implicit Intents
¤ Each implicit Intent contains information useful to the
receiver for performing the job:
¤ A...
Adding extra parameters
¤ It is possible to pass some parameters to the started
Activity by adding them as extra content ...
Receiving an Intent
35
Receiving an Intent
¤ Within the just-started Activity, you can inspect the
received Intent by using the getIntent() meth...
Receiving implicit Intents
¤ How to inform the system that an Activity can handle a
specific type of implict Intent?
¤ T...
Intent filter
¤ An Intent filter:
¤ declares the capability of an Activity of receiving implicit
Intents of a specific t...
Example:
dialing a number
¤ In order to declare an Intent filter, you need to add an
<intent-filter> element in your mani...
Example:
dialing a number
¤ Everytime an implicit intent of type
android.intent.action.DIAL is casted:
¤ Our Activity is...
Example:
launching an app
¤ Look in your AndroidManifest.xml file:
¤ You will find an intent filter associated with the ...
Returning data from an Activity
42
Requesting a result from an Activity
¤ Starting another Activity does not have to be one-way
¤ You can start a second Ac...
Requesting a result from an Activity
44
Activity 1
Camera
Activity
Implicit Intent:
“Take a picture”
Calling (first)
Activ...
Activity class
¤ Activity class:
45
public class Activity {
public Activity() {...}
public void startActivity(Intent inte...
Requesting a result from an Activity
¤ The Intent you pass is the same you could use to start
an Activity without requiri...
Returned data
¤ Once the user is done with the called Activity, the result
will be sent back to the first Activity
¤ Alo...
Returned data
48
Activity 1
Camera
Activity
(android.media.action.IMAGE_CAPTURE,
TAKE_PICTURE*)
Calling (first)
Activity
C...
Activity class
¤ Activity class:
49
public class Activity {
public Activity() {...}
public void startActivity(Intent inte...
Processing the returned data
¤ When the second Activity returns, the system invokes the
onActivityResult() method on the ...
Role of the request code
¤ The calling Activity uses different request codes to
distinguish between different types of re...
Role of the request code
52
Activity 1
Activity 4
Activity 3
Activity 2
Request code: 1
Request code: 2
Request code: 3
Role of the request code
53
Request code: 1
Activity 2
Activity 1
Activity 4
Activity 3
The data come
from Activity 2, as
...
Returning data from an Activity
¤ In case the called Activity is part of our application, we
have to deal also with the p...
Performing the operation in the
called Activity
¤ An Activity represents a task the user can achieve
¤ To this end, it m...
Packing the result into an Intent
object
¤ The called Activity produces a result of some kind
¤ These data can be packed...
Activity class
¤ Activity class:
57
public class Activity {
public Activity() {...}
public void startActivity(Intent inte...
Determining the result code
¤ The operation performed in the called Activity may
succeed or fail, and the resulting data ...
Terminating the called Activity
¤ The Intent containing the result will not be fired until the
called Activity is running...
Terminating the called Activity
¤ Usually, it is up to the Operating System to decide when
to terminate an Activity
¤ Ho...
Activity class
¤ Activity class:
61
public class Activity {
public Activity() {...}
public void startActivity(Intent inte...
TakeNotes V3
62
TakeNotes v3:
require a new to do
¤ When the + button is clicked, a new to do is required
¤ The input form containing th...
TakeNotes v3:
input the new to do
¤ The interface is built in the layout file (res/layout/)
64
<LinearLayout [...]>
<Edit...
TakeNotes v3:
returning the result
¤ When the user clicks on the button, the to-do item has to
return to the first activi...
TakeNotes v3:
returning the result
66
public void addToDoToList(View view) {
Intent intent = new Intent(this, ToDoListActi...
@Override
public void onActivityResult(int requestCode, int resultCode, Intent intent) {
super.onActivityResult(requestCod...
TakeNotes v3
adding to-do’s
¤ We retrieved the item description, not the checkbox
¤ The next steps are:
¤ Build the che...
References
69
References
¤ Android Training, Starting an Activity
http://developer.android.com/training/basics/firstapp/starting-
activ...
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Intents in Android 5.x

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An introduction to Intents in Android. First, the presentation introduces the concept of Intents as messages between application components. Then, the difference between implicit and explicit intents is clearly stated, along with a description of the Intent resolution mechanism. The presentation concludes with a step-by-step tutorial on how to cast and intercept Intents from Activities.

The source code is available at https://github.com/iliocatallo/takenotes

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Intents in Android 5.x

  1. 1. Intents Ilio Catallo, Eleonora Ciceri – Politecnico di Milano ilio.catallo@polimi.it, eleonora.ciceri@polimi.it
  2. 2. Activities are tasks ¤ Each Activity should be associated with a specific task 2 view activity I can take pictures from the camera
  3. 3. Applications are collection of activities ¤ Applications are made of different activities, each of which with its own responsibility 3 view activity view activity I can take pictures from the camera I allow users to compose new notes
  4. 4. Activities should cooperate ¤ From time to time, an activity may be required to perform a task out of its set of responsibilities 4 view activity view activity I can’t take a picture by myself
  5. 5. Activities should cooperate ¤ Wouldn’t it be great if Activities could cooperate? 5 view activity view activity I’ll back you up Let me ask for help help request
  6. 6. Activities should cooperate ¤ Wouldn’t it be great if Activities could cooperate? 6 view activity view activity Here’s your picture Thank you! Picture
  7. 7. Intents 7
  8. 8. Intents ¤ Intents are messages that are passed between components 8 intent view activity activity view
  9. 9. Intents ¤ A component may ask another component to execute some action on its behalf 9 intent view activity activity view
  10. 10. Intents ¤ The message contains a description of the action to be performed 10 intent view activity activity view
  11. 11. Intents ¤ As such, an intent always involves two subjects 11 intent view activity activity view
  12. 12. Intents ¤ The sender, which wants an action to be performed and thus publishes a message 12 intent activity view view activity
  13. 13. Intents ¤ The receiver, which is any component capable of performing the requested action 13 intent view activity view activity
  14. 14. What Intents are good at ¤ Actions can be of different type: ¤ Taking pictures from the camera ¤ Starting a new web search ¤ Sending e-mails ¤ Sending SMS’s ¤ Each of these actions requires the execution of the associated Activity 14
  15. 15. ExampleofAction 15 Intent Action: play video Browser application YouTube application
  16. 16. Types of Intents 16
  17. 17. Two ways of launching Activities... ¤ The calling Activity has only two ways of launching a second Activity 17 view activity 1 view activity 2
  18. 18. ...by name ¤ By name: The calling Activity knows the exact name of the Activity to launch 18 view activity 1 view activity 2 Start activity 2
  19. 19. ...or by action ¤ By action: The calling Activity wants something to be done (e.g., taking a picture) 19 view activity 1 view activity 2 I need to take a picture
  20. 20. Implicit and Explicit intents ¤ Consequently, Android provides two kinds of Intents: ¤ Explicit intents, which specify the exact Activity to which the Intent should be given ¤ Implicit Intents, which do not declare the name of the Activity to start, but instead declare an action to perform 20 Start activity 2 Explicit intent Take a picture Implicit intent
  21. 21. Intent resolution mechanism ¤ For implicit intents, the system determines which Activity is best to run based on the Intent’s fields ¤ If more than one Activity can handle the Intent, a dialog for the user to select which app to use is displayed 21
  22. 22. Intent resolution mechanism 22 Activity 1 Activity 2 Intent Resolution Explicit Intent: “Execute Activity3” Implicit Intent: “Send a new e-mail” Activity 3 Fancy e-mail client Humble e-mail client User decides which client to use
  23. 23. Launching new Activities 23
  24. 24. Launching a new Activity ¤ An Activity asks for the intervention of a second Activity by invoking on itself the startActivity() method 24 view activity view activity startActivity()
  25. 25. Launching a new Activity ¤ An Activity asks for the intervention of a second Activity by invoking on itself the startActivity() method 25 view activity view activity startActivity() intent
  26. 26. Launching a new Activity ¤ Activity class: 26 public class Activity { public Activity() {...} public void startActivity(Intent intent) {...} ... } startActivity() launches a new Activity based on the provided Intent object
  27. 27. Launching a new Activity ¤ First, the calling Activity forges an Intent object that contains the description of the needed action ¤ The Intent could be either implicit or explicit 27 Start activity 2 Explicit intent Take a picture Implicit intent
  28. 28. Launching a new Activity ¤ Once ready, the Intent object is passed to startActivity() 28 public void startActivity(Intent intent) {...} Start activity 2 Explicit intent Take a picture Implicit intent
  29. 29. Launching a new Activity ¤ Once the startActivity() method is invoked ¤ Implicit intents: the resolution mechanism identifies the Activity that best matches the intent fields ¤ Explicit intents: the system receives the call and starts an instance of the Activity specifiedby the Intent 29
  30. 30. 30 How to create Intents?
  31. 31. Intent class ¤ Intent class: 31 public class Intent { public Intent(Context context, Class<?> cls) {...} public Intent(String action, Uri uri) {...} ... } Explicit Intent constructor Implicit Intent constructor
  32. 32. Creating explicit Intents ¤ Explicit Intents are typically used for launching new Activities within the same application ¤ Nothing in the Intent object other than the name of the Activity to be launched matters ¤ All other fields (action, data, category) are null 32 Intent intent = new Intent(this, TargetActivity.class); Context The calling Activity is used as a Context object in order to determine the package where to look for the NewActivity class
  33. 33. Creating implicit Intents ¤ Each implicit Intent contains information useful to the receiver for performing the job: ¤ Action: the job that the sender can require to perform ¤ Data: the data on which the job will run ¤ Category: the execution requirements (e.g., of being executed in a browser) 33 Intent in = new Intent(Intent.ACTION_DIAL, Uri.parse("tel:0223993669")); Action Making a phone call Data The phone number
  34. 34. Adding extra parameters ¤ It is possible to pass some parameters to the started Activity by adding them as extra content of the Intent 34 Intent intent = ... intent.putExtra(EXTRA_MESSAGE, myMessage); startActivity(intent);
  35. 35. Receiving an Intent 35
  36. 36. Receiving an Intent ¤ Within the just-started Activity, you can inspect the received Intent by using the getIntent() method ¤ Useful to retrieve the data contained in the Intent ¤ This is usually done in the onCreate() method of the new Activity 36 Intent intent = getIntent(); String message = intent.getStringExtra(EXTRA_MESSAGE);
  37. 37. Receiving implicit Intents ¤ How to inform the system that an Activity can handle a specific type of implict Intent? ¤ This can be done by advertising that a given Activity can manage a specific implicit Intent, i.e., by associating the Activity with an Intent filter 37
  38. 38. Intent filter ¤ An Intent filter: ¤ declares the capability of an Activity of receiving implicit Intents of a specific type ¤ delimits the Intents an Activity can handle 38 view activity I can take pictures from the camera
  39. 39. Example: dialing a number ¤ In order to declare an Intent filter, you need to add an <intent-filter> element in your manifest file for the corresponding <activity> element ¤ Example: we declare that the Activity can act as a dialer 39 <intent-filter> <action android:name="android.intent.action.DIAL" /> <category android:name="android.intent.category.DEFAULT" /> <data android:scheme="tel"/> </intent-filter>
  40. 40. Example: dialing a number ¤ Everytime an implicit intent of type android.intent.action.DIAL is casted: ¤ Our Activity is listed as a candidate: 40 Intent in = new Intent(Intent.ACTION_DIAL, Uri.parse("tel:0223993669")); startActivity(in); Example: dialing a number (2) !  Everytime an implicit intent of type android.intent.action.DIAL is casted: !  Our Activity is listed as a candidate: 26 Intent in = new Intent(Intent.ACTION_DIAL, Uri.parse("tel:0223993669")); startActivity(in);
  41. 41. Example: launching an app ¤ Look in your AndroidManifest.xml file: ¤ You will find an intent filter associated with the main Activity ¤ The intent filters declares that the main activity can receive an implicit Intent coming from the Launcher ¤ Such a implicit intent corresponds to the user’s action of launching the application 41 <intent-filter> <action android:name="android.intent.action.MAIN" /> <category android:name="android.intent.category.LAUNCHER" /> </intent-filter>
  42. 42. Returning data from an Activity 42
  43. 43. Requesting a result from an Activity ¤ Starting another Activity does not have to be one-way ¤ You can start a second Activity and receive a result back ¤ Examples ¤ Start the camera and receive the photo as a result ¤ Start the People app and receive a contact details as a result 43
  44. 44. Requesting a result from an Activity 44 Activity 1 Camera Activity Implicit Intent: “Take a picture” Calling (first) Activity Called (second) Activity
  45. 45. Activity class ¤ Activity class: 45 public class Activity { public Activity() {...} public void startActivity(Intent intent) {...} public void startActivityForResult(Intent intent, int requestCode) {...} ... } startActivityForResult() launches a new Activity, expecting to receive a result back
  46. 46. Requesting a result from an Activity ¤ The Intent you pass is the same you could use to start an Activity without requiring a result back ¤ Just call startActivityForResult() instead of startActivity() ¤ You need to pass an additional integer argument, that represents a request code that identifies the request 46 Intent intent = ... startActivityForResult(intent, MY_REQUEST_CODE);
  47. 47. Returned data ¤ Once the user is done with the called Activity, the result will be sent back to the first Activity ¤ Along with the actual result, additional information is returned from the called Activity to the calling Activity: ¤ The request code originally sent by the calling Activity ¤ A result code to notify that the operation in the called Activity has been successful ¤ Insight: Activity result codes have the same role as the HTTP status codes 47
  48. 48. Returned data 48 Activity 1 Camera Activity (android.media.action.IMAGE_CAPTURE, TAKE_PICTURE*) Calling (first) Activity Called (second) Activity < , TAKE_PICTURE, RESULT_OK> * where TAKE_PICTURE is a int constant representing the request code
  49. 49. Activity class ¤ Activity class: 49 public class Activity { public Activity() {...} public void startActivity(Intent intent) {...} public void startActivityForResult(Intent intent, int requestCode) {...} protected void onActivityResult(int requestCode, int resultCode, Intent data) {...} ... } result is processed in onActivityResult()
  50. 50. Processing the returned data ¤ When the second Activity returns, the system invokes the onActivityResult() method on the calling Activity ¤ The data are sent back to the calling Activity as an Intent 50 @Override protected void onActivityResult(int requestCode, int resultCode,Intent data) { if (requestCode == MY_REQUEST_CODE) if (resultCode == RESULT_OK) [...] }
  51. 51. Role of the request code ¤ The calling Activity uses different request codes to distinguish between different types of requests ¤ When receiving a result, the calling Activity uses the request code to identify the provenance of the result ¤ The request codes only makes sense for the calling Activity, while they do not have any particular meaning for the called Activity 51
  52. 52. Role of the request code 52 Activity 1 Activity 4 Activity 3 Activity 2 Request code: 1 Request code: 2 Request code: 3
  53. 53. Role of the request code 53 Request code: 1 Activity 2 Activity 1 Activity 4 Activity 3 The data come from Activity 2, as the request code bundled with the result is 1
  54. 54. Returning data from an Activity ¤ In case the called Activity is part of our application, we have to deal also with the problem of returning data to the calling Activity ¤ We need to: ¤ Perform the operation for which we launched the second Activity ¤ Pack the result into an Intent object ¤ Determine the result code ¤ Terminate the called Activity 54
  55. 55. Performing the operation in the called Activity ¤ An Activity represents a task the user can achieve ¤ To this end, it may be possible for the calling Activity to provide some parameters to the second Activity ¤ These parameters are sent within the Intent as extra’s ¤ We already know how to inspect the triggering Intent within the second Activity: 55 Intent intent = getIntent(); String message = intent.getStringExtra(EXTRA_MESSAGE);
  56. 56. Packing the result into an Intent object ¤ The called Activity produces a result of some kind ¤ These data can be packed into an Intent by using the Intent’s putExtra() method 56 Intent intent = new Intent(); intent.putExtra("KeyForResult1", result1); intent.putExtra("KeyForResult2", result2); We use the default constructor, since we do not know anything about the calling Activity (but the OS knows for us)
  57. 57. Activity class ¤ Activity class: 57 public class Activity { public Activity() {...} public void startActivity(Intent intent) {...} public void startActivityForResult(Intent intent, int requestCode) {...} protected void onActivityResult(int requestCode, int resultCode, Intent data) {...} public final void setResult(int resultCode, Intent data) {...} } set the result that the Activity will return to the caller
  58. 58. Determining the result code ¤ The operation performed in the called Activity may succeed or fail, and the resulting data may change accordingly ¤ You can set the result and the corresponding result code as follows: 58 Intent data = new Intent(); data.putExtra(...) .... setResult(RESULT_OK, data); We use the default constructor, since we do not know anything about the calling Activity (but the OS knows for us)
  59. 59. Terminating the called Activity ¤ The Intent containing the result will not be fired until the called Activity is running ¤ It is the Operating System (OS) that passes Intents between different components ¤ No communication occurs as long as the control does not return to the OS ¤ Hence, when the user is done with the Activity, it has to be terminated 59
  60. 60. Terminating the called Activity ¤ Usually, it is up to the Operating System to decide when to terminate an Activity ¤ However, is some cases the user performs an operation of the UI that indicates that he is done with the current Activity ¤ To manually terminate an Activity we can use the finish() method 60 Intent data = new Intent(); data.putExtra(...) .... setResult(RESULT_OK, data); finish()
  61. 61. Activity class ¤ Activity class: 61 public class Activity { public Activity() {...} public void startActivity(Intent intent) {...} public void startActivityForResult(Intent intent, int requestCode) {...} protected void onActivityResult(int requestCode, int resultCode, Intent data) {...} public final void setResult (int resultCode, Intent data) {...} public void finish () {...} } Call this when your activity is done and should be closed.
  62. 62. TakeNotes V3 62
  63. 63. TakeNotes v3: require a new to do ¤ When the + button is clicked, a new to do is required ¤ The input form containing the to do data is launched in a second Activity (AddToDoActivity) ¤ We require the execution of the second Activity, expecting it to return a result (i.e., the to do description) 63 Intent intent = new Intent(this, AddToDoActivity.class); startActivityForResult(intent, REQUIRE_NEW_TODO); Request code, stored in ToDoListActivity.java: public static final int REQUIRE_NEW_TODO = 0;
  64. 64. TakeNotes v3: input the new to do ¤ The interface is built in the layout file (res/layout/) 64 <LinearLayout [...]> <EditText android:id="@+id/toDoContent" [...]/> <Button [...] android:onClick="addToDoToList"/> </LinearLayout>
  65. 65. TakeNotes v3: returning the result ¤ When the user clicks on the button, the to-do item has to return to the first activity (ToDoListActivity) ¤ To do so, we collect the result from the AddToDoActivity activity and send it back with an Intent ¤ The Intent will report the to-do description as extra content ¤ Once the Intent is sent, the AddToDoActivity activity is closed using the finish() instruction 65
  66. 66. TakeNotes v3: returning the result 66 public void addToDoToList(View view) { Intent intent = new Intent(this, ToDoListActivity.class); EditText editText = (EditText) findViewById(R.id.toDoContent); String newContent = editText.getText().toString(); intent.putExtra(TODO_CONTENT, newContent); setResult(RESULT_OK, intent); finish(); } Called when the button is pressed Terminate the Activity and release the intent
  67. 67. @Override public void onActivityResult(int requestCode, int resultCode, Intent intent) { super.onActivityResult(requestCode, resultCode, intent); if (requestCode == REQUIRE_NEW_TODO && resultCode == RESULT_OK) if (intent != null) addNewToDoToList( intent.getStringExtra(AddToDoActivity.TODO_CONTENT)); } TakeNotes v3: receiving the result 67 ¤ The result is now returned to ToDoListActivity and needs to be processed ¤ All the results coming from an Intent are collected by the onActivityResult method Verify the request code so as to identify the correct result
  68. 68. TakeNotes v3 adding to-do’s ¤ We retrieved the item description, not the checkbox ¤ The next steps are: ¤ Build the checkbox ¤ Add the checkbox to the list linear layout 68 CheckBox newToDoCheckBox = new CheckBox(this); newToDoCheckBox.setText(message); addOnClickListener(newToDoCheckBox); LinearLayout linearLayout = (LinearLayout) findViewById(R.id.checkBoxContainer); linearLayout.addView(newToDoCheckBox); See TakeNotes v2
  69. 69. References 69
  70. 70. References ¤ Android Training, Starting an Activity http://developer.android.com/training/basics/firstapp/starting- activity.html ¤ Android Training, Getting a Result from an Activity http://developer.android.com/training/basics/intents/result.html ¤ Android API Guides, Intent and Intent Filters http://developer.android.com/guide/components/intents- filters.html ¤ Android Training, Sending the User to Another App http://developer.android.com/training/basics/intents/sending.html ¤ Android Training, Allowing other Apps to Start Your Activity http://developer.android.com/training/basics/intents/filters.html 70

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