MobAppDev

Explicit & Implict Intents, Intent Resolution, Data Transfer with Intents, Getting Results from Started Activities

Vladimir Kulyukin

www.vkedco.blogspot.com

Outline● Explicit Intents● Implicit Intents● Intent Resolution● Data Transfer with Intents● Get Results from Started Activities

Review: Intents

● Intents are messages that allow Android components (e.g., activities, applications, etc) to send computational requests or notifications to other Android components via Android OS

● Intents are used for requesting information, computation, or event signaling

● All intents objects are instances of android.content.Intent

Explicit Intents

Explicit Intents

● If Android OS resolves an explicit intent via its Java class identifier & the component specified by the class identifier is accessible, the component will likely be launched

● Explicit intents are typically used within one application when the developers have control over the Java class identifiers of the components

Data Transfer with Intents

● Intents can bundle and transfer data to other components via Android

● An intent sender creates an intent, puts a few key-value pairs into its bundle and sends it to Android

● An intent receiver receives an intent, checks it for the presence/absence of specific keys and then does some action

● Keys are always Strings

Explicit Intent Examples

Explicit Intents: Example 01

● Develop an app that has two Activities: PixBrowserExpIntentsActivity_01 and ImageDisplayerAct_01

● The PixBrowserExpIntentsActivity_01 activity consumes a button click and launches the ImageDisplayerAct_01 activity via an explicit intent

● The ImageDisplayerAct_01 activity displays a random image with an ImageView object

● Source code is here

Sample Screenshots

Code Fragments

Image Displayer's Layout

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"

android:layout_width="match_parent"

android:layout_height="match_parent"

android:orientation="vertical" >

<ImageView

android:id="@+id/img_view_01"

android:layout_width="match_parent"

android:layout_height="match_parent"

android:src="@drawable/img_01" />

</LinearLayout>

Main Activity's Layout<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"

android:orientation="vertical" >

<ImageView

android:id="@+id/img_01"

android:layout_width="match_parent"

android:layout_height="match_parent"

android:layout_weight="1"

android:src="@drawable/img_01" />

<LinearLayout

android:layout_width="match_parent"

android:layout_height="wrap_content" >

<Button

android:id="@+id/btn_display"

android:layout_width="match_parent"

android:layout_height="wrap_content"

android:text="@string/btn_display_txt" />

</LinearLayout>

</LinearLayout>

AndroidManifest.xml<manifest xmlns:android="http://schemas.android.com/apk/res/android" >

<application >

<activity

android:name=".PixBrowserExpIntentsActivity_01"

android:label="@string/app_name" >

<intent-filter>

<action android:name="android.intent.action.MAIN" />

<category android:name="android.intent.category.LAUNCHER" />

</intent-filter>

</activity>

<activity android:name=".ImageDisplayerAct_01" android:label="Image Displayer 01">

<intent-filter>

<action android:name="android.intent.action.ImageDisplayerAct_01" />

<category android:name="android.intent.category.DEFAULT" />

</intent-filter>

</activity>

</application>

</manifest>

PixBrowserExpIntentsActivity_01.onCreate()protected Button mBtnDisplay = null;

protected Activity mThisAct = null;

public void onCreate(Bundle savedInstanceState) {

super.onCreate(savedInstanceState);

setContentView(R.layout.activity_pix_browser_exp_intents_activity_01);

mBtnDisplay = (Button) this.findViewById(R.id.btn_display);

mThisAct = this;

// Button click creates an explicit intent to request a launch of ImageDisplayerAct01

mBtnDisplay.setOnClickListener(

new OnClickListener() {

public void onClick(View v) {

// 1. Create an explicit intent

Intent i = new Intent(mThisAct, ImageDisplayerAct_01.class);

// 2. Request Android to consider it.

startActivity(i); }});

}

ImageDisplayerAct_01.onCreate()

protected ImageView mImgView = null;

protected void onCreate(Bundle savedInstanceState) {

mImgView = (ImageView) this.findViewById(R.id.img_view_01);

mImgView.setImageDrawable(null);

int img_num = new Random().nextInt(8) + 1;

switch ( img_num ) {

case 1: mImgView.setBackgroundResource(R.drawable.img_01); break;

case 2: mImgView.setBackgroundResource(R.drawable.img_02); break;

// Other cses for 3 to 8

}

Toast.makeText(this, getResources().getString(R.string.randome_img_msg) + " " + img_num,

Toast.LENGTH_SHORT).show();

}

Explicit Intents: Example 02● Develop an app that has two Activities: MainLauncher and

ImageDisplayerAct02

● The MainLauncher activity consumes a button click and launches the ImageDisplayerAct02 activity via an explicit intent AND passes to the ImageDisplayerAct02 the numerical ID of the image to display

● The ImageDisplayerAct02 activity displays the requested image in an ImageView object

● We will call this project ExplicitIntentsApp02

● Source code is here

Code Fragments

MainLauncherAct.onCreate() mBtnDisplay.setOnClickListener(

new OnClickListener() {

public void onClick(View v) {

// 1. Create an explicit intent

Intent i = new Intent(mThisAct, ImageDisplayerAct02.class);

// 2. Generate a random number in [1, 8]

int random_img_id = new Random().nextInt(8) + 1;

// 3. Put key-value pair into the intent.

i.putExtra(mRes.getString(R.string.img_id_key), random_img_id);

// 4. Toast which image is requested.

Toast.makeText(mThisAct,

mRes.getString(R.string.img_display_request_msg) + " "

+ random_img_id, Toast.LENGTH_SHORT).show();

// 5. Request Android to run it.

startActivity(i);}});

}

ImageDisplayerAct02.onCreate()

// 1. Get the intent

Intent i = this.getIntent();

// 2. Check to see if it contains an integer keyed on R.string.img_id_key.

// default to 0 if it does not.

int img_num = i.getIntExtra(mRes.getString(R.string.img_id_key), 0);

// 3. If it does, display it

if (img_num > 0) {

switch (img_num) {

case 1: mImgView.setBackgroundResource(R.drawable.img_01); break;

case 2: mImgView.setBackgroundResource(R.drawable.img_02); break;

// handle other cases from 3 to 8

}

// Set the text of the mTV text view to the appropriate message.

mTV.setText(mRes.getString(R.string.img_display_msg) + " " + img_num);

Implicit Intents

Implicit Intents● Implicit intents provide a mechanism for anonymous

(no explicit activity specification) requests ● An activity asks Android OS to do an action by

specifying high-level requirements but without knowing which activity or application will actually perform the action

● The requesting action can specify a URI to the data which the performing activity must process or add that data to the implicit intent

Native Android Actions● Android OS defines many actions that can be satisfied by

native or 3-rd party components and are launched via implicit intents

● Here are some examples: – ACTION_ANSWER – open an activity that handles incoming

calls– ACTION_CALL – open an activity with a phone dialer– ACTION_PICK – launch an activity to pick an item from a list of

items– ACTION_WEB_SEARCH – launch an activity to perform a web

search– ACTION_VIEW – launch an activity that can view the specified

data in a reasonable manner

Implicit Intents

● Implicit intents provide a mechanism for anonymous requests

● An implicit intent is launched with startActivity(Intent i)

● Activities and applications use intent filters to tell Android ecosystem about the actions they respond to and/or which data types they work with

● An component that has an intent filter is called an intent filter

Intent Filter's Structure● Action

– Name of the action being serviced– Name should be a unique string– Best practice is to use the Java package

naming convention ● Category

– When the action should be serviced● Data

– Data types that the action can work with

Intent Resolution

● Intent resolution starts when an activity calls startActivity() or startActivityForResult() with some intent Intnt

● Android OS gets the list of all intent filters available from the installed packages

● Intent filters that do not match Intnt's requested action or category are removed from the list

Intent Resolution● An action match occurs when Intnt's action

and the filter's action are the same or no action is specified

● A category match occurs when Intnt's categories and the filter's categories are the same

● A data match occurs when Intnt's data types are the same as the filter's data types

Intent Resolution

● Native Android components are part of the Intent resolution process

● Native components are no different from 3rd-part components

● Native components do not have higher-priorities and can be completely replaced with 3rd-party components with the user's consent

● Android ecosystem does not give anyone a priori advantages to any components

Implicit Intent Example

● Develop an app that uses an implicit intent to request Android ecosystem to find a component that can display the wiki entry about Kurt Gödel

● This implicit intent must contain two components: Intent.ACTION_VIEW and the URI (e.g., URL of a web page)

● The default native component that responds to this intent is the web browser

● Source code is here

Sample Screenshots

Code Fragments

Wiki Constant in /res/values/strings.xml

<resources>

...

<string name=”kurt_godel_wiki_url”>

http://en.wikipedia.org/wiki/Kurt_Godel

</string>

...

</resources>

Launching Intent for Viewing URL

public class GodelWikiPageBrowserAct extends Activity {

protected void onCreate(Bundle savedInstanceState) {

super.onCreate(savedInstanceState);

setContentView(R.layout.godel_wiki_page_browser_act_layout);

Intent myIntent = new Intent(Intent.ACTION_VIEW,

Uri.parse(getResources().getString(R.string.kurt_godel_wiki_url)));

this.startActivity(myIntent);

}

…

}

Getting Results from

Started Activities

startActivity() vs startActivityForResult()

● In some cases, a component wants to get a result from an activity that it wants to launch either explicitly or implicity

● In this case, the launching component must use startActivityForResult() and implement onActivityResult()

● startActivityForResult() takes an intent and a request code (intenger constant)

● onActivityResult() is called when Android ecosystem responds to the intent launcher

startActivity() vs startActivityForResult()

● In some cases, a component wants to get a result from an activity that it wants to launch either explicitly or implicity

● In this case, the launching component must use startActivityForResult() and implement onActivityResult()

● startActivityForResult() takes an intent and a request code (intenger constant)

● onActivityResult() is called when Android ecosystem responds to the intent launcher

Example

● Develop an app that attaches uses an implicit intent to check for the availability of the Text-to-Speech (TTS) engine

● If the the TTS data are not installed, another intent is launched to install the data

● If all installations are successful, the app displays an activity that allows the user to enter texts and send them to the TTS

● Source code is here

Sample Screenshots

Code Fragments

Launching Intent with Expected Result

// The request type constants.

private static final int REQ_TTS_STATUS_CHECK = 0;

// PICO TTS object

private TextToSpeech mTts = null;

// Create intent to check tts data

Intent checkTtsIntent =

new Intent(TextToSpeech.Engine.ACTION_CHECK_TTS_DATA);

startActivityForResult(checkTtsIntent, REQ_TTS_STATUS_CHECK);

Processing Intent's Resultprotected void onActivityResult(int requestCode, int resultCode, Intent data) {

if ( requestCode == REQ_TTS_STATUS_CHECK ) {

switch ( resultCode ) {

case TextToSpeech.Engine.CHECK_VOICE_DATA_PASS:

mTts = new TextToSpeech(this, this); break;

case TextToSpeech.Engine.CHECK_VOICE_DATA_BAD_DATA:

case TextToSpeech.Engine.CHECK_VOICE_DATA_MISSING_DATA:

case TextToSpeech.Engine.CHECK_VOICE_DATA_MISSING_VOLUME:

Intent installIntent = new Intent();

installIntent.setAction(TextToSpeech.Engine.ACTION_INSTALL_TTS_DATA);

startActivity(installIntent);

case TextToSpeech.Engine.CHECK_VOICE_DATA_FAIL:

default: Log.d(TAG, getResources().getString(R.string.tts_unavailable));

}

}}

Stopping or Shutting TTSpublic void onStop() {

super.onStop();

if ( mTts != null ) { mTts.stop(); }

}

public void onPause() {

super.onPause();

if ( mTts != null ) { mTts.stop(); }

}

public void onDestroy() {

super.onDestroy();

mTts.shutdown();

}

References● http://developer.android.com/reference/android/content/Intent.html

http://developer.android.com/reference/android/speech/tts/package-summary.html