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Friday, 9 March 2012

Chapter 3 Android Intents


Android Intents

Overview

Intents are asynchronous messages which allow Android components to request functionality from other components of the Android system. For example an Activity can send an Intents to the Android system which starts another Activity.
Therefore Intents allow to combine loosely coupled components to perform certain tasks.
Intents can be used to signal to the Android system that a certain event has occurred. Other components in Android can register to this event and will get notified.
Intents are instances of the android.content.Intent class.
Intents are send to the Android system. Depending on how the Intent was constructed the Android system will run an receiver determination and determine what to do.
An Intent can also contain data. This data can be used by the receiving component. For example your application can calls via an Intent a browser component. As data is it may send the URL to the browser component.
Android supports explicit and implicit Intents.

Explicit Intents

Explicit Intents explicitly names the component which should be called by the Android system, by using the Java class as identifier.
The following shows an explicit Intent. If that Intent is correctly send to the Android system, it will start the associated class. 
                                
Intent i = new Intent(this, ActivityTwo.class);
i.putExtra("Value1", "This value one for ActivityTwo ");
i.putExtra("Value2", "This value two ActivityTwo");
Explicit Intents are typically used within on application as the classes in an application are controlled by the application developer.


Implicit intents

An implicit Intent contains the Action and optional the URI. The receiving component can get this information via the getAction() and getData() methods.
Explicit and implicit Intents can also contain additional data. This data call be filled by the component which creates the Intent. It can and can get extracted by the component which receives the Intent.
The component which creates the Intent can add data to it via the overloaded putExtra() method. Extras are key/value pairs; the key is always a String. As value you can use the primitive data types (int, float,..), String, Bundle, Parceable and Serializable.
For example you can trigger all components which have been registered to send some data via the new Intent(Intent.ACTION_SEND) This Intent determines possible receivers via the type. What is send it defined via the putExtra method. You can use any String as key, the following uses the keys which are predefined for the ACTION_SEND intent. 
                                
Intent sharingIntent = new Intent(Intent.ACTION_SEND);
sharingIntent.setType("text/plain");
sharingIntent.putExtra(android.content.Intent.EXTRA_TEXT, "News for you!");
// createChooser is a convenience method to create
// an Chooser Intent with a Title
startActivity(Intent.createChooser(sharingIntent,"Share this using"));
The component which receives the Intent can use the getIntent().getExtras() method call to get the extra data. 
                                
Bundle extras = getIntent().getExtras();
if (extras == null) {
                return;
                }
// Get data via the key
String value1 = extras.getString(Intent.EXTRA_TEXT);
if (value1 != null) {
        // Do something with the data
}
Three of the core components of an application — activities, services, and broadcast receivers — are activated through messages, called intents. Intent messaging is a facility for late run-time binding between components in the same or different applications. The intent itself, an Intent object, is a passive data structure holding an abstract description of an operation to be performed — or, often in the case of broadcasts, a description of something that has happened and is being announced. There are separate mechanisms for delivering intents to each type of component:
In each case, the Android system finds the appropriate activity, service, or set of broadcast receivers to respond to the intent, instantiating them if necessary. There is no overlap within these messaging systems: Broadcast intents are delivered only to broadcast receivers, never to activities or services. An intent passed to startActivity() is delivered only to an activity, never to a service or broadcast receiver, and so on.

Defining Intent Filters

The following will register an Activity for the Intent which is triggered when someone wants to open a webpage. 
                                
<activity android:name=".BrowserActivitiy" android:label="@string/app_name">
  <intent-filter>
     <action android:name="android.intent.action.VIEW" />
     <category android:name="android.intent.category.DEFAULT" />
     <data android:scheme="http"/> 
  </intent-filter>
</activity>
This is an example how you could define an Intent receiver for the ACTION.SEND Intent. 
                                
<activity
        android:name=".ActivityTest"
    android:label="@string/app_name" >
    <intent-filter>
      <action android:name="android.intent.action.SEND" />
      
      <category android:name="android.intent.category.DEFAULT" />

      <data android:mimeType="text/plain" />
    
    </intent-filter>

</activity>
If a component does not define Intent filters, it can only be called by explicit Intents.
The following example will register an Activity for the ACTION_SEND intent for the "text/plain" mime type. 
                                
<activity
        android:name=".ActivityTest"
    android:label="@string/app_name" >
    <intent-filter>
      <action android:name="android.intent.action.SEND" />
      
      <category android:name="android.intent.category.DEFAULT" />

      <data android:mimeType="text/plain" />
    
    </intent-filter>

</activity>

Restrictions as of Android 3.1

As of Android 3.1 the Android system will per default exclude all BroadcastReceiver from receiving Intents if the corresponding application has never been started by the user or if the user explicitly stopped the application via the Android menu (in Manage Application).
This is an additional security features as the user can be sure that only the applications he started will receive broadcast Intents.

Intents as event triggers

Intents can also be used to send broadcast messages into the Android system. BroadcastReceivers can register to event and will get notified if such an event is triggered.
Your application can register to system events, e.g. a new email has arrived, system boot is complete or a phone call is received and react accordingly.
As said earlier, since Android version 3.1 the Android system will per default exclude all BroadcastReceiver from receiving Intents if the corresponding application has never been started by the user or if the user explicitly stopped the application via the Android menu (in Manage Application).

 Share Intent and ShareActionProvider

As of Android 4.0 you can also add an Action Provider to your ActionBar which allows to share. For this you have to define a special menu entry and assign an Intent which contain the sharing data to it in your Activity. 
                        
<?xml version="1.0" encoding="utf-8"?>
<menu xmlns:android="http://schemas.android.com/apk/res/android" >

   <item android:id="@+id/menu_share"
          android:title="Share"
          android:showAsAction="ifRoom"
          android:actionProviderClass="android.widget.ShareActionProvider" />
    <item
        android:id="@+id/item1"
        android:showAsAction="ifRoom"
        android:title="More entries...">
    </item>

</menu>
                
                        
@Override
public boolean onCreateOptionsMenu(Menu menu) {
        getMenuInflater().inflate(R.menu.mymenu, menu);
        // provider is a field in your Activity
        provider = (ShareActionProvider) menu.findItem(R.id.menu_share)
                .getActionProvider();
        setShareIntent();
        return true;
}


public void setShareIntent() {
        Intent intent = new Intent(Intent.ACTION_SEND);
        intent.setType("text/plain");
        intent.putExtra(Intent.EXTRA_TEXT, "Message");
        provider.setShareIntent(intent);
}
Unfortunately this does not seem to work in the Android emulator, see Bug report for details .

Finding out if an Intent is available

Sometimes you want to find if an application has registered for a certain intent. For example you want to check if a certain receiver is available and if you enable some functionality in your app.
This can be done via checking the PackageManager. The following code checks if an Intent exists. You can check via this method for Intent and change your application behavior accordingly for example disable or hide menu items. 
                        
public boolean isIntentAvailable(Context context, String action) {
    final PackageManager packageManager = context.getPackageManager();
    final Intent intent = new Intent(action);
    List<ResolveInfo> resolveInfo =
            packageManager.queryIntentActivities(intent,
                    PackageManager.MATCH_DEFAULT_ONLY);
   if (resolveInfo.size() > 0) {
                return true;
        }
   return false;
}

Explicit intents and data transfer between activities

The following tutorial demonstrates how to use explicit Intents and how to transfer data between two Activities.
The first Activity will call the second one via an explicit intent. Once the user select the "Back" button on his phone the first Activity will receive some hard-coded data from the Sub-Activity.
Create a new Android application "de.vogella.android.intent.explicit" with the Activity called "ActivityOne".
Change the layout "main.xml" to the following. 
                        
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:orientation="vertical" >

    <TextView
        android:id="@+id/TextView01"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:minHeight="60dip"
        android:text="First Activity. Press button to call second activity"
        android:textSize="20sp" >
    </TextView>

    <Button
        android:id="@+id/Button01"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:onClick="onClick"
        android:text="Calling an intent" >
    </Button>

</LinearLayout>
Create the layout "second.xml". 
                        
<?xml version="1.0" encoding="utf-8"?>
<GridLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:alignmentMode="alignBounds"
    android:columnCount="2"
    android:columnOrderPreserved="false"
    android:useDefaultMargins="true" >

    <TextView
        android:layout_gravity="center_horizontal"
        android:text="Input 1"
        android:textSize="32dip" />

    <EditText
        android:id="@+id/input1"
        android:layout_gravity="fill_horizontal"
        android:text="Default" />

    <TextView
        android:layout_gravity="center_horizontal"
        android:text="Input 2"
        android:textSize="32dip" />

    <EditText
        android:id="@+id/input2"
        android:layout_gravity="fill_horizontal"
        android:text="Default" />

 
    <Button
        android:layout_column="1"
        android:layout_gravity="right"
        android:width="80dp"
        android:onClick="onClick"
        android:text="Finish" />

</GridLayout>
Declare a new Activity called "ActivityTwo" via the "AndroidManifest.xml" file. 
                        <?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="de.vogella.android.intent.explicit"
    android:versionCode="1"
    android:versionName="1.0" >

    <uses-sdk android:minSdkVersion="15" />

    <application
        android:icon="@drawable/icon"
        android:label="@string/app_name" >
        <activity
            android:name=".ActivityOne"
            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="ActivityTwo"
            android:label="ActivityTwo" >
        </activity>
    </application>


</manifest>
Create the following coding for your two Activities. 
                        
package de.vogella.android.intent.explicit;

import android.app.Activity;
import android.content.Intent;
import android.os.Bundle;
import android.view.View;
import android.widget.Toast;

public class ActivityOne extends Activity {
        private static final int REQUEST_CODE = 10;

        
/** Called when the activity is first created. */

        @Override
        public void onCreate(Bundle savedInstanceState) {
                super.onCreate(savedInstanceState);
                setContentView(R.layout.main);
        }

        public void onClick(View view) {
                Intent i = new Intent(this, ActivityTwo.class);
                i.putExtra("Value1", "This value one for ActivityTwo ");
                i.putExtra("Value2", "This value two ActivityTwo");
                // Set the request code to any code you like, you can identify the
                // callback via this code
                startActivityForResult(i, REQUEST_CODE);
        }

        @Override
        protected void onActivityResult(int requestCode, int resultCode, Intent data) {
                if (resultCode == RESULT_OK && requestCode == REQUEST_CODE) {
                        if (data.hasExtra("returnKey1")) {
                                Toast.makeText(this, data.getExtras().getString("returnKey1"),
                                                Toast.LENGTH_SHORT).show();
                        }
                }
        }
}
                
                        
package de.vogella.android.intent.explicit;

import android.app.Activity;
import android.content.Intent;
import android.os.Bundle;
import android.view.View;
import android.widget.EditText;

public class ActivityTwo extends Activity {

        
/** Called when the activity is first created. */

        @Override
        public void onCreate(Bundle bundle) {
                super.onCreate(bundle);
                setContentView(R.layout.second);
                Bundle extras = getIntent().getExtras();
                if (extras == null) {
                        return;
                }
                String value1 = extras.getString("Value1");
                String value2 = extras.getString("Value2");
                if (value1 != null && value2 != null) {
                        EditText text1 = (EditText) findViewById(R.id.input1);
                        EditText text2 = (EditText) findViewById(R.id.input2);
                        text1.setText(value1);
                        text2.setText(value2);
                }
        }

        public void onClick(View view) {
                finish();
        }

        @Override
        public void finish() {
                Intent data = new Intent();
                // Return some hard-coded values
                data.putExtra("returnKey1", "Swinging on a star. ");
                data.putExtra("returnKey2", "You could be better then you are. ");
                setResult(RESULT_OK, data);
                super.finish();
        }
}
Run your application. The first will send data to the second Activity which will be shown on the user interface. If you select back on your phone, the first Activity will display a Toast with the data from the second Activity.

Implicit Intents

The following tutorial demonstrates the usage of implicit intents of the Android System.
Create a new Android application "de.vogella.android.intent.implicit" with a Activity called " CallIntentsActivity ".
In this example we will use a Spinner to select the Intent which should get triggered. For the content of the Spinner we will define static values.
Create the following "intents.xml" file in the "res/values" folder. 
                        
<resources>
    <string-array name="intents">
        <item>Open Browser</item>
        <item>Call Someone</item>
        <item>Dial</item>
        <item>Show Map</item>
        <item>Search on Map</item>
        <item>Take picture</item>
        <item>Show contacts</item>
        <item>Edit first contact</item>
    </string-array>
    
</resources>
Change the layout file "main.xml" to the following. 
                        
<?xml version="1.0" encoding="utf-8"?>
<GridLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:alignmentMode="alignBounds"
    android:columnCount="1" >

      <Spinner
        android:id="@+id/spinner"
        android:layout_gravity="fill_horizontal"
        android:drawSelectorOnTop="true"
        >
      </Spinner>
    
    <Button
        android:id="@+id/trigger"
        android:onClick="onClick"
        android:text="Trigger Intent">
    </Button>

  
</GridLayout>
To be able to use certain intents you need to register then for your application. Maintain the following "AndroidManifest.xml". 
                        
<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="de.vogella.android.intent.implicit"
    android:versionCode="1"
    android:versionName="1.0" >

    <uses-sdk android:minSdkVersion="15" />

    <uses-permission android:name="android.permission.CALL_PRIVILEGED" >
    </uses-permission>
    <uses-permission android:name="android.permission.CALL_PHONE" >
    </uses-permission>
    <uses-permission android:name="android.permission.CAMERA" >
    </uses-permission>
    <uses-permission android:name="android.permission.READ_CONTACTS" >
    </uses-permission>
    <uses-permission android:name="android.permission.INTERNET"/>

    <application
        android:icon="@drawable/icon"
        android:label="@string/app_name" >
        <activity
            android:name=".CallIntentsActivity"
            android:label="@string/app_name" >
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
    </application>

</manifest>
Change your activity to the following. We will start the new intent with the method startActivityForResult() which allow us to specify a desired result code. Once the intent is finished the method onActivityResult() is called and you can perform actions based on the result of the activity. 
                        
package de.vogella.android.intent.implicit;

import android.app.Activity;
import android.content.Intent;
import android.net.Uri;
import android.os.Bundle;
import android.view.View;
import android.widget.ArrayAdapter;
import android.widget.Spinner;
import android.widget.Toast;

public class CallIntentsActivity extends Activity {
        private Spinner spinner;

        
/** Called when the activity is first created. */

        @Override
        public void onCreate(Bundle savedInstanceState) {
                super.onCreate(savedInstanceState);
                setContentView(R.layout.main);
                spinner = (Spinner) findViewById(R.id.spinner);
                ArrayAdapter adapter = ArrayAdapter.createFromResource(this,
                                R.array.intents, android.R.layout.simple_spinner_item);
                adapter.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item);
                spinner.setAdapter(adapter);
        }

        public void onClick(View view) {
                int position = spinner.getSelectedItemPosition();
                Intent intent = null;
                switch (position) {
                case 0:
                        intent = new Intent(Intent.ACTION_VIEW,
                                        Uri.parse("http://www.vogella.de"));
                        break;
                case 1:
                        intent = new Intent(Intent.ACTION_CALL,
                                        Uri.parse("tel:(+49)12345789"));
                        break;
                case 2:
                        intent = new Intent(Intent.ACTION_DIAL,
                                        Uri.parse("tel:(+49)12345789"));
                        startActivity(intent);
                        break;
                case 3:
                        intent = new Intent(Intent.ACTION_VIEW,
                                        Uri.parse("geo:50.123,7.1434?z=19"));
                        break;
                case 4:
                        intent = new Intent(Intent.ACTION_VIEW,
                                        Uri.parse("geo:0,0?q=query"));
                        break;
                case 5:
                        intent = new Intent("android.media.action.IMAGE_CAPTURE");
                        break;
                case 6:
                        intent = new Intent(Intent.ACTION_VIEW,
                                        Uri.parse("content://contacts/people/"));
                        break;
                case 7:
                        intent = new Intent(Intent.ACTION_EDIT,
                                        Uri.parse("content://contacts/people/1"));
                        break;

                }
                if (intent != null) {
                        startActivity(intent);
                }
        }

        @Override
        public void onActivityResult(int requestCode, int resultCode, Intent data) {
                if (resultCode == Activity.RESULT_OK && requestCode == 0) {
                        String result = data.toURI();
                        Toast.makeText(this, result, Toast.LENGTH_LONG);
                }
        }

}
If you start your application you should see an list of buttons and if you press the button, different activities should be performed. Note that you didn't specify any receiving application only the thing that should be done.

Registering an IntentFilter

The following example will register a Activity for the Intent which is triggered to view a http webpage.
Our application downloads the HTML source of this page and display this in a TextView.
Create the Android project "de.vogella.android.intent. " with the activity "BrowserActivitiy". Register your Activity to the Intent via the Intent.Action_VIEW action and the scheme "http" via the following "AndroidManifest.xml". The manifest also declares the permission to access the Internet. 
                        
<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="de.vogella.android.intent.browserfilter"
    android:versionCode="1"
    android:versionName="1.0" >

    <uses-sdk android:minSdkVersion="15" />

    <uses-permission android:name="android.permission.INTERNET" >
    </uses-permission>

    <application
        android:icon="@drawable/icon"
        android:label="@string/app_name" >
        <activity
            android:name=".BrowserActivity"
            android:label="@string/app_name" >
            <intent-filter>
                <action android:name="android.intent.action.VIEW" />

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

                <data android:scheme="http" />
            </intent-filter>
        </activity>
    </application>

</manifest>
Change "main.xml" to the following. 
                        
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    >
<TextView  
    android:layout_width="fill_parent" 
    android:layout_height="wrap_content" 
    android:id="@+id/textView"/>
</LinearLayout>
Install your application. If you now trigger an Intent to open an URL your should be able to select your own component. You can for example trigger this Intent via the example from the implicit tutorials. 
                        
package de.vogella.android.intent.browserfilter;

import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.net.URL;

import android.app.Activity;
import android.content.Intent;
import android.net.Uri;
import android.os.Bundle;
import android.os.StrictMode;
import android.widget.TextView;

public class BrowserActivity extends Activity {
        
/** Called when the activity is first created. */

        @Override
        public void onCreate(Bundle savedInstanceState) {
                super.onCreate(savedInstanceState);

                // To keep this example simple, we allow network access
                // in the user interface thread
                StrictMode.ThreadPolicy policy = new StrictMode.ThreadPolicy.Builder()
                                .permitAll().build();
                StrictMode.setThreadPolicy(policy);

                setContentView(R.layout.main);
                Intent intent = getIntent();
                TextView text = (TextView) findViewById(R.id.textView);
                // To get the action of the intent use
                String action = intent.getAction();
                if (action != Intent.ACTION_VIEW) {
                        throw new RuntimeException("Should not happen");
                }
                // To get the data use
                Uri data = intent.getData();
                URL url;
                try {
                        url = new URL(data.getScheme(), data.getHost(), data.getPath());
                        BufferedReader rd = new BufferedReader(new InputStreamReader(
                                        url.openStream()));
                        String line = "";
                        while ((line = rd.readLine()) != null) {
                                text.append(line);
                        }

                } catch (Exception e) {
                        e.printStackTrace();
                }

        }
}
If you select you component the HTML code should be loaded into your text view.


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