Android - Intents

Objective #1: Understand the role of intents.

• An intent is a message that contains an abstract description of an operation to be performed. For example, when a user clicks a button, an intent can be sent to Google Maps to bring up a web page with the view of a specific GPS location. Or, a user could swipe an entry in a contacts list and the intent to dial a phone number from that contact list could be sent to the phone's dialer.
  
  
• An intent can be a broadcast that contains a description of something that has happened or is being announced.
  
  
• See the API for the Intent class. See this reference for info on intent filters. See this link for info on intents from Vogella. See this link for info on intents from Geetha.
  
  
• An intent must specify the name of the component that will be handling the intent. This includes the name of the app's package as well as the name of the class of the target component.
  
  
• An intent will also contain one or more of the following:
  
  
  • action -    such as <action android:name="android.intent.action.MAIN" />
    
    
  • type-    such as <action android:mimeType="vnd.android.cursor.dir/vnd.google.note" />
    
    
  • data -    which includes the URI of the data to be acted on and the MIME type of that data. For example, your app can send a URL as data to the device's browser component such as <action android:mimeType="vnd.android.cursor.item/vnd.google.note" />
    
    
  • categories - the kind of component that should handle the intent such as <category android:name="android.intent.action.LAUNCHER" />
    
    
• An intent can have additional data which the receiving component can get using the getAction and getData methods. Or, the putExtra method can bu used to add key/value pairs to the intent. The key is a String and the value is a data type.

Objective #2: Use Explicit Intents

• You can use an explicit intent to to launch a specific activity in your project. That is, one activity can open another activity.
  
  Technically, the invoking activity sends an internal message to an invoked activity (the target component) to respond to the intent.
  
  
• The following statement can be used to open a second activity named Activity2
  
  startActivity(new Intent(this, Activity2.class));
  
  Most likely this line of code would be placed in something like a button's click listener
  
  button1.setOnClickListener(new OnClickListener()
  {
  
      public void onClick(View v)
      {
        startActivity(new Intent(this, Activity2.class));
      }
  
  });
  
  
• In order to make use of a second activity in your project, you must declare your intent to use that activity in the project's manifest file. The following manifest file is an example of one in which the activity named MultipleActivitiesDemoActivity is designated as the activity that is first launched when the app is executed. But another activity named Activity2 is listed as an available activity that can be used by the project.
  
  
  

  ‹?xml version="1.0" encoding="utf-8"?>
  
  ‹manifest xmlns:android="http://schemas.android.com/apk/res/android"
        package="com.minich.multipleactivitiesdemo"
        android:versionCode="1"
        android:versionName="1.0">
  
  
      ‹uses-sdk android:minSdkVersion="10" />
  
     
      ‹application android:icon="@drawable/icon" android:label="@string/app_name">
  
          ‹activity android:name=".MultipleActivitiesDemoActivity"
                    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=".Activity2"
                    android:label="@string/app_name">
          ‹/activity>
          
      ‹/application>
  ‹/manifest>
  

  
  
• It's possible to send data such as a number or a string along with the intent when you open a second activity.
  
  Follow the "8. Tutorial: Explicit intents and data transfer between activities" at http://www.vogella.de/articles/AndroidIntent/article.html#explicitintents(Depending on your SDK version, you may need to change the GridLayout in second.xml to a TableLayout and remove all the GridLayout attributes except for layout_width and layout_height.)

Objective #3: Use Implicit Intents

• With an implicit intent, you simply declare an intent. Then, the your phone (or device) finds a matching activity that is able to respond to the intent. For example, you could declare an intent to open a web page but not specify which browser that is installed on the phone to use to open the web page. The phone will offer the user a choice of which browser to handle the intent and open the web page. In this way, the user will be able to choose another app (i.e. the target component) to use to seamlessly continue with the task initiated by the implicit intent. If there is only one other component (i.e. app) is found, then it automatically starts and handles the intent's action.
  
  
• An implicit intent allows for late runtime binding of code in different apps.
  
  
• The Intent is matched using the <intent-filter> descriptions in the installed app packages. See the IntentFilter class API for details.
  
  
• An implicit intent is passed to interested broadcast receivers using any of the broadcast methods (e.g. Context.sendBroadcast(), Context.sendOrderedBroadcast(), or Context.sendStickyBroadcast() )
  
  
• Example(s):
  
  Intent intent = new Intent(Intent.ACTION_VIEW, Uri.parse("http://www.minich.com"));
  
  
• This Geetha tutorial and download may be useful.
  
  Or, this Vogella tutorial may be helpful. (Depending on your SDK version, you may need to change the GridLayout in second.xml to a TableLayout and remove all the GridLayout attributes except for layout_width and layout_height.)
  
  
• The difference between explicit and implicit intents is that an explicit intent names the component (e.g. specific class) that will handle the desired action or service while an implicit intent does not specify a specific component. With an implicit content, the operating system is asked to perform the action or service and the user may be presented with a choice of apps to pick from to perform the action or service.
  
  With an implicit intent you would see code like this:
  
  Intent intent = new Intent(Intent.ACTION_VIEW, Uri.parse("http://www.minich.com"));
  startActivity(intent);
  
  while with an explicit content, you may see code like this:
  
  Intent intent = new Intent(this, Activity2.class);
  startActivity(intent);