程式設計 Club: Android Intents

Android Intents - Tutorial

資料來源 http://www.vogella.com/articles/AndroidIntent/article.html

Lars Vogel

Version 2.6

17.04.2012

Revision History
Revision 0.1	20.07.2010	LarsVogel
Created
Revision 0.2 - 2.6	19.07.2010 - 17.04.2012	LarsVogel
bug fixes and enhancements

Using Intents in Android

This tutorials describes what Intents are and how to use them in Android. It is based on Eclipse 3.7, Java 1.6 and Android 4.0.4 (Ice Cream Sandwich).

1. Android Intents

1.1. 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.

1.2. 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.

1.3. Implicit Intents

Implicit Intents do not specify the Java class which should be called. They specify the action which should be performed and optionally an URI which should be used for this action.
For example the following tells the Android system to view a webpage. Typically the web browser is registered to this Intent but other component could also register themself to this event.

Intent intent = new Intent(Intent.ACTION_VIEW, Uri.parse("http://www.vogella.com"));

If these Intents are send to the Android system it searches for all components which are registered for the specific action and the data type.
If only one component is found, Android starts this component directly. If several components are identifier by the Android system, the user will get an selection dialog and can decide which component should be used for the Intent.

1.4. Data Transfer

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 intent = new Intent(Intent.ACTION_SEND);
intent.setType("text/plain");
intent.putExtra(android.content.Intent.EXTRA_TEXT, "News for you!");
startActivity(intent);

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
}

2. Using Intents to call Activities

2.1. Calling Activities

If you send an Intent to the Android system, Android requires that you tell it to which type of component your Intent should be send.
To start an Activity use the method startActivity(Intent). This method is defined on the Context object and available in every Activity object.
If you call an Activity with the startActivity(Intent) method the caller requires no result from the called Activity.

2.2. Calling Sub-Activities for result data

If you need some information from the called Activity use the startActivityForResult() method.

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);
}

If you use the startActivityForResult() method then the started Activity is called a Sub-Activity.
If the Sub-Activity is finished it can send data back to its caller via Intent. This is done in the finish() method.

@Override
public void finish() {
 // Prepare data intent 
 Intent data = new Intent();
 data.putExtra("returnKey1", "Swinging on a star. ");
 data.putExtra("returnKey2", "You could be better then you are. ");
 // Activity finished ok, return the data
 setResult(RESULT_OK, data);
 super.finish();
}

Once the Sub-Activity finished, the onActivityResult() method in the calling Activity will be called.

@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();
  }
 }
}

3. Defining Intent Filters

3.1. Overview

If an Intent is send to the Android system, it will determine suitable applications for this Intent. If several components have been registered for this type of Intent, Android offers the user the choice to open one of them.
This determination is based on IntentFiltes. An IntentFilter specifies the types of Intent that an activity, service, or broadcast receiver can respond to. An IntentFilter declares the capabilities of a component. It specifies what an Activity or Service can do and what types of broadcasts a Rseceiver can handle. It allow the corresponding component to receive Intents of the declared type.
IntentFilters are typically defined via the AndroidManifest.xml file. For BroadcastReceiver it is also possible to define them in coding. An IntentFilters is defined by its category, action and data filters. It can also contain additional metadata.
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>

3.2. Restrictions as of Android 3.1

As of Android 3.1 the Android system will by 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.

4. 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).

5. 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 .

6. 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;
}

7. Prerequisites for this tutorial

The following assumes that you have already basic knowledge in Android development. Please check the Android development tutorial to learn the basics.

程式設計 Club

2012年7月13日星期五

Android Intents - Tutorial

Android Intents - Tutorial

Lars Vogel

1. Android Intents

1.1. Overview

1.2. Explicit Intents

1.3. Implicit Intents

1.4. Data Transfer

2. Using Intents to call Activities

2.1. Calling Activities

2.2. Calling Sub-Activities for result data

3. Defining Intent Filters

3.1. Overview

3.2. Restrictions as of Android 3.1

4. Intents as event triggers

5. Share Intent and ShareActionProvider

6. Finding out if an Intent is available

7. Prerequisites for this tutorial

沒有留言:

張貼留言

2012年7月13日 星期五

Android Intents - Tutorial

Android Intents - Tutorial

Lars Vogel

1. Android Intents

1.1. Overview

1.2. Explicit Intents

1.3. Implicit Intents

1.4. Data Transfer

2. Using Intents to call Activities

2.1. Calling Activities

2.2. Calling Sub-Activities for result data

3. Defining Intent Filters

3.1. Overview

3.2. Restrictions as of Android 3.1

4. Intents as event triggers

5. Share Intent and ShareActionProvider

6. Finding out if an Intent is available

7. Prerequisites for this tutorial

沒有留言:

張貼留言

2012年7月13日星期五