What is the different between Explicit and implici

问题:

What is the difference between explicit and implicit activity call in android? If you explain the answer with a simple example will be good.

回答1:

For example:

implicit activity call

In intent filter you create action for you activity, so other app can call your activity via this action as following:

<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>

And the other way to call implicit Intent is below:

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

Explicit activity call

You make a call that indicate exactly which activity class:

Intent intent = new Intent(this, ActivityABC.class);
i.putExtra("Value", "This value for ActivityABC");
startActivity(intent);

Hope this help you understand more about Explicit and implicit activity call in android.

You can get more detail about Android Intent here

回答2:

1. Explicit Intents are used to call a specific component. When you know which component you want to launch and you do not want to give the user free control over which component to use.For example, you have an application that has 2 activities. Activity A and activity B. You want to launch activity B from activity A. In this case you define an explicit intent targeting activityB and then use it to directly call it.
2. Implicit Intents are used when you have an idea of what you want to do, but you do not know which component should be launched. Or if you want to give the user an option to choose between a list of components to use. 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 the component directly. For example, you have an application that uses the camera to take photos. One of the features of your application is that you give the user the possibility to send the photos he has taken. You do not know what kind of application the user has that can send photos, and you also want to give the user an option to choose which external application to use if he has more than one. In this case you would not use an explicit intent. Instead you should use an implicit intent that has its action set to ACTION_SEND and its data extra set to the URI of the photo.

An explicit intent is always delivered to its target, no matter what it contains; the filter is not consulted. But an implicit intent is delivered to a component only if it can pass through one of the component's filters

回答3:

See Intent Resolution here

http://developer.android.com/guide/topics/intents/intents-filters.html

Explicit intents (activities) refer to a specific class, and in general, are only available to your packages. Implicit intents refer to intent filters where apps publicly announce that they can handle certain types of data or can provide specific services, e.g. send an email. With implicit intents, the users chooses which activity (typically a package) to use to handle the intent or if a default handler is set, it is launched.

回答4:

There are two types of intents:

• Explicit Intent:

  While creating an Intent object when we explicitly specify and pass on the target component name directly in the intent, it’s an explicit intent.

• Implicit Intent:

  In this case we delegate the task of evaluating the registered components (registration is usually done using intent filters that we’ll cover later) to Android based on the intent data and the intended action (like send an email, capture a photo, pin location on a map, etc.) that we pass. So Android will automatically fire up the component from the same app or some other app that can handle the intent message/job. The idea in this case is that, let’s say we have to pin a location on a map, for that we don’t have to code our own activity to handle that. Instead just pass on the location data to an app like Google maps that can do the job on our app’s behalf.

source : http://codetheory.in/android-intents/

回答5:

When to use which?

Explicit intent: When you know which component can handle your request. So you explicitly mention that component name in the intent.

Intent i = new Intent(context,DetailActivity.class);  // DetailActivity.class is the component name
startActivity(i);

Implicit intent: When you don't know which application can handle your request then you mention the action in intent and let the OS decide which application/s is/are suitable for your task.

Example: Play music

Intent intent = new Intent();
intent.setAction(Intent.ACTION_VIEW);
intent.setData(file);
startActivity(intent);

How OS decides?

When there is implicit call with an intent then OS takes out the action and then it matches with all the intent-filters of all the registered activities of all application using PackageManager and then populates the result as a list. It is called intent resolution
So there is a possibility that no application is available in your device which can handle your request. In that case, you will get NullPointer Exception.

So a safer way to call implicit intent would be this

Intent intent = new Intent(Intent.ACTION_VIEW);
intent.setData(file);
if (intent.resolveActivity(getPackageManager()) != null) {
    startActivity(intent);
}