Seven attributes of Android Intent, the key to building efficient communication between applications

In Android development, Intent is a core component used to pass messages and data within an application or between different applications. Intent has seven attributes:

• "Action": describes the operation to be performed.
• "Data": Uri object that specifies the data location of the operation.
• "Category": Provides additional information for the Intent to help the system more accurately determine how to handle the Intent.
• "Extras": A collection of key-value pairs used to pass additional data.
• 「Type」: Specifies the MIME type of the data.
• "Component": For explicit intents, specifies the class name of the component to be started.
• "Flags": Flags that specify how to handle the Intent.

Action Attributes

The Action property of an Intent is a string that describes the operation or action that the Intent wants to perform. It is particularly important in implicit Intents and helps the Android system determine which component (such as Activity, Service, or BroadcastReceiver) can handle this Intent.

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The Android SDK defines a series of standard Action strings, such as:

• Intent.ACTION_VIEW: Used to display data specified by a data URI (such as a web page, image, etc.).
• Intent.ACTION_EDIT: Used to edit the data specified by the data URI.
• Intent.ACTION_DIAL: Dials a phone number without actually establishing a connection.
• Intent.ACTION_CALL: Calls a phone number and attempts to establish a connection. Note: This action requires user confirmation and may require special permissions.
• Intent.ACTION_SEND: Send some kind of data. This is usually used to share text, pictures, etc.
• Intent.ACTION_SENDTO: Sends a message to a specific recipient (such as an email address).
• Intent.ACTION_GET_CONTENT: Allows the user to select an item of data to be returned to the caller.
• Intent.ACTION_SYNC: Triggers a synchronous operation.
• Intent.ACTION_SEARCH: Perform a search.
• Intent.ACTION_MAIN: This is a special category, usually used in conjunction with CATEGORY_LAUNCHER, to indicate the entry point of an application.

In addition to these standard actions, you can also define your own custom action strings. You can use the Intent constructor or the setAction() method:

 // 使用构造函数设置Action Intent intent = new Intent(Intent.ACTION_VIEW); // 自定义Action Intent intent = new Intent("自定义Action"); // 或使用setAction()方法设置Action Intent intent = new Intent(); intent.setAction(Intent.ACTION_VIEW);

When using implicit Intent, in addition to setting the Action, you may also need to set other properties (such as Category, Type, Data, etc.) to ensure that the Intent is handled correctly.

Data Attributes

The Data property of an Intent is a Uri object that specifies the location or type of data associated with the Intent. A Uri can be a reference to a file, web page, email address, etc.

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The Data attribute is particularly important in implicit Intents, and together with the Action attribute, it helps the Android system determine which component (such as Activity, Service, or BroadcastReceiver) can handle this Intent.

Use the setData() method to set the Data property of the Intent:

 Uri data = Uri.parse("https://www.baidu.com"); Intent intent = new Intent(); intent.setData(data);

Set the Data property directly when creating the Intent object:

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

The Action property of the Intent is set to Intent.ACTION_VIEW, indicating that you want to view some data. The Data property is set to a Uri pointing to a web page, indicating that the data to be viewed is a web page.

In addition to the Data attribute, Intent also has a Type attribute that specifies the MIME type of the data pointed to by the URI in the Data attribute. In most cases, when the Data attribute is set, the Android system automatically infers the value of the Type attribute. If you need to explicitly specify the MIME type, you can use the setType() method or the setDataAndType() method to set the Type attribute.

Category Property

The Category attribute of Intent is a string used to add additional information to the Intent or specify the environment in which the current action is executed. The Category attribute is used to further describe the nature or purpose of the Intent and help the system more accurately determine which component can handle the Intent.

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The Android SDK defines some standard Category constants:

• Intent.CATEGORY_DEFAULT: The default category, usually used with Action, indicating that there is no specific category information.
• Intent.CATEGORY_BROWSABLE: Indicates that this Intent can be used to browse data, such as launching a web browser to view a web page.
• Intent.CATEGORY_LAUNCHER: This is a special category, usually used with Action.MAIN, indicating the entry point of the application.

In addition to these standard categories, you can also define your own custom category strings. Use the addCategory() method to add a Category attribute to an Intent:

 Intent intent = new Intent(Intent.ACTION_VIEW); intent.setData(Uri.parse("https://www.baidu.com")); intent.addCategory(Intent.CATEGORY_BROWSABLE);

The Action property of the Intent is set to Intent.ACTION_VIEW, indicating that you want to view some data. The Data property is set to a Uri pointing to a web page, indicating that the data to be viewed is a web page. The Category property is set to Intent.CATEGORY_BROWSABLE, indicating that this Intent can be used to browse data, so the system may choose a web browser to handle this Intent.

The Category attribute is often used in conjunction with the Action attribute. In some cases, if the Intent does not specify the correct Category, then even if the Action matches, the system may not be able to find the right component to handle the Intent. When creating an implicit Intent, it is very important to ensure that the correct Action and Category attributes are set.

Extras

The Extras property of Intent is a collection of additional data required by the target component. Data is usually stored in the form of key-value pairs, which can be added to the Intent through the putExtra() method and retrieved in the target component using the getXXX() series of methods (such as getStringExtra(), getIntExtra(), getParcelableExtra(), etc.). Extras can be used to pass almost any type of data, such as strings, integers, Boolean values, serialized objects, etc.

 Intent intent = new Intent(MainActivity.this, UserActivity.class); intent.putExtra("name", "Reathin"); intent.putExtra("age", 29); startActivity(intent);

Get data in UserActivity:

 String name = getIntent().getStringExtra("name"); int age = getIntent().getIntExtra("age", 0);

Type Attribute

The Type attribute of an Intent (also called Data Type or MIME type) is used to specify the MIME type of the data associated with the Intent. The Type attribute is usually used together with the Data attribute, which specifies the URI of the data and the Type attribute specifies the MIME type of the data. By setting these two attributes, you can more precisely control which component should receive and process this Intent.

Use the setType() method to set the Type attribute of the Intent:

 Intent intent = new Intent(Intent.ACTION_VIEW); Uri imageUri = Uri.parse("content://media/external/images/media/4"); intent.setData(imageUri); intent.setType("image/jpeg"); startActivity(intent);

When this Intent is launched, the system will find an activity that can handle JPEG images to display the image.

Although the Type attribute is optional in some cases, it is required in some cases. For example, if you want to start a Service or BroadcastReceiver that can handle a specific type of data, you need to set the correct Type attribute to ensure that the system can find the correct target component.

Component Properties

The Component property of Intent is used to explicitly specify the target component to be started. It is usually a ComponentName object that contains the package name and class name of the target component.

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After setting the Component property, the Android system will directly start the specified component, and will no longer search for matching components based on other Intent properties (such as Action, Data, Category, etc.). The Intent parsing process is more clear and direct.

The ComponentName class can be created in the following way:

1. ComponentName(String pkg, String cls): Create a ComponentName object from the package name and class name.

 ComponentName componentName = new ComponentName("com.reathin.sample", "com.reathin.sample.MainActivity"); Intent intent = new Intent(); intent.setComponent(componentName);

2. ComponentName(Context pkg, String cls): Creates a ComponentName object using the Context object and class name.

 Context context = getApplicationContext(); ComponentName componentName = new ComponentName(context, MainActivity.class); Intent intent = new Intent(); intent.setComponent(componentName);

3. ComponentName(Context pkg, Class<?> cls): Creates a ComponentName object from a Context object and a class object.

An Intent with the Component attribute set is called an explicit Intent, and an Intent without the Component attribute set is called an implicit Intent. The system needs to find a matching component based on other attributes of the Intent.

Flags property

The Flags attribute of Intent is used in Android to set some behavioral characteristics when starting an Activity or other component. Flags can affect how Intent is parsed, how components are started, and how components interact with each other.

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1. 「FLAG_ACTIVITY_NEW_TASK」: Usually used to start an Activity from a non-Activity context (such as a broadcast receiver, service, or outside the application). When this flag is set, the system starts the Activity in a new task (Task), even if the Activity already exists in another task. If a task with the same affinity as the Activity is found, it is moved to the foreground as a whole, keeping the state in the stack unchanged, and then the Activity is pushed onto the stack.
2. "FLAG_ACTIVITY_CLEAR_TOP": When the Intent object contains this flag, if an Activity with the same instance as the Activity to be started is found in the stack, the system will clear all Activities above this instance and make the instance at the top of the stack. This is often used to ensure that the state of the application is clear when the user returns to a specific Activity in the application.
3. FLAG_ACTIVITY_SINGLE_TOP: If this flag is set, when a new Activity is started, if it is already at the top of the task stack, the system does not create a new Activity instance, but reuses the existing instance and passes the new Intent by calling its onNewIntent() method.
4. 「FLAG_ACTIVITY_EXCLUDE_FROM_RECENTS」: Indicates that you do not want the launched Activity to appear in the list of recently used applications. This may be useful for one-time or privacy-sensitive activities.

These flags can be used individually or in combination to define specific behavior. Use the addFlags() or setFlags() method of the Intent class to set the Flags attribute.

 Intent intent = new Intent(MainActivity.this, UserActivity.class); intent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK); startActivity(intent);

or:

 Intent intent = new Intent(MainActivity.this, UserActivity.class); intent.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK | Intent.FLAG_ACTIVITY_CLEAR_TOP); startActivity(intent);