[MUSIC] In our last lesson we talked about the activity class. And I showed you how one activity can programmatically start another activity by first creating an intent object and then by passing that intent to a method such as start activity or start activity for result. Today, we are going to take a deeper look at intents. We'll discuss how they are created and processed as they're used in starting other activities. And I'll begin by presenting the intent class itself. I'll talk about how they're created, what fields they have, and what information those fields contain. After that, I'll talk about the two ways in which Android decides which activities should be started when a method such as start activity is called. First, I'll talk about explicit activation in which intents specifically name the activity they want to start. And second, I'll start about implicit activation, where intents don't select a target activity but instead describe the operation they want to be performed. After which Android finds and then starts the activity that can perform that operation. The intent class is basically a data structure that serves at least two purposes in Android. One, it can specify an operation that you want to perform. And two, it can represent an event that has occurred in the system that you want to notify other components about. Today however we're going to focus on just the first of these. Using intents to specify operations that you want performed. And we'll leave the second of these, using intents for event notification, for a later lesson when we talk about broadcast receivers. Now you can think of intents as providing a kind of language for specifying operations that you want to have performed. In essence, intents give you an easy way to say things like, I want to select a contact. I want to take a photo. I want to dial a phone number. Or I want to display a map. So in practice, intents are usually constructed by one activity that wants some work to be done. And then Android uses that intent to start another activity that can perform the desired work. So let's talk now about the kinds of information that can be specified within an intent. For example, we'll talk about intent fields including action, data, category, MIME type, target component, extras, and flags. An intent's action field is a string that represents or names the operation to be performed. Some built in examples include, action_dial, which means I want someone to dial a phone number. Action_edit, I want someone to display some data for the user to edit. Action_sync, I would like to synchronize some data on my device with data on a server. An action_main, I want to start an activity as the initial activity of an application. You can set an intent's action in several ways. For example, you can pass an action string as a parameter to the intent constructor. Alternatively you can create an empty intent and then call the set action method on it, passing the action string again as a parameter. Intents also has a data field which represents data that is associated with the intent. Now that data is formatted as a uniform resource identifier, or URI. And one example of intent data is a geo-schemed URI which indicates map data. You might also remember that we saw something exactly like this in earlier lessons as part of those map location applications. Another example is a tell schemed URI, indicating a phone number that you want dialed. And note that the strings that represent the underlying data are first being passed through the Uri.parse method, which takes that string and then returns a URI object. You can set the intents data in a variety of ways. For instance, you can pass it directly to the intents constructor. Or you can create an empty intent and then use the setData method to set the data for that intent. The intent category represents additional information about the kinds of components that can handle or should handle this intent. Some examples include category_browsable. Which means that the activity can be invoked by a browser via a URI link. Another example is category_launcher. Which means that the target activity can be the initial activity of a task and is listed in the top level application launcher. Intents also have a type feel. Which specifies the MIME type of the intents data. Some examples of MIME types include, various image formats. Things like image/png for PNG images, and image/jpeg or JPG for JPEG images. There's, there are also various text formats, such as text/html for HTML text, or text/plain for arbitrary text data. And of course, if you don't specify a MIME type, Android will try to infer one for you. You can set an intents MIME type by using the setType method passing in a string that represents the desired mind type. You can also set both the data and the type together by calling the setDataAndType method. Intents also have a component field that identifies the intent's target activity. And you can set this field if you know that there is exactly one activity that should always receive this intent. You can set the intent's target component using one of the intent's constructors, by passing in a context object, and a class object which represents the activity that should perform the desired operation. And we'll see a code example of this in a few minutes. You can also create an empty intent and then use one of the setComponent(), setClass(), or setClassName() methods to set that target activity. Intents also have an extras field. Extras store additional information associated with the intent. Now extras are effectively a map of key-value pairs. So the target activity has to know both the name and the type of any extra data that it intends to use. For example, the intent class defines an extra called EXTRA_EMAIL. And this extra is used to pass a list of recipients when sending email. Now as you can see here, this code creates an intent, with the action Intent.ACTION_SEND. So we want to send some email. It also adds an extra to that intent. The extra is named EXTRA_EMAIL. And in this case it's values include me, aporter@cs.umd.edu, ceo@microsoft.com, potus@whitehouse.gov, and mozart@musician.org. Now there are several different methods for setting extras and the specific form of these methods will depend on the type of data you want to store. So, for example, there's one method for storing a string, one method for storing an array of floats, and so on. Another intent field is called flags. Flags represent information about how the intent should be handled. Some built in examples include flag_activity_no_history. Which means that when an activity is started based on this intent, it shouldn't be put in the history stack. Another flag is FLAG_DEBUG_LOG_RESOLUTION which tells Android to print out extra logging information when this intent is being processed, and this is a great tool to use if your intents are not starting up the activities that you want them to.
