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What is an Intent in Android?
(14 answers)
Closed 9 years ago.
I still don't understand what is so special about Intents. Why not just use a new thread or just call the function? I think I got the whole idea about intents wrong. A simple code showing why Intents are better or when are needed would be great!
Intents are get widely used in android to switch from one activity to other . it is good practice to use intents . Using intents we can pass/send values from one activity to another. So it can be used as value passing mechanism. Also its syntax is very simple.so why to think about threads ?
Intents are asynchronous messages which allow application components to request functionality from other Android components. Intents allow you to interact with components from the own and other applications. For example an activity can start an external activity for taking a picture.
Intents are objects of the android.content.Intent type. Your code can send them to the Android system defining the components you are targeting. For example via the startActivity() method you can define that the intent should be used to start an activity.
An intent can contain data via a Bundle. This data can be used by the receiving component.
To start an activity use the method startActivity(intent). This method is defined on the Context object which Activity extends.
The following code demonstrates how you can start another activity via an intent.
# Start the activity connect to the
# specified class
Intent i = new Intent(this, ActivityTwo.class);
startActivity(i);
Why not just use a new thread or just call the function?
No matter what Thread you use, there would still need to be a mechanism to direct the message which is what an Intent does. It is a way to send a message. Now, it needs to be called on the UI Thread or have an appropriate Context passed because it needs this to send the message. Call what function? You are. It calls a constructor of the Intent class.
A simple code showing why Intents are better or when are needed would be great!
I don't have a simple code to compare to because I'm not sure what you want to see. No one is saying that it is better than something else. As to why Intents are used opposed to something else? I don't know...you would have to ask the developers of the Android platform. That is what they decided to use.
When they are needed is when you want to pass a message from one Activity to another or from one application to another. From the docs
An intent is an abstract description of an operation to be performed
I said "message" but the docs say "description of an operation to be performed" (I guess can mean the same thing). You can use them to start an Activity, pass data between Activities, and more such as telling the OS what to do at boot time. Why is it better? Better than what? That is what the developers decided to use so I guess you would have to ask them but maybe they didn't think it was better rather different.
while I don't have an example, Intents encourage coupling components loosely.
It negates the necessity of you building your own Observer design patterns and enables Inter/Intra-Application communication.
Related
why we have to launch an activity using intent
why cannot we launch it using Activity a = new Activity ??
i tried searching in android developers but did not get any answer
You can, but it won't be fully initialized. There's a series of functions on the Activity that needs to be called in the right order to correctly initialize it, and a series of steps taken by the framework in order to display the activity. Calling startActivity will take those steps and call the Activity lifecycle functions in the correct order. So there's no reason to use new.
Looking at it another way- lets say you were to do Activity a = new Activity(). How would the OS know you did that, and know its supposed to display that Activity? How would it know that the Activity is now supposed to go into the foreground? It wouldn't. It needs to be told that a new Activity was launched. That's why startAcctivity exists. And since you have to call that anyway, why not just make it a factory method as well, so initialization and launching are one step?
Not only you can create an activity object using the new keyword, but also you can inflate its layout/sub-views and then even launch it.
But the approach of implementing things above is very tedious and error-prone. And it is not recommended if you are dealing with normal Android app development.
If you are curious on how it is done, have a look at these github projects:
https://github.com/singwhatiwanna/dynamic-load-apk
https://github.com/didi/VirtualAPK
https://github.com/DroidPluginTeam/DroidPlugin
These projects aim to load and launch external Activities/Services from UNINSTALLED apk files. One tiny aspect of the way they implement this is by creating activities with new.
While working with different activities and starting them for results I have no choice but to use intent. Now intent requires context and that makes no sense to me. I know that context allows access to the application resources but
why do you need to know about the application resources when an intent is just a simple messenger?
Also, I am not so sure why some people create intent with the getApplicationContext() while other use this for the activity context????
Lastly, I am not so sure how the activity that calls for startActivityResult() receive a call back on the method onActivityResult() when I don't pass the "this" for the context but instead the application context. I thought that you have to use the "this" or passing in the current activity context that called startActivityResult() in order to receive a callback. That is just straight up java right? If you pass in a class then the other activity class will have a reference to your class and hence allows it to call the method in your class which is onActivityForResult(). However, this is not the case so what am I missing?
Intent itself does not need the Context. The constructor Intent#Intent(Context, Class) is just a convenience constructor, that internally uses the provided arguments to derive a ComponentName. ComponentName is in turn just a package name of your app and a class name to target. So ComponentName might be something like:
com.foo.bar/com.foo.bar.ui.activity.MyActivity
However, you can as well just use an empty constructor Intent#Intent() and provide ComponentName yourself (Intent#setComponentName(ComponentName)).
Therefore it doesn't matter if you provide your Application's or your Activity's context (the latter is just simpler to type). Also keep in mind that classes that require application context can call Context#getApplicationContext themselves, so this is not something you need to worry about.
About startActivityForResult() - Android manages internally a stack of your Activity records. Therefore it delivers the result to the previous Activity on the stack. It is the same way it knows where to return, when you click "back".
Please note it doesn't mean it maintains a stack of your Activity instances. These instances might be long gone - destroyed and garbage collected to free the memory. However the stack contains the information that allows to recreate them and to restore their state.
Intent does not need Context for itself but as you yourself pointed out that Intent is just a messenger. It also passes the current state of application/object to the newly created object so that it can understand that what exactly is going on in the application. And that is why we need to pass the context.
And, I believe that you want to ask about startActivityForResult(). Android itself takes care of the callback in the same way other callbacks are handled. You can take the example of Activity Life-cycle. Whenever it is started onCreate(), onStart(), onResume() are called itself by Android.
Not been much deep into Android development but still let me try with an explanation. So basically, context is a reference to linking your resources to your program. Each object is given its own context, which contains the resources required to set that object up. It is required for many objects to be created, and to get program identifying information, among other purposes. This makes it invaluable to set up new views and activities, but it can also be used for other purposes. See this Android Context for more information.
According to this page Activity inherits context. Thus, if you are in an activity, you only need to pass itself to use the context. It also contains a pointer to getBaseContext(). You might occasionally need to reference that, if you need the entire application context, but most likely you won't for a while.
There are ways of creating an Intent which do not require a Context. But if you want to target a specific class of a specific package, then providing a context for the target package is a ready way to do that. Refer this Context Lesson.
As explained by snctin in his answer getApplicationContext() offers application context. Basically the Application context is associated with the Application and will always be the same throughout the life cycle of your app. Also refer this post.
See Android - How to start (display) a new Activity. According to it
startActivity(new Intent(this, ProjectsActivity.class));
assumes your current class extends one of the Android Activity classes, which gives you access to the startActivity method.
According to Getting result from a activity, Starting another activity doesn't have to be one-way. You can also start another activity and receive a result back. To receive a result, call startActivityForResult() (instead of startActivity()).
For example, your app can start a camera app and receive the captured photo as a result. Or, you might start the People app in order for the user to select a contact and you'll receive the contact details as a result. This post will help you understand the same more better way.
Hope that helps. And also thanks becuase of your question I had a refresh with Android.:)
Intents are taking too long to be process in my app. Is there a better way I can tell different elements of my app that something has happened? For example I use:
Intent i = new Intent("com.ftx.player_died");
I listen for that intent on two different places in my app. I would not like to make the same call twice.
Inventory.playerDied(true);
NotificationBar.playerDied(true);
Doesn't scale nicely.
Is there something I can use that is faster than intents but that I don't make same call twice or three times?
Using LocalBroadcastManager orEventBus is not only faster the normal BroadcastManager, but also is private to your app (that means other application cant listen to your Intent action)
It depends on a lot of factors. It is needed to see of your project to give a piece of advice.
But in some cases it is possible to use usual interface that declared and shared in the static Application class.
I'm working with the sample media player given by the android sdk. MainActivity starts Service MusicService with startService(new Intent(MusicService.ACTION_PLAY)).
I need to find a view by ID inside the Service but I don't know how to do it:
findViewById(R.id.playbutton).setVisibility(View.GONE);
I've found some similar questions but none provide a simple solution (the most similar question's accepted answer is actually "no you can't" and I'm sure it's possible). How can I make this line work inside the Service? Do I have to pass the context from MainActivity to it, how do I do it?
Since the service handles media playback the interface should be updated directly before playing/pausing that's why I need to update the ui from it
No, you do not. You need to let the UI know, if it exists, about the state change. The UI will affect its own changes how it sees fit. There may not be any UI at all, depending upon what the user has done.
For letting any affected bits of UI know about the state change, you can:
send a regular broadcast Intent, or
use LocalBroadcastManager to send a "narrowcast" Intent (works a lot like a broadcast, but it is completely within your process), or
use Otto as an event bus
I have two classes, one is a standard activity, the other a service. I would like the service to update a TextView (via .setText()) of the Activity on certain events that happen within the service.
I've tried to achieve this by programming something like a setter-method inside the Activity-Class, but TextViews don't accept a static-reference, and when I try invoking an instance of the Activity-class instead (via MyActivityClassName aVariable = new MyActivityClassName();), I get a NullPointer Exception, even though the View in questions is visible at the time of the call.
Could anyone tell me what I'm doing wrong :-)? It is probably more of a basic Java question than an Android-one, but since it might have to do with the nature of Android-services, I've still added the android-tag.
Thanks for your help!
I would like the service to update a
TextView (via .setText) of the
Activity on certain events that happen
within the service.
I would strongly recommend greater logical decoupling. Have the Service tell the Activity to update the TextView.
You can do that via a broadcast Intent, or by having the Activity provide a listener object that the Service calls. In either case, be sure the Activity detaches from the service (unregisters the listener or broadcast Intent receiver), lest you cause a memory leak.
And, of course, this only works if the activity is actually running.