I need a little help with my Interface. I think that i doesn't understand them at all.
So i created this interface to notify every classes that implements it when a certain event occurs.
public interface OnColorsChangeListener {
void onColorsChangeListener(ColorsProp colorsProp);
}
My class that hold the interface:
private OnColorsChangeListener mCallback;
... // other code
// the event occurs here so i call:
mCallback.onColorsChangeListener(mProps);
// but of course here i get an NPE becouse this is undefined in this class.. well, with some replies here i'll understand better how to use that for reach my point
The class that implements it:
public class ClassTest implements OnColorsChangeListener {
... // other code
#Override
public void onColorsChangeListener(ColorsProp colorsProp) {
Log.d(TAG, "Color changed! " + colorsProp.color);
}
i put this in 4/5 classes to be notified in the same time for the color change. I'm quite sure the reason is that I didn't understand very well how them works, so can anyone point me to the right direction? Thank you!
Explanation by example:
You have to instantiate your callback, & it has to be an instance of your class
private OnColorsChangeListener mCallback;
mCallback = new ClassTest();
mCallback.onColorsChangeListener(mProps);
However if you want multiple callbacks you will need to use the Observer pattern.
Simple example:
private List<OnColorsChangeListener> mCallbacks = new ArrayList<OnColorsChangeListener>();
mCallbacks.add(new ClassTest());
mCallbacks.add(new OtherClass());
for(OnColorsChangeListener listener : mCallbacks) {
listener.onColorsChangeListener(mProps);
}
Obviously if you have the class, somewhere else you would not new it up, you would use that reference:
mCallbacks.add(mClassTest);
Observer Pattern Wikipedia
An interface is just a way to group together a bunch of related methods. Implementing this interface then requires you to implement all the methods grouped together by the interface.
The Java Tutorials has a good read on the subject:
What is an interface?
Here's a Stackoverflow thread regarding listener interfaces in android:
How to create our own Listener interface in android?
In short, you don't use the interface directly since it only specifies which methods implementing classes are supposed to implement.
Related
I am creating a Listener class that a couple instances of a custom button in different Activities/Fragments are using. This class has listener methods that will update the respective ViewModel for that Activity/Fragment.
How do you define a ViewModel in a non-activity/fragment class? The documentation says to implement ViewModelStoreOwner, but I'm not really sure on how and what I should be implementing. I'm assuming if I don't implement it correctly, I'll have some sort of memory leak...
public class Listeners implements View.OnClickListener, ViewModelStoreOwner {
#NonNull
#org.jetbrains.annotations.NotNull
#Override
public ViewModelStore getViewModelStore() {
return // what do I do here, and how do I tell it to close the scope appropriately
// when the context is destroyed?
}
// Implement OnClick...
}
Am I just trying to abstract too much here? Does Android really just revolve around Activities and Fragments thus requiring me to have annoyingly long files? The above class is my attempt to reduce redundant implementations of a button listener between two activity/fragments
EDIT:
Is it wrong to just pass the store owner of the activity that this listener instance will eventually reside in? For example:
// Custom class constructor
public Listeners(ViewModelStoreOwner storeOwner) {
mModel = new ViewModelProvider(storeOwner).get(Model.class);
}
// Calling/parent activity/fragment/context
Listeners listeners = new Listeners(this);
mButton.setOnClickListener(listeners);
Unless someone posts an answer to this that says otherwise (and that this is a bad idea), I ended up utilizing the latter solution I updated my question with.
I passed the store owner into the custom Listener class as a parameter, then used this value to initialize my ViewModelProvider inside the custom class.
I believe this is safe, since the class is instantiated within the scope of that parent Fragment/Activity anyway.
So for instance, if you were calling this class from an activity/fragment:
// Calling from Activity
Listeners listeners = new Listeners(this);
// Calling from Fragment
Listeners listeners = new Listeners(requireActivity());
And the relevant class definition:
public Listeners(ViewModelStoreOwner storeOwner) {
mModel = new ViewModelProvider(storeOwner).get(Model.class);
}
I have multiple fragments which does the same thing but presented in different way to the user. However, the functionality in all those fragments more like same as in delete, add etc. That said, I do not want to duplicate the code. Therefore created a manager class so that I could have the centralized code. But, the problem now is, when the user is performing an action say, when the user is deleting an item, the fragment does not get refreshed. So, I need to send the message to the fragment from the manager class to refresh the list. I have the following pseudo code to give an insight...
public class MyFragment extends Fragment{
ManagerClass mManagerClass;
private void onItemSelected() {
mManagerClass = new ManagerClass(itemId);
}
public void refreshItems() {
ItemDao.query();
}
}
public class ManagerClass {
public ManagerClass(int itemId) {
DeleteItem(itemId);
}
private void DeleteItem(int itemId) {
//when this task completes it should call the MrFragment.refreshItems();
//Keep in mind that I cannot pass the Fragment becuse this ManagerClass is designed to handle more than two fragments
//MAY BE I SHOULD DO CALLBACK BUT HOW?.... When i try to implement there several callbacks but not sure which one should I use and how...
}
}
Any help would be greatly appreciated...
I was so stupid and dumb at the time. There are multiple ways to achieve this but the quick resolutions are:
1. Have a ABSTRACT class and extend the fragment out of it or
2. Use the OBSERVER pattern
I went with the second option, Observer pattern and works like a charm!
Well, I'm at a dilemma here. I made my own class that uses the Bluetooth class from android but I'm not sure where to put it. Extending the android Bluetooth class seems like a good idea but I need to override the onActivityResult() which is only available to an activity class. So, where would I put my class so that I have access to onActivityResult() (keeping in mind the idea here is to use as few dependencies as possible)?
In other words, I want to move the Bluetooth code from the main activity to a separate class.
You should to use separate file for each class. You can create a folder "engine". For example: com.mycorp.myapp.engine. You can get access to onActivityResult() very simple. For example: MainActivity.onActivityResult(). Note: function should be public.
Or you can pass your activity to your CustomBluetooth's constructor.
public class CustomBluetooth {
private Activity mActivity;
/* Constructor */
public CustomBluetooth (Activity pActivity ) {
super();
this.mActivity = pActivity;
}
/* Your functions */
public int getResult() {
return this.mActivity.onActivityResult();
}
}
Alex. P.S. Sorry for my English:)
Add an interface to your Bluetooth class and implement the interface in your activity.
I want to create an Android framework service and publish a listener interface to update clients on updates. I don't want to make register_XX() & unregister_XX() methods for applications to provide listener reference .What i want is , applications simply implement that listener interface and without calling any register/ unregister method gets notified of changes.
How can i implement that ?
is there any good reason against register/unregister? if you call an interface *Listener, i would expect to register/unregister it (from a dev point of view).
i think you can achieve your goal with some abstract class AbstractListener, that registers itself in the constructor - but that's not nice from an architectural point of view and doesn't work around the unregistering.
e.g.
public abstract class AbstractUpdateReceiver {
public AbstractUpdateReceiver() {
register();
}
public abstract void onReceive(Update anUpdate);
// optional, only the user should react on registering
public abstract void onRegistered();
// optional, only the user should react on unregistering
public abstract void onUnregistered();
private void register() {
//...do register
onRegistered();
}
public void unregister() {
//do unregister
onUnregistered();
}
}
the on(Un)Registered calls and implementation are not necessary, if the whole registration process should be hidden from the implementing application. but for the unregistering, the user still has to call unregister(), as long as your framework doesn't provide anything like clearReceivers() or clearReceiver(AbstractUpdateReceiver receiver)
another (much) nicer solution without anything like registering/unregistering the implementation into the framework from the implementing application would be a factory doing the whole job instead of an abstract class. in that case, you have to split the abstract class into a normal class and an interface, the normal class provides and handles the registration, while the interface provides a method for onUpdateReceived. then you'll have the factory, that creates and returns the class from the framework.
public UpdateReceiver createUpdateReceiver(IUpdateReceiver wrapped) {
UpdateReceiver receiver = new UpdateReceiver(wrapped);
receiver.register(); //or register directly into a non-static factory
return receiver;
}
I just switched over from iPhone to Android and am looking for something similar to where in the iPhone SDK, when a class finishes a certain task, it calls delegate methods in objects set as it's delegates.
I don't need too many details. I went through the docs and didn't find anything (the closest I got was "broadcast intents" which seem more like iOS notifications).
Even if someone can point me to the correct documentation, it would be great.
Thanks!
Never mind... found the answer here :)
http://www.javaworld.com/javaworld/javatips/jw-javatip10.html
Pasting from the article so as to preserve it:
Developers conversant in the event-driven programming model of MS-Windows and the X Window System are accustomed to passing function pointers that are invoked (that is, "called back") when something happens. Java's object-oriented model does not currently support method pointers, and thus seems to preclude using this comfortable mechanism. But all is not lost!
Java's support of interfaces provides a mechanism by which we can get the equivalent of callbacks. The trick is to define a simple interface that declares the method we wish to be invoked.
For example, suppose we want to be notified when an event happens. We can define an interface:
public interface InterestingEvent
{
// This is just a regular method so it can return something or
// take arguments if you like.
public void interestingEvent ();
}
This gives us a grip on any objects of classes that implement the interface. So, we need not concern ourselves with any other extraneous type information. This is much nicer than hacking trampoline C functions that use the data field of widgets to hold an object pointer when using C++ code with Motif.
The class that will signal the event needs to expect objects that implement the InterestingEvent interface and then invoke the interestingEvent() method as appropriate.
public class EventNotifier
{
private InterestingEvent ie;
private boolean somethingHappened;
public EventNotifier (InterestingEvent event)
{
// Save the event object for later use.
ie = event;
// Nothing to report yet.
somethingHappened = false;
}
//...
public void doWork ()
{
// Check the predicate, which is set elsewhere.
if (somethingHappened)
{
// Signal the even by invoking the interface's method.
ie.interestingEvent ();
}
//...
}
// ...
}
In that example, I used the somethingHappened predicate to track whether or not the event should be triggered. In many instances, the very fact that the method was called is enough to warrant signaling the interestingEvent().
The code that wishes to receive the event notification must implement the InterestingEvent interface and just pass a reference to itself to the event notifier.
public class CallMe implements InterestingEvent
{
private EventNotifier en;
public CallMe ()
{
// Create the event notifier and pass ourself to it.
en = new EventNotifier (this);
}
// Define the actual handler for the event.
public void interestingEvent ()
{
// Wow! Something really interesting must have occurred!
// Do something...
}
//...
}
That's all there is to it. I hope use this simple Java idiom will make your transition to Java a bit less jittery.
The pendant for kotlin.
Define your interface: In my example I scan a credit card with an external library.
interface ScanIOInterface {
fun onScannedCreditCard(creditCard: CreditCard)
}
Create a class where you can register your Activity / Fragment.
class ScanIOScanner {
var scannerInterface: ScanIOInterface? = null
fun startScanningCreditCard() {
val creditCard = Library.whichScanCreditCard() //returns CreditCard model
scannerInterface?.onScannedCreditCard(creditCard)
}
}
Implement the interface in your Activity / Fragment.
class YourClassActivity extends AppCompatActivity, ScanIOInterface {
//called when credit card was scanned
override fun onScannedCreditCard(creditCard: CreditCard) {
//do stuff with the credit card information
}
//call scanIOScanner to register your interface
override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
super.onViewCreated(view, savedInstanceState)
val scanIOScanner = ScanIOScanner()
scanIOScanner.scannerInterface = this
}
}
CreditCard is a model and could be define however you like. In my case it includes brand, digits, expiry date ...
After that you can call scanIOScanner.startScanningCreditCard() wherever you like.
The main content of this video tutorial is to show how to use interfaces to delegate methods / data exchange between different Fragments and activities, but it is great example to learn how delegate pattern can be implemented in Java for Android.
Java callback is not the same thing like ios delegate, in ios you can use a callback almost the same way like in Android. In Android there is startActivityForResult that can help you to implement the tasks for what ios delegate is used.
I believe ListAdapter is a example of delegation pattern in Android.
Kotlin's official Delegation pattern:
interface Base {
fun print()
}
class BaseImpl(val x: Int) : Base {
override fun print() { print(x) }
}
class Derived(b: Base) : Base by b
fun main() {
val b = BaseImpl(10)
Derived(b).print()
}
See: https://kotlinlang.org/docs/delegation.html