I'm a beginner in Android, so I apologize for the mistakes and I'd appreciate any constructive criticism.
I'm writing a basic application with a ListView of images, and when the user clicks on an item in the list, I want to display that image in a ViewPager, where the user can swipe back and forth to browse the whole list of images. Afterwards when the user presses the back button, I want to switch back to the ListView.
I manage the business logic in the MainActivity, which uses MainActivityFragment for the ListView and ImageHolderFragment for ViewPager.
The simplified code so far is as follows:
#Override
protected void onCreate(final Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
mListItems = new ArrayList<>();
mListItemAdapter = new ListItemAdapter(this, R.layout.list_item, R.id.list_item_name, mListItems);
mListView = (ListView) findViewById(R.id.list_view_content);
mListView.setAdapter(mListItemAdapter);
mDeletedListItems = new ArrayList<>();
mViewPager = (ViewPager) getLayoutInflater().inflate(R.layout.image_display, null, true);
mImageAdapter = new ImageAdapter(getSupportFragmentManager(), mListItems);
mViewPager.setAdapter(mImageAdapter);
mViewPager.setOffscreenPageLimit(3);
mListView.setOnItemClickListener(new AdapterView.OnItemClickListener() {
#Override
public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
mViewPager.setCurrentItem(position);
setContentView(mViewPager); // TODO: this is very wrong!
}
});
loadImages();
noContentText = (TextView) findViewById(R.id.no_content_text);
if (mListItems.isEmpty()) {
noContentText.setText(R.string.no_images);
} else {
mImageAdapter.notifyDataSetChanged();
}
}
Although this does work to some extent, meaning that it manages to display the ViewPager when an item in the list is clicked, there are two things about it ringing the alarm bells:
I've read that calling setContentView() for the second time in the same class is pretty much a sin. Nobody explained me why.
The back button doesn't work in this case. When it's pressed, the application is terminated instead of going back to the list view. I believe this is connected to the first point.
I would appreciate any help, explanations if my idea is completely wrong, and if my case is hopeless, I'd like to see a successful combination of ListView and ViewPager with transitions between each other.
Your activity already has R.layout.activity_main set as content view, which rightly displays the list view - that's what the responsibility of this activity is as you defined it. If we want to change what's shown on the screen, we should use a different instance of a building block (activity or fragment) to display the view pager images.
To say the least, imagine if you wanted to change the view to a third piece of functionality or UI, or a fourth... it would be a nightmare to maintain, extend and test as you're not separating functionality into manageable units. Fields that are needed in one view are mixed with those needed in another, your class file would grow larger and larger as each view brings its click listeners, callbacks, etc., you'd also have to override the back button so it does what you want - it's just not how the Android framework was designed to help you. And what if you wanted to re-use UI components in different contexts whilst tapping in to the framework's activity lifecycle callbacks? That's why fragments were introduced.
In your case, the list view could continue to run in your MainActivity and in your click listener, onItemClick you could start a new activity that will hold a viewPager:
Intent i = new Intent(MainActivity.this, MyLargePhotoActivityPager.class);
i.putExtra(KEY_POSITION, position);
// pass the data too
startActivityForResult(i, REQUEST_CODE);
Notice how you could pass the position to this activity as an int extra, in order for that second activity to nicely set the viewPager to the position that the user clicked on. I'll let you discover how to build the second activity and put the ViewPager there. You also get back button functionality assuming your launch modes are set accordingly, if needed. One thing to note is that when you do come back to the list View, you'd probably want to scroll to the position from the view pager, which is why you could supply that back as a result via a request code. The returned position can be supplied back to the list view.
Alternatively, you could use the same activity but have two fragments (see the link further above) and have an equivalent outcome. In fact, one of your fragments could store the list view, and the second fragment could be a fullscreen DialogFragment that stores a viewPager, like a photo gallery (some details here).
Hope this helps.
I've read that calling setContentView() for the second time in the
same class is pretty much a sin. Nobody explained me why.
Well, you kind of get an idea as to why.
When you use setContentView() to display another 'screen' you do no have a proper back stack.
You also keep references to Views (like mListView) that are not visible anymore and are therefore kind of 'useless' after you setContentView() for the second time.
Also keep in mind orientation changes or your app going to the background - you'll have to keep track of the state that your Activity was in which is way more complicated than it has to be if you have one Activity that does two different things.
You won't be arrested for doing things like you do right now, but it's just harder to debug and keep bug free.
I'd suggest using two different Activities for the two different things that you want to do, or use one Activity and two Fragments, swapping them back and forth.
If you insist on having it all in one Activity you need to override onBackPressed() (called when the user presses the back button) and restore the first state of your Activity (setContentView() again, pretty much starting all over).
Related
I know this is somewhat of a design question but I do have specific questions for it. I'm trying to understand how to handle a situation like this one:
Let's say I have a RecyclerViewFragment which loads a RecyclerView containing a bunch of Toy objects.
In one situation: Maybe this RecyclerViewFragment is part of a ViewPager on main display. There is a FloatingActionButton add-button present over this RecyclerView. You click the + button and you can add a new Toy to the list. Or you can click a Toy from the list directly and a floating menu pops up with Edit/Delete buttons, and pressing Edit lets you edit the Toy's details in a DialogFragment, or clicking Delete removes it from the RecyclerView.
In another situation: Now I am in a separate part of the app where I want to choose toys to use. So I press a button and a DialogFragment appears with a RecyclerView of Toys. I can click a Toy and it'll be added to my cart.
It seems like I should be re-using the same RecyclerView code in both situations, since they both involve a list of the same Toys. The only difference is that in one situation, I can add Toys and edit Toy details, and in the other situation, there is no Add button and clicking on a toy does something different (adding to a cart as opposed to bringing up an Edit/Delete dialog).
Is this the correct way to handle this:
Communication from Fragment to Activity: Interfaces? Have the RecyclerViewFragment, in the onAttach method, assign a listener of my design to the context. Then when a row of the RecyclerView is pressed, the callback is triggered. Now the underlying Activity can decide what to do with that press -- show the Edit/Delete dialog in one situation, add the Toy to a Cart in the other situation. Either way, the click item sends the Toy to the calling Activity so it can decide what to do with it.
Communication from Activity to Fragment: Now what about the situation with the Add button? This Add button would not be intrinsically part of the RecyclerViewFragment, so when I click Add, it would bring up the details dialog box where I can give the Toy details, and then press OK to add it. So somehow I have to transfer this new Toy to the Fragment to have it added to the RecyclerView. Would I simply do something like this:
RecyclerViewFragment recyclerViewFragment = (RecyclerViewFragment ) getSupportFragmentManager().findFragmentByTag("TOY_RECYCLERVIEW");
recyclerViewFragment.getNewToyAndRefreshList(newToy);
and then in the RecyclerViewFragment:
public void getNewToyAndRefreshList(Toy newToy) {
toyList.add(newToy);
Collections.sort(toyList); //Toy has Comparable implemented, sort by name
recyclerViewAdapter.notifyDataSetChanged();
}
Am I on the right track? Is there a different way to fix this situation?
That's certainly a design question, but IMHO there's a very specific issue on it and I believe it's a good question (reason I'm answering), but that also means other developers might have other approaches to solve the issue.
1. that is a totally fair and acceptable approach to it. You let the fragment be simple UI element and let someone else (the activity) implement the click behavior.
For this approach remember to code it only against the interface. That means, don't cast it to your activity. For example:
// do this
toyClickListener.onToyClicked(toy);
// don't do this
((MyActivity)getActivity()).onToyClicked(toy);
That way you keep the "simple UI element" be completely unaware of who is implementing the behavior.
2. IMO for this kind of scenario (specially on RecyclerView.Adapter) the best thing to do is to forget the UI and only focus on the data. And how speciafically you implement this, will vary on what is your data source.
But the base idea is that you have somewhere a data repo (DB?) and anyone using data from there, should subscribe to changes to it.
So you override RecyclerView.Adapter.registerAdapterDataObserver and unregisterAdapterDataObserver add the subscription/listener code, something like that:
#Override registerAdapterDataObserver(RecyclerView.AdapterDataObserver observer) {
super.registerAdapterDataObserver(observer);
db.subscribe(this, toyList);
}
#Override unregisterAdapterDataObserver(RecyclerView.AdapterDataObserver observer) {
db.unsubscribe(this);
super.unregisterAdapterDataObserver(observer);
}
#Override public void onDbDataUpdate(new Data comes here){
update the data, and call .notifyDataSetChanged();
}
that way once the FAB + and then dialog is clicked the new Toy gets added to the DB and the adapter gets "automatically" notified.
So if this data comes from a SQLite you can call on the cursor registerContentObserver if it's a RealmDB you'll use addChangeListener, even Android databinding libraries have a ObservableList
I have an app that hold post information in an activity. in this activity related posts listed in bottom of post. User by clicking on related post can go to post activity and see that post info and related posts too.
As you can see in image, I have Activity A that holds post and it's related posts. When user Click on post I send user to Activity A with new post id and fill activity by new data.
But I think this is not Right way!
should I used Fragment instead of Activity?
Opening another Instance of an Activity on top of another is simplest way of navigating a content graph. User can simply press back, and go to previously opened content, until user reaches back to starting Activity, then the application closes. Though pretty straight forward, this particular approach has two issues:
It may happen that a lot of Instances of same activity are on the stack, utilising a large amount of device resources like memory.
You don't have a fine grained control over Activity Stack. You can only launch more activities, finish some, or have to resort to intent flags like FLAG_CLEAR_TOP etc.
There is another approach, that re-uses the same Activity instance, loads new content in it while also remembering the history of content that was loaded. Much like a web browser does with web page urls.
The Idea is to keep a Stack of content viewed so far. Loading new content pushes more data to stack, while going back pops the top content from stack, until it is empty. The Activity UI always displays the content from top of the stack.
Rough Example:
public class PostActivity extends AppCompatActivity {
// keep history of viewed posts, with current post at top
private final Stack<Post> navStack = new Stack<>();
#Override
protected void onCreate(#Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
// get starting link from intent extras and call loadPost(link)
}
private void loadPost(String link){
// Load post data in background and then call onPostLoaded(post)
// this is also called when user clicks on a related post link
}
private void onPostLoaded(Post post){
// add new post to stack
navStack.push(post);
// refresh UI
updateDisplay();
}
private void updateDisplay(){
// take the top Post, without removing it from stack
final Post post = navStack.peek();
// Display this post data in UI
}
#Override
public void onBackPressed() {
// pop the top item
navStack.pop();
if(navStack.isEmpty()) {
// no more items in history, should finish
super.onBackPressed();
}else {
// refresh UI with the item that is now on top of stack
updateDisplay();
}
}
#Override
protected void onDestroy() {
super.onDestroy();
// cancel any background post load, release resources
}
}
I would choose:
activity/fragment depends on complexity with:
horizontal recyclerview with custom expanded card view
and inside this expanded card view second vertical recyclerview :)
Here's what you can try.
Create a PostActivity which is a shell for fragments. Inside this activity you can just replace fragments using FragmentTransaction.
Your PostActivity can now have a PostFragment which will hold post and related posts. Now on click of post you can replace PostFragment with PostDetailFragment with postID being sent to the new fragment as a bundle. The PostDetailFragment will now display details according to id.
Check here: http://www.vogella.com/tutorials/Android/article.html#components_fragments
By seeing the picture the way i would implement is i would have create an activity with a bottom listview for your items and on top there would be a framelayout for holding fragments . when user click on any list item i would load the respective fragment in the activity
It all depends on what you are trying to achieve. What would you expect to happen when the user touches the back button after going down a couple of levels? If you want to the application to exit, no matter how deep in the sequence they have gone, then the best solution in my opinion is to simply reload the same activity with the new data and invaliding the affected views. If you need the back button to take the user back to the previous data, then the next question would be if you are keeping track of the past data breadcrumb. If so, then just intercept the back button and load the previous data for as long as there is data in your stack, or exit if you get to the top. If you don't want to keep track of the previous data chain, then instead of loading one activity with the new data, you can start a new activity of the same class, but with the new data. Android with keep the track of activities and each back button touch would close the running activity and take the user to the previous activity. Choice of activity versus fragment is just yours. You can use fragments that hold the data that you want to change after each user touch, create new ones when needed, disconnect the previous ones, and connect the new ones. You will need to do some extra work to make sure the back button works correctly (depending on you want the back button to behave). Based on what I can tell, it is simpler to just have one activity and load new data when needed and keep a trail of data changes, if you need to be able to go back.
It can be achieved using activity alone. Though I preferred moving all related UI to fragment.
You can use Navigator class.
Here the steps:
1. Add Navigator Class
public class Navigator {
private static Navigator instance;
public synchronized static Navigator getInstance() {
if (instance == null) {
instance = new Navigator();
}
return instance;
}
public void navigateToActivityA(Context context) {
Intent activity= AActivity.getCallingIntent(context);
context.startActivity(activity);
}
}
2. Add the calling method to your Activity class.
public static Intent getCallingIntent(Context context) {
return new Intent(context, AActivity.class);
}
3. Call the activity with the following code in your caller activity.
Navigator.getInstance().navigateToActivityA(this);
I suggest that you read about AndroidCleanArchitecture
For this task...
0) Starting new activity
I read again about question, and understood that you need advice for starting activity. So, starting new activity it's Ok, your main problem will be with another things (see below).
But lets talk about starting another data. Using Fragment instead doesn't resolve your task, fragments helps with different screen work. Using for example just data refreshing as a variant. You may use just single activity and refresh only data, it will look much better, if you also add animation, but not better than starting activity.
Using Fragment helps your with different screen actions. And maybe, answering on your question - it will be most suitable solution. You just use single acitivity - PostActivity, and several fragments - FragmentMainPost, FragmentRelated - which will be replaced, each other, by selecting from related post.
1) Issues with returning back
Lets imagine, that users clicks to new one activity and we loaded new data. It's Ok, and when Users clicks over 100 activities and receiving a lot of information. It's Ok, too. But main question here it's returning back (also another about caching, but lets leave it, for now).
So everyone know, it's bad idea to save a lot of activities in stack. So for my every application, with similar behavior we override onBackPressed in this activity. But how, lets see the flow below:
//Activities most have some unique ID for saving, for ex, post number.
//Users clicks to 100 new activities, just start as new activity, and
//finish previous, via method, or setting parameter for activity in AndroidManifest
<activity
noHistory=true>
</activity>
....
//When activity loaded, save it's activity data, for ex, number of post
//in some special place, for example to our Application. So as a result
//we load new activity and save information about it to list
....
// User now want return back. We don't save all stack this activities,
// so all is Ok. When User pressing back, we start action for loading
//activities, saved on our list..
.....
onBackPressed () {
//Getting unique list
LinkedTreeSet<PostID> postList =
getMyApplication().getListOfHistory();
//Getting previous Post ID based on current
PostID previousPostID = postList.get(getCurrentPost());
//Start new activity with parameter (just for ex)
startActivity(new Intent().putExtra(previousPostID));
}
RESULT
I found this as the best solution for this tasks. Because in every time - we work only with single activity!
I'm not so good at Android but I'm working on my project. So please forgive my ignorance.
I'm making a set of similar items with need prefenrences setting, so I made them preference fragments for each, and put those preference fragments in a viewpager, for easier use. I want to go to fragment No.1 if I clicked item No.1, then maybe fragment No.3 if I clicked on item No.3. Simply put, I want the items can link to its own preference fragment in the viewpager. The items are in MainActivity, the viewpager is in its own activity and this viewpager activity would not start as the app starts unless an item was clicked.
I learnt that viewPager.setCurrentItem(position); should be used when I want to get to a specific page of a viewpager. But my items are in MainActivity, not inside the viewpager activity. I set the viewpager as static, and put viewPager.setCurrentItem(position); inside onClick method in MainActivity, but when I click a item, the app crashes. It shows error: java.lang.NullPointerException. The app had worked fine before I dicided to link those items to their own settings, and now I really have no idea...
Could you please tell me whether it is possible to get to a specific page of a viewpager from other activity? If it is possible, how to do? Thank you so much!
Renew:
I tried to mess with the code, I changed the adapter of the viewpager into protected static MyAdapter adapter;. There was no crash then, but I always went to the first page of the viewpager, whichever item I clicked...
I think you need to pass the data to the activity.
When you call the ViewPager in the MainActivity, set a parameter.
Intent intent = new Intent(getBaseContext(), ViewPagerActivity.class);
intent.putExtra("EXTRA_PAGE", (String) position);
startActivity(intent);
Then in the start of the View Pager Activity, get the object and change the currentItem.
Bundle extras = getIntent().getExtras();
if (extras != null) {
String value = extras.getString("EXTRA_PAGE");
int position= Integer.parseInt(value );
viewPager.setCurrentItem(position);
}
Android can only have 1 visible activity at the same time, so i think it's impossible in your way.
Another approach will be the use of Fragment, but it depends on your app design and structure.
You can check more about Fragments here (Android documentation).
Hope it helps ;)
Best Regards
I'm new to Android development.
I created a simple master-detail app that starts with a simple, vertical scrolling list of topics.
When the user selects a topic, a details screen appears, replacing the first screen, with a list of details that pertain to the selected topic.
I want the title for the details screen to show the topic the user has selected on the first page, but haven't been able to solve the problem after working for almost a week.
All I need to know is, Can this be done? Not looking for someone to solve this for me, but maybe a hint or a link to a tutorial that shows how this can be done.
Note: I'd post a drawing of what I want to do, but I'm new here and don't have 10 reputation yet.
Thanks,
SonCoder
Not exactly sure what you want but either way..
-You have a listview. Each view (the data) in the listview should be a represented by a model. (aka a separate class containing specific information that you want to represent for each listitem.
-Write a custom list adapter (extend from base adapter).
http://www.androidhive.info/2012/02/android-custom-listview-with-image-and-text/
In the getView method of this class you load the the String field of the model that you want in the textview.
-Make sure to use the viewholder pattern in the adapter above. I noticed the example doesnt use one. This speeds up scrolling in the list because there are much fewer calls to findViewById.
-in the list activity set up a View onClick listener. This should create an intent (for launching an activity) or a fragment transaction (for fragments). Send the instance of your entire model (will get from
parent.getAdapter().getItem(position);
in the on click method) into the detail activity.
-if you want to set a textview title just get the textview and set it from the model. It will be the same filed you inflated in the getView method of the adapter.
-if you want to set the titile in the actionbar set:
this.getActionBar().setTitle(title)
This is simple. Just send extra data in the intent that starts the activity and then in the activity's onCreate read the data and then use the setTitle(myString) method from the activity.
setTitle(String title) can be called from anywhere using the activity by the way.
So, your in your listadapter, then you set a listener on your view right? A simple onClickListener on the whole "root" view is just fine.
In the listener you say something in the ways of this:
Intent intent = new Intent(myActivity, MySubActivity.class);
intent.putExtra(key, titleName);
myActivity.startActivity(intent);
Note that the activity reference should be set in the constructor of the adapter and that the "key" String is something you get from your strings.xml. Do not duplicate these in code since if you change one and forget to change the others you might get some wierd NPEs.
Continue in your MySubActivity's onCreate()
#Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
Intent intent = getIntent();
String key = getString(R.string.my_title_key);
String title = intent.getString(key);
setTitle(title);
}
NOTE: I'm not sure of all method names are correct and such but something like this.
Normally I would use a separate activity for each "screen" I wish to display, using different XML files.
However I'm working with Dynamically loading jar files into an android application, so therefore at runtime, I am not aware of how many activities there will be, or how many screens there will be.
At the moment, using java reflection, I am able to return a list of strings from the dynamically loaded java file, and draw each list item, as a separate button onto the screen. If one of these buttons is clicked, i want to be able to load a different "screen" on the stack. So when I press back from this new screen, it goes to the previous screen that called it.
Is it possible to do this without creating a new activity and passing a new intent to it and of course making relevant changes to the android manifest file?
To use blackberry functionality as an example - Is there an equivalent in android to blackberry's push and pop screens? Where the screen ur pushing/popping, would simply extent MainScreen?
If anyone has questions, or If I've been vague, please comment and I will try my best to explain myself, any help is very much appreciated.
The Android equivalent to BB's push/pop screen is startActivity()/finish(). However, you can manage your own views in a single activity by either using a container view (such as ViewSwitcher, as #hasanghaforian suggests) or by simply calling setContentView() with a new view hierarchy whenever you want to change the screen. Be aware that when you call setContentView, any view references that you obtained by calling findViewById will be stale.
In my opinion you should use Fragment. I assume that you have some piece of code where you iterate over the strings:
for(String def : definitions) {
Fragment f = new CustomFragment();
Bundle b = new Bundle();
b.putString("STRING_DEF",def);
f.setArguments(b);
fragments.add(f);
}
in above piece of code a collection of Framents is just created. Let's look at the CustomFragment implementation:
CustomFragment extends Fragment {
#Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
String def = getArguments.getString("STRING_DEF");
//write code to create view
return view;
}
}
Now in your ListActivity you have to implement on click listener more or less like this like this
public void onListItemClick(ListView l, View v, int position, long id) {
FragmentManager fragMgr = getFragmentManager();
FragmentTransaction t = fragMgr.beginTransaction();
t.replace(R.id.id_of_view_place_holder_for_fragment,
fragments.get(position),"FRAGMENT_TAG");
t.commit();
}
you can use ViewSwitcher. ViewSwitcher is a ViewAnimator that switches between two views, and has a factory from which these views are created. You can either use the factory to create the views, or add them yourself. A ViewSwitcher can only have two child views, of which only one is shown at a time.Or you can use fragments.
If you refer to if it is possible to have different layouts in the same activity, the answer is yes.
Activities are independent of layouts, you don't assign the layout for an activity in the manifest, you define what layout to use in the activity calling setContentView() method from Activity class to set a layout.
So if you want to have some layouts (screens) the only thing you have to do is define various layouts and use them when you want calling setContentView(R.layout.the layout), after this call, the layout chosen will be displayed.
If you can't create the layout statically by xml, you can create it dinamically by code as needed by demand each time you want.
In addition you can have a stack of layouts, each time you need a new screen, build it, push it to the stack and call setContentView() method, when you don't need it more, pop off the stack and call setContentView() with the new layout in the top of the stack.
Hope it help you