Tag: android development

1. Don’t use platform fragments (android.app.Fragment), they have been deprecated and can trigger version-specific bugs. Use the support library fragments (android.support.v4.app.Fragment) instead.
2. A Fragment is created explicitly via your code or recreated implicitly by the FragmentManager. The FragmentManager can only recreate a Fragment if it’s a public non-anonymous class. To test for this, rotate your screen while the Fragment is visible.
3. FragmentTransaction#commit can fail if the activity has been destroyed.
   “java.lang.IllegalStateException: Activity has been destroyed”
   Why – This can happen in the wild where say right before FragmentTransaction#commit() executes, the user gets a phone call and your activity is backgrounded and destroyed.
   How to trigger manually – The easy way to manually test this is to add a call to Activity#finish() right before FragmentTransaction#commit.
   Fix – Before doing FragmentTransaction#commit(), check that the activity has not been destroyed –Activity#isDestroyed() should return false.
4. FragmentTransaction#commit can fail if onSaveInstanceState has been called.
   “java.lang.IllegalStateException: Can not perform this action after onSaveInstanceState”
   Why – This can happen in the wild where say right before FragmentTransaction#commit() executes, the user gets a phone call and your activity is backgrounded and paused.
   How to trigger manually – The easiest way to manually trigger this behavior is to call Activity#onSaveInstanceState in your uncommitted code right before the call to FragmentTransaction#commit
   Fix 1 – call FragmentTransaction#commitAllowingStateLoss but that implies that your fragment would be in a different state then the user expects it to be.
   Fix 2 – The better way is to ensure that the code path L which leads to FragmentTransaction#commit is not invoked once Activity’s onSaveInstanceState has been called but that’s not always easy to do.
5. FragmentManager is null after Activity is destroyed.
   “java.lang.NullPointerException: … at getSupportFragmentManager().beginTransaction()”
   Why – This can happen when the activity has been destroyed before getSupportFragmentManager() is invoked. The common cause of this is when a new fragment has to be added in response to a user action and the user immediately backgrounds the app, again, say due to a phone call, after clicking the button before getSupportFragmentManager() is invoked. Another common case is where an AsyncTask which will call getSupportFragmentManager() in onPostExecute and while the task is engaged in the background processing (doInBackground), the activity is destroyed.
   How to trigger manually – call `Activity#finish()` before `getSupportFragmentManager().beginTransaction()`
   Fix – If getSupportFragmentManager() is being invoked in the Activity, check if it’s null. If it is being invoked inside a Fragment check if isAdded() of the Fragment returns true before calling this.
6. Avoid UI modifications which are not related to a FragmentTransaction with FragmentTransaction committed using commitAllowStateLoss
   Why – Any UI modifications like modifications of the text in a TextView are synchronous while the execution of a FragmentTransaction via `FragmentTransaction#commitAllowStateLoss()` is asynchronous. If the activity’s onSaveInstanceState is invoked after the UI changes have been made but before commitAllowStateLoss is called then the user can end up seeing a UI state which you never expected them to see.
   Fix – use commitNow() or hook into FragmentManager.FragmentLifecycleCallbacks#onFragmentAttached(). I will admit this I haven’t found a simpler fix for this. And this issue is definitely an edge case.
7. Saving Fragment State
   As mentioned earlier, a Fragment is re-created on activity recreation by FragmentManager which will invoke it’s default no-parameter constructor. If you have no such constructor then on Fragment recreation, the app will crash with “java.lang.InstantiationException: MyFragment has no zero argument constructor”. If you try to fix this by adding a no argument constructor then the app will not crash but on activity recreation say due to screen rotation, the Fragment will lose its state. The right way to serialize a Fragment’s state is to pass arguments in a Bundle via setArguments.

   MyFragment myFragment = new MyFragment();
   myFragment.setArguments(mBundle);
   return myFragment;
   

   The Fragment code should then use getArguments() method to fetch the arguments. In fact, I would recommend a Builder pattern to hide all this complexity.

   Consider this complete example,

   public class MyFragment {
   	
   		private static final String KEY_NAME = "name";
   	
   		public static class MyFragment.Builder {
   		   private final Bundle mBundle = new Bundle();
   		   
   		   public Builder setName(String username) {
   		       mBundle.putInt(KEY_NAME, clickCount);
   		       return this;
   		   }
   		   
   		   public MyFragment build() {
   		      MyFragment myFragment = new MyFragment();
   		      // Set the username
   		      myFragment.setArguments(mBundle);
   		      return myFragment;
   		   }
   		}
   		
   		public MyFragment() {
   			// Get the user name
   			@Nullable String username = getArguments() != null ? getArguments().getString(KEY_NAME, null) : null;
   		}
   	}
   	
   	if (!isDestroyed()) {
   	   // Create the Fragment
   		MyFragment myFragment = new MyFragment().Builder().setName(username).build();
   		// Add the Fragment
   		FragmentTransaction ft = getSupportFragmentManager().beingTransaction();
   		ft.add(myFragment);
   		ft.commitNowAllowingStateLoss();
   	}

8. Inside your Fragment code, if you want to decide whether it is safe to execute a UI code or not, rely on isAdded(), if it returns true, it is safe to perform UI modifications, if it returns false, then your Fragment has been detached from the activity either because it has been removed or because the host (Fragment/Activity) is being destroyed.
9. Callbacks
   To callback into the parent activity/fragment in case of action inside your Fragment, say, a user click, provide an interface (say, MyFragmentListener) which the holding activity/Fragment should implement. In Fragment#onCreateView() get the host via getHost(), cast it to MyFragmentListener, and store it in the instance variable of your Fragment class. Set that instance variable to null in Fragment#onDestroyView(). Now, you can invoke callbacks on this MyFragmentListener instance variable.
10. Backstack
    Backstack is nuanced and my grasp of it is still limited. What I do understand is that if you want your Fragment to react to the back key press then you should call FragmentTransaction#addToBackStack(backStackStateName) while adding the Fragment via FragmentTransaction and remove it while removing it. Removal from the back stack is a bit more nuanced. Note that, manual removal of a fragment from the back stack is not required in Activity#onBackPressed() as long as your Activity inherits from FragmentActivity.

    SupportFragmentManager manager = getSupportFragmentManager();
    	FragmentManager.BackStackEntry entry = manager.getBackStackEntryAt(manager.getBackStackEntryCount() - 1);
    	if (backStackStateName.equals(entry.getName())) {
    	   manager.popBackStack(backStackStateName, FragmentManager.POP_BACK_STACK_INCLUSIVE);
    	}
    }

The core idea behind resumable upload is straightforward if you are uploading a big file, then you are going to encounter users in the network conditions where they cannot upload the file in a single network session. The client-side code, to avoid restarting the file upload from the beginning, must figure out what portion of the file was uploaded and “resume” the upload of the rest.

How to do resumable upload

Before starting the upload, send a unique ID generated from the file contents to the server like MD-5 or SHA-256. The server decides and declares what the format of that unique ID is. Next, the server responds with an offset which indicates how many bytes server already has. The client uploads rest of the bytes with a Content-Range header.

How to test resumable upload

The right way to verify that this code works is to break the upload intentionally and randomly in the middle and check that the next upload session does not start from zero. Note that, it might not start from the exact byte offset where it was disconnected since the client network stack can have its buffer size to fill and it might discard the buffered bytes in case of an exception.  Therefore, the ratio of the number of bytes read to the file size should be close to one but might not be one.

The right way to verify that this code works is to break the upload intentionally and randomly in the middle and check that the next upload session does not start from zero.

Sample skeleton codes

// SHA-256 or MD-5 whatever the server decides
String uniqueId = generateUniqueId(File file);
// Number of already uploaded bytes (could be zero)
int alreadyUploaded = getAlreadyUploadedByteCount(uniqueId);
// Upload rest of the bytes
upload(file, uniqueId, alreadyUploaded);

boolean upload(File file, String uniqueId, int alreadyUploaded)
 throws IOException {
  InputStream in = getInputStream(file);
  uploadViaHttp(file, uniqueId, alreadyUploaded);
}

int readCount;

boolean upload(File file, String uniqueId, int alreadyUploaded) 
throws IOException {
  InputStream in = getInputStream(file);
  readCount = 0;  // Testing only
  uploadViaHttp(in, uniqueId, alreadyUploaded);
  double ratio = ((double)readCount)/file.size();  // Testing only
}


// Standard method
InputStream getInputStream(File file) throws IOException {
    return new FileInputStream(file);
}

// Enhanced method for testing
InputStream getInputStream(File file) throws IOException {
    return new FileInputStream(file) {
        int read(byte[] b, int off, int len) throws IOException {
            if (random.nextBoolean()) {
                throw new IOException("Intentionally broke the stream");
            }
            int tmp = super.read(b, 0, b.length());
            readCount += tmp;
            return tmp;
        }

        int read(byte[] b) throws IOException {
            return read(b, 0, b.length);
        }

        int read() throws IOException {
            readCount++;
            return super.read();
        }
    }
}