To use monorepo or not is an eternal debate. Each has its pros and cons. Let’s say you decide to go with monorepo, one major issue you will face over time is slow testing. Imagine a monorepo, consisting of an Android app, an iOS app, some backend code, some web frontend code. In only very few occasions will someone modify more than one of those simultaneously.
In 2020, the web is still the most accessible permission-less platform. For the past few months, I have been playing and building side-projects to simplify my life. I started with a Calendar Bot for scheduling events, DeckSaver for downloading decks from Docsend, AutoSnoozer for email management, and StayInTouch for maintaining follow-ups.
When I started on this journey, I had the following in my mind.
Docker containers are small OS images in themselves which one can deploy and run without worrying about dependencies or interoperability. All the dependencies are packed in the same container file. And the docker runtime takes care of the interoperability. You are not tied to using a single language or framework. You can write code in Python, Go, Java, Node.js, or any of your favorite languages and pack it in a container.
The first statement is a Mac, GNU/Linux, and BSD portable way of finding the location of the bash interpreter. The second statement combines
“set -e” which ensures that your script stops on first command failure. By default, when a command fails, BASH executes the next command. Looking at the logs, you might feel that the script executed successfully while some commands might have failed. Caveat: Be careful about applying it to existing scripts.
“set -u” which ensures that your script exits on the first unset variable encountered. Otherwise, bash replaces the unset variables with empty default values.
“set -o pipefail” which ensures that if any command in a set of piped commands failed, the overall exit status is the status of the failed command. Otherwise, the exit status is the status of the last command.
Update: As of April 2020, I have switched over to GitHub Actions. Travis CI has become buggy and flaky over time and I got tired of trying to keep the builds green. My GitHub action scripts can be seen here.
Just like many software engineers, I maintain my config files for GNU/Linux and Mac OS in a git repository. Given that, I wrote a fair bit of them in interpreted code, notably, Bash, it is a bit hard to ensure that it is bug-free. The other problem I face is that packages on homebrew, the Mac OS package manager becomes obsolete and gets deleted from time to time.
I added CI testing on Travis CI to prevent these breakages and to ensure that my dotfiles are always in good shape for installation. The great thing about Travis CI is that it is entirely free for open-source repositories even for testing on Mac OS containers.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
A JPEG file can have Exif metadata which can provide the rotation/translation field information for a raw JPEG image. So, a landscape raw JPEG image could actually be a portrait because it’s EXIF orientation could be set to ORIENTATION_ROTATE_90, the best way to handle such scenarios is to either use a library like Picasso or Glide or at least learn from them. Here is a piece of code from Picasso which loads a JPEG as an in-memory bitmap and performs the right translation/rotation.
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
// Number of already uploaded bytes (could be zero)