Jonathan's Notes for Android

Jonathan's Notes is note/diary app for Android.

(playstore: https://play.google.com/store/apps/details?id=com.jonathan.jonathans.notes)

RoomDB-Repository-ViewModel pattern

app architecture guide: https://developer.android.com/jetpack/guide?hl=ko

ROOM DB

Add table with Foreign key (one-to-many)

New table entity:

@Entity(tableName = "image_table", foreignKeys = [ForeignKey(entity=Note::class, parentColumns = ["id"], childColumns = ["noteId"], onDelete = CASCADE)])
data class Image(
    @PrimaryKey
    var path: String,
    var noteId: Int
)

Migration:

    private val MIGRATION_5_6 = object : Migration(5,6) {
        override fun migrate(database: SupportSQLiteDatabase) {
            database.execSQL(
                "CREATE TABLE IF NOT EXISTS image_table(noteId INTEGER NOT NULL, path VARCHAR NOT NULL, PRIMARY KEY(path), FOREIGN KEY(noteId) REFERENCES note_table(id) ON DELETE CASCADE)"
            )
        }
    }

Migration

• For IllegalStateException from migration, stacktrace will print out two parts like below:

     Expected:
    TableInfo{name='image_table', columns={path=Column{name='path', type='TEXT', affinity='2', notNull=true, primaryKeyPosition=1, defaultValue='null'}, noteId=Column{name='noteId', type='INTEGER', affinity='3', notNull=true, primaryKeyPosition=0, defaultValue='null'}}, foreignKeys=[ForeignKey{referenceTable='note_table', onDelete='CASCADE', onUpdate='NO ACTION', columnNames=[noteId], referenceColumnNames=[id]}], indices=[]}
     Found:
    TableInfo{name='image_table', columns={path=Column{name='path', type='VARCHAR', affinity='2', notNull=false, primaryKeyPosition=1, defaultValue='null'}, noteId=Column{name='noteId', type='INTEGER', affinity='3', notNull=false, primaryKeyPosition=0, defaultValue='null'}}, foreignKeys=[ForeignKey{referenceTable='note_table', onDelete='NO ACTION', onUpdate='NO ACTION', columnNames=[noteId], referenceColumnNames=[id]}], indices=[]}

Here, Expected: defined data class Found: Migration scheme

Repository

• properties - service instance (ex. dao instance, webservice instance)
• methods - fetch specific data (ex. fun getUser(): LiveData)

ViewModel

• properties - LiveData objects on which views will observe.

LiveData

When NOT to use LiveData in ViewModel?

• when multiple views are not interacting each other there's no need to use LiveData. (UNLESS you want to recover view states on rotation)

Extra

• when updating collection of instances in adapter class with another list wrapped with LiveData container, be aware that this new list should be handled asynchronously. Thus, below code will trigger NullPointerException.

Bad:

    val newListLive: LiveData<List<User>> = myViewModel.getUsers()    //fetched from ROOM DB
    myAdapter.submitList(newListLive.value!!)    //NO NO!

newListLive.value is asynchronously updating but myAdapter tries to take the value of newListLive right away, before newListLive has finished fetching all data. By the time myAdapter tries to access newListLive.value the value is null so this will trigger NullPointerException. The right way to do is at below.

Good:

    val newListLive: LiveData<List<User>> = myViewModel.gerUsers()
    newListLive.observe(viewLifeCycleOwner){
        adapter.submitList(it)
    }

• If observer is not set on a LiveData object, the object won't update values. For example, if LiveData<List<User>> is returned by Dao but no observer is set then the value of the LiveData object will always be null.

UI components

Collapsing Toolbar with RecyclerView

• Remember: CoordinatorLayout → AppbarLayout → Toolbar

RecyclerView

• set layout manager type(e.g. LinearLayoutManager, GridLayoutManager)
• OnScrollListener can be added for various needs (ex. hiding FAB). See below:

        rv_main.addOnScrollListener(object: RecyclerView.OnScrollListener() {
            override fun onScrolled(recyclerView: RecyclerView, dx: Int, dy: Int) {
                if (dy>0)
                    fab.hide()
                else
                    fab.show()

                super.onScrolled(recyclerView, dx, dy)
            }
        })

AppBarLayout

• CORRECT: layout_height="wrap_content", WRONG: layout_height="match_parent"

• Child of (vertical)LinearLayout.

• Depends HEAVILY on being used as direct child of CoordinatorLayout

Toolbar

• setSupportActionBar(toolbar) is used along with onOptionsItemSelected() and override fun onCreateOptionsMenu(). If setSupportActionBar(toolbar) is called then toolbar.inflaterMenu() is unnecessary since Toolbar is working as ActionBar. If the former is not called the latter should be used with toolbar.setOnMenuItemClicklistener().

How it's done here:

1. setSupportActionBar(toolbar) in MainActivity
2. override fun onCreateOptionsMenu(...)
3. override fun onOptionsItemSelected(...)

• For some reason a small gap exists on the left of the left-end child view of Toolbar. Add below line to remove it. app:contentInsetStart="0dp"

• To disable shadow below toolbar, set app:elevation="0dp" in AppBarLayout like below.

        <com.google.android.material.appbar.AppBarLayout
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:orientation="vertical"
            app:elevation="0dp">

FragmentContainerView

• FOR NAVIGATION GRAPH USE , NOT THIS(in activity xml where HomeFragment is at). - Else, nav.xml does not acknowledge activity)
• Container for Fragments. Extends FrameLayout.
• Original FrameLayout had problem with properly showing exiting animation of Fragment. FCV fixes this issue.

Extra

• parent.supportActionBar is lost after rotation so set it back like below:

    val parent = requireActivity() as AppCompatActivity
    parent.supportActionBar = parent.findViewById<Toolbar>(R.id.my_toolbar)
    supportActionBar?.apply{
        doSomething()
    }

• Change visibility by fab.hide(), NOT fab.visibility = View.INVISIBLE. (Latter is buggy)

• To customize Dialog,

1. set layout_gravity in xml for position.
2. to set margin do below. (10% here)

        window?.apply {
            setBackgroundDrawableResource(android.R.color.transparent)
            val newX = context.resources.displayMetrics.widthPixels     //screen pixel
            val newY = context.resources.displayMetrics.heightPixels
            setLayout((newX*0.9).toInt(), (newY*0.9).toInt())
            Log.d("", "layout: $newX, $newY")
        }

3. to adjust 'parent view'(window) dimension do below.

        val lp = window?.attributes
        lp?.apply{
            flags = WindowManager.LayoutParams.FLAG_DIM_BEHIND
            dimAmount = 0.8f
            width = WindowManager.LayoutParams.MATCH_PARENT
            height = WindowManager.LayoutParams.MATCH_PARENT
        }

By default window is whole screen for app. (area except notification bar and bottom bar)

• fillAfter=true unnecessary if the animation is 'hiding' type of animation and target view's default visibility is visibility="invisible" in XML.

Furthermore..

• Iterating through Collection without iterators to remove entry will trigger ConcurrentModificationException.

Bad: (will trigger ConcurrentModificationException)

    myMap.forEach{
        myMap.remove(it.key)
    }

Good:

    val iter = myMap.iterator()
    while(iter.hasNext()){
        iter.next()
        iter.remove()
    }

• ViewPager2 glitch with ListAdapter - Calling .submitList(newList) after data deletion does not properly set viewPager2.currentItem. Thus if page transformer is attached page positions displayed are spoiled after the deletion. Solved simply by using RecyclerView.Adapter.