• Text -- TextView(label), EditText (textfield); Password etc., ...
• Button -- Button, FAB, Checkbox, Switch, Seekbar(slider), Radio; Chip, Toggle, ...
• List -- RecyclerView, Spinner, Toolbar, Tab, Menu, ...
• View -- ImageView, WebView, VideoView, CalendarView, ProgressBar, ...
• Container -- Scroll, ViewPager, Card, Navigation, Fragment,
• Layout -- Constraint, Linear, Frame, Table
• Legacy -- TabHost, ListView, GridView, RelelativeLayout, GridLayout

• Identify the challenge
• Create checklist of technique difficulities
• Create toy apps to complete unsolved difficulities
• With the help of toy apps, codebase, tutorials, etc.
• One at a time, keep intermediate versions

• One TextView, Three Buttons, FAB, Checkbox, Switch
• Button Click changes TextView Button color
• Button Ciick changes Background (and Button as white) when checked
• Checkbox sync with Switch
• FAB reset all

• Borrow from ios
• IBAction -- create actions, need callback (such as Button Click)
• IBOutlet -- receive (such as Button background set)
• UI Widgets usually can be both

-Start from 'Empty' template
-Layout with constraints 
-Button callback use Method #4 since 3 buttons
-FAB copy paste from earlier

• RED button changes TextView's background
• GREEN button changes itslef background
• BLUE button changes View's background

setTextColor(Color.RED); // import android.Graphics.Color
setBackgroundColor(Color.GREEN);

@Override
    public void onClick(View v) {
        TextView textView = (TextView) findViewById(R.id.textView);

        if(v.getId() == R.id.redbutton) {
            //// HERE
            textView.setTextColor(Color.RED);
        }
        else if(v.getId() == R.id.greenbutton) {
            //// HERE
            v.setBackgroundColor(Color.GREEN);
            // greenbutton.setBackgroundColor(Color.GREEN);
        }
        else if(v.getId() == R.id.bluebutton) {
            //// HERE
            rootView.setBackgroundColor(Color.BLUE);
        }
    }
		

• Checkbox at the corner
• Button changes which color based on checked
• Chceckbox click callback

//// HERE
checkBox = (CheckBox) findViewById(R.id.checkBox);
checkBox.setOnClickListener(this);

@Override
    public void onClick(View v) {
        TextView textView = (TextView) findViewById(R.id.textView);

        if(v.getId() == R.id.redbutton) {
            //// HERE
            if(checkBox.isChecked()) {
                v.setBackgroundColor(Color.WHITE);
                rootView.setBackgroundColor(Color.RED);
                textView.setTextColor(Color.BLACK);
            }
            else {
                v.setBackgroundColor(Color.RED); // set button's background
                rootView.setBackgroundColor(Color.WHITE);
                textView.setTextColor(Color.RED);
            }
        }
        else if(v.getId() == R.id.greenbutton) {
        }
        else if(v.getId() == R.id.bluebutton) {
        }
        //// HERE
        else if(v.getId() == R.id.checkBox) {
            textView.setText("CHECKBOX");
        }
    }	

• Switch right next to Checkbox
• Switch callback => sync with CheckBox
• Checkbox callback sync with Switch
• FAB reset

    //// HERE
    Switch switch1;
	
        //// HERE
        switch1 = (Switch) findViewById(R.id.switch1);
        switch1.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                checkBox.setChecked(switch1.isChecked()); <<== A commonly used 'trick'
            }
        });

       FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.floatingActionButton);
        fab.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                //// HERE
                rootView.setBackgroundColor(Color.WHITE);
                redbutton.setBackgroundColor(Color.LTGRAY);
                greenbutton.setBackgroundColor(Color.LTGRAY);
                bluebutton.setBackgroundColor(Color.LTGRAY);
                ((TextView) findViewById(R.id.textView)).setTextColor(Color.BLACK);
                ((CheckBox) findViewById(R.id.checkBox)).setChecked(false);
                ((Switch) findViewById(R.id.switch1)).setChecked(false);            }
        });
		
    //// HERE
    @Override
    public void onClick(View v) {
        TextView textView = (TextView) findViewById(R.id.textView);

        if(v.getId() == R.id.redbutton) {
            if(checkBox.isChecked()) {
                v.setBackgroundColor(Color.WHITE);
                rootView.setBackgroundColor(Color.RED);
                textView.setTextColor(Color.BLACK);
            }
            else {
                v.setBackgroundColor(Color.RED);
                rootView.setBackgroundColor(Color.WHITE);
                textView.setTextColor(Color.RED);
            }
        }
        else if(v.getId() == R.id.greenbutton) {
            if(checkBox.isChecked()) {
                v.setBackgroundColor(Color.WHITE);
                rootView.setBackgroundColor(Color.GREEN);
                textView.setTextColor(Color.BLACK);
            }
            else {
                v.setBackgroundColor(Color.GREEN);
                rootView.setBackgroundColor(Color.WHITE);
                textView.setTextColor(Color.GREEN);
            }
        }
        else if(v.getId() == R.id.bluebutton) {
            if(checkBox.isChecked()) {
                v.setBackgroundColor(Color.WHITE);
                rootView.setBackgroundColor(Color.BLUE);
                textView.setTextColor(Color.BLACK);
            }
            else {
                v.setBackgroundColor(Color.BLUE);
                rootView.setBackgroundColor(Color.WHITE);
                textView.setTextColor(Color.BLUE);
            }
        }
        else if(v.getId() == R.id.checkBox) {
            switch1.setChecked(checkBox.isChecked());
        }
    }			

• FAB reset, button background is NOT 'purple-500'
• Button Callback too many if-then-else, repeative too much

1. parseColor() 
2. ContextCompact.getColor()
3. getColor()

• Get it work first!
• Demand driven improvement
• SAVE the intermediate results!
• MUST do improvement -- KEY to improve your programming skills

@Override
    public void onClick(View v) {
        if(v.getId() == R.id.redbutton) {
            setColors(v, Color.RED);
        }
        else if(v.getId() == R.id.greenbutton) {
            setColors(v, Color.GREEN);
        }
        else if(v.getId() == R.id.bluebutton) {
                setColors(v, Color.BLUE);
        }
        else if(v.getId() == R.id.checkBox) {
            switch1.setChecked(checkBox.isChecked());
        }
    }

    void setColors(View v, int buttonColor) {
        TextView textView = (TextView) findViewById(R.id.textView);

        if(checkBox.isChecked()) {
            v.setBackgroundColor(Color.WHITE);
            rootView.setBackgroundColor(buttonColor);
            textView.setTextColor(Color.BLACK);
        }
        else {
            v.setBackgroundColor(buttonColor);
            rootView.setBackgroundColor(Color.WHITE);
            textView.setTextColor(buttonColor);
        }
    }

• Google "android colors.xml stackoverflow"
• Create colors.xml under /res -- right click 'values' then New -> Values resource file
• Here is a list of common colors, add some to colors.xml in future
  <color name="LightYellow">#FFFFE0</color>
  ootView.setBackgroundColor(getResources().getColor(android.R.color.LightYellow));
• An issue: getColor() is deprecated

• Four TextView, Three SeekBars, FAB
• Three Seekbars change TextViews and compose background RGB color
• SeekBar sync with TextView
• FAB reset all

• Rename the root folder -> A06RGB
• setting.gradle -> rootProject.name = "A06-RGB" then 'Sync Now'
• app/build.gradle -> defaultConfig applicationID -> "a06_rgb" then 'Sync Now'
• res/values/strings.xml -> app_name -> "A06-RGB"
• res/themes/themes.xml -> Refactor name="Theme.A06RGB"
• AndroidManifest.xml -> package Refactor, change package -> a06_rgb
• If errors in Java, File->Sync Project with Gradle Files

• Layout of Seekbar+TextView(x3)
• SeekBar binding

    <SeekBar
        android:id="@+id/seekBarGreen"
        android:layout_width="300dp"
        android:layout_height="9dp"
        android:max="255"
        android:progress="255"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.252"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/seekBarRed"
        app:layout_constraintVertical_bias="0.175" />

    <TextView
        android:id="@+id/textViewGreen"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="TextView"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.52"
        app:layout_constraintStart_toEndOf="@+id/seekBarGreen"
        app:layout_constraintTop_toBottomOf="@+id/seekBarRed"
        app:layout_constraintVertical_bias="0.149" />

        binding.textViewRed.setText("255");
        binding.textViewGreen.setText("255");
        binding.textViewBlue.setText("255");
		
        binding.floatingActionButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
				//// HERE
                int rgb = 255;
                binding.seekBarRed.setProgress(rgb);
                binding.seekBarGreen.setProgress(rgb);
                binding.seekBarBlue.setProgress(rgb);

                binding.textViewRed.setText(""+rgb);
                binding.textViewGreen.setText(""+rgb);
                binding.textViewBlue.setText(""+rgb);
            }
        });		

• Seekbar callback use #4
• RGB color

package com.example.a06_rgb;

import androidx.appcompat.app.AppCompatActivity;

import android.graphics.Color;
import android.os.Bundle;

import com.example.a06_rgb.databinding.ActivityMainBinding;
import android.view.View;
import android.widget.SeekBar;
import android.view.Menu;
import android.view.MenuItem;

public class MainActivity extends AppCompatActivity implements SeekBar.OnSeekBarChangeListener {
    private ActivityMainBinding binding;
    ////HERE
    View rootView;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        ////setContentView(R.layout.activity_main);

        binding = ActivityMainBinding.inflate(getLayoutInflater());
        setContentView(binding.getRoot());

        binding.textViewRed.setText("255");
        binding.textViewGreen.setText("255");
        binding.textViewBlue.setText("255");

        //// HERE
        rootView = getWindow().getDecorView();
        binding.seekBarRed.setOnSeekBarChangeListener(this);
        binding.seekBarGreen.setOnSeekBarChangeListener(this);
        binding.seekBarBlue.setOnSeekBarChangeListener(this);

        binding.floatingActionButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                //// HERE
                int rgb = 255;
                binding.seekBarRed.setProgress(rgb);
                binding.seekBarGreen.setProgress(rgb);
                binding.seekBarBlue.setProgress(rgb);

                binding.textViewRed.setText(""+rgb);
                binding.textViewGreen.setText(""+rgb);
                binding.textViewBlue.setText(""+rgb);

                //// HERE
                rootView.setBackgroundColor(Color.rgb(rgb, rgb, rgb));
            }
        });
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        getMenuInflater().inflate(R.menu.menu_main, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem menuItem) {
        ////Toast.makeText(this, "You chose : " + menuItem.getTitle(), Toast.LENGTH_SHORT).show();
        switch (menuItem.getItemId()) {
            case R.id.menu_first: binding.textview.setText("First Selected");
                return true;
            case R.id.menu_second: binding.textview.setText("Second Selected");
                return true;
            case R.id.menu_third: binding.textview.setText("Third Selected");
                return true;
            default:
                return super.onOptionsItemSelected(menuItem);
        }
    }
	//// HERE
    private int r=255, g=255, b=255;
    @Override
    public void onStopTrackingTouch(SeekBar seekBar) {
    }
    @Override
    public void onStartTrackingTouch(SeekBar seekBar) {
    }
    @Override
    public void onProgressChanged(SeekBar seekBar, int progress, boolean fromUser) {
        switch (seekBar.getId()) {
            case R.id.seekBarRed:
                r = progress;
                binding.textViewRed.setText(""+progress);
                break;
            case R.id.seekBarGreen:
                g = progress;
                binding.textViewGreen.setText(""+progress);
                break;
            case R.id.seekBarBlue:
                b = progress;
                binding.textViewBlue.setText(""+progress);
                break;
        }
        rootView.setBackgroundColor(Color.rgb(r, g, b));
    }
}