mini-cards/app/src/main/java/com/codigoparallevar/minicards/DrawView.java

package com.codigoparallevar.minicards;

import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.util.Log;
import android.view.MotionEvent;
import android.view.View;

import com.codigoparallevar.minicards.parts.Part;
import com.codigoparallevar.minicards.parts.Placeholder;
import com.codigoparallevar.minicards.parts.buttons.RoundButton;

import java.util.ArrayList;

class DrawView extends View {

    Paint paint = new Paint(Paint.ANTI_ALIAS_FLAG);
    ArrayList<Part> parts = new ArrayList<>();

    public DrawView(Context context) {
        super(context);
        this.setBackgroundColor(Color.rgb(4, 69, 99));
        parts.add(new RoundButton(500, 1200, 80, 100));
    }

    @Override
    public void onDraw(Canvas canvas){
        long time = System.currentTimeMillis();
        drawBackground(canvas);

        for (Part part : parts){
            part.draw(canvas);
        }

        Log.d("Render time", System.currentTimeMillis() - time + "ms");
    }

    private void drawBackground(Canvas canvas) {
        // Blueprint background
        final int width = getWidth() + getLeft();
        final int height = getHeight() + getTop();
        final int cellSize = 50;

        Paint blueprintLines = new Paint(Paint.ANTI_ALIAS_FLAG);
        blueprintLines.setColor(Color.argb(100, 255, 255, 255));

        // Vertical lines
        for (int x = cellSize; x < width; x += cellSize){
            canvas.drawLine(x, 0, x, height, blueprintLines);
        }

        // Horizontal lines
        for (int y = cellSize; y < height; y += cellSize){
            canvas.drawLine(0, y, width, y, blueprintLines);
        }
    }

    @Override
    public boolean onTouchEvent(MotionEvent event){
        int x = (int) event.getX() - this.getLeft();
        int y = (int) event.getY() - this.getTop();

        Part touchedPart = getPartOn(x, y);
        if (touchedPart == null){
            Log.d("Touched part", "not found");
            return false;
        }

        Log.d("Touched part", "Part: " + touchedPart);
        invalidate();
        return true;
    }

    private Part getPartOn(int x, int y) {
        // Look in the list, in reverse so top-most elements are checked before
        for (int i = parts.size() - 1; i >= 0; i--){
            Part part = parts.get(i);
            if ((x >= part.getLeft()) && (part.getRight() >= x)
                    && (y >= part.getTop()) && (part.getBottom() >= y)){
                return part;
            }
        }
        return null;
    }
}