Week 3 · Day 6

Libraries & Packages

Use code others already wrote. Save hours. Build real apps.

Split Files dart:math dart:convert DateTime int.parse pub.dev pubspec.yaml 3 Quizzes + Project

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Recap

Where We Left Off — Day 5

Quick reminder of everything you already know.

Modifiers

abstract, base, interface, final

abstract

force children to fill methods

sealed

fixed family + exhaustive switch

factory

smart object creation

mixins

share skills with with

extensions

add methods to built-ins

📌 Today: stop rewriting. Use existing code.

Topic 1 · Why Libraries?

One Big File = Chaos

❌ MEGA FILE

// main.dart — 2000 lines 😱

class MenuItem { ... }

class Order { ... }

class Payment { ... }

class Bill { ... }

// scroll forever

// can't find anything

// team fights over same file

📚

One Giant Book

Like one book with ALL subjects. Math, science, history mixed. Can't find anything.

📌 Break big file into small files. Each file = one topic.

Topic 1 · Solution

Split into Small Files

restaurant/

├── menu.dart ← menu code only

├── order.dart ← order code only

├── bill.dart ← bill code only

└── main.dart ← uses all 3

📑

Chapters in a Book

Like chapters in a book. Math chapter, science chapter. Find fast, organize easy.

📌 One concept = one file. One folder = one feature.

Big Picture

3 Types of Libraries

Every import is one of these three.

📁

Your Own Files

Files YOU write and split.

import 'menu.dart';

🧰

Dart Core

Already in Dart — FREE.

import 'dart:math';

🌐

pub.dev Packages

Shared by other devs.

import 'package:http/http.dart';

📌 We'll cover all 3. Starting with your own files.

Topic 2 · Your Files

Split Your Own Files

❌ GROWING FILE

// main.dart — getting big

List<String> menuItems = [...];

class Order { ... }

class Bill { ... }

// hard to find, hard to edit

🗃️

Folders on Your Laptop

Like moving files to folders on your computer. One topic per folder.

📌 Rule: one concept = one file.

Topic 2 · Your Files

How to Split Files

❌ BEFORE

// main.dart

class MenuItem {

final String name;

final int price;

MenuItem(this.name, this.price);

}

void main() {

var m = MenuItem('Momo', 120);

}

// menu.dart — NEW file

class MenuItem {

final String name;

final int price;

MenuItem(this.name, this.price);

}

// main.dart

import 'menu.dart'; // ← import it!

void main() {

var m = MenuItem('Momo', 120);

}

Topic 2 · import

`import` — Two Ways

Relative path

// same folder

import 'menu.dart';

// subfolder

import 'data/menu.dart';

// parent folder

import '../utils/helpers.dart';

Package path

// uses app name from

// pubspec.yaml

import 'package:restaurant/menu.dart';

📌 Small project → use relative. Big project → use package path.

Topic 2 · Real Use

Real Use — Restaurant Files

restaurant/

├── pubspec.yaml

├── lib/

│ ├── menu.dart

│ ├── order.dart

│ ├── bill.dart

│ └── main.dart

// menu.dart

class MenuItem {

final String name;

final int price;

MenuItem(this.name, this.price);

}

var menu = [

MenuItem('Momo', 120),

MenuItem('Dal Bhat', 150),

];

// main.dart

import 'menu.dart';

void main() {

for (var item in menu) {

print('${item.name} Rs.${item.price}');

}

🎮 Quiz 1 · Your Files

Test Your Import Knowledge

1. `import 'menu.dart';` — what does it do?

ADeletes menu.dart

BLoads code from menu.dart

CCopies the file

✅ import loads the classes/functions from that file so you can use them here.

2. Why split one file into many?

AFaster compile

BEasier to find, edit, share

CSaves memory

✅ Small files are easy to navigate and teams don't fight over one giant file.

3. `import '../utils.dart';` means?

ASame folder

BParent folder

CSubfolder

✅ .. goes up one folder. Just like the terminal!

Topic 3 · Dart Core

Dart Core — Already Included

🧰

Toolbox that comes with Dart

Hammer, screwdriver already inside. No buying. Just import 'dart:xxx'; and use.

🎲 dart:math

Random, sqrt, pi

📝 dart:convert

JSON, Base64

⏰ dart:core

String, int (auto-imported)

📂 dart:io

files, network

⚡ dart:async

Future, Stream (Day 7)

🗃️ dart:collection

extra collections

📌 Just import 'dart:xxx'; — free, always there.

Topic 3.1 · dart:math

dart:math — Random Numbers

❌ PROBLEM

// Want a random order ID 1..1000

// Dart core has no Random() by default!

var id = ??? // how?

🎲

A Dice

Like a dice. Roll → random number. dart:math gives you the dice.

📌 Random, sqrt, pi, pow — all need import 'dart:math';

Topic 3.1 · dart:math

How to Use dart:math

import 'dart:math'; // ← import

var rng = Random(); // create generator

rng.nextInt(100); // 0 to 99

rng.nextDouble(); // 0.0 to 1.0

rng.nextBool(); // true / false

✅ Do

Create Random() once, reuse.

💡 Note

nextInt(100) → 0..99 (not including 100).

✅ Also

Has sqrt, pi, pow, min, max.

Topic 3.1 · Real Use

Real Use — Order IDs

import 'dart:math';

String generateOrderId() {

var rng = Random();

var num = rng.nextInt(10000);

return 'ORD-$num';

}

int rollDice() => Random().nextInt(6) + 1;

void main() {

print(generateOrderId()); // ORD-4823

print(rollDice()); // 1..6

print(sqrt(16)); // 4.0

print(pi); // 3.14159...

}

Topic 3.2 · DateTime

DateTime — Time Travel

❌ PROBLEM

// When was the order placed?

// No time tracking = can't sort

// Can't show "2 hours ago"

📅

Clock + Calendar

Like a clock + calendar together. Knows current time + date.

📌 DateTime is auto-imported from dart:core. Free!

Topic 3.2 · DateTime

How to Use DateTime

var now = DateTime.now();

print(now); // 2026-04-21 14:30:00

now.hour; // 14

now.day; // 21

now.month; // 4

now.year; // 2026

now.weekday; // 2 (Tuesday)

// specific date

var bday = DateTime(2001, 5, 15);

// difference

var diff = now.difference(bday);

print(diff.inDays); // days between

✅ Do

DateTime.now() for current time.

💡 Note

weekday: 1 = Mon, 7 = Sun.

✅ Also

.difference() returns a Duration.

Topic 3.2 · Real Use

Real Use — Order Timestamp

class Order {

final String id;

final DateTime placedAt;

Order(this.id) : placedAt = DateTime.now();

String get timeInfo {

var hour = placedAt.hour;

var minute = placedAt.minute;

return '$hour:$minute';

}

void main() {

var o = Order('ORD-100');

print(o.timeInfo); // 14:30

}

Topic 3.3 · parse

int.parse — String to Number

❌ PROBLEM

// User types '500' (as text!)

String input = '500';

input + 10; // ❌ '50010' (concats!)

input * 2; // ❌ error

🔄

Translator

'500' is a word. int.parse turns it into a number you can add, multiply, tax.

Topic 3.3 · parse

How to Parse

// String → int

int x = int.parse('500');

print(x + 10); // 510 ✅

// String → double

double y = double.parse('99.5');

print(y * 2); // 199.0

// number → String

int price = 120;

String s = price.toString();

print(s + ' rupees'); // '120 rupees'

✅ Works

int.parse('500') → 500.

❌ Breaks

int.parse('abc') → crashes.

💡 Safer

int.tryParse('abc') → null.

Topic 3.3 · Real Use

Real Use — Tax Calculator

void takeInput(String userInput) {

var price = int.tryParse(userInput);

if (price == null) {

print('❌ Not a number');

return;

}

print('Price is Rs.$price');

print('With 13% tax: Rs.${price * 1.13}');

}

void main() {

takeInput('500'); // ✅ Price is Rs.500

takeInput('abc'); // ❌ Not a number

}

Topic 3.4 · dart:convert

dart:convert — Save Data as Text

❌ PROBLEM

var order = {

'id': 'ORD-1',

'items': ['Momo', 'Tea'],

'total': 150,

};

// Want to save to file or

// send over internet

// Need to turn it into TEXT

📝

Packing a Box

Like packing a box for shipping. Fold the object flat into text. Unpack later.

📌 JSON = standard text format for data. Used everywhere on the web.

Topic 3.4 · Real Use

Real Use — JSON Round Trip

import 'dart:convert';

void main() {

var order = {

'id': 'ORD-1',

'items': ['Momo', 'Tea'],

'total': 150,

};

// Map → JSON text

String text = jsonEncode(order);

print(text);

// {"id":"ORD-1","items":["Momo","Tea"],"total":150}

// JSON text → Map

var back = jsonDecode(text);

print(back['items']); // [Momo, Tea]

}

📌 jsonEncode → text. jsonDecode → Map. Round trip!

🎮 Quiz 2 · Dart Core

Test Your Core Library Knowledge

1. `Random().nextInt(10)` returns?

A1 to 10

B0 to 9

C0 to 10

✅ nextInt(n) returns 0 up to but not including n.

2. `int.parse('abc')` does?

Areturns 0

Bcrashes

Creturns null

✅ int.parse throws on bad input. Use int.tryParse if unsure.

3. Why use `jsonEncode`?

AEncrypt data

BTurn Map into text

CMake it faster

✅ JSON is text — perfect for saving to a file or sending over the network.

Topic 4 · pub.dev

pub.dev — Shared Code Store

❌ REINVENTING

// Want: format 15000 as "15,000"

// Build from scratch?

// Handle 1 lakh, 1 crore too?

// Hours of work 😱

🏪

Supermarket for Code

Like a supermarket for code. Someone already built it. Grab and use. Free.

📌 pub.dev has 100k+ packages. Someone already solved your problem.

Topic 4 · pub.dev

What pub.dev Looks Like

pub.dev

──────────────────────

📦 http — web requests

📦 intl — formatting

📦 shared_prefs — save settings

📦 provider — state mgmt

📦 dio — advanced http

Visit It

https://pub.dev

// Search for:

"http"

"intl"

"image picker"

Every Flutter dev lives here.

Topic 4 · pubspec.yaml

pubspec.yaml — The Recipe

name: restaurant # your app name

description: A demo app

version: 1.0.0

environment:

sdk: ">=3.0.0 <4.0.0"

dependencies:

intl: ^0.19.0 # ← package!

http: ^1.2.0 # ← another!

dev_dependencies:

test: ^1.24.0 # only for testing

✅ Do

List packages under dependencies.

💡 Note

^1.0.0 = compatible updates allowed.

✅ After edit

Run dart pub get.

Topic 4 · Install

How to Install a Package

Step 1

# Terminal

dart pub add intl

Adds intl to pubspec.yaml.

Step 2

# Auto after pub add

dart pub get

Downloads package.

Step 3

// in .dart file

import 'package:intl/intl.dart';

Use it!

📌 3 steps. Takes 10 seconds. Saves you hours.

Topic 4 · Real Use

Real Use — Format Money

import 'package:intl/intl.dart';

void main() {

var price = 15000;

// Without intl:

print(price); // 15000

// With intl:

var f = NumberFormat('#,###');

print(f.format(price)); // 15,000

// Nepali rupees

var rs = NumberFormat.currency(

symbol: 'Rs.',

);

print(rs.format(15000)); // Rs.15,000.00

}

Topic 4 · Versions

Version `^1.0.0` Meaning

intl: ^1.2.3

│ │ │

│ │ └─ PATCH (tiny fixes)

│ └─── MINOR (new features)

└───── MAJOR (breaking)

✅ `^1.2.3`

Means 1.2.3 up to <2.0.0.

💡 Safe

Auto-updates that won't break your app.

🔒 Lock

Use 1.2.3 (no caret) to pin exactly.

Topic 4 · Popular

Popular Packages

🌐 http

make web requests

🎨 flutter/material

UI widgets

💾 shared_preferences

save settings

📸 image_picker

pick photos

📍 geolocator

GPS location

🔔 notifications

push notifs

📌 Need something? Search pub.dev FIRST. Don't reinvent.

🎮 Quiz 3 · Packages

Test Your pub.dev Knowledge

1. What is `pub.dev`?

ADart docs

BStore of shared packages

CGitHub

✅ pub.dev hosts 100k+ open-source packages for Dart and Flutter.

2. `^1.2.0` allows?

AOnly 1.2.0

B1.2.0 up to <2.0.0

CAny version

✅ Caret lets minor + patch updates. Major bump is blocked (breaking changes).

3. Where do you list packages?

Amain.dart

Bpubspec.yaml dependencies

Cimport line

✅ pubspec.yaml is the recipe. The dependencies section is where packages live.

🎮 Project · Upgraded Restaurant

Build the Full Restaurant

Everything from today — split files, dart:math, DateTime, JSON, intl. Click each feature.

📁 Split Files

🎲 Random ID

📅 DateTime

📝 JSON

💵 intl

CONCEPTS USED

Quick Reference

When to Use What

Save code organized?

→ split files

Random number?

→ dart:math

Current time?

→ DateTime

String → number?

→ int.parse

Save as text?

→ jsonEncode

Format money?

→ intl package

Web request?

→ http package

Save settings?

→ shared_preferences

Summary

Today's Libraries Recap

Split Files

import, relative paths

dart:math

Random, sqrt, pi

DateTime

now, difference, format

int.parse

String → number

dart:convert

jsonEncode/Decode

pub.dev

shared packages

pubspec.yaml

list packages

^version

safe updates

intl

format money/dates

Assignment

Take-Home: Polish Restaurant

1. Split restaurant into 4 files: menu.dart, order.dart, bill.dart, main.dart

2. Add random order IDs using dart:math

3. Store DateTime on each Order

4. Save the order as JSON with dart:convert

5. Format all prices with the intl package

6. Push to GitHub via PR

Use branches! Create feature/your-name and submit a PR.

Coming Next

Day 7 — Async & Futures

await async Future Stream http package

We'll make a real API call and get live data back. Finish today's assignment first!

Cheat Sheet

Dart Commands

dart create myapp # new project

dart run # run main.dart

dart pub add <name> # add package

dart pub get # download packages

dart pub upgrade # update versions

dart format . # format code

dart analyze # find issues

📌 Save this slide. You'll use these commands every day.

Any Questions?

Practice at dartpad.dev — add import lines at the top to pull in packages!

Libraries & Packages

Where We Left Off — Day 5

Modifiers

abstract

sealed

factory

mixins

extensions

One Big File = Chaos

One Giant Book

Split into Small Files

Chapters in a Book

3 Types of Libraries

Your Own Files

Dart Core

pub.dev Packages

Split Your Own Files

Folders on Your Laptop

How to Split Files

import — Two Ways

Relative path

Package path

Real Use — Restaurant Files

Test Your Import Knowledge

1. import 'menu.dart'; — what does it do?

2. Why split one file into many?

3. import '../utils.dart'; means?

Dart Core — Already Included

Toolbox that comes with Dart

🎲 dart:math

📝 dart:convert

⏰ dart:core

📂 dart:io

⚡ dart:async

🗃️ dart:collection

dart:math — Random Numbers

A Dice

How to Use dart:math

✅ Do

💡 Note

✅ Also

Real Use — Order IDs

DateTime — Time Travel

Clock + Calendar

How to Use DateTime

✅ Do

💡 Note

✅ Also

Real Use — Order Timestamp

int.parse — String to Number

Translator

How to Parse

✅ Works

❌ Breaks

💡 Safer

Real Use — Tax Calculator

dart:convert — Save Data as Text

Packing a Box

Real Use — JSON Round Trip

Test Your Core Library Knowledge

1. Random().nextInt(10) returns?

2. int.parse('abc') does?

3. Why use jsonEncode?

pub.dev — Shared Code Store

Supermarket for Code

What pub.dev Looks Like

Visit It

pubspec.yaml — The Recipe

✅ Do

💡 Note

✅ After edit

How to Install a Package

Step 1

Step 2

Step 3

Real Use — Format Money

Version ^1.0.0 Meaning

✅ ^1.2.3

💡 Safe

🔒 Lock

`import` — Two Ways

1. `import 'menu.dart';` — what does it do?

3. `import '../utils.dart';` means?

1. `Random().nextInt(10)` returns?

2. `int.parse('abc')` does?

3. Why use `jsonEncode`?

Version `^1.0.0` Meaning

✅ `^1.2.3`

1. What is `pub.dev`?

2. `^1.2.0` allows?