python-crash-course-book/chap12_a_ship_that_fires_bullets.md

# Chapter 12. A Ship that Fires Bullets

## Starting the Game Project

```python
import sys
import pygame

class AlienInvasion:
    """Overall class to manage game assets and behaviour"""

    def __init__(self):
        """Initialize the game, and create game resources."""
        pygame.init()

        self.screen = pygame.display.set_mode((1200, 800))
        pygame.display.set_caption("Alien Invasion")

    def run_game(self):
        """Start the main loop for the game"""
        while True:

            # event loop, watch for mouse/keyboard events
            for event in pygame.event.get():
                if event.type == pygame.QUIT:
                    sys.exit()

            # Make the most recently draw screen visible
            pygame.display.flip()

if __name__ == '__main__':
    ai = AlienInvasion()
    ai.run_game()
```

* `pygame.display.set_mode()` assigned a **surface** to `self.screen`, which is a part of the screen where a game element can be displayed.
  * When we activate the game's animation loop, this surface will be redrawn every time.
* Game is contolled by `run_game()` method. containing
  * event loop block
  * code that manages screen updates.
* To access the events that Pygame detects, use `pygame.event.get()` function. In each loop, any keyboard/mouse event will cause the `for` loop to run.


### Creating a Settings Class

In order to manage the setting more efficiently, we move setting into a separate module `settings.py`

```python
class Settings:
    """A class to store all settings for Alien Invasion"""

    def __init__(self):
        """Initialize the game's settings."""

        # Screen settings
        self.screen_width = 1200
        self.screen_height = 800
        self.bg_color = (230,230,230)
```

`alien_invasion.py` will also be modified to fit it.

## Adding The Ship Image

* Verify it's free licensed image

### Creating the Ship Class

Create this `ship.py` module

```python
import pygame

class Ship:
    """A class to manage the ship."""

    def __init__(self, ai_game):
        """Initialize the ship and set its starting position"""
        self.screen = ai_game.screen
        self.screen_rect = ai_game.screen.get_rect()

        # Load the ship image and get its rect
        self.image = pygame.image.load('ship.bmp')
        self.rect = self.image.get_rect()
        # Start each new ship at the bottom center of the screen
        self.rect.midbottom = self.screen_rect.midbottom

    def blitme(self):
        """Draw the ship at its current location"""
        self.screen.blit(self.image,self.rect)
```

* Pygame treat all game elements like rectangles (rects). So, collision of objects can be calculated via `get_rect()` method
* `pygame.image.load()` is called to load the image
* When the image is loaded, we call `get_rect()` to access ship surface's `rect` attribute

How to work with a `rect` object: 
* Use x,y coord of top, bottom, left and right edges of the rect.
* We can set any of these values to establish the current position of the rect.
* When working center of a game element, use `center`, `centerx`, or `centery` attribute of a rect.
* When working with edge of the screen: work with `top`, `bottom`, `left` or `right` attributes.
* When adjust horizontal/vertical placement of rect, just use `x` & `y` attributes.
* `(0,0)` is the top-left corner of the screen.

### Drawing the Ship to the Screen

Modify `alien_invasion.py` by:

1. Add ship object in `__init__`
2. Add `self.ship.blitme()` in screen redraw code block

## Refactoring: The _check_events() and _update_screen() methods

Refactor code:
* simplifies the structure
* Make it easier to build on.

Here we refactor the code by break the `run_game()`:
* Move code for managing events to `_check_events()`
* Move code for updating the screen to `_update_screen()`

**helper method**:
* is a method in class but isn't meant to be called through an instance.
* syntax: a single leading underscore

### `_check_events()` Method

Create function in `AlienInvasion` and replace original code snippet in while loop

```python
def _check_events(self):
    """Respond to keypresses and mouse events."""
    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            sys.exit()
```

### `_update_screen()` Method

Create function `_update_screen()` and replace original code snippet

```python
def _update_screen(self):
    """Update images on the screen, and flip to the new screen"""
    self.screen.fill(self.settings.bg_color)
    self.ship.blitme()
```

```python
def run_game(self):
    """Start the main loop for the game"""
    while True:

        self._check_events()
        self._update_screen()

```

After replacing lengthy codes with helper function, `run_game` now become simpler. This is a way of refactoring the code

## Piloting the Ship

Now write code that responds when player presses the right or left arrow key.

### Responding to a Keypress

* Whenever the player presses a key, the keypress is registered in Pygame as an event.
* Each event is picked by `pygame.event.get()` method.
* We can specify in `_check_events()` method what kind of events we want the game to check for.
* Each keypress is registered as a `KEYDOWN` event
* If a `K_RIGHT` event is found, move the ship to right by increasing the value `self.ship.rect.x` by 1

```python
    def _check_events(self):
        """Respond to keypresses and mouse events."""
        for event in pygame.event.get():
            if event.type == pygame.QUIT:
                sys.exit()
            elif event.type == pygame.KEYDOWN:
                if event.key == pygame.K_RIGHT:
                    self.ship.rect.x += 1
```

### Allowing Continuous Movement

* We can detect whether the key is released using `KEYUP` event.
* We can use `KEYDOWN` & `KEYUP` events together with a flag called `moving_right` to implement continuous motion
  * When `moving_right` flag is `False`, the ship will be motionless.
  * Create an attribute called `moving_right` and `update()` method to check the status of the `moving_right` flag.

Change `alien_invasion.py`:

```python
    def run_game(self):
        """Start the main loop for the game"""
        while True:

            self._check_events()
            self.ship.update()
            self._update_screen()

    def _check_events(self):
        """Respond to keypresses and mouse events."""
        for event in pygame.event.get():
            if event.type == pygame.QUIT:
                sys.exit()
            elif event.type == pygame.KEYDOWN:
                if event.key == pygame.K_RIGHT:
                    self.ship.moving_right = True
            elif event.type == pygame.KEYUP:
                if event.key == pygame.K_RIGHT:
                    self.ship.moving_right = False
```

Change `ship.py`:

```python
    def __init__(self, ai_game):
        """Initialize the ship and set its starting position"""
        self.screen = ai_game.screen
        self.screen_rect = ai_game.screen.get_rect()

        # Load the ship image and get its rect
        self.image = pygame.image.load('ship.bmp')
        self.rect = self.image.get_rect()
        # Start each new ship at the bottom center of the screen
        self.rect.midbottom = self.screen_rect.midbottom

        # Movement flag
        self.moving_right = False

    def update(self):
        """Update the ship's position based on movement flag"""
        if self.moving_right:
            self.rect.x += 1
```

### Moving Both Left and Right

Apply the same logic on left movement

### Adjusting the Ship's Speed

1. Add `ship_speed` attribute to the `Settings` class
2. Modify `update()` & `__init__` method of `Ship`, so it move by defined speed

```python
class Settings:
    """A class to store all settings for Alien Invasion"""

    def __init__(self):
        """Initialize the game's settings."""

        # Screen settings
        self.screen_width = 1200
        self.screen_height = 800
        self.bg_color = (230,230,230)
        
        # Ship settings
        self.ship_speed = 1.5
```

Modify `ship.py`

```python
    def __init__(self, ai_game):
        """Initialize the ship and set its starting position"""
        self.screen = ai_game.screen
        self.screen_rect = ai_game.screen.get_rect()
        self.settings = ai_game.settings

        # Load the ship image and get its rect
        self.image = pygame.image.load('ship.bmp')
        self.rect = self.image.get_rect()
        # Start each new ship at the bottom center of the screen
        self.rect.midbottom = self.screen_rect.midbottom

        # Store a decimal value for the ship's horizontal position
        self.x = float(self.rect.x)

        # Movement flag
        self.moving_right = False
        self.moving_left = False

    def update(self):
        """Update the ship's position based on movement flag"""
        
        # Update the ship's x value, not the rect
        if self.moving_right:
            self.rect.x += self.settings.ship_speed
        if self.moving_left:
            self.rect.x -= self.settings.ship_speed
```

### Limiting the Ship's Range

Add limit of ship's range

```python
    def update(self):
        """Update the ship's position based on movement flag"""
        
        # Update the ship's x value, not the rect
        if self.moving_right and self.rect.right < self.screen_rect.right:
            self.rect.x += self.settings.ship_speed
        if self.moving_left and self.rect.left > 0:
            self.rect.x -= self.settings.ship_speed
```

### Refactoring _check_events()

As `_check_events()` is becoming lengthy, we should break/refactor it.

```python
    def _check_events(self):
        """Respond to keypresses and mouse events."""
        for event in pygame.event.get():
            if event.type == pygame.QUIT:
                sys.exit()
            elif event.type == pygame.KEYDOWN:
                self._check_keydown_events(event)
            elif event.type == pygame.KEYUP:
                self._check_keyup_events(event)

    def _check_keydown_events(self, event):
        """Respond to keypresses"""
        if event.key == pygame.K_RIGHT:
            self.ship.moving_right = True
        elif event.key == pygame.K_LEFT:
            self.ship.moving_left = True

    def _check_keyup_events(self, event):
        """Respond to key releases"""
        if event.key == pygame.K_RIGHT:
            self.ship.moving_right = False
        elif event.key == pygame.K_LEFT:
            self.ship.moving_left = False
```

### Pressing Q to Quit

* use `pygame.K_q` key event (q) to exit program


```python
    def _check_keydown_events(self, event):
        """Respond to keypresses"""
        if event.key == pygame.K_RIGHT:
            self.ship.moving_right = True
        elif event.key == pygame.K_LEFT:
            self.ship.moving_left = True
        elif event.key == pygame.K_q:
            sys.exit()
```

### Running the Game in Fullscreen Mode

To run the game in fullscreen mode

```python
class AlienInvasion:
    """Overall class to manage game assets and behaviour"""

    def __init__(self):
        """Initialize the game, and create game resources."""
        pygame.init()

        self.settings = Settings()

        # self.screen = pygame.display.set_mode((self.settings.screen_width, self.settings.screen_height))
        self.screen = pygame.display.set_mode((0,0), pygame.FULLSCREEN)
        self.settings.screen_width = self.screen.get_rect().width
        self.settings.screen_height = self.screen.get_rect().height
        pygame.display.set_caption("Alien Invasion")

        self.ship = Ship(self)
```

## Shooting Bullets

Write code that fires a bullet, which is represented by a small rectangle, when the player presses the spacebar.

### Adding the Bullet Settings

```python
    def __init__(self):
        """Initialize the game's settings."""

        # Screen settings
        ...
        # Ship settings
        ...
        # Bullet settings
        self.bullet_speed = 1.0
        self.bullet_width = 3
        self.bullet_height = 15
        self.bullet_color = (60,60,60)
```

`bullet.py` module

```python
import pygame
from pygame.sprite import Sprite

class Bullet(Sprite):
    """
    A class to manage bullets fired from the ship
    """


    def __init__(self, ai_game):
        """Create a bullet object at the ship's current position."""
        super().__init__()
        self.screen = ai_game.screen
        self.settings = ai_game.settings
        self.color = self.settings.bullet_color

        # Create a bullet rect at (0,0) and then set correct position
        self.rect = pygame.Rect(0,0,self.settings.bullet_width, self.settings.bullet_height)
        self.rect.midtop = ai_game.ship.rect.midtop

        # Store the bullet's position as a decimal value
        self.y = float(self.rect.y)
```

* `self.rect = ...` created bullet's `rect` attribute. 
  * First, initialize at `(0,0)`, with width/height stored in `self.settings`
  * Then, move it to the correct location via `self.rect.midtop`, as the bullet's position depends on the ship's position

Then add `update()` & `draw_bullet()` for Bullet class

```python
    def update(self):
        """Move the bullet up the screen"""
        
        # Update the decimal position of the bullet
        self.y -= self.settings.bullet_speed
        # Update the rect position
        self.rect.y = self.y # When a bullet is fired, it moves up the screen

    def draw_bullet(self):
        """Draw the bullet to the screen"""
        pygame.draw.rect(self.screen, self.color, self.rect)
```

### Storing Bullets in a Group

After defining the `Bullet` class and its necessary settings, we can write code to fire a bullet each time the player presses the spacebar.

First, we'll create a group in `AlienInvasion` to store all the live bullets, so we can manage the bullets that have already been fired.

`pygame.sprite.Group` class behave like a list with some extra functionality that's helpful when building games. So we directly draw the group (instead of individual bullets) to the screen.

`alien_invasion.py`

```python
def __init__(self):
    ...
    
    self.ship = Ship(self)
    self.bullets = pygame.sprite.Group()

...

def run_game(self):
    """Start the main loop for the game"""
    while True:
        self._check_events()
        self.ship.update()
        self.bullets.update() # initiate the bullets group
        self._update_screen()
```

### Firing Bullets

1. Add key press event in `_check_keydown_events(self,event)`, which will trigger `_fire_bullet()` by `K_SPACE`
2. Create function `_fire_bullet(self)`
3. Add bullets group's drawing action in `_update_screen()`

```python
from bullet import Bullet

class AlienInvasion:

    def _check_keydown_events(self, event):
        """Respond to keypresses"""
        ...
        elif event.key == pygame.K_q:
            sys.exit()
        elif event.key == pygame.K_SPACE:
            self._fire_bullet()

    def _check_keyup_events(self, event):
        ...

    def _fire_bullet(self):
        """Create a new bullet and add it to the bullets group"""
        new_bullet = Bullet(self)
        self.bullets.add(new_bullet)

    def _update_screen(self):
        """Update images on the screen, and flip to the new screen"""
        ...
        for bullet in self.bullets.sprites():
            bullet.draw_bullet()

        pygame.display.flip()
```

### Deleting Old Bullets

Currently, bullets don't delete themselves after moving out of frame. Only their y-coordinate values grow increasingly negative.

* Con: bullets continue to consume memory and processing power.

Solution: need recycle these objects during running the game

```python
    def run_game(self):
        """Start the main loop for the game"""
        while True:
            ...

            # Delete bullets out of screen
            for bullet in self.bullets.copy():
                if bullet.rect.bottom <= 0:
                    self.bullets.remove(bullet)
            print(len(self.bullets))
```

### Limiting the Number of Bullets

To limit the number of bullets.

```python
class Settings:
    """A class to store all settings for Alien Invasion"""

    def __init__(self):
        """Initialize the game's settings."""
        ...
        self.bullet_allowed = 3
```

Modify `AlienInvasion` class as shown below

```python
class AlienInvasion:
    """Overall class to manage game assets and behaviour"""
    ...

    def _fire_bullet(self):
        """Create a new bullet and add it to the bullets group"""
        if len(self.bullets) < self.settings.bullets_allowed:
            new_bullet = Bullet(self)
            self.bullets.add(new_bullet)
```

### Creating the _update_bullets() Method

Need refactor `run_game` function in `AlienInvasion` class again, to make bullets update and deletion independent of `run_game()`

```python
...
class AlienInvasion:
    """Overall class to manage game assets and behaviour"""
    def run_game(self):
        """Start the main loop for the game"""
        while True:

            self._check_events()
            self.ship.update()
            self._update_bullets()

            self._update_screen()

    def _update_bullets(self):
        """Update position of bullets and get rid of old bullets"""
        # Update bullet positions.
        self.bullets.update()

        # Delete bullets out of screen
        for bullet in self.bullets.copy():
            if bullet.rect.bottom <= 0:
                self.bullets.remove(bullet)
        print(len(self.bullets))
...
```