Creating Android App UI Programmatically

Creating the Example Project in Android Studio

• The onCreate() method is initially designed to use a resource layout file for the user interface.
• To add views to an activity, the line related to the resource layout file must be deleted from the onCreate() method.

Adding Views to an Activity

• A RelativeLayout object with a single Button view child is created programmatically in the onCreate() method.
• The Button view is configured to display the text "Press Me".

Setting View Properties

• The background of the RelativeLayout view is set to blue.
• The Button view is set to display the text "Press Me".

Adding Layout Parameters and Rules

• A LayoutParams object is created and initialized with appropriate values to position the button.
• The LayoutParams object is used to center the button both horizontally and vertically within the layout.
• The LayoutParams object is passed as an argument when adding the child view to the parent.

Drawing 2D Graphics

• To draw custom 2D graphics, create a custom View subclass with the drawing code. If the app is animated, use a thread to periodically update the graphics and redraw them.
• Use the onDraw method to draw on the view, which is called when the view is redrawn.

Custom View

• Create a custom view by extending the View class and overriding the onDraw method.
• Insert the custom view into an activity's layout XML.

Canvas Object Methods

• drawARGB - fill the window with a color (RGB values: 0-255)
• drawArc - draw a partial ellipse
• drawBitmap - draw an image
• drawCircle - draw a circle
• drawLine - draw a line segment
• drawOval - draw an oval or circle (Android 5.0 and above)
• drawPoint - color a single pixel
• drawRect - draw a rectangle (Android 5.0 and above)
• drawRoundRect - draw a rounded rectangle (Android 5.0 and above)
• drawText - draw a text string
• getWidth and getHeight - get the dimensions of the drawing area

Paint

• Create a Paint object by specifying an alpha (opacity) value and RGB integer values.
• Set the paint style, stroke width, and text size using various methods.
• Use the Paint object to set the color, font, and style of the drawn shapes.

Typeface

• In Android, a font is called a Typeface.
• Set a font inside a Paint object.
• Create a Typeface based on a specific font name or a general font family.
• Load a Typeface from a file in the assets directory.

Bitmap Images

• Draw an image using the Bitmap class.
• Decode a Bitmap from a resource using BitmapFactory.decodeResource.
• Decode a Bitmap from an input stream using BitmapFactory.decodeStream.

Animation via Redrawing

• To animate a view, redraw it at regular intervals and change the variables or positions of the shapes.
• Force a view to redraw itself by calling its invalidate method.
• Use a thread to redraw the view at regular intervals, but avoid locking up the app's UI.

Threads

• A thread is a lightweight process or isolated sub-task within a program.
• Use a thread to implement concurrency and perform multiple tasks simultaneously.
• Variables created in one thread can be seen by other threads.

Using a Thread

• Create a thread by passing a Runnable object with a run method.
• Use the start method to start the thread.
• Use the stop method to stop the thread.
• Use the sleep method to pause the thread.
• Use the isRunning method to check if the thread is running.
• Use the join method to wait for the thread to finish.

Redrawing a View in a Thread

• Use a Handler object to make the invalidate call on the view from the thread.
• Create a Handler object and post a Runnable to it to redraw the view.

Mouse Touch Events

• Handle finger presses from the user by writing an onTouchEvent method in the custom View class.
• Use the getAction method to determine the type of event (e.g., ACTION_DOWN, ACTION_UP, ACTION_MOVE).

Keyboard Events

• Handle key presses by setting the app to receive keyboard focus in the View constructor.
• Write onKeyDown and onKeyUp methods in the custom View class.
• Use the getKeyCode method to determine the key pressed.

Sprite Class

• A sprite is an object of interest in a game.
• A sprite class can represent the sprites in the game.
• The sprite class can contain data such as location, size, velocity, shape/image, and points.

Collision Detection

• Collision detection is determining whether sprites in the game world are touching each other.
• Use RectF and other shapes to check for collisions using methods such as contains, intersects, and collide.
• Use a smaller collision rectangle to give the collisions a bit of slack.

Wake Lock

• Use a wake lock to prevent the screen from blanking.

List Adapter Example

• An ArrayList is loaded from a file and used to create an ArrayAdapter with the layout android.R.layout.simple_list_item_1.
• The adapter is then set to a ListView with the ID mylist.

Handling List Events

• List views respond to several events, including setOnItemClickListener, setOnItemLongClickListener, and setOnItemSelectedListener.
• Event listeners must be attached in Java code, not in XML, and require the use of anonymous inner classes.

Attaching Event Listeners in Java

• Event listeners can be attached using anonymous inner classes, which are declared and constructed on the fly.
• An example of attaching an event listener to a button is provided.

List Events

• List views respond to several events, including setOnItemClickListener, setOnItemLongClickListener, and setOnItemSelectedListener.
• An example of attaching a listener to a ListView is provided, which responds to item clicks.

Anatomy of an Android App and the App Lifecycle

Application Components

• Android apps consist of four primary components: Activities, Services, Content Providers, and Broadcast Receivers.
• Each component has a different purpose and lifecycle.

Activity

• An Activity is a single screen with a user interface, and an app may have several Activities.
• Activities are subclasses of the Activity class.

Service

• A Service is an application component that performs long-running operations in the background with no UI.
• An example of a Service is an app that automatically responds to texts when driving.

Content Providers

• A Content Provider is a bridge between applications to share data.
• An example of a Content Provider is the device's contacts information.

Broadcast Receivers

• A Broadcast Receiver is a component that responds to system-wide announcements.
• Examples of broadcasts include battery low, screen off, and date changed.

Intents

• Intents are used to pass information between applications.

Activity Stack

• The most recently created Activity is at the top of the stack.
• When the user presses the Back button or an Activity is destroyed, the previous Activity is brought to the top.

Saving and Restoring State

• The onSaveInstanceState method is used to save the state of an Activity.
• The onRestoreInstanceState method is used to restore the state of an Activity.

Starting Your Own Activities

• New Activities can be started within an Activity to accomplish a task or get some data.
• An example of starting a new Activity to get a name is provided.

Drawing 2D Graphics

• Custom 2D graphics can be drawn on screen by creating a custom View subclass with the drawing code.
• If the app is animated, a thread can be used to periodically update the graphics and redraw them.

Canvas Object Methods

• Several methods can be used to draw on a Canvas object, including drawARGB, drawArc, drawBitmap, and more.

Paint

• A Paint object can be used to specify a color to use for drawing.
• Many methods accept a Paint object as a parameter.

Touch Events

• The onTouchEvent method can be overridden to handle touch events.

Keyboard Events

• Keyboard events can be handled by setting the app to receive keyboard "focus" in the View constructor.
• The onKeyDown and onKeyUp methods can be overridden to handle key presses.

Sprite Class

• A Sprite class can be used to represent objects of interest in a game.
• Possible data for a Sprite include location, size, velocity, shape/image, and points.

Collision Detection

• Collision detection involves determining whether sprites in the game world are touching each other.
• Android's RectF and other shapes have methods to check whether they touch.
• Collision detection can be more complex for non-rectangular sprites.

Learn how to add views to an activity in Android Studio by creating a RelativeLayout and Button programmatically in the onCreate() method.