Finding Game Objects

Learning Goals:

• Learn how to access current and interact with scene GameObjects and Components within custom class scripts.

• Learn how to use image files as Unity 2D sprites and UI elements.

• Learn how to create UI-Buttons from images, and to use those button to toggle visibility of UI-Panels.

Object References:

When we create object-reference variables, we're creating a variable that stores a memory address for an object. For primitive data-types, variables are a labeled space in memory that stores data of the type that we specified when we defined the variable. However, objects are more complex data types, the system can't know in advance how large of a space in memory we might need for our object, so instead the variable we create for an object holds a pointer or address of the place in memory where our object will be stored. This is very nice because it allows us to share the address, we can use it as in input variable for a function, then within the function, changes made actually do change the object itself, since we are referring directly to the object via it's address. This is different than when we pass primitive types into a function, in that case, only a copy of the variable in actually passed in, this is part of the reason that variable scope is important for primitive data types. Variable scope behaves somewhat differently for object references, since we're passing the address

    int someInteger;   //primitive type variable stores  integer in variable's memory space
    someInteger=5;  //assign a value

    private GameObject mainPanel; // declaring a variable for objects of GameObject type.
    private Button endBtn;  //Button component reference variable

    mainPanel=GameObject.Find ("MainPanel1");  //assign address of gameObject to referenceVariable so we can interact with it throughout the state class code

    // initialize object reference to EndButton, Button component
    endBtn = GameObject.Find("EndButton").GetComponent<Button>();
    //set our custom method: LoadEndScene so it's executed when the EndButton is clicked
    endBtn.onClick.AddListener (LoadEndScene);

Errors - Null-Reference Exception

When we initialize our object-reference variables, we are creating connections to GameObjects in our scenes from within our custom classes. There can be some problems the object can't be found in the scene when we're trying to make this object-reference connection.