Android中的ModelLayer（Context appContext）Singleton构造函数…下面的带有注释的代码到底能做什么？

Question

In the model layer of the Android project I am learning from the Big nerd Ranch Android Programming book, there is a specific singleton - model layer class, which goes like this:

 public class ModelLayerClass          
 {
 private static ModelLayerClass class_instance;   //its a clear singleton here ! 
 private Context context_instance;

 private ModelLayerClass(Context appContext)  //why this parameter is being passed?
 {
 context_instance = appContext;             //how this helps  here ?    
 }
 public static ModelLayerClass get(Context c)
 {
 if(class_instance=null)
  {
     class_instance = new ModelLayerClass(c.getApplicationContext());
   }
return class_instance;
 }
} 

When I went through the book, it was saying that, it is common practice in Android to have a Context parameter which allows the singleton to "start activities", access project resources, find your apps private storage and more.....doesnt the classes in our project have a default access to all of these (except for starting activities). Can anyone direct me to proper online resources or could give me a good explanation about this...thnx :)

Answer 1

From your activity, you do have access to everything within your application, but generally through the use of context. Here's the Android documentation for Context .

For example, when you write

startActivity(new Intent(MainActivity.this, NewActivity.class));

You only have access to the startActivity method because your Activity class is extends Activity. If you want to start an activity from a singleton or from another class or anything like that, you need to have a Context to start an activity.

For example, if you wanted to start the same activity above, but from outside of your Activity class, you must have a context:

context.startActivity(new Intent(context, NewActivity.class));

Same goes for any number of other things you might want to do outside of your Activity class.

Accessing resources:

Bitmap imageFromRes = BitmapFactory.decodeResource(context.getResources(), R.drawable.image);

Creating new Android views:

ImageView iv = new ImageView(context);

Basically, Context is a good "catch-all" parameter that allows your non-Android classes to still utilize methods that you would use in Android classes (like Activity, Service, Dialog, etc).