CODEWITHSUNDEEP
androidandroid-context
hqt
hqt

Reputation: 30284

Android: Context object

When programming on Android, we use context object everywhere (maybe context keyword or this keyword), but I don't really understand what its purpose.

For example, when we add UI Component such as TextView:

TextView textView = new TextView(this);  //this simple line make me headache
setContentView(textView);

The first time I think above line is : this keyword mean: this textView will be assign to current screen. But after that, I see that is a wrong thinking because line setContentView(textView) do what I think.

So, who can explain for me, what purpose when we declare context object in above example. (and others case, if you please, tell me more :D)

thanks :)

Upvotes: 4

Views: 6337

Answers (3)

user1204873
user1204873

Reputation:

You will need the Context class is when creating a view dynamically in an activity. For example, you may want to dynamically create a TextView from code. To do so, you instantiate the TextView class. The constructor for the TextView class takes a Context object, and because the Activity class is a subclass of Context, you can use the this keyword to represent the Context object.

Upvotes: 7

sinharaj
sinharaj

Reputation: 1103

Why does it need a context? The documentation says that every view needs the context to access the right resources (e.g. the theme, strings etc.).

But why in the constructor and not through setContentView(View)?

  1. Because the resources must be accessible while the view is being constructed (the constructor will need some resources to fully initialise the view).

  2. This allows the flexibility of using a context that is different from the one of the current activity (imagine a view that uses some other string resources and not the ones from the current activity).

  3. The designers of the Android SDK seem to have chosen that the context must be set only once and then stay the same throughout the lifetime of the view.

Why isn't the context determined automatically at construction point?

  1. Because there exists no static variable that would tell you the current global context of your application. The method getApplicationContext() is the closest to this, but it's not static, so you need an instance of the Activity object to call it.

  2. The Java language provides an option to look through the call stack and find whether the View has been constructed in a Context class. But what if there are many? Or what if there are none? This method is very expensive and error prone. So the designers of the API decided that a context must be manually provided.

Upvotes: 3

Ted Hopp
Ted Hopp

Reputation: 234857

A Context object provides access to the application's resources and other features. Each Activity is a Context and each View needs a Context so it can retrieve whatever resources it needs (including things like system-defined resources).

The second line tells the Activity object to use that particular View (a TextView) as the top-level user interface element to display for that Activity. There is no conflict between that and using the Activity as a Context for constructing the TextView in the first place. They are different things.

Upvotes: 7

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