Getting Started - Building a Simple User Interface

http://developer.android.com/training/basics/firstapp/building-ui.html

1, View/View Group

2, LinearLayout is a view group (a subclass of ViewGroup) that lays out child views in either a vertical or horizontal orientation, as specified by the android:orientation attribute. Each child of a LinearLayoutappears on the screen in the order in which it appears in the XML.

3,"match_parent"fill the entire screen area that's available to the app by setting the width and height."wrap_content" specifies that the view should be only as big as needed to fit the contents of the view. eg. just fit the words showed on a button.

Declaring your UI layout in XML rather than runtime code is useful for several reasons, but it's especially important so you can create different layouts for different screen sizes.

3,

android:id

This provides a unique identifier for the view, which you can use to reference the object from your app code, such as to read and manipulate the object (you'll see this in the next lesson).

The at sign (@) is required when you're referring to any resource object from XML. It is followed by the resource type (id in this case), a slash, then the resource name (edit_message).

The plus sign (+) before the resource type is needed only when you're defining a resource ID for the first time.

android:hint

This is a default string to display when the text field is empty.

4, Make the Input Box Fill in the Screen Width

The weight value is a number that specifies the amount of remaining space each view should consume, relative to the amount consumed by sibling views. This works kind of like the amount of ingredients in a drink recipe: "2 parts vodka, 1 part coffee liqueur" means two-thirds of the drink is vodka. For example, if you give one view a weight of 2 and another one a weight of 1, the sum is 3, so the first view fills 2/3 of the remaining space and the second view fills the rest. If you add a third view and give it a weight of 1, then the first view (with weight of 2) now gets 1/2 the remaining space, while the remaining two each get 1/4.

The default weight for all views is 0, so if you specify any weight value greater than 0 to only one view, then that view fills whatever space remains after all views are given the space they require. So, to fill the remaining space in your layout with the EditText element, give it a weight of 1 and leave the button with no weight.

In order to improve the layout efficiency when you specify the weight, you should change the width of theEditText to be zero (0dp).