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It is technically possible to create and attach widgets to our activity purely through Java code, but the more common approach is to use an XML-based layout file. Dynamic instantiation of widgets is reserved for more complicated scenarios, where the widgets are not known at compile-time.
An XML-based layout is a specification of widgets' relationships to each other encoded in XML format. Specifically, Android considers XML-based layouts to be resources, and as such layout files are stored in the res/layout directory inside each Android project.
Each XML file contains a tree of elements specifying a layout of widgets and containers that make up one View. The attributes of the XML elements are properties, describing how a widget should look or how a container should behave. For example, if a Button element has an attribute value of android:textStyle = "bold", that means that the text appearing on the face of the button should be rendered in a boldface font style.
Most everything you do using XML layout files can be achieved through Java code. For example, you could use setTypeface() to have a button render its text in bold, instead of using a property in an XML layout. Since XML layouts are yet another file for you to keep track of, we need good reasons for using such files.
Perhaps the biggest reason is to assist in the creation of tools for view definition, such as a GUI builder in an IDE like Eclipse. Such GUI builders could, in principle, generate Java code instead of XML. The challenge is re-reading the definition in to support edits – that is far simpler if the data is in a structured format like XML than in a programming language. Moreover, keeping the generated bits separated out from hand-written code makes it less likely that somebody's custom-crafted source will get clobbered by accident when the generated bits get re-generated. XML forms a nice middle ground between something that is easy for tool-writers to use and easy for programmers to work with by hand as needed.
An XML-based layout is a specification of widgets' relationships to each other encoded in XML format. Specifically, Android considers XML-based layouts to be resources, and as such layout files are stored in the res/layout directory inside each Android project.
Each XML file contains a tree of elements specifying a layout of widgets and containers that make up one View. The attributes of the XML elements are properties, describing how a widget should look or how a container should behave. For example, if a Button element has an attribute value of android:textStyle = "bold", that means that the text appearing on the face of the button should be rendered in a boldface font style.
Most everything you do using XML layout files can be achieved through Java code. For example, you could use setTypeface() to have a button render its text in bold, instead of using a property in an XML layout. Since XML layouts are yet another file for you to keep track of, we need good reasons for using such files.
Perhaps the biggest reason is to assist in the creation of tools for view definition, such as a GUI builder in an IDE like Eclipse. Such GUI builders could, in principle, generate Java code instead of XML. The challenge is re-reading the definition in to support edits – that is far simpler if the data is in a structured format like XML than in a programming language. Moreover, keeping the generated bits separated out from hand-written code makes it less likely that somebody's custom-crafted source will get clobbered by accident when the generated bits get re-generated. XML forms a nice middle ground between something that is easy for tool-writers to use and easy for programmers to work with by hand as needed.
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