The following is the first few sections of a chapter from The Busy Coder's Guide to Android Development, plus headings for the remaining major sections, to give you an idea about the content of the chapter.
There is a decent chance that you have already done work with widget-based UI frameworks. In that case, much of this chapter will be review, though checking out the section on the absolute positioning anti-pattern should certainly be worthwhile.
There is a chance, though, that your UI background has come from places where you have not been using a traditional widget framework, where either you have been doing all of the drawing yourself (e.g., game frameworks) or where the UI is defined more in the form of a document (e.g., classic Web development). This chapter is aimed at you, to give you some idea of what we are talking about when discussing the notion of widgets and containers.
Wikipedia has a nice definition of a widget:
In computer programming, a widget (or control) is an element of a graphical user interface (GUI) that displays an information arrangement changeable by the user, such as a window or a text box. The defining characteristic of a widget is to provide a single interaction point for the direct manipulation of a given kind of data. In other words, widgets are basic visual building blocks which, combined in an application, hold all the data processed by the application and the available interactions on this data.
(quote from the 7 March 2014 version of the page)
Take, for example, this Android screen:
Figure 64: A Sample Android Screen
Here, we see:
Everything listed above is a widget. The user interface for most Android screens (“activities”) is made up of one or more widgets.
This does not mean that you cannot do your own drawing. In fact, all the existing widgets are implemented via low-level drawing routines, which you can use for everything from your own custom widgets to games.
This also does not mean that you cannot use Web technologies. In fact, we will see later in this book a widget designed to allow you to embed Web content into an Android activity.
However, for most non-game applications, your Android user interface will be made up of several widgets.
Widgets have some sort of size, since a zero-pixel-high, zero-pixel-wide widget is not
especially user-friendly. Sometimes, that size will be dictated by what is inside the
widget itself, such as a label (
TextView) having a size dictated by the text in the
label. Sometimes, that size will be dictated by the size of whatever holds the
widget (a “container”, described in the next section), where the widget wants to take
up all remaining width and/or height. Sometimes, that size will be a specific set of
Widgets can have margins. As with CSS, margins provide separation between a widget and anything adjacent to it (e.g., other widgets, edges of the screen). Margins are really designed to help prevent widgets from running right up next to each other, so they are visually distinct. Some developers, however, try to use margins as a way to hack “absolute positioning” into Android, which is an anti-pattern that we will examine later in this chapter.
Widgets can have padding. As with CSS, padding provides separation between the contents of a widget and the widget’s edges. This is mostly used with widgets that have some sort of background, like a button, so that the contents of the widget (e.g., button caption) does not run right into the edges of the button, once again for visual distinction.
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