As mentioned in section 2.1, there are two major types of components in Android XMLs: views and view groups. A
ViewGroupessentially describes the layout and structure of a set of
Viewobjects. Typically, if you want to display a set of elements in a unique way in relation to one another, a parent
ViewGroupobject is created to define the relationship.
Some of the most common
ViewGroupsyou’ll run into are the following:
These are the most light-weight and commonly used
ViewGroup. The default orientation of these layouts is horizontal, i.e. any child elements placed inside them will be displayed in a horizontal line.
Changing the attribute
LinearLayoutto "vertical" will force the
LinearLayoutto display the child views vertically rather than horizontally.
Linear Layout Horizontal vs Vertical Orientations
Because this layout is fairly inexpensive for the compiler to build, you’ll see this layout used everywhere, especially for layouts with list-like properties.
In contrast to
LinearLayout, which can only place views in a line, this
ViewGroupgives the developer more control over the precise positioning of child views. Positioning attributes are added to the child views of
RelativeLayout, like so:
This attribute will position the child component immediately to the right of the element with id “elementA” (elementA should be a direct child of the same
RelativeLayout). We can do the same thing for positioning elements to the left, above, and below.
We can also center the child elements to the middle of the parent view (vertically), with the attribute
android:layout_centerVertical="true". Remember, these constraints can only be added to child components who are encompassed inside a
RelativeLayoutin the hierarchical tree of XML.
RelativeLayoutitself, adding the attribute
android:gravitymoves all child elements into one pattern or location within the
RelativeLayoutis convenient to use, it is important to note that it is also the most expensive layout to build (meaning that the compiler has to make extra computations to create the layout). This is why we try to avoid using
ConstraintLayoutis similar to
RelativeLayout, but has better performance and (generally) more flexibility (not to mention that it is the layout most recommended by Google). Each direct child view in a
ConstraintLayoutneeds both a horizontal and a vertical constraint. Let’s take a look at the following pair of attributes:
The first constraint will constrain the child to be vertically attached to the bottom of the
ConstraintLayout, and the second constraint will position the right side of the child to be in line with the right side of element bigButton. Note here that the terms Start and End refer to Left and Right constraints, respectively.
If we didn’t want the constraints to be directly in line with these elements, we can add the following attributes to the same child component above:
These four properties in combination will now position the right side of our element 50 density-independent pixels (dp) to the left of the right side of bigButton, and will position our element 70dp above the bottom of the screen. Margins are a very useful attribute that are used all over the place, not just in
FrameLayoutis a flexible layout that allows child views to be stacked on top of each other based on the order of when the views are added, similar to how someone may layer photographs inside of a picture frame.
Example of how FrameLayout stacks views
FrameLayoutis very similar to a
ViewGroupand doesn’t have any custom attributes like
RelativeLayout, it is used often as a fragment container (will be explained in chapter 4) or as a container to hold multiple views that may be swapped out.
These layouts all display some sort of scrollable content to users. A
ScrollViewis similar to a vertical
LinearLayoutin their organization of displaying child elements, but simply makes the child content scrollable if the total height of the child elements extends beyond the height of the
List views and recycler views are used to display a scrollable list of elements that are set dynamically. They are meant to create a list of varying lengths based on an array of information, whereas scroll views children are normally set in the
ViewGroupobjects also are more efficient in that they only render as much child content as needed to display to the user, and only when the user scrolls to new areas are those child elements rendered.
As for comparing these two, recycler views are generally thought of as a better version (simpler, more efficient, easier to use) of list views. These views become very important to development, and will be elaborated upon in Chapter 4.
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