When an Activity receives focus, it will be requested to draw its layout.
The Android framework will handle the procedure for drawing, but the Activity must provide
the root node of its layout hierarchy.
Drawing begins with the root node of the layout. It is requested to measure and
draw the layout tree. Drawing is handled by walking the tree and rendering each View that
intersects the invalid region. In turn, each View group is responsible for requesting
each of its children to be drawn (with the draw() method)
and each View is responsible for drawing itself.
Because the tree is traversed in-order,
this means that parents will be drawn before (i.e., behind) their children, with
siblings drawn in the order they appear in the tree.
The framework will not draw Views that are not in the invalid region, and also
will take care of drawing the Views background for you.
You can force a View to draw, by calling invalidate().
Drawing the layout is a two pass process: a measure pass and a layout pass. The measuring
pass is implemented in measure(int, int) and is a top-down traversal
of the View tree. Each View pushes dimension specifications down the tree
during the recursion. At the end of the measure pass, every View has stored
its measurements. The second pass happens in
layout(int, int, int, int) and is also top-down. During
this pass each parent is responsible for positioning all of its children
using the sizes computed in the measure pass.
When a View's measure() method returns, its getMeasuredWidth() and
getMeasuredHeight() values must be set, along with those for all of
that View's descendants. A View's measured width and measured height values
must respect the constraints imposed by the View's parents. This guarantees
that at the end of the measure pass, all parents accept all of their
children's measurements. A parent View may call measure() more than once on
its children. For example, the parent may measure each child once with
unspecified dimensions to find out how big they want to be, then call
measure() on them again with actual numbers if the sum of all the children's
unconstrained sizes is too big or too small (i.e., if the children don't agree among themselves
as to how much space they each get, the parent will intervene and set the rules on the second pass).
To intiate a layout, call requestLayout(). This method is typically
called by a View on itself when it believes that is can no longer fit within
its current bounds.
The measure pass uses two classes to communicate dimensions. The
View.MeasureSpec class is used by Views to tell their parents how they
want to be measured and positioned. The base LayoutParams class just
describes how big the View wants to be for both width and height. For each
dimension, it can specify one of:
- an exact number
- FILL_PARENT, which means the View wants to be as big as its parent
(minus padding)
- WRAP_CONTENT, which means that the View wants to be just big enough to
enclose its content (plus padding).
There are subclasses of LayoutParams for different subclasses of ViewGroup.
For example, RelativeLayout has its own subclass of LayoutParams, which includes
the ability to center child Views horizontally and vertically.
MeasureSpecs are used to push requirements down the tree from parent to
child. A MeasureSpec can be in one of three modes:
- UNSPECIFIED: This is used by a parent to determine the desired dimension
of a child View. For example, a LinearLayout may call
measure() on its child
with the height set to UNSPECIFIED and a width of EXACTLY 240 to find out how
tall the child View wants to be given a width of 240 pixels.
- EXACTLY: This is used by the parent to impose an exact size on the
child. The child must use this size, and guarantee that all of its
descendants will fit within this size.
- AT_MOST: This is used by the parent to impose a maximum size on the
child. The child must gurantee that it and all of its descendants will fit
within this size.
