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 +
When an {@link android.app.Activity} receives focus, it will be requested to +draw its layout. +The Android framework will handle the procedure for drawing, but the +{@link android.app.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 {@link android.view.View#draw(Canvas) draw()} method)
-and each View is responsible for drawing itself.
+draw the layout tree. Drawing is handled by walking the tree and rendering each
+{@link android.view.View} that intersects the invalid region. In turn, each
+{@link android.view.ViewGroup} is responsible for requesting
+each of its children to be drawn
+(with the {@link android.view.View#draw(Canvas) draw()} method)
+and each {@link android.view.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.
@@ -20,76 +24,107 @@ and each View is responsible for drawing itself.
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 {@link android.view.View#invalidate()}.
+
The framework will not draw {@link android.view.View} objects that are not +in the invalid region, and also + will take care of drawing the {@link android.view.View} background for you.
+You can force a {@link android.view.View} to draw, by calling +{@link android.view.View#invalidate()}.
- Drawing the layout is a two pass process: a measure pass and a layout pass. The measuring
- pass is implemented in {@link android.view.View#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
+ Drawing the layout is a two pass process: a measure pass and a layout pass.
+The measuring pass is implemented in {@link android.view.View#measure(int, int)}
+and is a top-down traversal of the {@link android.view.View} tree. Each {@link android.view.View}
+pushes dimension specifications down the tree
+ during the recursion. At the end of the measure pass, every
+{@link android.view.View} has stored
its measurements. The second pass happens in
- {@link android.view.View#layout(int,int,int,int)} and is also top-down. During
+ {@link android.view.View#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 {@link android.view.View#getMeasuredWidth()} and
- {@link android.view.View#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
+ When a {@link android.view.View} object's
+{@link android.view.View#measure(int, int) measure()} method
+returns, its {@link android.view.View#getMeasuredWidth()} and
+ {@link android.view.View#getMeasuredHeight()} values must be set, along
+ with those for all of that {@link android.view.View} object's descendants.
+A {@link android.view.View} object's measured width and
+measured height values must respect the constraints imposed by the
+{@link android.view.View} object'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
+ children's measurements. A parent {@link android.view.View} may call
+{@link android.view.View#measure(int, int) 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).
+ {@link android.view.View#measure(int, int) measure()} on them again with
+actual numbers if the sum of all the children's
+ unconstrained sizes is too big or too small (that is, 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 initiate a layout, call {@link android.view.View#requestLayout}. This method is typically
- called by a View on itself when it believes that is can no longer fit within
+ To initiate a layout, call {@link android.view.View#requestLayout}.
+This method is typically
+ called by a {@link android.view.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 - {@link android.view.ViewGroup.LayoutParams} 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 + {@link android.view.ViewGroup.LayoutParams} class is used by +{@link android.view.View} objects to tell their parents how they + want to be measured and positioned. The base +{@link android.view.ViewGroup.LayoutParams} class just + describes how big the {@link android.view.View} wants to be for both +width and height. For each dimension, it can specify one of:
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. +
There are subclasses of {@link android.view.ViewGroup.LayoutParams} for +different subclasses of {@link android.view.ViewGroup}. + For example, {@link android.widget.RelativeLayout} has its own subclass of +{@link android.view.ViewGroup.LayoutParams}, which includes + the ability to center child {@link android.view.View} objects +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:
+ {@link android.view.View.MeasureSpec MeasureSpec} objects are used to push +requirements down the tree from parent to + child. A {@link android.view.View.MeasureSpec MeasureSpec} can be in one of +three modes: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.