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am 161e423e: Merge "docs: Fixed deprecated value. Bug: 10641906" into jb-mr2-docs

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* commit '161e423ead0e2272ebb0b73957abf0c916ad57ff':
  docs: Fixed deprecated value. Bug: 10641906
This commit is contained in:
quddusc
2013-09-11 10:09:45 -07:00
committed by Android Git Automerger
parent b096bee155 161e423ead
commit a0b259a917
1 changed files with 75 additions and 40 deletions
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115
docs/html/guide/topics/ui/how-android-draws.jd
View File

@@ -4,15 +4,19 @@ parent.link=index.html
@jd:body
<p>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
<p>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.</p>
<p>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 <code>{@link android.view.View#draw(Canvas) draw()}</code> 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.
<div class="sidebox-wrapper">
<div class="sidebox">
<p>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.</p>
<p>You can force a View to draw, by calling <code>{@link android.view.View#invalidate()}</code>.
<p>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.</p>
<p>You can force a {@link android.view.View} to draw, by calling
{@link android.view.View#invalidate()}.
</p>
</div>
</div>
<p>
Drawing the layout is a two pass process: a measure pass and a layout pass. The measuring
pass is implemented in <code>{@link android.view.View#measure(int, int)}</code> 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
<code>{@link android.view.View#layout(int,int,int,int)}</code> 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.
</p>
<p>
When a View's <code>measure()</code> method returns, its <code>{@link android.view.View#getMeasuredWidth()}</code> and
<code>{@link android.view.View#getMeasuredHeight()}</code> 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 <code>measure()</code> 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
<code>measure()</code> 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).
</p>
<div class="sidebox-wrapper">
<div class="sidebox"><p>
To initiate a layout, call <code>{@link android.view.View#requestLayout}</code>. 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.</p>
</div>
</div>
<p>
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:</p>
<ul>
<li> an exact number
<li><var>FILL_PARENT</var>, which means the View wants to be as big as its parent
<li>{@link android.view.ViewGroup.LayoutParams#MATCH_PARENT MATCH_PARENT},
which means the {@link android.view.View} wants to be as big as its parent
(minus padding)</li>
<li><var>WRAP_CONTENT</var>, which means that the View wants to be just big enough to
<li>{@link android.view.ViewGroup.LayoutParams#WRAP_CONTENT WRAP_CONTENT},
which means that the {@link android.view.View} wants to be just big enough to
enclose its content (plus padding).</li>
</ul>
<p>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.
<p>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.
</p>
<p>
MeasureSpecs are used to push requirements down the tree from parent to
child. A MeasureSpec can be in one of three modes:</p>
{@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:</p>
<ul>
<li><var>UNSPECIFIED</var>: This is used by a parent to determine the desired dimension
of a child View. For example, a LinearLayout may call <code>measure()</code> on its child
with the height set to <var>UNSPECIFIED</var> and a width of <var>EXACTLY</var> 240 to find out how
tall the child View wants to be given a width of 240 pixels.</li>
<li><var>EXACTLY</var>: This is used by the parent to impose an exact size on the
<li>{@link android.view.View.MeasureSpec#UNSPECIFIED UNSPECIFIED}: This is
used by a parent to determine the desired dimension
of a child {@link android.view.View}. For example, a
{@link android.widget.LinearLayout} may call
{@link android.view.View#measure(int, int) measure()} on its child
with the height set to {@link android.view.View.MeasureSpec#UNSPECIFIED UNSPECIFIED}
and a width of {@link android.view.View.MeasureSpec#EXACTLY EXACTLY} 240 to
find out how tall the child {@link android.view.View} wants to be given a
width of 240 pixels.</li>
<li>{@link android.view.View.MeasureSpec#EXACTLY 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.</li>
<li><var>AT_MOST</var>: This is used by the parent to impose a maximum size on the
<li>{@link android.view.View.MeasureSpec#AT_MOST AT MOST}: This is used by
the parent to impose a maximum size on the
child. The child must guarantee that it and all of its descendants will fit
within this size.</li>
</ul>