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Diffstat (limited to 'docs/html/guide/topics/ui/how-android-draws.jd')
| -rw-r--r-- | docs/html/guide/topics/ui/how-android-draws.jd | 115 |
1 files changed, 75 insertions, 40 deletions
diff --git a/docs/html/guide/topics/ui/how-android-draws.jd b/docs/html/guide/topics/ui/how-android-draws.jd index 6a8cd86..168f77b 100644 --- a/docs/html/guide/topics/ui/how-android-draws.jd +++ b/docs/html/guide/topics/ui/how-android-draws.jd @@ -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> |