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