android view 绘制流程

custom_view

关于view的绘制流程

View
ViewParent（interface）
ViewGroup（extends View implements ViewParent）
ViewRoot（implements ViewParent）

Being a root parent of all views in the activity, the ViewRootschedules traversals of all the views in 
order to first lay them out at the right place with the right size; this is called the layout phase. The 
ViewRoot then traverses the view hierarchy to draw them; this phase is called the drawing phase. 
We will talk about each of these phases now.（ViewRoot是每个activity视图层级的最顶层。负责整个视图的layout和draw两个过程）

Layout Phase: Measurement and Layout
布局阶段：确定视图层级中视图的大小和位置。


Measure pass
The measure pass is implemented by the measure( ) function of the Viewclass.The signature of this function is
public final void measure(int widthMeasureSpec, int heightMeasureSpec)

Make a note of this method’s signature. This signature will help you to easily locate this method 
measure( ) in the source code of the large View.javasource file. This method, measure( ), does some 
housekeeping and calls the onMeasure( ) of the derived views. The derived views need to set their 
dimensions by calling setMeasuredDimension(). These measured dimensions set on each view are 
then subsequently used in the layout pass. In this context, your primary override is View.onMeasure(). 
Keep in mind that there is a default implementation for onMeasure(). The default implementation of 
onMeasure( ) decides the size of your view based on suggestions from the layout files, including an 
exact size passed in. We will cover this later in the chapter.

Although it is the onMeasure( ) that you care about when you are a creating custom view like the 
CircleView, there are times when measure( ) is important as well. If the inherited custom view is a 
collection of other views, as in a ViewGroup, then you need to call measure( ) on child views in your 
onMeasure( ) method. The signature of measure( ) earlier clearly supports the idea that you can’t 
override it by being final, but you are expected to call it by being public. We will cover the measure( ) 
method arguments widthMeasureSpecand heightMeasureSpecwhen we work with onMeasure()later 
in this chapter.


Layout pass
After the measure pass, each view knows its dimensions. The control then passes to the layout pass. 
This layout pass is implemented in the layout( ) method, whose signature in the base Viewclass is:
public void layout(int left, int top, int right, int bottom)
Much like the measure( ) method, the layout( ) method carries out an internal protocol for the base 
Viewand result in calling the overridden methods in Listing 1-1, in that order.
Listing 1-1.  Overridden Methods Called by a View’s Layout( ) Method
protected void onSizeChanged(int w, int h, int oldw, int oldh);
protected void onLayout(boolean changed, int left, int top, int right, int bottom)

The layout pass, implemented in layout( ), will take the dimensions measured by the measure 
pass into account and give out the starting position for each view and the dimension each view 
needs to use. The base layout( ) method actually sets these dimensions on the view on which it is 
called. It then calls the onSizeChanged( ) if there is actually a change in size or position. The default 
implementation of onSizeChanged( ) exists in the Viewclass but it is a no-op.
After calling the onSizeChanged( ) method, the layout( ) method calls the onLayout( ) to allow 
for something like a view group to call layout( ) on its children. The default implementation for 
onLayout( ) exists but it is a no-op. To apply this to our CircleView, we don’t need to do anything in 
the onLayout( ) because our position and dimensions are already fixed, and we have no children to 
advise their layouts by calling their layout( ) method.
Once both of the passes of the layout phase are completed, the traversal initiated by the view root 
will move to the drawing phase.


Drawing Phase: Mechanics of onDraw
The draw traversal is implemented in the View's draw( ) method. The protocol implemented by this 
method is:
Draw the background
Draw view's content by delegating to onDraw()
Draw children by delegating to dispatchDraw()
Draw decorations such as scroll bars

Because the draw traversal happens after the layout traversal, you already know the position and the 
size of your views. If your view like the CircleViewdoesn’t have children, you don’t care much about 
dispatchDraw( ). The default implementation for this method in the base Viewclass exists but is empty.

You could ask: If my custom view has children, why am I not choosing to draw them in onDraw? 
Perhaps because, in a framework, the base class View'sfixed protocol of draw( ) may choose 
to do something between your onDraw( ) and your children’s onDraw( ). So, it is suggested to the 
programmer, by dispatchDraw( ) of the View, that the View’s drawing is complete and the derived 
implementation could choose whatever is needed.