The drawing process of view in Android

The view's drawing process is performed separately: Onmeasure (multiple times) calculates the size of the view, OnLayout (), determines the size and position of the control OnDraw () draws the view

When the activity receives the focus, it will be asked to draw its own layout, and the Android framework will handle the drawing process, and the activity simply provides the root node of its layout.

The drawing process starts at the root node of the layout and measures and plots the entire layout tree from the root node.

Each of the ViewGroup is responsible for asking every child of it to be drawn, and each View is responsible for drawing itself.

Because the entire tree is traversed sequentially, the parent node is drawn first, and the sibling nodes are drawn in the order in which they appear in the tree.

　　

　　Drawing is a two-pass process: A measure pass and a layout pass.

　　The measurement process (measuring pass) is implemented in measure (int, int) , from top to bottom of the tree.

In this recursive process, each view will pass on its own dimension specifications.

At the end of the measure pass, each view stores its own measurements, the measurement result.

The second is the layout pass, which occurs in layout (int, int , int, int) and still from top to bottom (Top-down).

In this time, each parent will be responsible for the size of the measurement process, and put all of their children in the right place.

Dimension Parent-child relationship processing

When the measure () method of a View object returns, its getMeasuredWidth() and getMeasuredHeight() values should be set, and the values of all its descendants should be set together as well.

　The value of the measured width and measured height of a view object must take into account the restrictions that its parent container gives it.

This ensures that at the end of the measure pass, all the parent accepts the measurements result of all of its children.

　　Note: A parent may call the measure () method more than once to its child.

For example, the first time a parent might measure every child of it, and did not specify the size, the parent just to find out how much they want;

If, after the first pass, the unlimited size sum of all the children is too large or too small, the parent will again call the measure () method on its child, when the parent will set the rules, intervene in the process, and use the actual values.

(That is, to allow children to develop freely, so parents intervene ).

Layout Property Description

　　layoutparams is a parameter that the view uses to tell its parent container how it wants to be placed.

The most basic Layoutparams base class simply describes how large the view wants to be, indicating the size attribute.

That is, when the view is in the XML layout, it usually needs to specify the width and Height properties.

　　Each dimension can be specified as one of the following three values:

1.fill_parent (renamed to match_parentafter API level 8) indicates that the view wants to be as large as its PARENT (minus the margins).

2.wrap_content, which means that the view wants to be just as large as it can contain (including margins).

3. specific values .

Different subclasses of ViewGroup (different layout classes) have corresponding Layoutparams subclasses, which contain more layout-related properties.

Onmeasure Method

　　The onmeasure method is to measure the view and its contents, determining measured width and measured height, which is measure(int, int) evoked by the method, and subclasses can overwrite onmeasure to provide more accurate and effective measurements.

There is a convention: when covering the Onmeasure method, it must be called setMeasuredDimension(int,int) to store the view's measured measured width and height.

If this is not done, a illegalstateexceptionwill be thrown by the measure (int, int) method.

The declaration of the Onmeasure method is as follows:

protected void onmeasure (intint heightmeasurespec)

Two of these input parameters:

　　Widthmeasurespec

　　Heightmeasurespec

The horizontal and vertical space requirements proposed by the parent are respectively.

These two requirements are coded according to the View.measurespec class.

See View.measurespec for a description of this class: This class wraps the layout request passed down from the parent and passes it to the child.

Each of the MEASURESPEC represents a requirement for width or height.

　　Each of the MEASURESPEC has a size (size) and a pattern (mode).

Measurespecs This class provides a way to wrap a <size, mode> tuple into an int type, thus reducing the allocation of objects. Of course, the inverse parsing method is provided, and the size and mode are solved from the int value.

　　There are three types of modes:

　　UNSPECIFIED

This means that the parent does not impose any restrictions on child, and the child can be any size it wants.

　　Exactly

The parent determines an absolute size for child, and child will be given these boundaries, regardless of how much she wants.

　　At_most

The child can be of any size, but has an absolute size limit.

when covering the Onmeasure method, subclasses are responsible for ensuring that measured height and width are at least the minimum height and width of this view.

( getSuggestedMinimumHeight() and getSuggestedMinimumWidth() ).

OnLayout

This method is called in the layout pass to determine the placement and size of the view. Method declaration:

protected void OnLayout (booleanint int int int bottom)

The upper and lower left and right parameters are relative to the parent.