【移动应用开发技术】Android UI绘制流程的示例分析

【移动应用开发技术】AndroidUI绘制流程的示例分析

这篇文章主要介绍AndroidUI绘制流程的示例分析，文中介绍的非常详细，具有一定的参考价值，感兴趣的小伙伴们一定要看完！一、绘制流程源码路径1、Activity加载ViewRootImplActivityThread.handleResumeActivity()

-->

WindowManagerImpl.addView(decorView,

layoutParams)

-->

WindowManagerGlobal.addView()2、ViewRootImpl启动View树的遍历ViewRootImpl.setView(decorView,

layoutParams,

parentView)

-->ViewRootImpl.requestLayout()

-->scheduleTraversals()

-->TraversalRunnable.run()

-->doTraversal()

-->performTraversals()（performMeasure、performLayout、performDraw）二、View绘制流程1、measure（1）MeasureSpec是什么？重写过onMeasure()方法都知道，测量需要用到MeasureSpec类获取View的测量模式和大小，那么这个类是怎样存储这两个信息呢？留心观察的话会发现，onMeasure方法的两个参数实际是32位int类型数据，即：0000000000000000000000000000000000000000000000000000000000000000而其结构为mode+size，前2位为mode，而后30位为size。==>getMode()方法（measureSpec-->mode）：private

static

final

int

MODE_SHIFT

=

30;

//

0x3转换为二进制即为：11

//

左移30位后：11000000

00000000

00000000

00000000

private

static

final

int

MODE_MASK

=

0x3

<<

MODE_SHIFT;

public

static

int

getMode(int

measureSpec)

{

//

与MODE_MASK按位与运算后，即将低30位清零，结果为mode左移30位后的值

return

(measureSpec

&

MODE_MASK);

}getSize()方法同理。==>makeMeasureSpec()方法（mode+size-->measureSpec）：public

static

int

makeMeasureSpec(

@IntRange(from

=

0,

to

=

(1

<<

MeasureSpec.MODE_SHIFT)

-

1)

int

size,

@MeasureSpecMode

int

mode)

{

if

(sUseBrokenMakeMeasureSpec)

{

return

size

+

mode;

}

else

{

return

(size

&

~MODE_MASK)

|

(mode

&

MODE_MASK);

}

}这里解释一下，按位或左侧为size的高2位清零后的结果，右侧为mode的低30位清零后的结果，两者按位或运算的结果正好为高2位mode、低30位size，例：01000000

00000000

00000000

00000000

|

00001000

00001011

11110101

10101101

=

01001000

00001011

11110101

10101101二进制计算规则可参考：/article/166892.htm==>测量模式：public

static

final

int

UNSPECIFIED

=

0

<<

MODE_SHIFT;

public

static

final

int

EXACTLY

=

1

<<

MODE_SHIFT;

public

static

final

int

AT_MOST

=

2

<<

MODE_SHIFT;UNSPECIFIED：父容器不对View作任何限制，系统内部使用。EXACTLY：精确模式，父容器检测出View大小，即为SpecSize；对应LayoutParams中的match_parent和指定大小的情况。AT_MOST：最大模式，父容器指定可用大小，View的大小不能超出这个值；对应wrap_content。（2）ViewGroup的测量流程回到ViewRootImpl的performMeasure方法，这里传入的参数为顶层DecorView的测量规格，其测量方式为：private

static

int

getRootMeasureSpec(int

windowSize,

int

rootDimension)

{

int

measureSpec;

switch

(rootDimension)

{

case

ViewGroup.LayoutParams.MATCH_PARENT:

measureSpec

=

MeasureSpec.makeMeasureSpec(windowSize,

MeasureSpec.EXACTLY);

break;

case

ViewGroup.LayoutParams.WRAP_CONTENT:

measureSpec

=

MeasureSpec.makeMeasureSpec(windowSize,

MeasureSpec.AT_MOST);

break;

default:

measureSpec

=

MeasureSpec.makeMeasureSpec(rootDimension,

MeasureSpec.EXACTLY);

break;

}

return

measureSpec;

}match_parent和具体数值大小为EXACTLY模式，wrap_content则为AT_MOST模式。往下走，performMeasure方法中调用了DecorView的onMeasure方法，而DecorView继承自FrameLayout，可以看到FL的onMeasure方法中调用了measureChildWithMargins方法，并传入自身的测量规格：protected

void

measureChildWithMargins(View

child,

int

parentWidthMeasureSpec,

int

widthUsed,

int

parentHeightMeasureSpec,

int

heightUsed)

{

final

MarginLayoutParams

lp

=

(MarginLayoutParams)

child.getLayoutParams();

final

int

childWidthMeasureSpec

=

getChildMeasureSpec(parentWidthMeasureSpec,

mPaddingLeft

+

mPaddingRight

+

lp.leftMargin

+

lp.rightMargin

+

widthUsed,

lp.width);

final

int

childHeightMeasureSpec

=

getChildMeasureSpec(parentHeightMeasureSpec,

mPaddingTop

+

mPaddingBottom

+

lp.topMargin

+

lp.bottomMargin

+

heightUsed,

lp.height);

child.measure(childWidthMeasureSpec,

childHeightMeasureSpec);

}即测量子控件的大小，测量规则详情可看getChildMeasureSpec方法，总结如下：回到onMeasure方法，测完子控件之后，ViewGroup会经过一些计算，得出自身大小：//

加上padding

maxWidth

+=

getPaddingLeftWithForeground()

+

getPaddingRightWithForeground();

maxHeight

+=

getPaddingTopWithForeground()

+

getPaddingBottomWithForeground();

//

检查是否小于最小宽度、最小高度

maxHeight

=

Math.max(maxHeight,

getSuggestedMinimumHeight());

maxWidth

=

Math.max(maxWidth,

getSuggestedMinimumWidth());

//

检查Drawable的最小高度和宽度

final

Drawable

drawable

=

getForeground();

if

(drawable

!=

null)

{

maxHeight

=

Math.max(maxHeight,

drawable.getMinimumHeight());

maxWidth

=

Math.max(maxWidth,

drawable.getMinimumWidth());

}

setMeasuredDimension(resolveSizeAndState(maxWidth,

widthMeasureSpec,

childState),

resolveSizeAndState(maxHeight,

heightMeasureSpec,

childState

<<

MEASURED_HEIGHT_STATE_SHIFT));综上，ViewGroup的测量需要先测量子View的大小，而后结合padding等属性计算得出自身大小。（3）View的测量流程View.performMeasure()

-->onMeasure(int

widthMeasureSpec,

int

heightMeasureSpec)

-->setMeasuredDimension(int

measuredWidth,

int

measuredHeight)

-->setMeasuredDimensionRaw(int

measuredWidth,

int

measuredHeight)可以看到setMeasuredDimensionRaw()方法：private

void

setMeasuredDimensionRaw(int

measuredWidth,

int

measuredHeight)

{

//

存储测量结果

mMeasuredWidth

=

measuredWidth;

mMeasuredHeight

=

measuredHeight;

//

设置测量完成的标志位

mPrivateFlags

|=

PFLAG_MEASURED_DIMENSION_SET;

}View不需要考虑子View的大小，根据内容测量得出自身大小即可。另外，View中的onMeasure方法中调用到getDefaultSize方法：protected

void

onMeasure(int

widthMeasureSpec,

int

heightMeasureSpec)

{

setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(),

widthMeasureSpec),

getDefaultSize(getSuggestedMinimumHeight(),

heightMeasureSpec));

}

public

static

int

getDefaultSize(int

size,

int

measureSpec)

{

int

result

=

size;

int

specMode

=

MeasureSpec.getMode(measureSpec);

int

specSize

=

MeasureSpec.getSize(measureSpec);

switch

(specMode)

{

case

MeasureSpec.UNSPECIFIED:

result

=

size;

break;

case

MeasureSpec.AT_MOST:

case

MeasureSpec.EXACTLY:

//

最终测量的结果都是父容器的大小

result

=

specSize;

break;

}

return

result;

}这里看到精确模式和最大模式，最终测量的结果都是父容器的大小，即布局中的wrap_content、match_parent以及数值大小效果都一样，这也就是自定义View一定要重写onMeasure方法的原因。2、layout布局相对测量而言要简单许多，从ViewRootImpl的performLayout方法出发，可以看到其中调用了DecorView的layout方法：//

实则为DecorView的left,

top,

right,

bottom四个信息

host.layout(0,

0,

host.getMeasuredWidth(),

host.getMeasuredHeight());进入layout方法，发现l、t、r、b被传递到了setFrame方法中，并设置给了成员变量：mLeft

=

left;

mTop

=

top;

mRight

=

right;

mBottom

=

bottom;所以，布局实际为调用View的layout方法，设置自身的l、t、r、b值。另外，layout方法中往下走，可以看到调用了onLayout方法，进入后发现为空方法。因而查看FrameLayout的onLayout方法：@Override

protected

void

onLayout(boolean

changed,

int

left,

int

top,

int

right,

int

bottom)

{

layoutChildren(left,

top,

right,

bottom,

false

/*

no

force

left

gravity

*/);

}

void

layoutChildren(int

left,

int

top,

int

right,

int

bottom,

boolean

forceLeftGravity)

{

final

int

count

=

getChildCount();

//

省略

for

(int

i

=

0;

i

<

count;

i++)

{

final

View

child

=

getChildAt(i);

if

(child.getVisibility()

!=

GONE)

{

final

LayoutParams

lp

=

(LayoutParams)

child.getLayoutParams();

//

省略

child.layout(childLeft,

childTop,

childLeft

+

width,

childTop

+

height);

}

}

}可以看到，进行一系列计算后，调用了child的layout方法，对子控件进行布局，同时子控件又会继续往下对自己的子控件布局，从而实现遍历。综上，布局实际为调用layout方法设置View位置，ViewGroup则需要另外实现onLayout方法摆放子控件。3、d