FrameLayout绘制 源码详解
一直以来都想搞清楚Android中View控件的绘制过程，怎奈自己以前能力不济再加上懒惰，自制力欠缺导致这件事情一直处于半将半就的状态。模模糊糊的了解到绘制分为measure测量、layout布局、draw绘制三步，但是针对其中涉及到的一些点一直有些恍惚，像方法measure()和onMeasure()、layout()和onLayout()、draw()和onDraw()相互之间的关系，它们各自的职责以及相互是如何配合的。其中尤以onMeasure在父子之间的调用顺序最让自己疑惑，今天通过分析FragmentLayout的绘制过程，让我们将这些疑问统统解决掉。也许你会问，为什么是FrameLayout?因为我觉得他最简单，是个软柿子，要怎滴！哈哈，言归正传！
这里有一点需要强调一下：分析的源码是Android 6.0的（API 23），如果跟您手边的源码有出入，大概是版本不同导致的！毕竟，Google在一直重构、优化Android的SDK嘛！
首先看一下FrameLayout的继承关系
FrameLayout –> ViewGroup –> View
onMeasure过程分析
在分析FrameLayout的onMeasure之前，我们先陈述一些基础知识点：我们知道跟一个控件尺寸相关的属性有(这里一height举例)layout_height、maxHeight、minHeight等，在这里我们都知道layout_height有三种可能的取值：MATCH_PARENT=-1、WRAP_CONTENT=-2和具体的数值，相信具体的含义大家都知道；还有onMeasure的int型参数高2位代表measureMode,低30位代表measureSize，而measureMode有三种可能的取值：MeasureSpec.EXACTLY、MeasureSpec.AT_MOST、MeasureSpec.UNSPECIFIED，MeasureSpec.EXACTLY代表子控件应该严格按照measureSize大小赋值，MeasureSpec.AT_MOST代表子控件最大为measureSize的大小而MeasureSpec.UNSPECIFIED代表不做任何限制。现在看来还一目了然，等到父控件套子控件，子控件再套孙子控件，父、子、孙子的layout_height再赋予不同的值得时候，是父控件先确定measure结果，还是子控件先确定measure结果，父子之间如何协商、onMeasure调用几次、layout_margin、padding、minHeight和maxHeight如何起作用？等等问题袭来，真是一团乱麻，下面从对FrameLayout的解剖中探寻究竟吧！
onMeasure(int widthMeasureSpec, int heightMeasureSpec)源码(==代码的说明在代码的注释中==)：
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
int count = getChildCount();
final boolean measureMatchParentChildren =
MeasureSpec.getMode(widthMeasureSpec) != MeasureSpec.EXACTLY ||
MeasureSpec.getMode(heightMeasureSpec) != MeasureSpec.EXACTLY;//是否需要重新测绘属性为MATCH_PARENT的子控件，在下面会用到
mMatchParentChildren.clear();
int maxHeight = 0;
int maxWidth = 0;
int childState = 0;
//首先，将父控件的padding和子控件的margin考虑在内，先来测绘子控件的大小，measureChildWithMargins函数会在下边展开
for (int i = 0; i < count; i++) {
final View child = getChildAt(i);
if (mMeasureAllChildren || child.getVisibility() != GONE) {
measureChildWithMargins(child, widthMeasureSpec, 0, heightMeasureSpec, 0);
final LayoutParams lp = (LayoutParams) child.getLayoutParams();
maxWidth = Math.max(maxWidth,
child.getMeasuredWidth() + lp.leftMargin + lp.rightMargin);
maxHeight = Math.max(maxHeight,
child.getMeasuredHeight() + lp.topMargin + lp.bottomMargin);
childState = combineMeasuredStates(childState, child.getMeasuredState());
if (measureMatchParentChildren) {
if (lp.width == LayoutParams.MATCH_PARENT ||
lp.height == LayoutParams.MATCH_PARENT) {
mMatchParentChildren.add(child);
}
}
}
}
// Account for padding too
maxWidth += getPaddingLeftWithForeground() + getPaddingRightWithForeground();
maxHeight += getPaddingTopWithForeground() + getPaddingBottomWithForeground();
// Check against our minimum height and width
maxHeight = Math.max(maxHeight, getSuggestedMinimumHeight());
maxWidth = Math.max(maxWidth, getSuggestedMinimumWidth());
// Check against our foreground's minimum height and width
final Drawable drawable = getForeground();
if (drawable != null) {
maxHeight = Math.max(maxHeight, drawable.getMinimumHeight());
maxWidth = Math.max(maxWidth, drawable.getMinimumWidth());
}
//从子控件的最大值（子控件的大小加上padding和foregroundPadding综合考虑的大小），背景的最小值、前景的最小值和控件的最小值属性综合考虑，取得其中的最大值，再考虑到父控件的measureSpec得到父控件也就是FrameLayout的大小。resolveSizeAndState在下边展开
setMeasuredDimension(resolveSizeAndState(maxWidth, widthMeasureSpec, childState),
resolveSizeAndState(maxHeight, heightMeasureSpec,
childState << MEASURED_HEIGHT_STATE_SHIFT));
//如果需要重新测量属性为match_parent的子控件则重新测量
count = mMatchParentChildren.size();
if (count > 1) {
for (int i = 0; i < count; i++) {
final View child = mMatchParentChildren.get(i);
final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();
final int childWidthMeasureSpec;
if (lp.width == LayoutParams.MATCH_PARENT) {
final int width = Math.max(0, getMeasuredWidth()
- getPaddingLeftWithForeground() - getPaddingRightWithForeground()
- lp.leftMargin - lp.rightMargin);
childWidthMeasureSpec = MeasureSpec.makeMeasureSpec(
width, MeasureSpec.EXACTLY);
} else {
childWidthMeasureSpec = getChildMeasureSpec(widthMeasureSpec,
getPaddingLeftWithForeground() + getPaddingRightWithForeground() +
lp.leftMargin + lp.rightMargin,
lp.width);
}
final int childHeightMeasureSpec;
if (lp.height == LayoutParams.MATCH_PARENT) {
final int height = Math.max(0, getMeasuredHeight()
- getPaddingTopWithForeground() - getPaddingBottomWithForeground()
- lp.topMargin - lp.bottomMargin);
childHeightMeasureSpec = MeasureSpec.makeMeasureSpec(
height, MeasureSpec.EXACTLY);
} else {
childHeightMeasureSpec = getChildMeasureSpec(heightMeasureSpec,
getPaddingTopWithForeground() + getPaddingBottomWithForeground() +
lp.topMargin + lp.bottomMargin,
lp.height);
}
child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}
}
}ViewGroup.measureChildWithMargins源码
protected void measureChildWithMargins(View child,
int parentWidthMeasureSpec, int widthUsed,
int parentHeightMeasureSpec, int heightUsed) {
final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();
//根据父控件的padding和子控件margin以及父控件的measureSpec和子控件的lp.width决定子控件的measureSpec，getChildMeasureSpec会在下边展开
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);
//进行子控件的测量工作
child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}ViewGroup.getChildMeasureSpec的源码
/**
*这个函数有几个遵从的原则，或许能帮助更好的理解代码：1、如果子控件指定了具体的大小则MeasureSpec的模式为EXACTLY值为指定大小；2、如果子控件的需求跟父控件的需求有冲突则以父控件为准；
*/
public static int getChildMeasureSpec(int spec, int padding, int childDimension) {
int specMode = MeasureSpec.getMode(spec);
int specSize = MeasureSpec.getSize(spec);
int size = Math.max(0, specSize - padding);
int resultSize = 0;
int resultMode = 0;
switch (specMode) {
// Parent has imposed an exact size on us
case MeasureSpec.EXACTLY:
if (childDimension >= 0) {
resultSize = childDimension;
resultMode = MeasureSpec.EXACTLY;
} else if (childDimension == LayoutParams.MATCH_PARENT) {
// Child wants to be our size. So be it.
resultSize = size;
resultMode = MeasureSpec.EXACTLY;
} else if (childDimension == LayoutParams.WRAP_CONTENT) {
// Child wants to determine its own size. It can't be
// bigger than us.
resultSize = size;
resultMode = MeasureSpec.AT_MOST;
}
break;
// Parent has imposed a maximum size on us
case MeasureSpec.AT_MOST:
if (childDimension >= 0) {
// Child wants a specific size... so be it
resultSize = childDimension;
resultMode = MeasureSpec.EXACTLY;
} else if (childDimension == LayoutParams.MATCH_PARENT) {
// Child wants to be our size, but our size is not fixed.
// Constrain child to not be bigger than us.
resultSize = size;
resultMode = MeasureSpec.AT_MOST;
} else if (childDimension == LayoutParams.WRAP_CONTENT) {
// Child wants to determine its own size. It can't be
// bigger than us.
resultSize = size;
resultMode = MeasureSpec.AT_MOST;
}
break;
// Parent asked to see how big we want to be
case MeasureSpec.UNSPECIFIED:
if (childDimension >= 0) {
// Child wants a specific size... let him have it
resultSize = childDimension;
resultMode = MeasureSpec.EXACTLY;
} else if (childDimension == LayoutParams.MATCH_PARENT) {
// Child wants to be our size... find out how big it should
// be
resultSize = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size;
resultMode = MeasureSpec.UNSPECIFIED;
} else if (childDimension == LayoutParams.WRAP_CONTENT) {
// Child wants to determine its own size.... find out how
// big it should be
resultSize = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size;
resultMode = MeasureSpec.UNSPECIFIED;
}
break;
}
return MeasureSpec.makeMeasureSpec(resultSize, resultMode);
}View.resolveSizeAndState源码
/**
*还是需求跟供给冲突了，以供给为准（FrameLayout想要的大小跟measureSpec冲突，以measureSpec为准）
*/
public static int resolveSizeAndState(int size, int measureSpec, int childMeasuredState) {
final int specMode = MeasureSpec.getMode(measureSpec);
final int specSize = MeasureSpec.getSize(measureSpec);
final int result;
switch (specMode) {
case MeasureSpec.AT_MOST:
if (specSize < size) {
result = specSize | MEASURED_STATE_TOO_SMALL;
} else {
result = size;
}
break;
case MeasureSpec.EXACTLY:
result = specSize;
break;
case MeasureSpec.UNSPECIFIED:
default:
result = size;
}
return result | (childMeasuredState & MEASURED_STATE_MASK);
}onLayout过程分析
onMeasure测量出控件自身的大小，而onLayout通过调用子控件的layout决定子控件的位置和大小(这里的大小是控件最终呈现的状态，一般就是控件测量的大小)，onLayout的过程跟FrameLayout的gravity和子控件的layout_gravity属性有关，下边看一下FrameLayout的onLayout过程：
onLayout(boolean changed, int left, int top, int right, int bottom源码
/**
*这里需要明确一下坐标系，子控件的所在的坐标系是由父控件确定的。拿这里举例，FrameLayout的onLayout的参数值，是以FrameLayout的父控件左上角为坐标原点的坐标系中的值。而接下调用child.layout确定FrameLayout的子控件的位置和大小的函数中的参数值是以FrameLayout左上角为原点的坐标系中的值
*/
protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
layoutChildren(left, top, right, bottom, false /* no force left gravity */);
}layoutChildren源码
void layoutChildren(int left, int top, int right, int bottom,
boolean forceLeftGravity) {
final int count = getChildCount();
final int parentLeft = getPaddingLeftWithForeground();
final int parentRight = right - left - getPaddingRightWithForeground();
final int parentTop = getPaddingTopWithForeground();
final int parentBottom = bottom - top - getPaddingBottomWithForeground();
for (int i = 0; i < count; i++) {
final View child = getChildAt(i);
if (child.getVisibility() != GONE) {
final LayoutParams lp = (LayoutParams) child.getLayoutParams();
final int width = child.getMeasuredWidth();
final int height = child.getMeasuredHeight();
int childLeft;
int childTop;
int gravity = lp.gravity;
if (gravity == -1) {
gravity = DEFAULT_CHILD_GRAVITY;
}
//布局是从右到左还是从左到右
final int layoutDirection = getLayoutDirection();
//将布局中的start、end转化为left、right
final int absoluteGravity = Gravity.getAbsoluteGravity(gravity, layoutDirection);
final int verticalGravity = gravity & Gravity.VERTICAL_GRAVITY_MASK;
switch (absoluteGravity & Gravity.HORIZONTAL_GRAVITY_MASK) {
case Gravity.CENTER_HORIZONTAL:
childLeft = parentLeft + (parentRight - parentLeft - width) / 2 +
lp.leftMargin - lp.rightMargin;
break;
case Gravity.RIGHT:
if (!forceLeftGravity) {
childLeft = parentRight - width - lp.rightMargin;
break;
}
case Gravity.LEFT:
default:
childLeft = parentLeft + lp.leftMargin;
}
switch (verticalGravity) {
case Gravity.TOP:
childTop = parentTop + lp.topMargin;
break;
case Gravity.CENTER_VERTICAL:
childTop = parentTop + (parentBottom - parentTop - height) / 2 +
lp.topMargin - lp.bottomMargin;
break;
case Gravity.BOTTOM:
childTop = parentBottom - height - lp.bottomMargin;
break;
default:
childTop = parentTop + lp.topMargin;
}
child.layout(childLeft, childTop, childLeft + width, childTop + height);
}
}
}总结
FrameLayout是几大布局中最简单的一个，比较适合入门。通过分析FrameLayout对控件的绘制过程有个大概的了解。其他Layout的measure过程可能更复杂，需要更多的父子控件的协商。