小知识，大挑战！本文正在参与“程序员必备小知识”创作活动。
一 概述
1.1 Android的View继承关系图
View是安卓所有控件的父基类。
1.2 Android的视图坐标
1.2.1 安卓坐标系
获取办法：MotionEvent提供的getRawX()和getRawY()；
1.2.2 视图坐标系
1.2.3 常用方法
View获取自身宽高
| getHeight() | 获取View自身高度 |
|---|---|
| getWidth() | 获取View自身宽度 |
View到其父控件（ViewGroup）的距离：
| getTop() | 获取View自身顶边到其父布局顶边的距离 |
|---|---|
| getLeft() | 获取View自身左边到其父布局左边的距离 |
| getRight() | 获取View自身右边到其父布局左边的距离 |
| getBottom() | 获取View自身底边到其父布局顶边的距离 |
MotionEvent提供的方法
我们看上图那个深蓝色的点，假设就是我们触摸的点，我们知道无论是View还是ViewGroup，最终的点击事件都会由onTouchEvent(MotionEvent event)方法来处理，MotionEvent也提供了各种获取焦点坐标的方法：
| getX() | 获取点击事件距离控件左边的距离，即视图坐标 |
|---|---|
| getY() | 获取点击事件距离控件顶边的距离，即视图坐标 |
| getRawX() | 获取点击事件距离整个屏幕左边距离，即绝对坐标 |
| getRawY() | 取点击事件距离整个屏幕顶边的的距离，即绝对坐标 |
二 View的事件分发
点击事件分发：点击屏幕就会产生触摸事件，这个事件被封装成了一个类：MotionEvent。当这个MotionEvent产生后，系统就会将这个MotionEvent传递给View的各个层级，MotionEvent在View层级传递的过程，就是点击事件分发。
点击事件有三个重要的方法：
| dispatchTouchEvent(MotionEvent ev) | 用来进行事件的分发 |
|---|---|
| onInterceptTouchEvent(MotionEvent ev) | 用来进行事件的拦截，在dispatchTouchEvent()中调用，需要注意的是View没有提供该方法 |
| onTouchEvent(MotionEvent ev) | 用来处理点击事件，在dispatchTouchEvent()方法中进行调用 |
点击事件分发的这三个重要方法的关系，用伪代码来简单表示就是：
public boolean dispatchTouchEvent(MotionEvent ev) {
boolean result=false;
if(onInterceptTouchEvent(ev)){
result=super.onTouchEvent(ev);
}else{
result=child.dispatchTouchEvent(ev);
}
return result;
这个U型图就很好的解释了事件分发：
总结：dispatchTouchEvent代表分发事件，onInterceptTouchEvent()代表拦截事件，onTouchEvent()代表消耗事件，由自己处理。
三 View的工作流程
3.1 概述
measure：测量View的宽高；测量；
layout：用来确定View的位置；布局；
draw：则用来绘制View；绘制；
3.2 测量
3.2.1 预备知识MeasureSpec（重要）
MeasureSpec是View的静态内部类；View的测量就是测量View的宽高，而View的宽高被封装到了MeasureSpec里面。
测量的过程实际上就是如何确定View的MeasureSpec的过程，而MeasureSpec主要包含两个参数：模式+尺寸。
MeasureSpec是一个32位int值，前2位表示SpecMode即模式 ，后30位表示SpecSize即尺寸。
3.2.2 MeasureSpec的SpecMode分类（模式）
UNSPECIFIED
public static final int UNSPECIFIED = 0 << MODE_SHIFT; 00000000000000000000000000000000
父容器不对View做任何限制，系统内部使用
EXACTLY
public static final int EXACTLY = 1 << MODE_SHIFT; 01000000000000000000000000000000
父容器检测出View的大小，Vew的大小就是SpecSize LayoutPamras match_parent 固定大小
AT_MOST public static final int AT_MOST = 2 << MODE_SHIFT; 10000000000000000000000000000000 父容器指定一个可用大小，View的大小不能超过这个值，LayoutPamras wrap_content
3.2.3 测绘过程
ViewGroup的测绘过程 ：
measure --> onMeasure(测量子控件的宽高)--> setMeasuredDimension -->setMeasuredDimensionRaw(保存自己的宽高)
View的测绘过程：
measure --> onMeasure --> setMeasuredDimension -->setMeasuredDimensionRaw(保存自己的宽高)
3.2.4 测绘源码分析，以DecorView为例
首先在ViewRootImpl中，执行performMeasure方法：
private void performMeasure(int childWidthMeasureSpec, int childHeightMeasureSpec) {
if (mView == null) {
return;
}
Trace.traceBegin(Trace.TRACE_TAG_VIEW, "measure");
try {
mView.measure(childWidthMeasureSpec, childHeightMeasureSpec);
} finally {
Trace.traceEnd(Trace.TRACE_TAG_VIEW);
}
}
点击mView.measure方法，会进入View的measure方法（标注1）（view的该方法为final，不能被重写），如下：
public final void measure(int widthMeasureSpec, int heightMeasureSpec) {
boolean optical = isLayoutModeOptical(this);
if (optical != isLayoutModeOptical(mParent)) {
Insets insets = getOpticalInsets();
int oWidth = insets.left + insets.right;
int oHeight = insets.top + insets.bottom;
widthMeasureSpec = MeasureSpec.adjust(widthMeasureSpec, optical ? -oWidth : oWidth);
heightMeasureSpec = MeasureSpec.adjust(heightMeasureSpec, optical ? -oHeight : oHeight);
}
// Suppress sign extension for the low bytes
long key = (long) widthMeasureSpec << 32 | (long) heightMeasureSpec & 0xffffffffL;
if (mMeasureCache == null) mMeasureCache = new LongSparseLongArray(2);
final boolean forceLayout = (mPrivateFlags & PFLAG_FORCE_LAYOUT) == PFLAG_FORCE_LAYOUT;
// Optimize layout by avoiding an extra EXACTLY pass when the view is
// already measured as the correct size. In API 23 and below, this
// extra pass is required to make LinearLayout re-distribute weight.
final boolean specChanged = widthMeasureSpec != mOldWidthMeasureSpec
|| heightMeasureSpec != mOldHeightMeasureSpec;
final boolean isSpecExactly = MeasureSpec.getMode(widthMeasureSpec) == MeasureSpec.EXACTLY
&& MeasureSpec.getMode(heightMeasureSpec) == MeasureSpec.EXACTLY;
final boolean matchesSpecSize = getMeasuredWidth() == MeasureSpec.getSize(widthMeasureSpec)
&& getMeasuredHeight() == MeasureSpec.getSize(heightMeasureSpec);
final boolean needsLayout = specChanged
&& (sAlwaysRemeasureExactly || !isSpecExactly || !matchesSpecSize);
if (forceLayout || needsLayout) {
// first clears the measured dimension flag
mPrivateFlags &= ~PFLAG_MEASURED_DIMENSION_SET;
resolveRtlPropertiesIfNeeded();
int cacheIndex = forceLayout ? -1 : mMeasureCache.indexOfKey(key);
if (cacheIndex < 0 || sIgnoreMeasureCache) {
// measure ourselves, this should set the measured dimension flag back
//这里！重要的onMeasure方法，终于找到你！
//这里！重要的onMeasure方法，终于找到你！
//这里！重要的onMeasure方法，终于找到你！
onMeasure(widthMeasureSpec, heightMeasureSpec);
mPrivateFlags3 &= ~PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
} else {
long value = mMeasureCache.valueAt(cacheIndex);
// Casting a long to int drops the high 32 bits, no mask needed
setMeasuredDimensionRaw((int) (value >> 32), (int) value);
mPrivateFlags3 |= PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
}
// flag not set, setMeasuredDimension() was not invoked, we raise
// an exception to warn the developer
if ((mPrivateFlags & PFLAG_MEASURED_DIMENSION_SET) != PFLAG_MEASURED_DIMENSION_SET) {
throw new IllegalStateException("View with id " + getId() + ": "
+ getClass().getName() + "#onMeasure() did not set the"
+ " measured dimension by calling"
+ " setMeasuredDimension()");
}
mPrivateFlags |= PFLAG_LAYOUT_REQUIRED;
}
mOldWidthMeasureSpec = widthMeasureSpec;
mOldHeightMeasureSpec = heightMeasureSpec;
mMeasureCache.put(key, ((long) mMeasuredWidth) << 32 |
(long) mMeasuredHeight & 0xffffffffL); // suppress sign extension
}
代码有点多，不用全看。前半部分主要是取了缓存，然后执行了 onMeasure(widthMeasureSpec, heightMeasureSpec)方法。如果缓存没读到会进行缓存的一个设置。（关于缓存可以先忽略不用管，主要是对测量做一些优化）
紧接着上面的代码段中，点击主要的onMeasure方法一探究竟：
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),
getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));
}
会发现，onMeasure方法里最主要调用了两个方法：setMeasuredDimension方法和getDefaultSize方法。先看下getDefaultSize方法：
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:
result = specSize;
break;
}
return result;
}
注意，可以看到，这里无论是AT_MOST模式还是EXACTLY模式，都是默认的specSize，所以我们在自定义View的时候，一定要重写onMeasure方法，否则match_parent和wrap_content属性时，将不起作用。
我们再来看看setMeasuredDimension方法里是什么：
protected final void setMeasuredDimension(int measuredWidth, int measuredHeight) {
boolean optical = isLayoutModeOptical(this);
if (optical != isLayoutModeOptical(mParent)) {
Insets insets = getOpticalInsets();
int opticalWidth = insets.left + insets.right;
int opticalHeight = insets.top + insets.bottom;
measuredWidth += optical ? opticalWidth : -opticalWidth;
measuredHeight += optical ? opticalHeight : -opticalHeight;
}
setMeasuredDimensionRaw(measuredWidth, measuredHeight);
}
可以看到，setMeasuredDimension方法里继续调用了setMeasuredDimensionRaw(measuredWidth, measuredHeight)方法，继续查看这个方法的详细：
private void setMeasuredDimensionRaw(int measuredWidth, int measuredHeight) {
mMeasuredWidth = measuredWidth;
mMeasuredHeight = measuredHeight;
mPrivateFlags |= PFLAG_MEASURED_DIMENSION_SET;
}
谢天谢地，终于到头了！可以看到，该方法对宽高成员变量赋值以及设置了一个标记位。
不要因为走得太远而忘记了为何出发，我们是在ViewRootImpl中执行performMeasure方法中的mView.measure(childWidthMeasureSpec, childHeightMeasureSpec)不断追溯来到这边的，所以measure（测量）实质上就是确定控件的宽和高。那么控件的宽和高，他们的值又是如何确定的呢？现在我们就需要查看mView.measure(childWidthMeasureSpec, childHeightMeasureSpec)中的childWidthMeasureSpec, childHeightMeasureSpec这两个参数！
这边打个分割线便于大家理解，上方是mView.measure方法测量的源码过程解析。
接下来我们需要知道mView.measure方法的两个参数如何获取。
在ViewRootlmpl中，继续解读源码：
int childWidthMeasureSpec = getRootMeasureSpec(mWidth, lp.width);
int childHeightMeasureSpec = getRootMeasureSpec(mHeight, lp.height);
if (DEBUG_LAYOUT) Log.v(mTag, "Ooops, something changed! mWidth="
+ mWidth + " measuredWidth=" + host.getMeasuredWidth()
+ " mHeight=" + mHeight
+ " measuredHeight=" + host.getMeasuredHeight()
+ " coveredInsetsChanged=" + contentInsetsChanged);
// Ask host how big it wants to be
performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
可以看到，childWidthMeasureSpec,和childHeightMeasureSpec这两个参数由getRootMeasureSpec(mWidth, lp.width)方法和getRootMeasureSpec(mHeight, lp.height)得出。以第一个为例，mWidth表示的是窗口（PhoneWindow）的宽，第二个参数表示的是顶层View的本身的宽。参数有了，我们接下来再详细进入getRootMeasureSpec这个方法里一探究竟：
/**
* Figures out the measure spec for the root view in a window based on it's
* layout params.
*
* @param windowSize
* The available width or height of the window
*
* @param rootDimension
* The layout params for one dimension (width or height) of the
* window.
*
* @return The measure spec to use to measure the root view.
*/
private static int getRootMeasureSpec(int windowSize, int rootDimension) {
int measureSpec;
switch (rootDimension) {
case ViewGroup.LayoutParams.MATCH_PARENT:
// Window can't resize. Force root view to be windowSize.
measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.EXACTLY);
break;
case ViewGroup.LayoutParams.WRAP_CONTENT:
// Window can resize. Set max size for root view.
measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.AT_MOST);
break;
default:
// Window wants to be an exact size. Force root view to be that size.
measureSpec = MeasureSpec.makeMeasureSpec(rootDimension, MeasureSpec.EXACTLY);
break;
}
return measureSpec;
}
注释写的很详细了，可以看到，该方法里对测量的view就行了switch判断，根据view的match_parent属性/wrap_content属性设置了measureSpec的不同模式(EXACTLY/AT_MOST)。
又这个方法我们可以得出结论：DecorView的MeasureSpec方法由窗口大小和自己的LayoutParams决定。具体的决定规则逻辑在如上代码中，具象下来可以总结如下：
继续分析。刚刚我们已经确定了这两个参数的由来。实际上这两个参数确定了DccorView的模式和尺寸（注意子view的测量还没有，过程才刚开始）。
继续分割一下避免混乱，上面是mView.measure方法的两个参数由来。
接下来我们继续分析mView.measure方法。
mView.measure（也就是View的measure方法,这里的mView就是DecorView）中调用onMeasure方法。我们知道DecorView实际上是集成FrameLayout的，所以我们直接去FrameLayout中看看这个onMeasure方法的具体实现：
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) { int count = getChildCount();
final boolean measureMatchParentChildren =
MeasureSpec.getMode(widthMeasureSpec) != MeasureSpec.EXACTLY ||
MeasureSpec.getMode(heightMeasureSpec) != MeasureSpec.EXACTLY;
mMatchParentChildren.clear();
int maxHeight = 0;
int maxWidth = 0;
int childState = 0;
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());
}
setMeasuredDimension(resolveSizeAndState(maxWidth, widthMeasureSpec, childState),
resolveSizeAndState(maxHeight, heightMeasureSpec,
childState << MEASURED_HEIGHT_STATE_SHIFT));
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);
}
}
}
很符合我们心理预期，FrameLayout作为容器，肯定会包含很多的子View，可以看到上面的源码方法里，也对其所有的子View进行了遍历，不难想象，它将对它的所有子View进行测量。为了验证猜想，继续点击该方法中的measureChildWithMargins中一探究竟：
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);
}
该方法中增加了对padding的处理，很容易理解不赘述。
看child.measure(childWidthMeasureSpec, childHeightMeasureSpec)方法。验证了我们的猜想，从名字就可以看出，开始对FrameLayout中的子View进行宽高 测量了。
这里有个非常重要的点：getChildMeasureSpec方法，即对子View的MeasureSpec的获取，点击看详细的getChildMeasureSpec方法实现：
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;
}
//noinspection ResourceType
return MeasureSpec.makeMeasureSpec(resultSize, resultMode);
}
这个方法有点长，不要怕。可以看到该方法的第一个参数表示父容器的MeasureSpec的值（父容器的模式和尺寸），第二个参数padding就不说了，第三个参数表示该view本身的尺寸。该方法就通过对父容器的模式的判断，确定该子View的模式和尺寸。具体的判断逻辑可看如上代码，不详细赘述了，总结如下：
在对所有的子控件测量完成后，FrameLayout开始需要最终确定自己的宽高。继续看FrameLayout中的onMeasure方法，看它的后半段，我再粘贴下关键代码：
// 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());
}
setMeasuredDimension(resolveSizeAndState(maxWidth, widthMeasureSpec, childState),
resolveSizeAndState(maxHeight, heightMeasureSpec,
childState << MEASURED_HEIGHT_STATE_SHIFT));
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);
}
}
可以看到，FrameLayout找到所有子控件中，最底部子控件的高，和最右边子控件的高。由此我们最终确定了Framelayout的宽高！
3.3 布局
源码看起来总是费力些，先总结如下：
View ：
调用View.layout来确定自己的位置，即确定mLeft,mTop,mRight,mBottom的值，获得4个点的位置。
ViewGroup：
调用View.layout来确定自己的位置，再调用onLayout确定子View的位置。
3.3.1 布局源码分析，以FrameLayout为例
查看FrameLayout的onLayout方法如下：
protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
layoutChildren(left, top, right, bottom, false /* no force left gravity */);
}
点击查看 layoutChildren方法：
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;
}
final int layoutDirection = getLayoutDirection();
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);
}
}
}
可以看到， layoutChildren方法中对子View进行了遍历，对子控件进行位置的摆放。而对子控件的摆放，调用的是子控件的layout方法，也就是上面代码的最后一行。布局就是这么简单。
3.4 绘制
ViewGroup 绘制步骤：
绘制背景 drawBackground(canvas)
绘制自己onDraw(canvas)
绘制子View dispatchDraw(canvas)
绘制前景，滚动条等装饰onDrawForeground(canvas)
View绘制步骤：
绘制背景 drawBackground(canvas)
绘制自己onDraw(canvas)
绘制前景，滚动条等装饰onDrawForeground(canvas)
绘制自定义控件的步骤：
onMeasure --> onLayout(自定义的是容器时需要重写该方法，自定义View时可不重写该方法) --> onDraw（可选，若容器里都是系统控件，可不重写）
3.4.1 绘制 源码分析，以FrameLayout为例
找到ViewRootlmpl中的performDraw方法：
private void performDraw() {
if (mAttachInfo.mDisplayState == Display.STATE_OFF && !mReportNextDraw) {
return;
} else if (mView == null) {
return;
}
final boolean fullRedrawNeeded = mFullRedrawNeeded || mReportNextDraw;
mFullRedrawNeeded = false;
mIsDrawing = true;
Trace.traceBegin(Trace.TRACE_TAG_VIEW, "draw");
boolean usingAsyncReport = false;
if (mReportNextDraw && mAttachInfo.mThreadedRenderer != null
&& mAttachInfo.mThreadedRenderer.isEnabled()) {
usingAsyncReport = true;
mAttachInfo.mThreadedRenderer.setFrameCompleteCallback((long frameNr) -> {
// TODO: Use the frame number
pendingDrawFinished();
});
}
try {
boolean canUseAsync = draw(fullRedrawNeeded);
if (usingAsyncReport && !canUseAsync) {
mAttachInfo.mThreadedRenderer.setFrameCompleteCallback(null);
usingAsyncReport = false;
}
} finally {
mIsDrawing = false;
Trace.traceEnd(Trace.TRACE_TAG_VIEW);
}
// For whatever reason we didn't create a HardwareRenderer, end any
// hardware animations that are now dangling
if (mAttachInfo.mPendingAnimatingRenderNodes != null) {
final int count = mAttachInfo.mPendingAnimatingRenderNodes.size();
for (int i = 0; i < count; i++) {
mAttachInfo.mPendingAnimatingRenderNodes.get(i).endAllAnimators();
}
mAttachInfo.mPendingAnimatingRenderNodes.clear();
}
if (mReportNextDraw) {
mReportNextDraw = false;
// if we're using multi-thread renderer, wait for the window frame draws
if (mWindowDrawCountDown != null) {
try {
mWindowDrawCountDown.await();
} catch (InterruptedException e) {
Log.e(mTag, "Window redraw count down interrupted!");
}
mWindowDrawCountDown = null;
}
if (mAttachInfo.mThreadedRenderer != null) {
mAttachInfo.mThreadedRenderer.setStopped(mStopped);
}
if (LOCAL_LOGV) {
Log.v(mTag, "FINISHED DRAWING: " + mWindowAttributes.getTitle());
}
if (mSurfaceHolder != null && mSurface.isValid()) {
SurfaceCallbackHelper sch = new SurfaceCallbackHelper(this::postDrawFinished);
SurfaceHolder.Callback callbacks[] = mSurfaceHolder.getCallbacks();
sch.dispatchSurfaceRedrawNeededAsync(mSurfaceHolder, callbacks);
} else if (!usingAsyncReport) {
if (mAttachInfo.mThreadedRenderer != null) {
mAttachInfo.mThreadedRenderer.fence();
}
pendingDrawFinished();
}
}
}
代码很多不用都看，找到其中的draw方法，点进去，draw()方法里代码也很多，不粘贴了，找到draw()方法里的drawSoftware方法，然后是drawSoftware方法中的mView.draw（）方法。点进去即进入了View的draw()方法，如下：
public void draw(Canvas canvas) {
final int privateFlags = mPrivateFlags;
final boolean dirtyOpaque = (privateFlags & PFLAG_DIRTY_MASK) == PFLAG_DIRTY_OPAQUE &&
(mAttachInfo == null || !mAttachInfo.mIgnoreDirtyState);
mPrivateFlags = (privateFlags & ~PFLAG_DIRTY_MASK) | PFLAG_DRAWN;
/*
* Draw traversal performs several drawing steps which must be executed
* in the appropriate order:
*
* 1. Draw the background
* 2. If necessary, save the canvas' layers to prepare for fading
* 3. Draw view's content
* 4. Draw children
* 5. If necessary, draw the fading edges and restore layers
* 6. Draw decorations (scrollbars for instance)
*/
// Step 1, draw the background, if needed
int saveCount;
if (!dirtyOpaque) {
drawBackground(canvas);
}
// skip step 2 & 5 if possible (common case)
...
...
可以看到，注释写的非常详细，已经把绘制的步骤一一列举出来了，省略的没有粘贴的代码就是一步步执行步骤的具体代码逻辑。
最后，来张神图：
参考文章
View的平滑移动：blog.csdn.net/itachi85/ar…
Android View体系：blog.csdn.net/itachi85/ar…
Android事件分发机制详解：juejin.im/post/5d3e52…
View的工作流程：blog.csdn.net/qian520ao/a…
