View的工作原理
文章目录
1、ViewRoot 和 DecorView
2、 MeasureSpec
2.1MeasureSpec
2.2MeasureSpec和LayoutParams的对应关系
3、View的工作流程
3.1 Measure过程
3.1.1 view的测量过程
3.1.2 ViewGroup的测量过程
3.1.3 获取View宽高的时机
1、Activity/View # onWindowFocusChanged
2、view.post(runnable)
3、ViewTreeObserver
3.2Layout过程
3.3Draw过程
4、自定义View
4.1自定义view的分类
4.2 自定义view 注意点
4.3 例子
4.4 自定义view的思想
1、ViewRoot 和 DecorView
- ViewRoot对应ViewRootImpl类，是连接WindowManager和DecorView的纽带。View的三大流程是通过ViewRoot完成的。 在ActivityThread中，当Activity对象被创建完毕时，会将DecorView添加到Window中，同时会创建ViewRootImpl，且ViewRootImpl和DecorView会建立关联。如下代码，WindowManagerGlobal的addView()方法：
public void addView(View view, ViewGroup.LayoutParams params,Display display, Window parentWindow) {
...
root = new ViewRootImpl(view.getContext(), display);
root.setView(view, wparams, panelParentView);
...
}
- View绘制流程从 performTraversals开始，经过Measure、layout、draw。流程图如下
3. DecorView是顶级View，是一个FrameLayout，上面是标题栏、下面是内容栏。内容栏就是setContengView的内容view，id是content。事件 经过DecorView 然后传给我们自己的View。
2、 MeasureSpec
MeasureSpec封装了从父级传递到子级的布局要求。系统把view的LayoutParams 根据 父容器施加的规则（父容器的SpecMode） 转换成 view的MeasureSpec，然后使用这个MeasureSpec确定view的测量宽高（不一定是最终宽高）。
2.1MeasureSpec
1.MeasureSpec—view的测量规格：高2位的SpecMode，低30位的SpecSize。
2.SpecMode的分类：
UNSPECIFIED父容器对view不限制，要多大给多大，一般系统内部使用。
EXACTLY，父容器检测出view所需大小，view最终大小就是SpecSize的值。对应 LayoutParams中的matchParent、具体数值 两种模式。
AT_MOST，父容器制定了可用大小即SpecSize，view的大小不能大于这个值，具体要看view的具体实现。对应LayoutParams中的wrap_content。
2.2MeasureSpec和LayoutParams的对应关系
前面说了View的MeasureSpec是由LayoutParams和父容器的MeasureSpec共同决定。顶级view，即DecorView，是由窗口尺寸和自身LayoutParams决定。
1、DecorView，ViewRootImpl中measureHierarchy()方法（performTraversals中执行），代码如下，desiredWindowWidth、desiredWindowHeight是屏幕的尺寸。
private boolean measureHierarchy(final View host, final WindowManager.LayoutParams lp,
final Resources res, final int desiredWindowWidth, final int desiredWindowHeight) {
...
childWidthMeasureSpec = getRootMeasureSpec(desiredWindowWidth, lp.width);
childHeightMeasureSpec = getRootMeasureSpec(desiredWindowHeight, lp.height);
performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
...
}
performMeasure()内部是调用mView.measure(childWidthMeasureSpec, childHeightMeasureSpec)，mView就是DecorVIew。继续看getRootMeasureSpec（）方法如下：
/**
* 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;
}
- DecorView的MeasureSpec就明确了，根据其LayoutParams：
- MATCH_PARENT：精确模式，就是窗口大小；
- WRAP_CONTENT：最大值模式，最大值不能超过窗口大小；
- 固定值（如100dp）：精确模式，就是LayoutParams的指定值。
2、普通View，测量过程从ViewGroup传递下来，看ViewGroup的measureChildWithMargins()方法：
/**
* Ask one of the children of this view to measure itself, taking into
* account both the MeasureSpec requirements for this view and its padding
* and margins. The child must have MarginLayoutParams The heavy lifting is
* done in getChildMeasureSpec.
*
* @param child The child to measure
* @param parentWidthMeasureSpec The width requirements for this view
* @param widthUsed Extra space that has been used up by the parent
* horizontally (possibly by other children of the parent)
* @param parentHeightMeasureSpec The height requirements for this view
* @param heightUsed Extra space that has been used up by the parent
* vertically (possibly by other children of the parent)
*/
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);
}
即先获取child的MeasureSpec，再调child.measure()。可以看到，child的MeasureSpec是由父容器的MeasureSpec、父容器的padding、child的LayoutParams、child的marging 共同决定。继续看getChildMeasureSpec()方法：
/**
* Does the hard part of measureChildren: figuring out the MeasureSpec to
* pass to a particular child. This method figures out the right MeasureSpec
* for one dimension (height or width) of one child view.
*
* The goal is to combine information from our MeasureSpec with the
* LayoutParams of the child to get the best possible results. For example,
* if the this view knows its size (because its MeasureSpec has a mode of
* EXACTLY), and the child has indicated in its LayoutParams that it wants
* to be the same size as the parent, the parent should ask the child to
* layout given an exact size.
*
* @param spec The requirements for this view
* @param padding The padding of this view for the current dimension and
* margins, if applicable
* @param childDimension How big the child wants to be in the current
* dimension
* @return a MeasureSpec integer for the child
*/
public static int getChildMeasureSpec(int spec, int padding, int childDimension) {
int specMode = MeasureSpec.getMode(spec);
int specSize = MeasureSpec.getSize(spec);
//padding,就是已被占用的空间，就是 父容器的padding+child的marging
//size，是ViewGroup本身size减去已使用的空间，是ViewGroup能提供给child的最大值。
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);
}
可见，view的MeasureSpec由 viewParent的MeasureSpec和自身layoutParams确定。另外，child的可利用的尺寸是parent尺寸减去padding，上面代码已有注释，这很好理解。
梳理如下：
| parentSpecMode /childLayoutParams |
EXACTLY | AT_MOST | UNSPECIFIED |
|---|---|---|---|
| dp/px | EXACTLY childSize |
EXACTLY childsize |
EXACTLY childsize |
| match_parent | EXACTLY parentSize |
AT_MOST parentSize |
UNSPECIFIED 0 |
| wrap_content | AT_MOST parentSize |
AT_MOST parentSize |
UNSPECIFIED 0 |
注意，parentSize是父容器可使用的大小。
更新，看到鸿洋公众号的文章关于UNSPECIFIED说明：
MeasureSpec.UNSPECIFIED是不是真的不常见？
在日常定制View时，确实很少会专门针对这个模式去做特殊处理，大多数情况下，都会把它当成MeasureSpec.AT_MOST一样看待，就比如最最常用的TextView，它在测量时也是不会区分UNSPECIFIED和AT_MOST的。
不过，虽说这个模式比较少直接接触到，但很多场景下，我们已经在不知不觉中用上了，比如RecyclerView的Item，如果Item的宽/高是wrap_content且列表可滚动的话，那么Item的宽/高的测量模式就会是UNSPECIFIED。
还有就是NestedScrollView和ScrollView，因为它们都是扩展自FrameLayout，所以它们的子View会测量两次，第一次测量时，子View的heightMeasureSpec的模式是写死为UNSPECIFIED的。
我们在自定义ViewGroup过程中，如果允许子View的尺寸比ViewGroup大的话，在测量子View时就可以把Mode指定为UNSPECIFIED。
看到ScrollView重写了measureChild方法，指定高度的mode是UNSPECIFIED
3、View的工作流程
View的三大流程，measure、layout、draw。measure确定view的测量宽高，layout确定view的最终宽高和四个顶点位置，draw绘制到屏幕。
3.1 Measure过程
view的测量过程，由measure()方法完成。viewGroup测量自身后，还需调用child.measure()遍历测量子view。
3.1.1 view的测量过程
/**
*
* This is called to find out how big a view should be. The parent
* supplies constraint information in the width and height parameters.
*
*
*
* The actual measurement work of a view is performed in
* {@link #onMeasure(int, int)}, called by this method. Therefore, only
* {@link #onMeasure(int, int)} can and must be overridden by subclasses.
*
*
*
* @param widthMeasureSpec Horizontal space requirements as imposed by the
* parent
* @param heightMeasureSpec Vertical space requirements as imposed by the
* parent
*
* @see #onMeasure(int, int)
*/
public final void measure(int widthMeasureSpec, int heightMeasureSpec) {
...
// measure ourselves, this should set the measured dimension flag back
onMeasure(widthMeasureSpec, heightMeasureSpec);
...
}
可见view的measure()方法是final，不可被子类重写。里面调用onMeasure()，实际真正的测量过程在onMeasure()中。所以只有onMeasure()可以且必须被子类重写。另外，参数widthMeasureSpec、heightMeasureSpec就是上一节最后的表格中的值。继续看onMeasure()：
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),
getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));
}
从名字就可以看出，setMeasuredDimension()就是设置测量的尺寸，且在onMeasure()中必须被调用，否则在测量时会发送异常。getDefaultSize()获取默认的宽/高。所以View类中的onMeasure() 是设置默认的宽高。 继续看getDefaultSize()具体实现：
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;
}
UNSPECIFIED，一般是系统使用，不需要关心。这里view大小直接取size，就是getSuggestedMinimumWidth()/getSuggestedMinimumHeight()，意思是 建议的 最小宽高。看下实现：
protected int getSuggestedMinimumWidth() {
return (mBackground == null) ? mMinWidth : max(mMinWidth, mBackground.getMinimumWidth());
}
没有背景，就取mMinWidth，就是xml中设置的minWidth属性值；有背景，取 mMinWidth 、背景的MinimumWidth 的较大值。drawable的getMinimumWidth()如下，有固有宽度就取固有宽度（如BitmapDrawable），没有就是0（如ShadeDrawable）。
public int getMinimumWidth() {
final int intrinsicWidth = getIntrinsicWidth();
return intrinsicWidth > 0 ? intrinsicWidth : 0;
}
AT_MOST、EXACTLY，直接取specSize，就是上一节最后的表格中的值，作为测量宽高。那这样取specSize是否合适呢？ 再来看一遍specSize的来源。
| parentSpecMode /childLayoutParams |
EXACTLY | AT_MOST | UNSPECIFIED |
|---|---|---|---|
| dp/px | 1EXACTLY childSize |
2EXACTLY childsize |
EXACTLY childsize |
| match_parent | 3EXACTLY parentSize |
4AT_MOST parentSize |
UNSPECIFIED 0 |
| wrap_content | 5AT_MOST parentSize |
6AT_MOST parentSize |
UNSPECIFIED 0 |
1、2的情况，具体dp值，取SpecSize没问题，因为是EXACTLY，就是给定的的尺寸。
3的情况，match_parent，取SpecSize，即parentSize，也没问题，因为是EXACTLY，也是确定的尺寸。
4的情况，match_parent，但父容器又是wrap_content，系统就给了AT_MOST+parentSize，限制最大尺寸为parentSize。而这里直接取specSize即parentSize，似乎也没问题。这个看一个例子一，如下，view是match_parent，可见view取得确实是parentSize。
5、6的情况，wrapContent即AT_MOST+parentSize，取specSize也就是parentSize，所以和3、4一样都是parentSize，即View类 中 默认wrapContent等同于match_parent。
再看一个情况例子二，如下，View换成TextView（继承View），尺寸就不是parentSize了，而是内容尺寸，说明TextView在onMeasure中做了处理。
继续看，例子三如下，同时有TextView、View，此时textView又是取parentSize（可用空间）：
所以得出结论：
通常直接继承View的自定义View，在onMeasure()需要处理 ：
a、wrap_content的情况，否则wrap_content就等同于match_parent；
b、match_parent+父容器wrap_content的情况，否则就像例子一，父容器wrap_content是无效的，处理方式就是例子二中的textView。
总结就是，直接继承View的自定义View，需要处理AT_MOST时的宽高。
处理方式如下：
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
super.onMeasure(widthMeasureSpec, heightMeasureSpec);
int widthSize = MeasureSpec.getSize(widthMeasureSpec);
int widthMode = MeasureSpec.getMode(widthMeasureSpec);
int heightSize = MeasureSpec.getSize(heightMeasureSpec);
int heightMode = MeasureSpec.getMode(heightMeasureSpec);
if (widthMode == MeasureSpec.AT_MOST && heightMode == MeasureSpec.AT_MOST) {
setMeasuredDimension(mWidth, mHeight);
} else if (widthMode == MeasureSpec.AT_MOST) {
setMeasuredDimension(mWidth, heightSize);
} else if (heightMode == MeasureSpec.AT_MOST) {
setMeasuredDimension(widthSize, mHeight);
}
}
实际就是在 AT_MOST时 设置一个指定的尺寸mWidth、mHeight，其他情况沿用系统。至于mWidth、mHeight是多少，则要具体看你的view的逻辑了。例如TextView，可以参考其源码的实现。
3.1.2 ViewGroup的测量过程
ViewGroup需要完成自身的测量，还要遍历子view调用measure()方法进行测量。
ViewGroup是抽象类，没有重写onMeasure，因为无法做到统一，是让具体继承ViewGroup的子类重写自己的逻辑。但是提供一些方便的方法给子类调用。如measureChildren()、measureChild()、measureChildWithMargins()，上面第二节分析过measureChildWithMargins()，这里我们看下measureChildren()：
/**
* Ask all of the children of this view to measure themselves, taking into
* account both the MeasureSpec requirements for this view and its padding.
* We skip children that are in the GONE state The heavy lifting is done in
* getChildMeasureSpec.
*
* @param widthMeasureSpec The width requirements for this view
* @param heightMeasureSpec The height requirements for this view
*/
protected void measureChildren(int widthMeasureSpec, int heightMeasureSpec) {
final int size = mChildrenCount;
final View[] children = mChildren;
for (int i = 0; i < size; ++i) {
final View child = children[i];
if ((child.mViewFlags & VISIBILITY_MASK) != GONE) {
measureChild(child, widthMeasureSpec, heightMeasureSpec);
}
}
}
就是遍历子view，调用measureChild()，继续看：
protected void measureChild(View child, int parentWidthMeasureSpec,
int parentHeightMeasureSpec) {
final LayoutParams lp = child.getLayoutParams();
final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec,
mPaddingLeft + mPaddingRight, lp.width);
final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec,
mPaddingTop + mPaddingBottom, lp.height);
child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}
通过getChildMeasureSpec()获取child的MeasureSpec，然后调用child.measure()，测量就传到child内部了，很好理解。measureChild()相比measureChildWithMargins() 没有考虑child的margin值。
上面说了，ViewGroup没有重写onMeasure，因为无法做到统一，让具体继承ViewGroup的子类重写自己的逻辑。具体看下LinearLayout的测量过程。
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
if (mOrientation == VERTICAL) {
measureVertical(widthMeasureSpec, heightMeasureSpec);
} else {
measureHorizontal(widthMeasureSpec, heightMeasureSpec);
}
}
继续看measureVertical()：
void measureVertical(int widthMeasureSpec, int heightMeasureSpec) {
...
//下面这句官方注释：看每个人多高，也记住最大宽度。想想这不就是计算竖向LinearLayout宽高的思路嘛！
// See how tall everyone is. Also remember max width.
for (int i = 0; i < count; ++i) {
...
final View child = getVirtualChildAt(i);
...
final LayoutParams lp = (LayoutParams) child.getLayoutParams();
...
// Determine how big this child would like to be. If this or
// previous children have given a weight, then we allow it to
// use all available space (and we will shrink things later
// if needed).
final int usedHeight = totalWeight == 0 ? mTotalLength : 0;
//这里测量child（里面就是measureChildWithMargins()）
measureChildBeforeLayout(child, i, widthMeasureSpec, 0,
heightMeasureSpec, usedHeight);
final int childHeight = child.getMeasuredHeight();
...
final int totalLength = mTotalLength;
//这里mTotalLength加上child的高度、margin，就是child高度累积。
mTotalLength = Math.max(totalLength, totalLength + childHeight + lp.topMargin +
lp.bottomMargin + getNextLocationOffset(child));
...
//这里记录最大宽度（包含margin）
final int margin = lp.leftMargin + lp.rightMargin;
final int measuredWidth = child.getMeasuredWidth() + margin;
maxWidth = Math.max(maxWidth, measuredWidth);
...
}
//遍历完了：高度加上自身的上下padding
// Add in our padding
mTotalLength += mPaddingTop + mPaddingBottom;
int heightSize = mTotalLength;
// Check against our minimum height
heightSize = Math.max(heightSize, getSuggestedMinimumHeight());
//这里很重要：调用resolveSizeAndState--决定 计算的高度（高度累加）和 LinearLayout的父容器约束的高度，取哪一个。
// Reconcile our calculated size with the heightMeasureSpec
int heightSizeAndState = resolveSizeAndState(heightSize, heightMeasureSpec, 0);
heightSize = heightSizeAndState & MEASURED_SIZE_MASK;
...
//最大宽度加上左右margin
maxWidth += mPaddingLeft + mPaddingRight;
// Check against our minimum width
maxWidth = Math.max(maxWidth, getSuggestedMinimumWidth());
//设置最终的测量尺寸（宽也也同样调用resolveSizeAndState决定取哪个）
setMeasuredDimension(resolveSizeAndState(maxWidth, widthMeasureSpec, childState),
heightSizeAndState);
}
所以，简单概括就是：
1.先测量所有child；
2.根据child的情况获取自身宽高（累加高度、最大宽度）。
那么，是否就取 累加高度、最大宽度？再看下resolveSizeAndState():
/**
* Utility to reconcile a desired size and state, with constraints imposed
* by a MeasureSpec. Will take the desired size, unless a different size
* is imposed by the constraints. The returned value is a compound integer,
* with the resolved size in the {@link #MEASURED_SIZE_MASK} bits and
* optionally the bit {@link #MEASURED_STATE_TOO_SMALL} set if the
* resulting size is smaller than the size the view wants to be.
*
* @param size How big the view wants to be. --想要的尺寸
* @param measureSpec Constraints imposed by the parent. --父布局给的measureSpec
* @param childMeasuredState Size information bit mask for the view's
* children.
* @return Size information bit mask as defined by
* {@link #MEASURED_SIZE_MASK} and
* {@link #MEASURED_STATE_TOO_SMALL}.
*/
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:
//AT_MOST时，想要的尺寸大于约束的尺寸，就只能取 约束的尺寸。
if (specSize < size) {
result = specSize | MEASURED_STATE_TOO_SMALL;
} else {
result = size;
}
break;
case MeasureSpec.EXACTLY:
//dp值、match_parent且父EXACTLY，就是SpecSize
result = specSize;
break;
case MeasureSpec.UNSPECIFIED:
default:
result = size;
}
return result | (childMeasuredState & MEASURED_STATE_MASK);
}
这个过程就是限制 AT_MOST时，即wrap_content（或match_parent且父wrap_content）时高度不能大于parent的剩余空间。
3.1.3 获取View宽高的时机
Measure过程完成，就可通过getMeasuredWidth()、getMeasuredHeight()获取测量宽高。但某些极端情况
需要多次Measure才能确定最终宽高。所以在onLayout方法中获取测量宽高是真正ok的。
我们知道，activity的onCreate中无法获取到view的宽高。实际onCreate、onStart、onResume都不能保证view已完成测量，所以可能获取的都是0。因为view的measure和activity生命周期不是同步的。
以下是保证可以获取view测量宽高的方法：
1、Activity/View # onWindowFocusChanged
onWindowFocusChanged：View已初始化完毕，宽高已准备ok。 但会多次调用，获取焦点、失去焦点都回调用。(这个回调是ViewRootIml中分发到DecorView，接着到Activity、到各级View。)
@Override
public void onWindowFocusChanged(boolean hasFocus) {
super.onWindowFocusChanged(hasFocus);
if (hasFocus) {
int measuredWidth = scoreView.getMeasuredWidth();
int measuredHeight = scoreView.getMeasuredHeight();
}
}
2、view.post(runnable)
view.post可以把runnable放入消息队列，等待looper到此runnable是view已经初始化完成。v详细原理参考【Android源码解析】View.post()到底干了啥
@Override
protected void onStart() {
super.onStart();
scoreView.post(new Runnable() {
@Override
public void run() {
int measuredWidth = scoreView.getMeasuredWidth();
int measuredHeight = scoreView.getMeasuredHeight();
}
});
}
3、ViewTreeObserver
ViewTreeObserver有很多回调，其中有个OnGlobalLayoutListener，当View树的状态发生改变或者View树内部view的可见性发生改变时 方法 onGlobalLayout()都会被调用。所以是会回调多次。 此时也可以获取view的宽高：
ViewTreeObserver observer = view.getViewTreeObserver();
observer.addOnGlobalLayoutListener(new ViewTreeObserver.OnGlobalLayoutListener() {
@Override
public void onGlobalLayout() {
mDefaultControlLayout.getViewTreeObserver().removeGlobalOnLayoutListener(this);
if (mIsGroupListAnimating) {
mIsGroupListAnimationPending = true;
} else {
updateLayoutHeightInternal(animate);
}
}
});
3.2Layout过程
layout()的作用是View用来确定view本身位置，内部调用onLayout()来确定子view的位置。 layout过程比measure过程简单很多。看View的layout方法：
public void layout(int l, int t, int r, int b) {
if ((mPrivateFlags3 & PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT) != 0) {
onMeasure(mOldWidthMeasureSpec, mOldHeightMeasureSpec);
mPrivateFlags3 &= ~PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
}
int oldL = mLeft;
int oldT = mTop;
int oldB = mBottom;
int oldR = mRight;
//使用setFrame方法设置4个顶点，就确定位置了~
boolean changed = isLayoutModeOptical(mParent) ?
setOpticalFrame(l, t, r, b) : setFrame(l, t, r, b);
if (changed || (mPrivateFlags & PFLAG_LAYOUT_REQUIRED) == PFLAG_LAYOUT_REQUIRED) {
//这里调用onLayout，是个空实现。ViewGroup中重写了，还是空实现，但加了abstract，即ViewGroup的子类必须重写onLayout确定子View的位置。
onLayout(changed, l, t, r, b);
if (shouldDrawRoundScrollbar()) {
if(mRoundScrollbarRenderer == null) {
mRoundScrollbarRenderer = new RoundScrollbarRenderer(this);
}
} else {
mRoundScrollbarRenderer = null;
}
mPrivateFlags &= ~PFLAG_LAYOUT_REQUIRED;
ListenerInfo li = mListenerInfo;
if (li != null && li.mOnLayoutChangeListeners != null) {
ArrayList listenersCopy =
(ArrayList)li.mOnLayoutChangeListeners.clone();
int numListeners = listenersCopy.size();
for (int i = 0; i < numListeners; ++i) {
listenersCopy.get(i).onLayoutChange(this, l, t, r, b, oldL, oldT, oldR, oldB);
}
}
}
...
}
先是用setFrame方法设置4个顶点，就确定位置了，即mLeft、mTop、mBottom、mRight确定了。 然后调用onLayout，是个空实现。ViewGroup中重写了onLayout，还是空实现，但加了abstract，即ViewGroup的子类必须重写onLayout确定子View的位置。
那就看看LinearLayout的onLayout：
@Override
protected void onLayout(boolean changed, int l, int t, int r, int b) {
if (mOrientation == VERTICAL) {
layoutVertical(l, t, r, b);
} else {
layoutHorizontal(l, t, r, b);
}
}
继续看layoutVertical()：
void layoutVertical(int left, int top, int right, int bottom) {
final int paddingLeft = mPaddingLeft;
int childTop;
int childLeft;
// Where right end of child should go
final int width = right - left;
int childRight = width - mPaddingRight;
// Space available for child
int childSpace = width - paddingLeft - mPaddingRight;
final int count = getVirtualChildCount();
final int majorGravity = mGravity & Gravity.VERTICAL_GRAVITY_MASK;
final int minorGravity = mGravity & Gravity.RELATIVE_HORIZONTAL_GRAVITY_MASK;
switch (majorGravity) {
case Gravity.BOTTOM:
// mTotalLength contains the padding already
childTop = mPaddingTop + bottom - top - mTotalLength;
break;
// mTotalLength contains the padding already
case Gravity.CENTER_VERTICAL:
childTop = mPaddingTop + (bottom - top - mTotalLength) / 2;
break;
case Gravity.TOP:
default:
childTop = mPaddingTop;
break;
}
//遍历子view
for (int i = 0; i < count; i++) {
final View child = getVirtualChildAt(i);
if (child == null) {
childTop += measureNullChild(i);
} else if (child.getVisibility() != GONE) {
//获取child的测量宽高
final int childWidth = child.getMeasuredWidth();
final int childHeight = child.getMeasuredHeight();
final LinearLayout.LayoutParams lp =
(LinearLayout.LayoutParams) child.getLayoutParams();
int gravity = lp.gravity;
if (gravity < 0) {
gravity = minorGravity;
}
final int layoutDirection = getLayoutDirection();
final int absoluteGravity = Gravity.getAbsoluteGravity(gravity, layoutDirection);
switch (absoluteGravity & Gravity.HORIZONTAL_GRAVITY_MASK) {
case Gravity.CENTER_HORIZONTAL:
childLeft = paddingLeft + ((childSpace - childWidth) / 2)
+ lp.leftMargin - lp.rightMargin;
break;
case Gravity.RIGHT:
childLeft = childRight - childWidth - lp.rightMargin;
break;
case Gravity.LEFT:
default:
childLeft = paddingLeft + lp.leftMargin;
break;
}
if (hasDividerBeforeChildAt(i)) {
childTop += mDividerHeight;
}
childTop += lp.topMargin;
//以上就是获取子view的左、上的位置，即宽高，然后调用setChildFrame
setChildFrame(child, childLeft, childTop + getLocationOffset(child),
childWidth, childHeight);
//top位置加上高度和margin，就是下一个view的top
childTop += childHeight + lp.bottomMargin + getNextLocationOffset(child);
i += getChildrenSkipCount(child, i);
}
}
}
就是遍历子view，确认childLeft、childTop，调用setChildFrame确认子view的位置：
private void setChildFrame(View child, int left, int top, int width, int height) {
//这里width、height就是 上面获取的 测量宽高
child.layout(left, top, left + width, top + height);
}
也就是调用child的layout方法，这样就走child的layout过程了。
一个问题：getMeasuredWidth() 与 getWidth()有何区别？
答曰：一般情况，getMeasuredWidth() 与 getWidth()两者无区别。
先看，getWidth():
public final int getWidth() {
return mRight - mLeft;
}
在上面分析LinearLayout时，child.layout的参数中 mRight就是mLeft + measuredWidth，所以getWidth()就是measuredWidth。只不过是measuredWidth在测量过程产生，getWidth()在layout过程产生。 只要不重写view的layout()方法（也不需要重写）改变顶点位置就不会出现不同的情况，例如下面这个最终宽高比测量宽高大100。
public void layout(int l, int t, int r, int b) {
super.layout(l,t,r+100,b+100);
}
3.3Draw过程
draw过程：
1、画背景
2、画自己-- onDraw，自己实现
3、画子view-- dispatchDraw
4、画装饰
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)
final int viewFlags = mViewFlags;
boolean horizontalEdges = (viewFlags & FADING_EDGE_HORIZONTAL) != 0;
boolean verticalEdges = (viewFlags & FADING_EDGE_VERTICAL) != 0;
if (!verticalEdges && !horizontalEdges) {
// Step 3, draw the content
if (!dirtyOpaque) onDraw(canvas);
// Step 4, draw the children
dispatchDraw(canvas);
drawAutofilledHighlight(canvas);
// Overlay is part of the content and draws beneath Foreground
if (mOverlay != null && !mOverlay.isEmpty()) {
mOverlay.getOverlayView().dispatchDraw(canvas);
}
// Step 6, draw decorations (foreground, scrollbars)
onDrawForeground(canvas);
// Step 7, draw the default focus highlight
drawDefaultFocusHighlight(canvas);
if (debugDraw()) {
debugDrawFocus(canvas);
}
// we're done...
return;
}
ViewGroup一般不用onDraw画自己，只需要画子View就可以了。但明确需要画自己的话，需要调用setViewNotDraw(false);
以上View的三大流程就分析完了。
4、自定义View
自定义view涉及view层次结构、事件分发、工作原理，有一定复杂度，但也是有章可循的。
4.1自定义view的分类
- 继承View：重写onDraw，要处理wrap_content、padding。
- 继承ViewGroup：重写onMeasure测量自己、子View，重写onLayout布局子View。
- 继承特定View（如TextView）：扩展自己的功能。
- 继承特定ViewGroup（如LinearLayout）：扩展自己的功能。
4.2 自定义view 注意点
- 支持wrap_content：直接继承View或ViewGroup的，要在onMeasure中处理wrap_content的情况。
- 支持padding：直接继承View在onDraw中处理；直接继承ViewGroup，在onMeasure、onLayout中处理padding和子view的margin。
- 不要在View中使用handler，因为本身提供了post方法。
- 在View#onDetachedFromWindow中停止动画或线程。
- 处理好嵌套滑动。
4.3 例子
自定义ViewGroup实例：横向滑动HorizontalView
4.4 自定义view的思想
先掌握基本功，弹性滑动、滑动冲突、绘制原理等，然后选择自定义的类别，按照注意事项多做就可以了。
转载：https://blog.csdn.net/hfy8971613/article/details/102570685