JVM部分基础回忆
- jdk8取消了永久代，jdk7常量池移到堆中
- 新生代中Eden和Survivor内存比为8：1:1,每次只有一个survivor可以用，实际可用为90% 。新时代：老年代为1：2。
- 大对象直接进入老年代，小对象优先进入Eden，GC发生时没有被回收，则进入survivor，熬过一定次数进入老年代，当然也并非绝对。survivor内存不足时会进入老年代，老年代不足会进行GC
- JVM对象存活判断采用可达性分析法，GCRoot通常为永久代的对象，对象不可达时不会立刻被杀死，finalize()有机会使对象“复活”，但是此方法最好不要用。
- 垃圾回收器基本都基于标记-清除、标记-整理和复制算法。GC发生时，用户线程停止，client模式垃圾收集器为serial和serial old（串行），server模式为PS scanvage（并发）和serial od。AndroidStudio本身的收集器默认使用CMS(并发，用户线程不会停止缩短，基于复制和标记-清除算法)，Eclipse本身的收集器默认使用G1（目前最先进的收集器，使用标记-整理和复制算法，stop the world时间进一步缩短）
View/ViewGroup部分疑惑解决记录
- VelocietyTracker滑动速度追踪
- TouchSlop最小滑动距离（与设备有关
- GestureDetector(这个不用多说
- 自定义View时padding无效
- 事件分发一旦DOWN没有被消费，后续事件不会下发，直接回调Activity.OnTounchEvent。一旦有事件被拦截，后续事件直接调用onTouchEven，不再调用onInteceptTouchEcent。除了DOWN，其他事件一旦没有被消费，直接回调Activity.onTouchEvent。
正题
关于measure(),onMeasure()和getWithMeasureDimenson,getWith()等的关系。经查看源码，实际上父ViewGroup调用onMeasure里面调用measureChild(WithMargins),再调用到getChildMeasureSpec(p1,p2,p3),p1为父View MeasureSpec,p2为padding(+margin+usedSpace),p3为layoutparameter.with，之后进一步调用子view measure(spec，spec)再调用onMeasure(spec,spec),onMeasure setMeasureDismenson并不包括margin信息！如果ViewGroup要支持margin,必须定义一个extends的MarginLayparameter,并使用measureChildWithMargin()，才可以正确获取到padding之外的margin信息。Layout()完成View布局，onLayout()完成子View布局。Draw()先绘制背景，在调用onDraw(）绘制自己，最后绘制子View(dispaDraw())。
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);
}
这里的withUsed和heightUsed实在不是很懂
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);
}
/**
* 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
* 可以看出这个方法判断ViewGroup除了padding,子View的
* margin，还有多少空间给子View内容（包括ziView的padding）
*/
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);
}