目录
什么情况下会onmeasure会执行?
进入view的measure方法:
void measure(){
boolean forcelayout = (mprivateflags & pflag_force_layout) == pflag_force_layout;
boolean specchanged = widthmeasurespec != moldwidthmeasurespec
|| heightmeasurespec != moldheightmeasurespec;
boolean issepcexactly = measurespec.getmode(widthmeasurespec) == measurespec.exactly
&& measurespec.getmode(heightmeasurespec) == measurespec.exactly;
boolean matchesspecsize = getmeasuredwidth() == measurespec.getsize(widthmeasurespec)
&& getmeasuredheight() == measurespec.getsize(heightmeasurespec);
final boolean needslayout = specchanged
&& (salwaysremeasureexactly || !isspecexactly || !matchesspecsize);
if(forcelayout || needlayout){
int cacheindex = forcelayout ? -1 : mmeasurecache.indexofkey(key);
if (cacheindex < 0 || signoremeasurecache) {
onmeasure(widthmeasurespec, heightmeasurespec);
mprivateflags3 &= ~pflag3_measure_needed_before_layout;
} else {
long value = mmeasurecache.valueat(cacheindex);
setmeasureddimensionraw((int) (value >> 32), (int) value);
mprivateflags3 |= pflag3_measure_needed_before_layout;
}
}
}
什么时候forcelayout=true:
- 调用
requestlayout - 调用
forcerequestlayout
什么时候needslayout=true:
- 当长宽发生改变
什么时候调用了onmeasure>方法:
forcelayouy=true- 或者
mmeasurecache没有当前的缓存
总结:
当调用了requestlayout一定会测发重测过程.当forcelayout=false的时候会去判断mmeasurecache值.现在研究下这个mmeasurecache
class view{
longsparselongarray mmeasurecache;
void measure(widthspec,heightspec){
---
long key = (long) widthmeasurespec << 32 | (long) heightmeasurespec & 0xffffffffl;
int cacheindex = forcelayout ? -1 : mmeasurecache.indexofkey(key);
if(cacheindex<0){
onmeasure(widthspec,heightspec);
}
moldwidthmeasurespec = widthmeasurespec;
moldheightmeasurespec = heightmeasurespec;
mmeasurecache.put(key,widhspec|heightspec);
---
}
}
这里可以看到oldwidthmeasurespec和mmeasurecache都是缓存上一次的值,那他们有什么不同呢?不同点就是,oldwidthmeasurespec>不仅仅缓存了测量的spec模式而且缓存了size.但是mmeasurecache只缓存了size.从这行代码可以看出:
long key = (long) widthmeasurespec << 32 | (long) heightmeasurespec & 0xffffffffl;
这里一同运算就为了排除掉spec造成的影响.
//不信你可以试下下面的代码
public class test {
public static void main(string[] args) {
long widthmeasurespec = makemeasurespec(10,0);
long heightmeasurespec = makemeasurespec(20,0);
long ss = widthmeasurespec << 32 | (long) heightmeasurespec & 0xffffffffl;
system.out.println("==========" ss);
}
private static final int mode_mask = 0x3 << 30;
public static int makemeasurespec(int size,
int mode) {
return (size & ~mode_mask) | (mode & mode_mask);
}
}
//42949672980
//42949672980
//42949672980
什么时候mprivateflags会被赋值pflag_force_layout.
在view viewgrouup的构造函数里面会主动赋值一次,然后在viewgroup.addview时候会给当前view的mprovateflags赋值pflag_force_layout.
为什么onmeasure会被执行两次?
void measure(int widthmeasurespec,int heightmeasurespec){
----
boolean forcelayout = (mprivateflags & pflag_force_layout) == pflag_force_layout;
if(forcelayout | needslayout){
onmeasure()
}
----
}
public void layout(int l, int t, int r, int b){
---
mprivateflags &= ~pflag_force_layout;
---
}
在第一次触发到measure方法时,forcelayoyt=true needslayout=true,但是layout方法还没触发到.
在第二次触发到measure>方法时,forcelayout=true needslayout=false,所以还是会进入onmeasure方法.这次会执行layout方法.然后我们在下次的时候forcelayout就等于false了.上面的这一段分析是分析的measure内部如何防止多次调用onmeasure.
分析外部是如何多次调用measure方法的
在activity执行到onresume生命周期的时候,会执行windowmanager.addview操作,windowmanager的具体实现类是windowmanagerimpl然后addview操作交给了代理类windowmanagerglobal,然后在windowmanagerglobal的addview里面执行了viewrootimpl.setview操作(viewrootimpl对象也是在这个时候创建的),在viewrootimpl会主动调用一次requestlayout,也就开启了第一次的视图 测量 布局 绘制.
在setview的时候主动调用了一次viewrootimpl.requestlayout,注意这个requestlayout是viewrootimpl的内部方法,和view viewgroup那些requestlayout不一样.在viewrootimpl.requestlayout内部调用了performtraversals方法:
class viewrootimpl{
void performtraversals(){
if(layoutresuested){
//标记1
windowsizemaychanged |= measurehierarchy(host,lp,res,desiredwindowwidth,desiredwindowheight);
}
//标记2
performmeasure()
performlayout()
}
void measurehierarchy(){
performmeasure()
}
}
从viewrootimpl的执行逻辑你可以看出,在执行performlayout之前,他自己就已经调用了两次performmeasure方法.所以你现在就知道为啥了.
以上就是onmeasure被执行两次原理解析的详细内容，更多关于onmeasure被执行两次的资料请关注其它相关文章！
