高级面试题：首先看案例
public class MainActivity extends AppCompatActivity {
private TextView mTextView;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
mTextView = (TextView) findViewById(R.id.text_view);
Log.e("TAG", "height1 -> " + mTextView.getMeasuredHeight()); // 0
mTextView.post(new Runnable() {
// 保存到Queue中，什么都没干，会在dispatchAttachedToWindow会在测量完毕之后调用中执行，executeActions（）
@Override
public void run() {
Log.e("TAG", "height2 -> " + mTextView.getMeasuredHeight()); // 高度
}
});
}
@Override
protected void onResume() {
super.onResume();
Log.e("TAG", "height3 -> " + mTextView.getMeasuredHeight()); // 0
}
}
上面我们可以看到oncreate和onResume中获得高度为0，即没有获得高度，而开个线程后会获得高度，这是为什么?
首先我们需要了解activity的启动流程，我以前写过一篇文章，但是只有流程图，今天也不详细说，若有需要则会抽时间写一篇文章，android插件化架构-activity的启动流程https://www.jianshu.com/p/911ec7004301也写过一篇文章invalidate和postinvalidate源码分析https://www.jianshu.com/p/6c5d65009ba1
我们直接从Activity的启动流程的handleResumeActivity源码开始分析
//首先会走这个方法，这个方法会走向activity的onResume方法，这时候还是没有调用onMeasure方法
r = performResumeActivity(token, clearHide, reason);
....
//之后会调用这个方法,我们点击进去后会发现这是个抽象方法，我们看wm的实例怎么创建的
wm.addView(decor, l);
//我们会发现这个方法会创建一个WindowManager
ViewManager wm = a.getWindowManager();
public WindowManager getWindowManager() {
return mWindowManager;
}
我们都知道WindowManager的实现类是PhoneWindow，查看其getWindowManager源码，我们会发现，实际这个走的是Window中的getWindowManager()方法
//实际最终创建的是WindowManagerImpl这个实例，查看这个源码中的addView方法
mWindowManager = ((WindowManagerImpl)wm).createLocalWindowManager(this);
@Override
public void addView(@NonNull View view, @NonNull ViewGroup.LayoutParams params) {
applyDefaultToken(params);
mGlobal.addView(view, params, mContext.getDisplay(), mParentWindow);
}
WindowMangerImp中的addView源码分析
ViewRootImpl root;
root = new ViewRootImpl(view.getContext(), display);
try {
//设置view进去
root.setView(view, wparams, panelParentView);
} catch (RuntimeException e) {
// BadTokenException or InvalidDisplayException, clean up.
if (index >= 0) {
removeViewLocked(index, true);
}
throw e;
}
调用的ViewRootImpl中的setView方法,调用其中的 requestLayout()-> scheduleTraversals();->mTraversalRunnable->doTraversal->performMeasure->view的measure-.onMeasure方法这时候我们就看到了onMeasure实际是在onResume方法之后调用的
以LinearLayout为例:LinearLayout的onMeasure->measureVertical->measureChildBeforeLayout->measureChildWithMargins->ViewGroup的measureChildWithMargins
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);
}
我们自定义view的时候widthMeasureSpec=childWidthMeasureSpec
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
super.onMeasure(widthMeasureSpec, heightMeasureSpec);
// 指定宽高
// widthMeasureSpec = childWidthMeasureSpec
// heightMeasureSpec = childHeightMeasureSpec
// wrap_content = AT_MOST
// match_parent fill_parent 100dp = EXACTLY
// 模式和大小是由父布局和自己决定的
// 比方 父布局是包裹内容 就算子布局是match_parent，这个时候计算的测量模式还是 AT_MOST
// 比方 父布局是match_parent 子布局是match_parent，这个时候计算的测量模式还是 EXACTLY
// setMeasuredDimension();
}
getChildMeasureSpec源码分析
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) {
//父布局是EXACTLY模式
case MeasureSpec.EXACTLY:
//子布局是精确的值，则子布局是EXACTLY
if (childDimension >= 0) {
resultSize = childDimension;
resultMode = MeasureSpec.EXACTLY;
} else if (childDimension == LayoutParams.MATCH_PARENT) {//子布局如果是MATCH_PARENT，子布局是EXACTLY
// Child wants to be our size. So be it.
resultSize = size;
resultMode = MeasureSpec.EXACTLY;
} else if (childDimension == LayoutParams.WRAP_CONTENT) {//子布局是WRAP_CONTENT,则子布局是AT_MOST
// Child wants to determine its own size. It can't be
// bigger than us.
resultSize = size;
resultMode = MeasureSpec.AT_MOST;
}
break;
//父布局是AT_MOST
case MeasureSpec.AT_MOST:
if (childDimension >= 0) {
//子布局是EXACTLY
resultSize = childDimension;
resultMode = MeasureSpec.EXACTLY;
} else if (childDimension == LayoutParams.MATCH_PARENT) {//子布局是MATCH_PARENT,返回的是AT_MOST
resultSize = size;
resultMode = MeasureSpec.AT_MOST;
} else if (childDimension == LayoutParams.WRAP_CONTENT) {//子布局是WRAP_CONTENT，返回的是AT_MOST
resultSize = size;
resultMode = MeasureSpec.AT_MOST;
}
break;
//父布局是UNSPECIFIED的模式
case MeasureSpec.UNSPECIFIED:
if (childDimension >= 0) {//子布局还是EXACTLY
// Child wants a specific size... let him have it
resultSize = childDimension;
resultMode = MeasureSpec.EXACTLY;
}
//其他啊不管是什么布局都是UNSPECIFIED
else if (childDimension == LayoutParams.MATCH_PARENT) {
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);
}
子布局设置为精确的值，无论父布局什么布局返回的都是精确的值。其他和父布局设置是一样的
View的绘制流程.png
