前言
最近加了移动开发前线的安卓学习群，看到他们在讨论onMeasure相关的内容，于是写个文章来学习和记录一下onMeasure相关的内容。
Activity的装载过程
从Activity的setContentView方法切入
setContentView
Window mWindow;
public void setContentView(@LayoutRes int layoutResID) {
getWindow().setContentView(layoutResID);
initWindowDecorActionBar();
}
attach
这里的getWindow()返回的是mWindow对象。
关于这个Window对象，Activity在创建时，会调用一个attach方法。
final void attach(Context context, ActivityThread aThread,
Instrumentation instr, IBinder token, int ident,
Application application, Intent intent, ActivityInfo info,
CharSequence title, Activity parent, String id,
NonConfigurationInstances lastNonConfigurationInstances,
Configuration config, String referrer, IVoiceInteractor voiceInteractor) {
……
mWindow = new PhoneWindow(this);
mWindow.setCallback(this);
……
}
Window类主要用来从WindowManagerService接收触摸、键盘等事件，然后分发给Activity，并且Window类还承载了UI的显示。Actiivty的Window实际上是个PhoneWindow对象。
接着分析PhoneWindow.setContentView(id)。
@Override
public void setContentView(int layoutResID) {
// Note: FEATURE_CONTENT_TRANSITIONS may be set in the process of installing the window
// decor, when theme attributes and the like are crystalized. Do not check the feature
// before this happens.
if (mContentParent == null) {
installDecor();
} else if (!hasFeature(FEATURE_CONTENT_TRANSITIONS)) {
mContentParent.removeAllViews();
}
if (hasFeature(FEATURE_CONTENT_TRANSITIONS)) {
final Scene newScene = Scene.getSceneForLayout(mContentParent, layoutResID,
getContext());
transitionTo(newScene);
} else {
mLayoutInflater.inflate(layoutResID, mContentParent);
}
……
}
installDecor
private void installDecor() {
if (mDecor == null) {
mDecor = generateDecor();
……
}
if (mContentParent == null) {
mContentParent = generateLayout(mDecor);
……
}
}
mDecor是一个DecorView对象，DecorView继承自FrameLayout。
generateLayout
protected ViewGroup generateLayout(DecorView decor) {
TypedArray a = getWindowStyle();
if (a.getBoolean(R.styleable.Window_windowNoTitle, false)) {
requestFeature(FEATURE_NO_TITLE);
} else if (a.getBoolean(R.styleable.Window_windowActionBar, false)) {
// Don't allow an action bar if there is no title.
requestFeature(FEATURE_ACTION_BAR);
}
int layoutResource;
int features = getLocalFeatures();
View in = mLayoutInflater.inflate(layoutResource, null);
decor.addView(in, new ViewGroup.LayoutParams(MATCH_PARENT, MATCH_PARENT));
mContentRoot = (ViewGroup) in;
ViewGroup contentParent = (ViewGroup)findViewById(ID_ANDROID_CONTENT);
return contentParent;
}
generateDecor函数就简单地调用了DecorView的构造函数，generateLayout函数主要工作是根据Window的Features来决定要加载什么样的布局文件，这里以R.layout.screen_simple这个布局文件为例子，这个布局文件位置在framework\core\res\res\layout目录中，具体布局文件如下。
在generateLayout函数中，加载了布局文件之后，添加到了mDecor中，并且LayoutParams皆为MATCH_PARENT,mContentParent为id为android.R.id.content的FrameLayout。
继续分析setContentView，在这个函数的后边的地方调用了mLayoutInflater.inflate,把我们指定的布局文件加载到了mContentParent中。
至此，我们可以大致的发现，在一个简单的Activity中，真正的布局文件如下：
Activity启动过程
handleLaunchActivity->performLaunchActivity->和ActivityServiceManager交互->handleResumeActivity->performResumeActivity。
handleResumeActivity
final void handleResumeActivity(IBinder token,
boolean clearHide, boolean isForward, boolean reallyResume) {
……
ActivityClientRecord r = performResumeActivity(token, clearHide);
if (r != null) {
final Activity a = r.activity;
……
if (r.window == null && !a.mFinished && willBeVisible) {
r.window = r.activity.getWindow();
View decor = r.window.getDecorView();
decor.setVisibility(View.INVISIBLE);
ViewManager wm = a.getWindowManager();
WindowManager.LayoutParams l = r.window.getAttributes();
a.mDecor = decor;
l.type = WindowManager.LayoutParams.TYPE_BASE_APPLICATION;
l.softInputMode |= forwardBit;
if (a.mVisibleFromClient) {
a.mWindowAdded = true;
wm.addView(decor, l);
}
} else if (!willBeVisible) {
……
}
……
}
}
在handleResumeActivity方法中，把PhoneWindow中生成的DecorView添加到了WindowManager中。
WindowManagerImpl.addView
@Override
public void addView(@NonNull View view, @NonNull ViewGroup.LayoutParams params) {
applyDefaultToken(params);
mGlobal.addView(view, params, mDisplay, mParentWindow);
}
mGlobal是一个WindowManagerGlobal对象。
WindowManagerGlobal.addView
public void addView(View view, ViewGroup.LayoutParams params,
Display display, Window parentWindow) {
final WindowManager.LayoutParams wparams = (WindowManager.LayoutParams) params;
ViewRootImpl root;
View panelParentView = null;
……
synchronized (mLock) {
// Start watching for system property changes.
……
root = new ViewRootImpl(view.getContext(), display);
……
view.setLayoutParams(wparams);
mViews.add(view);
mRoots.add(root);
mParams.add(wparams);
}
……
root.setView(view, wparams, panelParentView);
}
在这个方法中新建了一个ViewRootImpl对象，并把DecorView对象传入ViewRootImpl中，由ViewRootImpl来掌管整个视图的measure，layout。
ViewRootImpl.getRootMeasureSpec
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;
}
这个函数用来给DecorView赋MeasureSpec值，这个函数在measureHierarchy中被调用。
measureHierarchy
private boolean measureHierarchy(final View host, final WindowManager.LayoutParams lp,
final Resources res, final int desiredWindowWidth, final int desiredWindowHeight) {
int childWidthMeasureSpec;
int childHeightMeasureSpec;
boolean windowSizeMayChange = false;
boolean goodMeasure = false;
if (lp.width == ViewGroup.LayoutParams.WRAP_CONTENT) {
……
goodMeasure = true;
……
}
if (!goodMeasure) {
childWidthMeasureSpec = getRootMeasureSpec(desiredWindowWidth, lp.width);
childHeightMeasureSpec = getRootMeasureSpec(desiredWindowHeight, lp.height);
performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
if (mWidth != host.getMeasuredWidth() || mHeight != host.getMeasuredHeight()) {
windowSizeMayChange = true;
}
}
return windowSizeMayChange;
}
这个函数有个参数是WindowManager.LayoutParams，通过查看这个类可知，lp.width和lp.height都是match_parent，再结合getRootMeasureSpec函数，可知，DecorView传给子View的MeasureSpec是xxxdp+EXACTLY。
ViewGroup getChildMeasure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);
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 = 0;
resultMode = MeasureSpec.UNSPECIFIED;
} else if (childDimension == LayoutParams.WRAP_CONTENT) {
// Child wants to determine its own size.... find out how
// big it should be
resultSize = 0;
resultMode = MeasureSpec.UNSPECIFIED;
}
break;
}
return MeasureSpec.makeMeasureSpec(resultSize, resultMode);
}
FrameLayout等ViewGroup子类在measure子View时，会调用getChildMeasure方法，来决定子View的MeasureSpec。
总结
| Parent的MeasureSpec | 子View的LayoutParams | 子View的MeasureSpec |
|---|---|---|
| EXACTLY |
xxxdp match_parent wrap_content |
xxxdp + EXACTLY parent大小 + EXACTLY parent大小 + AT_MOST |
| AT_MOST |
xxxdp match_parent wrap_content |
xxxdp + EXACTLY parent大小 + AT_MOST parent大小 + AT_MOST |
| UNSPECIFIED |
xxxdp match_parent wrap_content |
xxxdp + EXACTLY 0 + UNSPECIFIED 0 + UNSPECIFIED |
DecorView的onMeasure函数
DecorView继承自FrameLayout，所以这里列出FrameLayout代码
/**
* {@inheritDoc}
*/
@Override
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();
int childWidthMeasureSpec;
int childHeightMeasureSpec;
if (lp.width == LayoutParams.MATCH_PARENT) {
childWidthMeasureSpec = MeasureSpec.makeMeasureSpec(getMeasuredWidth() -
getPaddingLeftWithForeground() - getPaddingRightWithForeground() -
lp.leftMargin - lp.rightMargin,
MeasureSpec.EXACTLY);
} else {
childWidthMeasureSpec = getChildMeasureSpec(widthMeasureSpec,
getPaddingLeftWithForeground() + getPaddingRightWithForeground() +
lp.leftMargin + lp.rightMargin,
lp.width);
}
if (lp.height == LayoutParams.MATCH_PARENT) {
childHeightMeasureSpec = MeasureSpec.makeMeasureSpec(getMeasuredHeight() -
getPaddingTopWithForeground() - getPaddingBottomWithForeground() -
lp.topMargin - lp.bottomMargin,
MeasureSpec.EXACTLY);
} else {
childHeightMeasureSpec = getChildMeasureSpec(heightMeasureSpec,
getPaddingTopWithForeground() + getPaddingBottomWithForeground() +
lp.topMargin + lp.bottomMargin,
lp.height);
}
child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}
}
}
接下来验证我们的猜想，首先是DecorView，这个Layout的MeasureSpec是xxxdp + EXACTLY,接着是LinearLayout，这个Layout获得的MeasureSpec是 parent大小+EXACTLY。接着是FrameLayout，它的MeasureSpec也是parent大小+EXACTLY，再接着就是我们的布局文件了。
TestView
public class TestView extends View{
public TestView(Context context, AttributeSet attrs) {
super(context, attrs);
}
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
int mode = MeasureSpec.getMode(widthMeasureSpec);
int size = MeasureSpec.getSize(widthMeasureSpec);
Log.d("xxj","size "+size);
switch (mode){
case MeasureSpec.EXACTLY:
Log.d("xxj","mode exactly");
break;
case MeasureSpec.AT_MOST:
Log.d("xxj","mode at_most");
break;
default:
Log.d("xxj","mode unspecified");
}
super.onMeasure(widthMeasureSpec, heightMeasureSpec);
}
}
activity_main.xml
按照之前的猜想，FrameLayout获得的MeasureSpec是parent大小 + EXACTLY。TestView获得的MeasureSpec也应该是parent大小+EXACTLY。
运行结果
运行结果
接下来将TestView的width和height改为wrap_content，运行结果应为parent大小 + AT_MOST。
运行结果
接下来将TestView的width和height改为50dp，结果应为50dp + EXACTLY
运行结果