本片文章讲解：UI绘制流程-源码讲解（基于API28的源码分析）
文章大纲：
part1:View是如何被添加到屏幕窗口上
part2:View的绘制流程
本片先讲part1:View是如何被添加到屏幕窗口上，part2：放在下一篇文章讲解。
part1:View是如何被添加到屏幕窗口上
首先，MainActivity中调用了setContentView(R.layout.activity_main);方法，我们跟进去，来到了Activity的源码当中：
public class Activity extends ContextThemeWrapper
implements LayoutInflater.Factory2,
Window.Callback, KeyEvent.Callback,
OnCreateContextMenuListener, ComponentCallbacks2,
Window.OnWindowDismissedCallback{
//省略....
/**
* Set the activity content from a layout resource. The resource will be
* inflated, adding all top-level views to the activity.
*
* @param layoutResID Resource ID to be inflated.
*
* @see #setContentView(android.view.View)
* @see #setContentView(android.view.View, android.view.ViewGroup.LayoutParams)
*/
public void setContentView(@LayoutRes int layoutResID) {
getWindow().setContentView(layoutResID);
initWindowDecorActionBar();
}
//省略....
}
其中getWindow()方法返回了Window对象：
class Activity{
/**
* Retrieve the current {@link android.view.Window} for the activity.
* This can be used to directly access parts of the Window API that
* are not available through Activity/Screen.
*
* @return Window The current window, or null if the activity is not
* visual.
*/
public Window getWindow() {
return mWindow;
}
}
继续查找Window这个类，发现类注释上面有这样一段话：
/**
* Abstract base class for a top-level window look and behavior policy. An
* instance of this class should be used as the top-level view added to the
* window manager. It provides standard UI policies such as a background, title
* area, default key processing, etc.
*
* <p>The only existing implementation of this abstract class is
* android.view.PhoneWindow, which you should instantiate when needing a
* Window.
*/
public abstract class Window {
//...
}
The only existing implementation of this abstract class is android.view.PhoneWindow，也就是说PhoneWindow是Window类的唯一继承实现。那么我们可以找到PhoneWindow类的setContentView方法了：
public class PhoneWindow extends Window implements MenuBuilder.Callback {
//....
@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();//注意这个方法 ①
} 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);//注意这个方法 ②
}
mContentParent.requestApplyInsets();
final Callback cb = getCallback();
if (cb != null && !isDestroyed()) {
cb.onContentChanged();
}
}
//....
}
在PhoneWindow类中有两个比较重要的方法installDecor()和mLayoutInflater.inflate(layoutResID, mContentParent)。接下来我们分别说一下这个两个方法的作用：
installDecor方法：
class PhoneWinow{
private void installDecor() {
if (mDecor == null) {
mDecor = generateDecor();//注意这里 方法①
mDecor.setDescendantFocusability(ViewGroup.FOCUS_AFTER_DESCENDANTS);
mDecor.setIsRootNamespace(true);
if (!mInvalidatePanelMenuPosted && mInvalidatePanelMenuFeatures != 0) {
mDecor.postOnAnimation(mInvalidatePanelMenuRunnable);
}
}
if (mContentParent == null) {
mContentParent = generateLayout(mDecor);//注意这里 方法②
}
}
}
这里先提一句，就是在installDecor方法里面有一个mContentParent = generateLayout(mDecor);这里给mContentParent赋值之后，回到setContentView方法中也有一个mContentParent对象，调用的方法为 mLayoutInflater.inflate(layoutResID, mContentParent)，这两个mContentParent是同一个对象。
然后我们跟进到generateDecor方法里面：
class PhoneWindow{
protected DecorView generateDecor(int featureId) {
// System process doesn't have application context and in that case we need to directly use
// the context we have. Otherwise we want the application context, so we don't cling to the
// activity.
Context context;
if (mUseDecorContext) {
Context applicationContext = getContext().getApplicationContext();
if (applicationContext == null) {
context = getContext();
} else {
context = new DecorContext(applicationContext, getContext());
if (mTheme != -1) {
context.setTheme(mTheme);
}
}
} else {
context = getContext();
}
return new DecorView(context, featureId, this, getAttributes());
}
}
实际上就是生成了一个DecorView对象，我们看看DecorView到底是个什么？
public class DecorView extends FrameLayout implements RootViewSurfaceTaker, WindowCallbacks {
//...
}
可以发现DecorView 实际上是一个FrameLayout，因此installDecor方法就是生成了一个DecorView容器对象，接下来分析：
mContentParent = generateLayout(mDecor);方法
class PhoneWindow{
protected ViewGroup generateLayout(DecorView decor) {
// Apply data from current theme.
TypedArray a = getWindowStyle();
mIsFloating = a.getBoolean(R.styleable.Window_windowIsFloating, false);
int flagsToUpdate = (FLAG_LAYOUT_IN_SCREEN|FLAG_LAYOUT_INSET_DECOR)
& (~getForcedWindowFlags());
if (mIsFloating) {
setLayout(WRAP_CONTENT, WRAP_CONTENT);
setFlags(0, flagsToUpdate);
} else {
setFlags(FLAG_LAYOUT_IN_SCREEN|FLAG_LAYOUT_INSET_DECOR, flagsToUpdate);
}
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);
}
//...
a.getValue(R.styleable.Window_windowMinWidthMajor, mMinWidthMajor);
a.getValue(R.styleable.Window_windowMinWidthMinor, mMinWidthMinor);
if (DEBUG) Log.d(TAG, "Min width minor: " + mMinWidthMinor.coerceToString()
+ ", major: " + mMinWidthMajor.coerceToString());
if (a.hasValue(R.styleable.Window_windowFixedWidthMajor)) {
if (mFixedWidthMajor == null) mFixedWidthMajor = new TypedValue();
a.getValue(R.styleable.Window_windowFixedWidthMajor,
mFixedWidthMajor);
}
if (a.hasValue(R.styleable.Window_windowFixedWidthMinor)) {
if (mFixedWidthMinor == null) mFixedWidthMinor = new TypedValue();
a.getValue(R.styleable.Window_windowFixedWidthMinor,
mFixedWidthMinor);
}
//...
// Inflate the window decor.
int layoutResource;//注意这个地方 ①
int features = getLocalFeatures();
// System.out.println("Features: 0x" + Integer.toHexString(features));
if ((features & (1 << FEATURE_SWIPE_TO_DISMISS)) != 0) {
layoutResource = R.layout.screen_swipe_dismiss;
setCloseOnSwipeEnabled(true);
} else if ((features & ((1 << FEATURE_LEFT_ICON) | (1 << FEATURE_RIGHT_ICON))) != 0) {
if (mIsFloating) {
TypedValue res = new TypedValue();
getContext().getTheme().resolveAttribute(
R.attr.dialogTitleIconsDecorLayout, res, true);
layoutResource = res.resourceId;
} else {
layoutResource = R.layout.screen_title_icons;
}
// XXX Remove this once action bar supports these features.
removeFeature(FEATURE_ACTION_BAR);
// System.out.println("Title Icons!");
} else if ((features & ((1 << FEATURE_PROGRESS) | (1 << FEATURE_INDETERMINATE_PROGRESS))) != 0
&& (features & (1 << FEATURE_ACTION_BAR)) == 0) {
// Special case for a window with only a progress bar (and title).
// XXX Need to have a no-title version of embedded windows.
layoutResource = R.layout.screen_progress;
// System.out.println("Progress!");
} else if ((features & (1 << FEATURE_CUSTOM_TITLE)) != 0) {
// Special case for a window with a custom title.
// If the window is floating, we need a dialog layout
if (mIsFloating) {
TypedValue res = new TypedValue();
getContext().getTheme().resolveAttribute(
R.attr.dialogCustomTitleDecorLayout, res, true);
layoutResource = res.resourceId;
} else {
layoutResource = R.layout.screen_custom_title;
}
// XXX Remove this once action bar supports these features.
removeFeature(FEATURE_ACTION_BAR);
} else if ((features & (1 << FEATURE_NO_TITLE)) == 0) {
// If no other features and not embedded, only need a title.
// If the window is floating, we need a dialog layout
if (mIsFloating) {
TypedValue res = new TypedValue();
getContext().getTheme().resolveAttribute(
R.attr.dialogTitleDecorLayout, res, true);
layoutResource = res.resourceId;
} else if ((features & (1 << FEATURE_ACTION_BAR)) != 0) {
layoutResource = a.getResourceId(
R.styleable.Window_windowActionBarFullscreenDecorLayout,
R.layout.screen_action_bar);
} else {
layoutResource = R.layout.screen_title;
}
// System.out.println("Title!");
} else if ((features & (1 << FEATURE_ACTION_MODE_OVERLAY)) != 0) {
layoutResource = R.layout.screen_simple_overlay_action_mode;
} else {
// Embedded, so no decoration is needed.
layoutResource = R.layout.screen_simple;
// System.out.println("Simple!");
}
mDecor.startChanging();
mDecor.onResourcesLoaded(mLayoutInflater, layoutResource);//注意这里 ②
ViewGroup contentParent = (ViewGroup)findViewById(ID_ANDROID_CONTENT);//注意这里 ③
if (contentParent == null) {
throw new RuntimeException("Window couldn't find content container view");
}
if ((features & (1 << FEATURE_INDETERMINATE_PROGRESS)) != 0) {
ProgressBar progress = getCircularProgressBar(false);
if (progress != null) {
progress.setIndeterminate(true);
}
}
if ((features & (1 << FEATURE_SWIPE_TO_DISMISS)) != 0) {
registerSwipeCallbacks(contentParent);
}
// Remaining setup -- of background and title -- that only applies
// to top-level windows.
if (getContainer() == null) {
final Drawable background;
if (mBackgroundResource != 0) {
background = getContext().getDrawable(mBackgroundResource);
} else {
background = mBackgroundDrawable;
}
mDecor.setWindowBackground(background);
final Drawable frame;
if (mFrameResource != 0) {
frame = getContext().getDrawable(mFrameResource);
} else {
frame = null;
}
mDecor.setWindowFrame(frame);
mDecor.setElevation(mElevation);
mDecor.setClipToOutline(mClipToOutline);
if (mTitle != null) {
setTitle(mTitle);
}
if (mTitleColor == 0) {
mTitleColor = mTextColor;
}
setTitleColor(mTitleColor);
}
mDecor.finishChanging();
return contentParent;//注意这里 ④
}
}
这个方法狠多，我们只捡重点的说几个：
方法①处的含义为，layoutResource的字段赋值是依旧features等字段，这里我们以最后一个else为例，给layoutResource 赋值：
layoutResource = R.layout.screen_simple;
找到screen_simple布局：
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:fitsSystemWindows="true"
android:orientation="vertical">
<ViewStub android:id="@+id/action_mode_bar_stub"
android:inflatedId="@+id/action_mode_bar"
android:layout="@layout/action_mode_bar"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:theme="?attr/actionBarTheme" />
<FrameLayout
android:id="@android:id/content"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:foregroundInsidePadding="false"
android:foregroundGravity="fill_horizontal|top"
android:foreground="?android:attr/windowContentOverlay" />
</LinearLayout>
注意一下FrameLayout的id是@android:id/content，后面会用到。
所以总结一下方法①的作用：根据设置的主题、features等字段的不同找到不同的对应布局给layoutResource赋值。
接下来进到方法②的分析，即：mDecor.onResourcesLoaded(mLayoutInflater, layoutResource);，mDecor就是前面生成的DecorView对象，跟进到onResourcesLoaded方法里面去：
class DecorView{
void onResourcesLoaded(LayoutInflater inflater, int layoutResource) {
if (mBackdropFrameRenderer != null) {
loadBackgroundDrawablesIfNeeded();
mBackdropFrameRenderer.onResourcesLoaded(
this, mResizingBackgroundDrawable, mCaptionBackgroundDrawable,
mUserCaptionBackgroundDrawable, getCurrentColor(mStatusColorViewState),
getCurrentColor(mNavigationColorViewState));
}
mDecorCaptionView = createDecorCaptionView(inflater);
final View root = inflater.inflate(layoutResource, null);//注意这里 ①
if (mDecorCaptionView != null) {
if (mDecorCaptionView.getParent() == null) {
addView(mDecorCaptionView,
new ViewGroup.LayoutParams(MATCH_PARENT, MATCH_PARENT));//注意这里 ②
}
mDecorCaptionView.addView(root,
new ViewGroup.MarginLayoutParams(MATCH_PARENT, MATCH_PARENT));
} else {
// Put it below the color views.
addView(root, 0, new ViewGroup.LayoutParams(MATCH_PARENT, MATCH_PARENT));//注意这里③
}
mContentRoot = (ViewGroup) root;
initializeElevation();
}
}
这个方法有三处需要注意的，方法①：final View root = inflater.inflate(layoutResource, null);
意思就是根据上面赋值的layoutResource然后inflate成一个View对象名称为root，方法②③addView方法，含义就是将生成的root这个View添加到DecorView里面，DecorView是一个FragmeLayout对象。
到此我们也知道了onResourcesLoaded的作用就是将找到的layoutResource布局生成View然后添加到DecorView身上。
接下来回去继续分析PhoneWindow类中的generatorLayout方法中第二处：ViewGroup contentParent = (ViewGroup)findViewById(ID_ANDROID_CONTENT);
ID_ANDROID_CONTENT是Window类中的定义的一个静态常量：
class Window{
/**
* The ID that the main layout in the XML layout file should have.
*/
public static final int ID_ANDROID_CONTENT = com.android.internal.R.id.content;
}
我们可以发现这个id就是之前我们在布局screen_simple里面FrameLayout的id！
所以通过findViewById方法之后找到了contentParent容器，然后返回到调用之处。回到调用之处：
class PhoneWinow{
private void installDecor() {
if (mDecor == null) {
mDecor = generateDecor();//注意这里 方法①
mDecor.setDescendantFocusability(ViewGroup.FOCUS_AFTER_DESCENDANTS);
mDecor.setIsRootNamespace(true);
if (!mInvalidatePanelMenuPosted && mInvalidatePanelMenuFeatures != 0) {
mDecor.postOnAnimation(mInvalidatePanelMenuRunnable);
}
}
if (mContentParent == null) {
mContentParent = generateLayout(mDecor);//注意这里 方法②
}
}
}
继续回到installDecor调用之处：
public class PhoneWindow extends Window implements MenuBuilder.Callback {
//....
@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();//注意这个方法 ①
} 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);//注意这个方法 ②
}
mContentParent.requestApplyInsets();
final Callback cb = getCallback();
if (cb != null && !isDestroyed()) {
cb.onContentChanged();
}
}
//....
}
此时installDecor方法走完，mContentParent也赋值完毕，mContentParent就等于布局screen_simple里面FrameLayout！
接下来走方法②： mLayoutInflater.inflate(layoutResID, mContentParent);这个就比较好理解了，即将传进来的布局layoutResID加入到mContengParent里面，即screen_simple布局中的FrameLayout容器里面！
以上就是View是如何被添加到屏幕窗口全部分析，接下来我们总结一下：
首先，系统会创建一个顶层布局容器DecorView，它是一个ViewGroup，继承FrameLayout,是PhoneWindow持有的一个实例，它(PhoneWindow)是所有应用程序的顶层View，在系统内部初始化。
当DecorView初始化完成之后，系统会根据应用程序的主题特性去加载一个基础容器，比如dart actionbar、noactionbar等主题加载的基础容器也是不同的。但无论如何这个基础容器里面一定会有一个com.android.internal.R.id.content的容器，这个容器就是FragmeLayout。
而我们开发者通过setContentView(R.layout.activity_main)的XML布局文件就是通过解析之后添加到上面的FragmeLayout中。
接下来我们用不同类的形式再汇总说一下：
涉及到的类如下：
MainActivity：
public class MainActivity extends AppCompatActivity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
/**
* 加载XML布局activity_main
*/
setContentView(R.layout.activity_main);
}
}
Activity：
public class Activity {
/**
* MainActivity的setContentView方法会调到这里来
* @param layoutResID 布局id
*/
public void setContentView(@LayoutRes int layoutResID) {
//getWindow()返回PhoneWindow对象，实际会走到PhoneWindow.setContentView方法中
getWindow().setContentView(layoutResID);
initWindowDecorActionBar();
}
/**
* 返回Window对象，实际是PhoneWindow对象
* @return Window对象
*/
public Window getWindow() {
return mWindow;
}
}
Window：
/**
* PhoneWindow是Window的唯一实现
*/
public abstract class Window {
/**
* 这个id是布局文件中FragmeLayout中的id
*/
public static final int ID_ANDROID_CONTENT = com.android.internal.R.id.content;
}
PhoneWindow：
public class PhoneWindow extends Window implements MenuBuilder.Callback {
/**
* MainActivity.setContentView-->Activity.setContentView-->PhoneWindow.setContentView
*
* @param layoutResID 传入的布局id，此处可以理解为R.layout.activity_main
*/
@Override
public void setContentView(int layoutResID) {
if (mContentParent == null) {
/**
* 这个方法作用：
* 1.生成DecorView对象（DecorView是继承FrameLayout）
* 2.初始化mContentParent对象
*/
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 {
/**
*这个方法作用：
* 将传进来的布局加载到mContentParent容器中
*/
mLayoutInflater.inflate(layoutResID, mContentParent);
}
mContentParent.requestApplyInsets();
final Callback cb = getCallback();
if (cb != null && !isDestroyed()) {
cb.onContentChanged();
}
mContentParentExplicitlySet = true;
}
/**
* 里面有两个重要的方法：
* 1. generateDecor：如果mDecor为空，则生成DecorView
* 2. generateLayout：
*/
private void installDecor() {
mForceDecorInstall = false;
if (mDecor == null) {
//生成DecorView,DecorView是继承FrameLayout
mDecor = generateDecor(-1);
mDecor.setDescendantFocusability(ViewGroup.FOCUS_AFTER_DESCENDANTS);
mDecor.setIsRootNamespace(true);
if (!mInvalidatePanelMenuPosted && mInvalidatePanelMenuFeatures != 0) {
mDecor.postOnAnimation(mInvalidatePanelMenuRunnable);
}
} else {
mDecor.setWindow(this);
}
if (mContentParent == null) {
//
mContentParent = generateLayout(mDecor);
}
}
/**
* 生成DecorView 对象
*/
protected DecorView generateDecor(int featureId) {
Context context;
if (mUseDecorContext) {
Context applicationContext = getContext().getApplicationContext();
if (applicationContext == null) {
context = getContext();
} else {
context = new DecorContext(applicationContext, getContext());
if (mTheme != -1) {
context.setTheme(mTheme);
}
}
} else {
context = getContext();
}
return new DecorView(context, featureId, this, getAttributes());
}
/**
*找相应的加载布局，加载到DecorView中，然后返回布局中FragmeLayout对象
* @param decor
* @return 布局中id为ID_ANDROID_CONTENT的容器
*/
protected ViewGroup generateLayout(DecorView decor) {
TypedArray a = getWindowStyle();
//省略...
//解析属性值
if (a.getBoolean(R.styleable.Window_windowNoTitle, false)) {
requestFeature(FEATURE_NO_TITLE);
} else if (a.getBoolean(R.styleable.Window_windowActionBar, false)) {
requestFeature(FEATURE_ACTION_BAR);
}
if (a.getBoolean(R.styleable.Window_windowActionBarOverlay, false)) {
requestFeature(FEATURE_ACTION_BAR_OVERLAY);
}
if (a.getBoolean(R.styleable.Window_windowActionModeOverlay, false)) {
requestFeature(FEATURE_ACTION_MODE_OVERLAY);
}
// 根据features和属性的不同，来给layoutResource赋值，实际就是找相应的加载布局
int layoutResource;
int features = getLocalFeatures();
if ((features & (1 << FEATURE_SWIPE_TO_DISMISS)) != 0) {
layoutResource = R.layout.screen_swipe_dismiss;
setCloseOnSwipeEnabled(true);
} else if {
//省略...
}else{
layoutResource = R.layout.screen_simple;
}
mDecor.startChanging();
//将找到的布局layoutResource加入到DecorView中去
mDecor.onResourcesLoaded(mLayoutInflater, layoutResource);
//找到layoutResource布局中id为ID_ANDROID_CONTENT的控件，然后返回
ViewGroup contentParent = (ViewGroup) findViewById(ID_ANDROID_CONTENT);
//省略...
return contentParent;
}
}
DecorView：
public class DecorView extends FrameLayout implements RootViewSurfaceTaker, WindowCallbacks {
/**
* 根据传入的layoutResource生成View，然后将它加入到DecorView里面
*
* @param inflater 布局生成器
* @param layoutResource 布局id
*/
void onResourcesLoaded(LayoutInflater inflater, int layoutResource) {
if (mBackdropFrameRenderer != null) {
loadBackgroundDrawablesIfNeeded();
mBackdropFrameRenderer.onResourcesLoaded(
this, mResizingBackgroundDrawable, mCaptionBackgroundDrawable,
mUserCaptionBackgroundDrawable, getCurrentColor(mStatusColorViewState),
getCurrentColor(mNavigationColorViewState));
}
mDecorCaptionView = createDecorCaptionView(inflater);
//根据layoutResource生成view对象
final View root = inflater.inflate(layoutResource, null);
if (mDecorCaptionView != null) {
if (mDecorCaptionView.getParent() == null) {
addView(mDecorCaptionView, new ViewGroup.LayoutParams(MATCH_PARENT, MATCH_PARENT));
}
//将生成的布局View加入到DecorView中
mDecorCaptionView.addView(root, new ViewGroup.MarginLayoutParams(MATCH_PARENT, MATCH_PARENT));
} else {
//将生成的布局View加入到DecorView中
addView(root, 0, new ViewGroup.LayoutParams(MATCH_PARENT, MATCH_PARENT));
}
mContentRoot = (ViewGroup) root;
initializeElevation();
}
}
最后画一个图来说一下：
相信看到这里，大家都应该明白View视图是如何被添加到屏幕上了。