Android应用程序窗口View的创建过程

View类是android中非常重要的一个类.view是应用程序界面的直观体现,我们看到的应用程序界面就可以看作是View(视图)组成的.

那么我们应用程序的界面是怎么创建的呢,也就是应用程序的View是什么时候创建的?

在android中与界面直接相关的就是Activity了.

我们平时在Activity的onCreate()函数中,通过调用它的setContentView()函数,将我们应用程序的界面资源设置进去.然后运行程序就可以看到我们布局文件里描述的界面了.

从我们调用setContentView()函数将界面资源设置进去,到运行完成界面完全显示出来,其中经过了很多过程.

这里我主要是通过源码来分析一下其中最终界面到底是什么样的View? 然后分析一下View的measure,layout,draw过程.

因为我们设置界面是setContentView()中设置的,所以就从该函数开始来分析.

在 

分析点击android桌面app图标启动应用程序的过程一文中我们知道Activity的onCreate()函数最先被调用.第五十步  

private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) {
....
Activity activity = null;
try {
java.lang.ClassLoader cl = r.packageInfo.getClassLoader();
activity = mInstrumentation.newActivity(
cl, component.getClassName(), r.intent);
StrictMode.incrementExpectedActivityCount(activity.getClass());
r.intent.setExtrasClassLoader(cl);
if (r.state != null) {
r.state.setClassLoader(cl);
}
} catch (Exception e) {
....
}

try {
Application app = r.packageInfo.makeApplication(false, mInstrumentation);
....
if (activity != null) {
Context appContext = createBaseContextForActivity(r, activity);
CharSequence title = r.activityInfo.loadLabel(appContext.getPackageManager());
Configuration config = new Configuration(mCompatConfiguration);
...
activity.attach(appContext, this, getInstrumentation(), r.token,
r.ident, app, r.intent, r.activityInfo, title, r.parent,
r.embeddedID, r.lastNonConfigurationInstances, config);

if (customIntent != null) {
activity.mIntent = customIntent;
}
r.lastNonConfigurationInstances = null;
activity.mStartedActivity = false;
int theme = r.activityInfo.getThemeResource();
if (theme != 0) {
activity.setTheme(theme);
}

activity.mCalled = false;
mInstrumentation.callActivityOnCreate(activity, r.state);
.....
}

这里首先创建Activity 的实例,然后mInstrumentation.callActivityOnCreate(activity, r.state)该函数最终就会调用Activity的onCreate()函数.

好了,看setContentView()函数

第一步:setContentView()

在frameworks/base/core/java/android/app/Activity.java中

public void setContentView(int layoutResID) {
getWindow().setContentView(layoutResID);
initActionBar();
}
public void setContentView(View view) {
getWindow().setContentView(view);
initActionBar();
}
public void setContentView(View view, ViewGroup.LayoutParams params) {
getWindow().setContentView(view, params);
initActionBar();
}

Activity中setContentView()函数有三个重载函数,一般用第一个比较多,这里就按第一个继续往下分析,其实它们最终实现都一样.

首先看getWindow()函数

第二步:getWindow()

在frameworks/base/core/java/android/app/Activity.java中

public Window getWindow() {
return mWindow;
}

Activity的成员变量mWindow是Window类型,它是什么时候被赋值的呢?

这里还是到分析点击android桌面app图标启动应用程序的过程一文中第五十步 看看

private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) {
....
Activity activity = null;
try {
java.lang.ClassLoader cl = r.packageInfo.getClassLoader();
activity = mInstrumentation.newActivity(
cl, component.getClassName(), r.intent);
StrictMode.incrementExpectedActivityCount(activity.getClass());
r.intent.setExtrasClassLoader(cl);
if (r.state != null) {
r.state.setClassLoader(cl);
}
} catch (Exception e) {
...
}

try {
Application app = r.packageInfo.makeApplication(false, mInstrumentation);
...
if (activity != null) {
Context appContext = createBaseContextForActivity(r, activity);
CharSequence title = r.activityInfo.loadLabel(appContext.getPackageManager());
Configuration config = new Configuration(mCompatConfiguration);
...
activity.attach(appContext, this, getInstrumentation(), r.token,
r.ident, app, r.intent, r.activityInfo, title, r.parent,
r.embeddedID, r.lastNonConfigurationInstances, config);

....
}

这里创建Activity的实例后,就通过activity.attach()函数给activity内部变量赋值,所以进attach()函数里面看看

第三步:attach()

在frameworks/base/core/java/android/app/Activity.java中

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) {
...
mWindow = PolicyManager.makeNewWindow(this);//
....
}
mWindow = PolicyManager.makeNewWindow(this);这里就是给activity中mWindow赋值.那继续看PolicyManager.makeNewWindow(this)这个函数
第四步:makeNewWindow()

在frameworks/base/core/java/com/android/internal/policyPolicyManager.java中

// The static methods to spawn new policy-specific objects
public static Window makeNewWindow(Context context) {
return sPolicy.makeNewWindow(context);
}

<pre name="code" class="java">

public final class PolicyManager {
private static final String POLICY_IMPL_CLASS_NAME =
"com.android.internal.policy.impl.Policy";
private static final IPolicy sPolicy;
static {
// Pull in the actual implementation of the policy at run-time
try {
Class policyClass = Class.forName(POLICY_IMPL_CLASS_NAME);
sPolicy = (IPolicy)policyClass.newInstance();
} catch (ClassNotFoundException ex) {
....
}

这里继续调用sPolicy.makeNewWindow(context);由上面代码可以知道这里的sPolicy其实是Policy类型

第五步:makeNewWindow()

在frameworks/base/policy/src/com/android/internal/policy/impl/Policy.java中

public class Policy implements IPolicy {
.
public Window makeNewWindow(Context context) {
return new PhoneWindow(context);
}

这里直接new PhoneWindow(context)返回,可知PhoneWindow类是Window类的子类,进入PhoneWindow类看看
第六步:PhoneWindow()

在frameworks/base/policy/src/com/android/internal/policy/impl/PhoneWindow.java中

public class PhoneWindow extends Window implements MenuBuilder.Callback {
.
public PhoneWindow(Context context) {
super(context);
mLayoutInflater = LayoutInflater.from(context);
}来看一下PhoneWindow类的构造函数,首先是调用了其父类(Window)的构造函数,然后创建了一个LayoutInflater对象mLayoutInflater,这个对象根据它的名字大概可以知道它是渲染布局资源的
去Window类的构造函数看看

第七步:Window()

在frameworks/base/core/java/android/view/Window.java中
public Window(Context context) {
mContext = context;
}回到第二步中,这是我们知道getWindow()返回其实是一个PhoneWindow对象,即Activity的成员变量mWindow是PhoneWindow类型.
然后回到第一步中,那么接着其实是调用PhoneWindow.setContentView()了

第八步:setContentView()

在frameworks/base/policy/src/com/android/internal/policy/impl/PhoneWindow.java中
public void setContentView(int layoutResID) {
if (mContentParent == null) {
installDecor();
} else {
mContentParent.removeAllViews();
}
mLayoutInflater.inflate(layoutResID, mContentParent);
final Callback cb = getCallback();
if (cb != null && !isDestroyed()) {
cb.onContentChanged();
}
}
PhoneWindow的成员变量mContentParent是ViewGroup类型,第一次进来为null,所以调用installDecor()函数,那我们首先看看该函数

第九步:installDecor()

在frameworks/base/policy/src/com/android/internal/policy/impl/PhoneWindow.java中
private void installDecor() {
if (mDecor == null) {
mDecor = generateDecor();
mDecor.setDescendantFocusability(ViewGroup.FOCUS_AFTER_DESCENDANTS);
...
}
if (mContentParent == null) {
mContentParent = generateLayout(mDecor);

// Set up decor part of UI to ignore fitsSystemWindows if appropriate.
mDecor.makeOptionalFitsSystemWindows();

mTitleView = (TextView)findViewById(com.android.internal.R.id.title);
if (mTitleView != null) {
mTitleView.setLayoutDirection(mDecor.getLayoutDirection());
if ((getLocalFeatures() & (1 << FEATURE_NO_TITLE)) != 0) {
View titleContainer = findViewById(com.android.internal.R.id.title_container);
if (titleContainer != null) {
titleContainer.setVisibility(View.GONE);
} else {
mTitleView.setVisibility(View.GONE);
}
if (mContentParent instanceof FrameLayout) {
((FrameLayout)mContentParent).setForeground(null);
}
} else {
mTitleView.setText(mTitle);
}
} else {
mActionBar = (ActionBarView) findViewById(com.android.internal.R.id.action_bar);
if (mActionBar != null) {
mActionBar.setWindowCallback(getCallback());
if (mActionBar.getTitle() == null) {
mActionBar.setWindowTitle(mTitle);
}
final int localFeatures = getLocalFeatures();
if ((localFeatures & (1 << FEATURE_PROGRESS)) != 0) {
mActionBar.initProgress();
}
if ((localFeatures & (1 << FEATURE_INDETERMINATE_PROGRESS)) != 0) {
mActionBar.initIndeterminateProgress();
}

boolean splitActionBar = false;
final boolean splitWhenNarrow =
(mUiOptions & ActivityInfo.UIOPTION_SPLIT_ACTION_BAR_WHEN_NARROW) != 0;
if (splitWhenNarrow) {
splitActionBar = getContext().getResources().getBoolean(
com.android.internal.R.bool.split_action_bar_is_narrow);
} else {
splitActionBar = getWindowStyle().getBoolean(
com.android.internal.R.styleable.Window_windowSplitActionBar, false);
}
final ActionBarContainer splitView = (ActionBarContainer) findViewById(
com.android.internal.R.id.split_action_bar);
if (splitView != null) {
mActionBar.setSplitView(splitView);
mActionBar.setSplitActionBar(splitActionBar);
mActionBar.setSplitWhenNarrow(splitWhenNarrow);

final ActionBarContextView cab = (ActionBarContextView) findViewById(
com.android.internal.R.id.action_context_bar);
cab.setSplitView(splitView);
cab.setSplitActionBar(splitActionBar);
cab.setSplitWhenNarrow(splitWhenNarrow);
} else if (splitActionBar) {
Log.e(TAG, "Requested split action bar with " +
"incompatible window decor! Ignoring request.");
}
.
}
}
}
}
PhoneWindow的成员变量mDecor是DecorView类型,看一下DecorView的类图结构



由上图可知,DecorView也是View,其实这个DecorView也是应用程序窗口根View.
第一次进来mDecor为null,所以会执行下面:

mDecor = generateDecor();
mDecor.setDescendantFocusability(ViewGroup.FOCUS_AFTER_DESCENDANTS);//焦点先交给mDecor的子view处理,如果子View没有处理自己再处理
首先来看一下generateDecor()这个函数

第十步:generateDecor()

在frameworks/base/policy/src/com/android/internal/policy/impl/PhoneWindow.java中
protected DecorView generateDecor() {
return new DecorView(getContext(), -1);
}这里就直接新建了DecorView实例
回到第九步中,mContentParent == null成立,所以执行:
mContentParent = generateLayout(mDecor);进入generateLayout(mDecor)函数看看,传进去的参数就是第十步创建的DecorView对象

第十一步:generateLayout()

在frameworks/base/policy/src/com/android/internal/policy/impl/PhoneWindow.java中
protected ViewGroup generateLayout(DecorView decor) {
....
mDecor.startChanging();

View in = mLayoutInflater.inflate(layoutResource, null);
decor.addView(in, new ViewGroup.LayoutParams(MATCH_PARENT, MATCH_PARENT));

ViewGroup contentParent = (ViewGroup)findViewById(ID_ANDROID_CONTENT);
...
return contentParent;
}这个函数内容还是挺多的,不过不难,首先收集在清单配置文件中,给该activity配置的theme属性值,然后根据这些属性值去加载系统的布局文件,设置这些theme属性值也可以在代码中设置,不过要setContentView()之前,不过就不起作用了.
else {
// Embedded, so no decoration is needed.
layoutResource = com.android.internal.R.layout.screen_simple;
// System.out.println("Simple!");
}这里假设需要加载的布局文件id是com.android.internal.R.layout.screen_simple,系统的布局文件在frameworks/base/core/res/res/layout/目录下,



进去screen_simple.xml看一下
<?xml version="1.0" encoding="utf-8"?>
<!--
/* //device/apps/common/assets/res/layout/screen_simple.xml
**
** Copyright 2006, The Android Open Source Project
**
** Licensed under the Apache License, Version 2.0 (the "License");
** you may not use this file except in compliance with the License.
** You may obtain a copy of the License at
**
** http://www.apache.org/licenses/LICENSE-2.0 **
** Unless required by applicable law or agreed to in writing, software
** distributed under the License is distributed on an "AS IS" BASIS,
** WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
** See the License for the specific language governing permissions and
** limitations under the License.
*/

This is an optimized layout for a screen, with the minimum set of features
enabled.
-->

<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" />
<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>有一个id为content的控件,这个很关键.
在确定好加载哪个系统布局文件后,接下来:
View in = mLayoutInflater.inflate(layoutResource, null);
decor.addView(in, new ViewGroup.LayoutParams(MATCH_PARENT, MATCH_PARENT));mLayoutInflater在第六步创建的,这里将一个布局文件渲染成一个View,这个view的具体类型就是布局文件的根节点的对象类型,像上面的screen_simple.xml它的根节点就是LinearLayout.
接着将这个渲染成的LinearLayout添加到decor中,因为DecorView是ViewGroup类型,能添加子view.
接着往下看:
ViewGroup contentParent = (ViewGroup)findViewById(ID_ANDROID_CONTENT);
这里ID_ANDROID_CONTENT为com.android.internal.R.id.content,我们进到findViewById()函数进去看看

第十二步:findViewById()

在frameworks/base/policy/src/com/android/internal/policy/impl/PhoneWindow.java中
public View findViewById(int id) {
return getDecorView().findViewById(id);
}

public final View getDecorView() {
if (mDecor == null) {
installDecor();
}
return mDecor;
}

这个函数就去查找decorView中id为com.android.internal.R.id.content的子view,在上面的布局文件中就是一个FrameLayout了,所以说系统布局文件要有一个id为content的控件.
好了,回到第十一步,找到了这控件后就返回到第九步中,将它赋值给了mContentParent.
现在整理一下思路,mDecor赋值了,mContentParent也赋值了,它们的关系是:



回到第九步,继续往下分析:
mTitleView = (TextView)findViewById(com.android.internal.R.id.title);
if (mTitleView != null) {
mTitleView.setLayoutDirection(mDecor.getLayoutDirection());
if ((getLocalFeatures() & (1 << FEATURE_NO_TITLE)) != 0) {
View titleContainer = findViewById(com.android.internal.R.id.title_container);
if (titleContainer != null) {
titleContainer.setVisibility(View.GONE);
} else {
mTitleView.setVisibility(View.GONE);
}
if (mContentParent instanceof FrameLayout) {
((FrameLayout)mContentParent).setForeground(null);
}
} else {
mTitleView.setText(mTitle);
}
} else {
mActionBar = (ActionBarView) findViewById(com.android.internal.R.id.action_bar);
if (mActionBar != null) {
mActionBar.setWindowCallback(getCallback());
if (mActionBar.getTitle() == null) {
mActionBar.setWindowTitle(mTitle);
}
final int localFeatures = getLocalFeatures();
if ((localFeatures & (1 << FEATURE_PROGRESS)) != 0) {
mActionBar.initProgress();
}
if ((localFeatures & (1 << FEATURE_INDETERMINATE_PROGRESS)) != 0) {
mActionBar.initIndeterminateProgress();
}

boolean splitActionBar = false;
final boolean splitWhenNarrow =
(mUiOptions & ActivityInfo.UIOPTION_SPLIT_ACTION_BAR_WHEN_NARROW) != 0;
if (splitWhenNarrow) {
splitActionBar = getContext().getResources().getBoolean(
com.android.internal.R.bool.split_action_bar_is_narrow);
} else {
splitActionBar = getWindowStyle().getBoolean(
com.android.internal.R.styleable.Window_windowSplitActionBar, false);
}
final ActionBarContainer splitView = (ActionBarContainer) findViewById(
com.android.internal.R.id.split_action_bar);
if (splitView != null) {
mActionBar.setSplitView(splitView);
mActionBar.setSplitActionBar(splitActionBar);
mActionBar.setSplitWhenNarrow(splitWhenNarrow);

final ActionBarContextView cab = (ActionBarContextView) findViewById(
com.android.internal.R.id.action_context_bar);
cab.setSplitView(splitView);
cab.setSplitActionBar(splitActionBar);
cab.setSplitWhenNarrow(splitWhenNarrow);
} else if (splitActionBar) {
Log.e(TAG, "Requested split action bar with " +
"incompatible window decor! Ignoring request.");
}


PhoneWindow类的成员函数installDecor()还会检查前面加载的窗口布局文件是否包含有一个id值为“title”的TextView控件。如果包含有的话，就会将它保存在PhoneWindow类的成员变量mTitleView中，用来描述当前应用程序窗口的标题栏。但是，如果当前应用程序窗口是没有标题栏的，即它的Feature位FEATURE_NO_TITLE的值等于1，那么PhoneWindow类的成员函数installDecor()就需要将前面得到的标题栏隐藏起来。注意，PhoneWindow类的成员变量mTitleView所描述的标题栏有可能是包含在一个id值为“title_container”的容器里面的，在这种情况下，就需要隐藏该标题栏容器。另一方面，如果当前应用程序窗口是设置有标题栏的，那么PhoneWindow类的成员函数installDecor就会设置它的标题栏文字。应用程序窗口的标题栏文字保存在PhoneWindow类的成员变量mTitle中，我们可以调用PhoneWindow类的成员函数setTitle来设置.如果没有id值为“title”的TextView控件,就去检查前面加载的窗口布局文件是否包含有一个id值为“action_bar”的ActionBarView控件.下面就不分析了.

回到第八步setContentView()函数里,
往下接着到
mLayoutInflater.inflate(layoutResID, mContentParent);这里的layoutResID,是在activity的onCreate()方法里面,通过setContentView()设置的应用程序的窗口布局资源id.
这里mLayoutInflater.inflate()方法,将应用程序的窗口布局资源渲染成一个view,然后添加到mContentParent这个ViewGroup中.
所以应用程序窗口的界面的View结构如下:



平时我们写应用,只需要写上图中setContentView()的布局就可以,其他android已经实现好了.

好了,应用程序窗口的布局结构就分析完了.我们知道一个应用程序的窗口的显示区域,其实就是DecorView及其包含的子view.

设置好应用程序的布局文件后,就要将DecorView包含内容渲染显示到屏幕上了.

至于如何渲染不打算分析了.

在显示出来之前,DecorView还要经过measure(测量),layout(布局),draw(绘制)三个过程.后面打算分析源码,对这三个过程加一分析下.