一.概述 在Activity中，我们基本都会用到setContentView方法，这个方法是干啥的想必大家都知道，把我们写好的布局文件显示到界面上。今天我们就去看看底层的源码，分析一下到底是如何实现的。 二.分析
1.Activity$getWindow()
public class MainActivity extends Activity {
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main)；
}我们先点进去MainActivity 中的setContentView方法，可以进入到Activity中，看到如下的代码：
public void setContentView(@LayoutRes int layoutResID) {
getWindow().setContentView(layoutResID);
initWindowDecorActionBar();
}在Activity的setContentView中调用了两个方法，我们只分析第一个，我们看看getWindow()返回的是一个什么东西，
public Window getWindow() {
return mWindow;
}是一个Window对象mWindow,大家注意，Window是一个抽象类，并且Window中的setContentView是一个抽象方法，那么Activity中肯定就有一个Window抽象类的实现，我们查找代码发现 在Activity的attach方法中为mWindow对象赋值：
mWindow = PolicyManager.makeNewWindow(this);
我们可以看到这里调用了PolicyManager中的静态方法makeNewWindow，我们继续点进去，
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) {
throw new RuntimeException(
POLICY_IMPL_CLASS_NAME + " could not be loaded", ex);
} catch (InstantiationException ex) {
throw new RuntimeException(
POLICY_IMPL_CLASS_NAME + " could not be instantiated", ex);
} catch (IllegalAccessException ex) {
throw new RuntimeException(
POLICY_IMPL_CLASS_NAME + " could not be instantiated", ex);
}
}
// Cannot instantiate this class
private PolicyManager() {}
// The static methods to spawn new policy-specific objects
public static Window makeNewWindow(Context context) {
return sPolicy.makeNewWindow(context);
}我们看到其实是调用了sPolicy对象的makeNewWindow方法，那么这个sPolicy对象是什么呢？很明显的可以看到是在上面通过反射得到的Policy的一个对象，所以我们接下来进入到Policy这个类中，寻找makeNewWindow这个方法，我们看到如下的代码，
public Window makeNewWindow(Context context) {
return new PhoneWindow(context);
}最终返回的是PhoneWindow这样一个对象，通过着一系列的查找，也就是说Activity中的getWindow()方法得到的其实是一个PhoneWindow对象，其实PhoneWindow是Window的唯一实现类，我们在进入到Window这个类的源码中的时候官方已经告诉我们了，
* <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 {
2.PhoneWindow$setContentView
由于我们平时写的setContentView最终是调用了PhoneWindow里面的
setContentView，所以我们看看这里面的代码，
@Override
public void setContentView(int layoutResID) {
if (mContentParent == null) {
installDecor();
} else {
mContentParent.removeAllViews();
}
//将我们的布局文件添加到mContentParent中，这个mContentParent是根布局中id为content的一个FrameLayout
mLayoutInflater.inflate(layoutResID, mContentParent);
final Callback cb = getCallback();
if (cb != null && !isDestroyed()) {
cb.onContentChanged();
}
}
首先判断mContentParent是否为null，mContentParent是什么呢？接下来会分析，一开始条件成立，进入installDecor()方法。
private void installDecor() {
if (mDecor == null) {
///创建DecorView
mDecor = generateDecor();
mDecor.setDescendantFocusability(ViewGroup.FOCUS_AFTER_DESCENDANTS);
mDecor.setIsRootNamespace(true);
if (!mInvalidatePanelMenuPosted && mInvalidatePanelMenuFeatures != 0) {
mDecor.postOnAnimation(mInvalidatePanelMenuRunnable);
}
}
if (mContentParent == null) {
// 找到系统默认的布局文件，添加到DecorView中，返回该布局文件中id为content的一个容器，我们通过setContentView设置的布局就是添加到这个容器中
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.");
}
// Post the panel invalidate for later; avoid application onCreateOptionsMenu
// being called in the middle of onCreate or similar.
mDecor.post(new Runnable() {
public void run() {
// Invalidate if the panel menu hasn't been created before this.
PanelFeatureState st = getPanelState(FEATURE_OPTIONS_PANEL, false);
if (!isDestroyed() && (st == null || st.menu == null)) {
invalidatePanelMenu(FEATURE_ACTION_BAR);
}
}
});
}
}
}在代码的第三行，我们看到
mDecor = generateDecor();
我们进入generateDecor这个方法，
protected DecorView generateDecor() {
return new DecorView(getContext(), -1);
}这里创建了一个DecorView对象，DecorView是PhoneWindow里面的一个内部类，继承自FrameLayout,到目前为止，setContentView
方法里生成一个FrameLayout类型的DecorView组件,我们继续向下看，有这样的代码
if (mContentParent == null) {
mContentParent = generateLayout(mDecor);把 DecorView 对象 mDecor 作为参数传递给 generateLayout方法得到 mContentParent。generateLayout()方法中的代码实现如下：
protected ViewGroup generateLayout(DecorView decor) {
// Apply data from current theme.
TypedArray a = getWindowStyle();
//省去一些代码
/**以下这些是Activity 窗口属性特征的设置*/
//窗口是否浮动，一般用于Dialog窗口是否浮动：是否显示在布局的正中间。
mIsFloating = a.getBoolean(com.android.internal.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);
}
//设置窗口是否支持标题栏，隐藏显示标题栏操作在此处。
if (a.getBoolean(com.android.internal.R.styleable.Window_windowNoTitle, false)) {
requestFeature(FEATURE_NO_TITLE);
} else if (a.getBoolean(com.android.internal.R.styleable.Window_windowActionBar, false)) {
// Don't allow an action bar if there is no title.
requestFeature(FEATURE_ACTION_BAR);
}
//ActionBar导航栏是否不占布局空间叠加显示在当前窗口之上。
if (a.getBoolean(com.android.internal.R.styleable.Window_windowActionBarOverlay, false)) {
requestFeature(FEATURE_ACTION_BAR_OVERLAY);
}
if (a.getBoolean(com.android.internal.R.styleable.Window_windowActionModeOverlay, false)) {
requestFeature(FEATURE_ACTION_MODE_OVERLAY);
}
//当前Activity是否支持全屏
if (a.getBoolean(com.android.internal.R.styleable.Window_windowFullscreen, false)) {
setFlags(FLAG_FULLSCREEN, FLAG_FULLSCREEN & (~getForcedWindowFlags()));
}
if (a.getBoolean(com.android.internal.R.styleable.Window_windowOverscan, false)) {
setFlags(FLAG_LAYOUT_IN_OVERSCAN, FLAG_LAYOUT_IN_OVERSCAN&(~getForcedWindowFlags()));
}
if (a.getBoolean(com.android.internal.R.styleable.Window_windowShowWallpaper, false)) {
setFlags(FLAG_SHOW_WALLPAPER, FLAG_SHOW_WALLPAPER&(~getForcedWindowFlags()));
}
if (a.getBoolean(com.android.internal.R.styleable.Window_windowEnableSplitTouch,
getContext().getApplicationInfo().targetSdkVersion
>= android.os.Build.VERSION_CODES.HONEYCOMB)) {
setFlags(FLAG_SPLIT_TOUCH, FLAG_SPLIT_TOUCH&(~getForcedWindowFlags()));
}
a.getValue(com.android.internal.R.styleable.Window_windowMinWidthMajor, mMinWidthMajor);
a.getValue(com.android.internal.R.styleable.Window_windowMinWidthMinor, mMinWidthMinor);
if (a.hasValue(com.android.internal.R.styleable.Window_windowFixedWidthMajor)) {
if (mFixedWidthMajor == null) mFixedWidthMajor = new TypedValue();
a.getValue(com.android.internal.R.styleable.Window_windowFixedWidthMajor,
mFixedWidthMajor);
}
if (a.hasValue(com.android.internal.R.styleable.Window_windowFixedWidthMinor)) {
if (mFixedWidthMinor == null) mFixedWidthMinor = new TypedValue();
a.getValue(com.android.internal.R.styleable.Window_windowFixedWidthMinor,
mFixedWidthMinor);
}
if (a.hasValue(com.android.internal.R.styleable.Window_windowFixedHeightMajor)) {
if (mFixedHeightMajor == null) mFixedHeightMajor = new TypedValue();
a.getValue(com.android.internal.R.styleable.Window_windowFixedHeightMajor,
mFixedHeightMajor);
}
if (a.hasValue(com.android.internal.R.styleable.Window_windowFixedHeightMinor)) {
if (mFixedHeightMinor == null) mFixedHeightMinor = new TypedValue();
a.getValue(com.android.internal.R.styleable.Window_windowFixedHeightMinor,
mFixedHeightMinor);
}
final Context context = getContext();
final int targetSdk = context.getApplicationInfo().targetSdkVersion;
final boolean targetPreHoneycomb = targetSdk < android.os.Build.VERSION_CODES.HONEYCOMB;
final boolean targetPreIcs = targetSdk < android.os.Build.VERSION_CODES.ICE_CREAM_SANDWICH;
final boolean targetHcNeedsOptions = context.getResources().getBoolean(
com.android.internal.R.bool.target_honeycomb_needs_options_menu);
final boolean noActionBar = !hasFeature(FEATURE_ACTION_BAR) || hasFeature(FEATURE_NO_TITLE);
if (targetPreHoneycomb || (targetPreIcs && targetHcNeedsOptions && noActionBar)) {
addFlags(WindowManager.LayoutParams.FLAG_NEEDS_MENU_KEY);
} else {
clearFlags(WindowManager.LayoutParams.FLAG_NEEDS_MENU_KEY);
}
if (mAlwaysReadCloseOnTouchAttr || getContext().getApplicationInfo().targetSdkVersion
>= android.os.Build.VERSION_CODES.HONEYCOMB) {
if (a.getBoolean(
com.android.internal.R.styleable.Window_windowCloseOnTouchOutside,
false)) {
setCloseOnTouchOutsideIfNotSet(true);
}
}
WindowManager.LayoutParams params = getAttributes();
if (!hasSoftInputMode()) {
params.softInputMode = a.getInt(
com.android.internal.R.styleable.Window_windowSoftInputMode,
params.softInputMode);
}
if (a.getBoolean(com.android.internal.R.styleable.Window_backgroundDimEnabled,
mIsFloating)) {
/* All dialogs should have the window dimmed */
if ((getForcedWindowFlags()&WindowManager.LayoutParams.FLAG_DIM_BEHIND) == 0) {
params.flags |= WindowManager.LayoutParams.FLAG_DIM_BEHIND;
}
if (!haveDimAmount()) {
params.dimAmount = a.getFloat(
android.R.styleable.Window_backgroundDimAmount, 0.5f);
}
}
if (params.windowAnimations == 0) {
params.windowAnimations = a.getResourceId(
com.android.internal.R.styleable.Window_windowAnimationStyle, 0);
}
// The rest are only done if this window is not embedded; otherwise,
// the values are inherited from our container.
if (getContainer() == null) {
if (mBackgroundDrawable == null) {
if (mBackgroundResource == 0) {
mBackgroundResource = a.getResourceId(
com.android.internal.R.styleable.Window_windowBackground, 0);
}
if (mFrameResource == 0) {
mFrameResource = a.getResourceId(com.android.internal.R.styleable.Window_windowFrame, 0);
}
if (false) {
System.out.println("Background: "
+ Integer.toHexString(mBackgroundResource) + " Frame: "
+ Integer.toHexString(mFrameResource));
}
}
mTextColor = a.getColor(com.android.internal.R.styleable.Window_textColor, 0xFF000000);
}
// Inflate the window decor.
int layoutResource;
int features = getLocalFeatures();
// System.out.println("Features: 0x" + Integer.toHexString(features));
if ((features & ((1 << FEATURE_LEFT_ICON) | (1 << FEATURE_RIGHT_ICON))) != 0) {
if (mIsFloating) {
TypedValue res = new TypedValue();
getContext().getTheme().resolveAttribute(
com.android.internal.R.attr.dialogTitleIconsDecorLayout, res, true);
layoutResource = res.resourceId;
} else {
layoutResource = com.android.internal.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 = com.android.internal.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(
com.android.internal.R.attr.dialogCustomTitleDecorLayout, res, true);
layoutResource = res.resourceId;
} else {
layoutResource = com.android.internal.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(
com.android.internal.R.attr.dialogTitleDecorLayout, res, true);
layoutResource = res.resourceId;
} else if ((features & (1 << FEATURE_ACTION_BAR)) != 0) {
layoutResource = com.android.internal.R.layout.screen_action_bar;
} else {
layoutResource = com.android.internal.R.layout.screen_title;
}
// System.out.println("Title!");
} else if ((features & (1 << FEATURE_ACTION_MODE_OVERLAY)) != 0) {
layoutResource = com.android.internal.R.layout.screen_simple_overlay_action_mode;
} else {
//默认布局文件.
layoutResource =
com.android.internal.R.layout.screen_simple;
// System.out.println("Simple!");
}
mDecor.startChanging();
//通过布局添加器LayoutInflater获取layoutResource布局，
View in = mLayoutInflater.inflate(layoutResource, null);
//将XML资源为layoutResource的布局添加到decor容器里面，至此PhoneWindow 内部类DecorView就添加了子布局
decor.addView(in, new ViewGroup.LayoutParams(MATCH_PARENT, MATCH_PARENT));
//此处很重要，通过findViewById找到 contentParent容器，也是该方法的返回值。
ViewGroup contentParent = (ViewGroup)findViewById(ID_ANDROID_CONTENT);
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);
}
}
// Remaining setup -- of background and title -- that only applies
// to top-level windows.
if (getContainer() == null) {
Drawable drawable = mBackgroundDrawable;
if (mBackgroundResource != 0) {
drawable = getContext().getResources().getDrawable(mBackgroundResource);
}
mDecor.setWindowBackground(drawable);
drawable = null;
if (mFrameResource != 0) {
drawable = getContext().getResources().getDrawable(mFrameResource);
}
mDecor.setWindowFrame(drawable);
// System.out.println("Text=" + Integer.toHexString(mTextColor) +
// " Sel=" + Integer.toHexString(mTextSelectedColor) +
// " Title=" + Integer.toHexString(mTitleColor));
if (mTitleColor == 0) {
mTitleColor = mTextColor;
}
if (mTitle != null) {
setTitle(mTitle);
}
setTitleColor(mTitleColor);
}
mDecor.finishChanging();
return contentParent;
}以上代码代比较复杂，主要做了以下几件事情。
(1)初始化窗口的特征，是否显示标题栏，是否全屏，是否支持ActionBar浮动等等。
(2)通过LayoutInflater 将默认根xml布局转换成view
(3)将找到的View添加到DecorView根布局当中。
(4)从根布局中找到id为R.id.content的 contentParent 容器。也就是当前方法的返回值。
接下来我们看看这个com.android.internal.R.layout.screen_simple布局文件
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="vertical"
android:fitsSystemWindows="true">
<!-- Popout bar for action modes -->
<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/title_container"
android:layout_width="match_parent"
android:layout_height="?android:attr/windowTitleSize"
style="?android:attr/windowTitleBackgroundStyle">
</FrameLayout>
<FrameLayout android:id="@android:id/content"
android:layout_width="match_parent"
android:layout_height="0dip"
android:layout_weight="1"
android:foregroundGravity="fill_horizontal|top"
android:foreground="?android:attr/windowContentOverlay" />
</LinearLayout>
我们的DecorView就添加了上面的布局，线性布局LinearLaout里包含两个组件，ViewStub是懒加载，默认不显示，FrameLayout是什么呢？看看id=content，就是我们下面这行找到的contentParent
ViewGroup contentParent = (ViewGroup)findViewById(ID_ANDROID_CONTENT);
那么这个父容器 contentParent有什么作用呢？
我们回到下面的两行代码，都是之前出现过的
//获得父容器
mContentParent = generateLayout(mDecor);
mLayoutInflater.inflate(layoutResID, mContentParent);
这里通过LayoutInflater将 setContentView（layoutResID）传进来的布局id加载到 父容器mContentParent中，至此，setContentView就将布局添加到Activity里面了。
现在我们来梳理一下流程：
Activity setContentView—>Window setContentView—>PhoneWindow setContentView—->PhoneWindow installDecor—–>PhoneWindow generateLayout——>PhoneWindow mLayoutInflater.inflate(layoutResID, mContentParent);
Activity 类中有一个Window抽象类的实现PhoneWindow类，该类中有个内部类DecorView，继承自FrameLayout，在DecorView容器中添加了根布局，根布局中包含了一个id为 contnet的FrameLayout 内容布局，我们的Activity加载的布局xml最后添加到 id为content的FrameLayout布局当中了。用一个图来描述，如下：
