本文主要探讨了Android中setContentView的工作原理，从Activity的源码分析了setContentView的重载方法，尤其是setContentView(int layoutResId)。通过一步步跟踪源码，解释了布局文件如何被加载到Activity中，涉及PhoneWindow、DecorView、LayoutInflater等多个关键概念，帮助读者理解XML布局到Activity的加载过程。
每次在Github上看到炫酷效果的demo时，都会想，什么时候自己也能实现这样的效果。要想有所收获，就必须得付出。于是，我决定花时间来学习自定义View，虽说自定义View水很深，但凡事得一步步来，终会有所收获的。要实现自定义View，必须得先了解View的绘制流程，但在学习View的绘制流程之前，先来了解下setContentView这个方法时如何将XML布局加载到当前Activity？
从Activity源码可以看出，setContentView方法有三个重载方法，分别是：
setContentView(int layoutResId)：将布局文件加载到当前Activity，并且在Activity充当父布局角色。
setContentView(View view)：将View加载到当前Activity。当调用这个方法设置View时，自己对View设置布局参数宽高会被忽略，默认的宽高是android.view.ViewGroup.LayoutParams.MATCH_PARENT。如果想使用自己定义的布局参数，可以使用setContentView(View view, ViewGroup.LayoutParams params)这个方法代替。
setContentView(View view, ViewGroup.LayoutParams params)：将View加载到当前Activity。
从源码来看，这个三个重载方法的实现逻辑都差不多，所以我只分析setContentView(int layoutResId).
Step 1
进入到Activity源码，找到setContentView(int layoutResId)方法如下：
public void setContentView(int layoutResID) {
getWindow().setContentView(layoutResID);
initWindowDecorActionBar();只有两行代码，有木有一种挺简单的感觉。其实不然，往下看就知道有多复杂了。从第2行看起，进入getWindow()这个方法，代码如下
public Window getWindow() {
return mWindow;
}获取Activity当前Window对象，Window是抽象类，不能实例化对象，那么是怎么获取到Window对象？在Activity源码中attach方法有这样的一行代码
mWindow = PolicyManager.makeNewWindow(this);很明显，这是获取Window对象的，继续往下看。进入PolicyManager类makeNewWindow()方法
public static Window makeNewWindow(Context context) {
return sPolicy.makeNewWindow(context);
}那么sPolicy又是什么呢，代码如下：
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);
}
}从以上代码可以看出，IPolicy是接口，通过反射机制获取到sPolicy，该接口的实现类是Policy，所以定位到该类makeNewWindow()方法
public Window makeNewWindow(Context context) {
return new PhoneWindow(context);
}终于获取到Window对象，该对象的类型是PhoneWindow。那么PhoneWindow又是啥呢？ 进入PhoneWindow该类一看，原来该类是继承抽象类Window，并重写Window类抽象方法。可以这么理解，Activity的窗口是PhoneWindow。获取到Window对象也就结束了。
Step2
进入PhoneWindow类setContentView(int layoutResId)方法中
@Override
public void setContentView(int layoutResID) {
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);
}
final Callback cb = getCallback();
if (cb != null && !isDestroyed()) {
cb.onContentChanged();
}
}一开始进入就判断mContentParent(类型是ViewGroup)是否为null，由于一开始加载布局，所以mContentParent为null。于是执行if语句，即执行installDecor()方法，于是进入到该方法代码实现
private void installDecor() {
if (mDecor == null) {
mDecor = generateDecor();
mDecor.setDescendantFocusability(ViewGroup.FOCUS_AFTER_DESCENDANTS);
mDecor.setIsRootNamespace(true);
}
if (mContentParent == null) {
mContentParent = generateLayout(mDecor);
mTitleView =(TextView)findViewById(com.android.internal.R.id.title);
if (mTitleView != null) {
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) {
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();
}
// 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() {
if (!isDestroyed()) { invalidatePanelMenu(FEATURE_ACTION_BAR);
}
}
});
}
}
}
}这个方法有点长，慢慢来看吧。（PS：说真的，我不是百分之百理解，大概理解流程而已）。刚进入，就判断mDecor（类型为DecorView）是否为null。由于一开始mDecor为null，所以执行if语句，即执行
mDecor = generateDecor();这里调用generateDecor()方法获取DecorView对象，那么是如何获取呢，进去瞧一瞧
protected DecorView generateDecor() {
return new DecorView(getContext(), -1);
}很容易理解吧，直接new一个对象，就获取到DecorView实例。再回到installDecor()方法中，对于if语句后面的几行代码，主要是对获取到DecorView对象进行设置。接下来看第7行代码，也是if语句，判断mContentParent是否为null，由于一开始进入为null，所以执行if语句，即执行
mContentParent = generateLayout(mDecor);这里将在之前获取到的mDecor作为参数传递到generateLayout()方法中，然后返回ViewGroup赋值给mContentParent。那么该方法是如何实现呢，进入该方法代码实现
protected ViewGroup generateLayout(DecorView decor) {
// Apply data from current theme.
TypedArray a = getWindowStyle();
if (false) {
System.out.println("From style:");
String s = "Attrs:";
for (int i = 0; i < com.android.internal.R.styleable.Window.length; i++) {
s = s + " " + Integer.toHexString(com.android.internal.R.styleable.Window[i]) + "="
+ a.getString(i);
}
System.out.println(s);
}
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);
}
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);
}
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);
}
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_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 (getContext().getApplicationInfo().targetSdkVersion
< android.os.Build.VERSION_CODES.HONEYCOMB) {
addFlags(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;
}
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) {
if ((features & (1 << FEATURE_ACTION_BAR_OVERLAY)) != 0) {
layoutResource = com.android.internal.R.layout.screen_action_bar_overlay;
} else {
layoutResource = com.android.internal.R.layout.screen_action_bar;
}
} else {
layoutResource = com.android.internal.R.layout.screen_title;
}
// System.out.println("Title!");
} else {
// Embedded, so no decoration is needed.
layoutResource = com.android.internal.R.layout.screen_simple;
// System.out.println("Simple!");
}
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);
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;
}这个方法真的很长，我根据自己的理解总结下这个方法的主要功能：
- 设置窗口属性
- 设置对应布局文件layoutResource
- 加载布局文件layoutResource
- 将加载获取到View添加到mDecor
- 设置窗口背景
由于个人能力有限，所以只能简单地分析下。定位到代码
View in = mLayoutInflater.inflate(layoutResource, null);根据之前各种条件获取到对应布局文件id，即layoutResource，再调用LayoutInflater.inflate加载布局获取到对应View（如果想理解LayoutInflater原理，可以参考这篇博客Android LayoutInflater原理分析，带你一步步深入了解View(一)）。接着再看
decor.addView(in, new ViewGroup.LayoutParams(MATCH_PARENT, MATCH_PARENT));把上一步加载布局文件获取到的View添加到mDecor，接着
ViewGroup contentParent =(ViewGroup)findViewById(ID_ANDROID_CONTENT);通过findViewById**(ID_ANDROID_CONTENT = com.android.internal.R.id.content)获取到contentParent，最终将**contentParent作为返回值，赋值给mContentParent（记得id），这样就获取到mContentParent。再回到installDecor()方法中，剩下的逻辑就自己看吧。再回到setContentView(int layoutResId)，有一行关键的代码
mLayoutInflater.inflate(layoutResID, mContentParent);这代码挺熟悉吧，这行代码说明加载XML布局到当前Activity还有一个父布局，那就是mContentParent，它是一个FrameLayout布局，资源id为R.id.content。到此也就了解了XML布局是怎样加载到当前Activity的流程。
以上就是自己对该知识点的理解，把它记录下来作为笔记。若哪里有错误，欢迎指出，大家一起学习，共同进步！
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