在Activity中，我们都会使用该方法设置自己需要的xml布局，比如：

@Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(getLayoutId());
    }

下面分析一下，自己写的布局，是怎么通过setContentView()来生效的。

首先放一张Android布局图：

Android窗口布局.png

其实我们平常写的layout布局，是加载到上图红色的框(ContentView)中。

• Window是一个抽象类，定义了一些规范(API)，其中就有setContentView方法

public abstract class Window{
  ...
 /**
     * Convenience for
     * {@link #setContentView(View, android.view.ViewGroup.LayoutParams)}
     * to set the screen content from a layout resource.  The resource will be
     * inflated, adding all top-level views to the screen.
     *
     * @param layoutResID Resource ID to be inflated.
     * @see #setContentView(View, android.view.ViewGroup.LayoutParams)
     */
    public abstract void setContentView(@LayoutRes int layoutResID);

  ...
}

• PhoneWindow 继承自 Window，属于window的具体实现类。
• DecorView 继承自 FrameLayout ，是所有应用窗口的根View。其中包含一个显示标题的TitleView，和一个包含内容的ContentView

public class DecorView extends FrameLayout

源码分析

1，首先从我们经常设置布局的setContentView()分析：

@Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(getLayoutId());
    }

这里的setContentView最终调用的是Activity的setContentView。
2，Activity.java 中的setContentView

public void setContentView(@LayoutRes int layoutResID) {
        //这里的getWindow() , 就是拿到了mWindow 对象
        //并调用了window的setContentView方法
        getWindow().setContentView(layoutResID);
        initWindowDecorActionBar();
    }

在Activity中，mWindow 是由下面的的代码创建：

final void attach(Context context, ActivityThread aThread,
        ...
        //在这里创建了PhoneWindow，
        mWindow = new PhoneWindow(this, window, activityConfigCallback);
        mWindow.setWindowControllerCallback(this);
        mWindow.setCallback(this);
        mWindow.setOnWindowDismissedCallback(this);
        mWindow.getLayoutInflater().setPrivateFactory(this);
        ...

因为PhoneView是Window的具体实现类，所以其实调用的是PhoneView中的setContentView

3，PhoneWindow.java 中的 setContentView方法：

@Override
    public void setContentView(int layoutResID) {
        // mContentParent，也就是我们设置Layout的父布局
        if (mContentParent == null) {
            installDecor();//注意该方法
        } else if (!hasFeature(FEATURE_CONTENT_TRANSITIONS)) {
            mContentParent.removeAllViews();
        }

        if (hasFeature(FEATURE_CONTENT_TRANSITIONS)) {
            final Scene newScene = Scene.getSceneForLayout(mContentParent, layoutResID,
                    getContext());
            transitionTo(newScene);
        } else {
            //在这里进行布局初始化，mContentParent 是我们传入布局的父布局！
            mLayoutInflater.inflate(layoutResID, mContentParent);
        }
        mContentParent.requestApplyInsets();
        final Callback cb = getCallback();
        if (cb != null && !isDestroyed()) {
            cb.onContentChanged();
        }
        mContentParentExplicitlySet = true;
    }

跟到 installDecor()方法(注意：该方法其实就是在mContentParent == null的时候，调用，也就是为了创建mContentParent)：
注意： generateLayout(mDecor);

private void installDecor() {
        .......
        if (mContentParent == null) {
            //注意这个方法
            mContentParent = generateLayout(mDecor);

            mDecor.makeOptionalFitsSystemWindows();

            final DecorContentParent decorContentParent = (DecorContentParent) mDecor.findViewById(
                    R.id.decor_content_parent);

            if (decorContentParent != null) {
                mDecorContentParent = decorContentParent;
                mDecorContentParent.setWindowCallback(getCallback());
                if (mDecorContentParent.getTitle() == null) {
                    mDecorContentParent.setWindowTitle(mTitle);
                }

                final int localFeatures = getLocalFeatures();
                for (int i = 0; i < FEATURE_MAX; i++) {
                    if ((localFeatures & (1 << i)) != 0) {
                        mDecorContentParent.initFeature(i);
                    }
                }

                mDecorContentParent.setUiOptions(mUiOptions);
        
       .............         
    }

跟到generateLayout()方法中：

protected ViewGroup generateLayout(DecorView decor) {
    ....
  int layoutResource;
        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);

        //ID_ANDROID_CONTENT 为：com.android.internal.R.id.content;
        ViewGroup contentParent = (ViewGroup)findViewById(ID_ANDROID_CONTENT);
......
}

在这个方法中，生成了contentParent

/**
     * 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;

这里的R.id.content也就是上面generateLayout中layoutResource布局文件中的id，例如打开R.layout.screen_simple可以看到：

R.layout.screen_simple.png

总结：

• Activity 包含了一个PhoneWindow
• PhoneWindow 继承自 Window
• Activity 通过 setContentView 将布局文件设置到了PhoneWindow上
• PhoneWindow 中又包含了 DecorView ，也就是布局最终添加到了 DecorView 上。