Android View体系（六）从源码解析Activity的构成

Android View体系（六）从源码解析Activity的构成

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Android View体系（五）从源码解析View的事件分发机制

前言

本来这篇是要讲View的工作流程的，View的工作流程主要指的measure、layout、draw这三大流程，在讲到这三大流程之前我们有必要要先了解下Activity的构成，所以就有了这篇文章。

1.从源码解析Activity的构成

当我们写Activity时会调用setContentView()方法，来加载布局，来看看setContentView()方法是怎么实现的（Activity.java）：

public void setContentView(@LayoutRes int layoutResID) {
        getWindow().setContentView(layoutResID);
        initWindowDecorActionBar();
    }

这里调用了getWindow().setContentView(layoutResID)，getWindow()指的是什么呢？接着往下看，getWindow()返回mWindow：

    public Window getWindow() {
        return mWindow;
    }

在Activity的attach()方法发现mWindow：

  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, String referrer, IVoiceInteractor voiceInteractor) {
        attachBaseContext(context);

        mFragments.attachHost(null /*parent*/);

        mWindow = new PhoneWindow(this);
...省略
｝

原来mWindow指的就是PhoneWindow，PhoneWindow是继承抽象类Window的，这样就知道getWindow()得到的是一个PhoneWindow，我们来看看PhoneWindow.java的setContentView()方法（PhoneWindow.java）：

 @Override
    public void setContentView(View view, ViewGroup.LayoutParams params) {
        // 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();
        } else if (!hasFeature(FEATURE_CONTENT_TRANSITIONS)) {
            mContentParent.removeAllViews();
        }

        if (hasFeature(FEATURE_CONTENT_TRANSITIONS)) {
            view.setLayoutParams(params);
            final Scene newScene = new Scene(mContentParent, view);
            transitionTo(newScene);
        } else {
            mContentParent.addView(view, params);
        }
        final Callback cb = getCallback();
        if (cb != null && !isDestroyed()) {
            cb.onContentChanged();
        }
    }

在第5行看到了 installDecor()方法，来看看这个方法里写了什么：

 if (mDecor == null) {
            mDecor = generateDecor();
            mDecor.setDescendantFocusability(ViewGroup.FOCUS_AFTER_DESCENDANTS);
            mDecor.setIsRootNamespace(true);
            if (!mInvalidatePanelMenuPosted && mInvalidatePanelMenuFeatures != 0) {
                mDecor.postOnAnimation(mInvalidatePanelMenuRunnable);
            }
        }
        if (mContentParent == null) {
            mContentParent = generateLayout(mDecor);  
        ｝
        ...省略
  ｝ 
  ...省略          
 ｝

再接着追踪看看generateDecor()方法里写了什么：

   protected DecorView generateDecor() {
        return new DecorView(getContext(), -1);
    }

这里创建了一个DecorView，这个DecorView就是Activity中的根View。接着查看DecorView的源码，发现DecorView是PhoneWindow类的内部类，并且继承FrameLayout。我们再来看看第10行generateLayout()方法：

 protected ViewGroup generateLayout(DecorView decor) {
...省略
        //根据不同的情况加载不同的布局给layoutResource
        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);
...省略

 mDecor.startChanging();
        //将layoutResource加载到View中并添加到DecorView中
        View in = mLayoutInflater.inflate(layoutResource, null);
        decor.addView(in, new ViewGroup.LayoutParams(MATCH_PARENT, MATCH_PARENT));
...省略

}

第42行加载layoutResource的布局，来看看其中的一种布局R.layout.screen_title，这个文件在frameworks\base\core\res\res\layout目录中（screen_title.xml）

<code class=" hljs xml"><linearlayout xmlns:android="http://schemas.android.com/apk/res/android" android:orientation="vertical" android:fitssystemwindows="true">
    <!--{cke_protected}{C}%3C!%2D%2D%20Popout%20bar%20for%20action%20modes%20%2D%2D%3E-->
    <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:layout_width="match_parent" android:layout_height="?android:attr/windowTitleSize">
        <textview android:id="@android:id/title" android:background="@null" android:fadingedge="horizontal" android:gravity="center_vertical" android:layout_width="match_parent" android:layout_height="match_parent">
    </textview></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"