Android GUI之Activity、Window、View

　　相信大家在接触Android之初就已经知道了Activity中的setContentView方法的作用了，很明显此方法是用于为Activity填充相应的布局的。那么，Activity是如何将填充的布局绘制出来的呢？实际上Activity将View的绘制与显示交给了Window对象来处理，下面我们通过源码来进行跟踪分析。

　　Activity的源码如下，只给出我们关注的部分：

public class Activity extends ContextThemeWrapper
        implements LayoutInflater.Factory2,
        Window.Callback, KeyEvent.Callback,
        OnCreateContextMenuListener, ComponentCallbacks2,
        Window.OnWindowDismissedCallback {
      ……
    ……
    private Window mWindow;
    private WindowManager mWindowManager;
    ……

  /**
     * Retrieve the current {@link android.view.Window} for the activity.
     * This can be used to directly access parts of the Window API that
     * are not available through Activity/Screen.
     *
     * @return Window The current window, or null if the activity is not
     *         visual.
     */
    public Window getWindow() {
        return mWindow;
    }
    ……
    /**
     * Set the activity content from a layout resource.  The resource will be
     * inflated, adding all top-level views to the activity.
     *
     * @param layoutResID Resource ID to be inflated.
     *
     * @see #setContentView(android.view.View)
     * @see #setContentView(android.view.View, android.view.ViewGroup.LayoutParams)
     */
    public void setContentView(int layoutResID) {
        getWindow().setContentView(layoutResID);
        initWindowDecorActionBar();
    }

    /**
     * Set the activity content to an explicit view.  This view is placed
     * directly into the activity's view hierarchy.  It can itself be a complex
     * view hierarchy.  When calling this method, the layout parameters of the
     * specified view are ignored.  Both the width and the height of the view are
     * set by default to {@link ViewGroup.LayoutParams#MATCH_PARENT}. To use
     * your own layout parameters, invoke
     * {@link #setContentView(android.view.View,android.view.ViewGroup.LayoutParams)}
     * instead.
     *
     * @param view The desired content to display.
     *
     * @see #setContentView(int)
     * @see #setContentView(android.view.View, android.view.ViewGroup.LayoutParams)
     */
    public void setContentView(View view) {
        getWindow().setContentView(view);
        initWindowDecorActionBar();
    }
    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, IVoiceInteractor voiceInteractor) {
        attachBaseContext(context);
        mFragments.attachActivity(this, mContainer, null);
        mWindow = PolicyManager.makeNewWindow(this);
      
        ……
    }
……
}

PolicyManager的部分源码：

public final class PolicyManager {
    ……
private static final IPolicy sPolicy;
    static {
        // Pull in the actual implementation of the policy at run-time
       ……
            sPolicy = (IPolicy)policyClass.newInstance();
      ……
    }
    // 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);
    }
    ……
}

Policy的部分源码：

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

　　从给出的源码我们可以看到，Activity内部含有一个Window类型的对象mWindow，当我们调用setContentView方法时，实际上是委托给了Window对象进行处理。Window本身是一个抽象类，它描述了android窗口的基本属性和行为特征。在activity的attach方法中通过mWindow = PolicyManager.makeNewWindow(this)创建了Window对象。通过追踪代码可知， PolicyManager.makeNewWindow(this)实际上是调用Policy中的makeNewWindow方法，在此方法中创建了一个PhoneWindow对象。而PhoneWindow正是Window的子类。他们的关系图如下：

继续追踪源码，PhoneWindow对Window的抽象方法setContentView(int layoutResId)进行了实现,具体源码如下：

@Override
    public void setContentView(int layoutResID) {
        // 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)) {
            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();
        }
    }

　　在这个方法中我们可以看到首先对mContentParent进行了判断，如果为空的话则调用installDecor方法，通过hasFeature判断window是否具备某些特征，如果窗口不含有FEATURE_CONTENT_TRANSITIONS特征，则清空mContentParent中的所有子元素,为后面加载布局文件到mContentParent中做好准备。通过后面的判断，我们也可以看出无论走那个分支，其实都是对mContentParent布局内容做了更新。由此我们可以推断出mContentParent其实就是我们自己的布局的存放容器，它在PhoneWindow中定义如下：

    // This is the view in which the window contents are placed. It is either
    // mDecor itself, or a child of mDecor where the contents go.
    private ViewGroup mContentParent;

　　那么mContentParent是在哪里被创建的呢，很显然是在方法installDecor中，方法installDecor的关键代码如下：

    private void installDecor() {
        if (mDecor == null) {
            mDecor = generateDecor();
           ……
        }
        if (mContentParent == null) {
mContentParent = generateLayout(mDecor);
            ……
       }
}

　　在这个方法中，我们可以看到，首先对mDecor进行判断，如果为空在调用generateDecor方法生成mDecor对象，那么mDecor对象是什么呢？通过查看代码，可以知道mDecor的类型为DecorView，此类型是定义在PhoneWindow中的一个内部类，它继承了FrameLayout。紧接着判断mContentParent是否为空，为空则调用generateLayout并通过传入参数mDecor生成了mContentParent对象。在这个方法中通过应用的主题、窗口特征等来确定使用的布局资源并将使用的布局添加mDecor中，而这些布局中都会含有一个id为content的ViewGroup(FrameLayout)，此ViewGroup正是mContentParent，方法关键代码如下：

protected ViewGroup generateLayout(DecorView decor) {
        ……
        View in = mLayoutInflater.inflate(layoutResource, null);
        decor.addView(in, new ViewGroup.LayoutParams(MATCH_PARENT, MATCH_PARENT));
        mContentRoot = (ViewGroup) in;
        ViewGroup contentParent = (ViewGroup)findViewById(ID_ANDROID_CONTENT);
       ……
        return contentParent;
    }

　　由此我们可以确定，view的显示处理顺序为Activity->PhoneWindow->DecorView->ViewGroup(mContentView)->自定义的View(布局)。

　　Activity中显示视图的层次结构，具体如下：