Android View工作原理

通常我们都是通过Activity中的一个setContentView方法进行初始化布局的。现在就从Activity的setContentView开始进行讨论。

/**
 * 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(@LayoutRes int layoutResID) {
    getWindow().setContentView(layoutResID);
    initWindowDecorActionBar();
}

getWindow()方法返回了Window对象，可以看出setContentView其实是调用了Window.setContentView进行添加布局的。

/**
 * 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;
}

Window是一个抽象类,根据注释我们能够看到PhoneWindow是唯一的实现类。其实在Activity的attach方法中也可以看到mWindow其实是PhoneWindow类型。

/**
 * Abstract base class for a top-level window look and behavior policy.  An
 * instance of this class should be used as the top-level view added to the
 * window manager. It provides standard UI policies such as a background, title
 * area, default key processing, etc.
 *
 * <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 {
    ...........
    ...........
    
    /**
     * 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，该类继承自Window是Window类的具体实现，即我们可以通过该类具体去绘制窗口。并且，该类内部包含了一个DecorView对象，改DecorView对象时所有应用窗口(Activity界面)的根View。简而言之，PhoneWindow类是把一个FrameLayout类即DecorView对象进行一定的包装，将它作为应用窗口的根View,并且提供一组通用的窗口操作接口。其中还有一个ViewGroup成员mContentParent。因为一个Activity显示的内容还有状态栏和ActionBar，mContentParent就是指放置我们布局文件的区域，他也是DecorView的一个子视图。

/**
 * Android-specific Window.
 * <p>
 * todo: need to pull the generic functionality out into a base class
 * in android.widget.
 *
 * @hide
 */
public class PhoneWindow extends Window implements MenuBuilder.Callback {
    ......
    ......
    ......
    // This is the top-level view of the window, containing the window decor.
    private DecorView mDecor;

    // 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;

    private ViewGroup mContentRoot;
    ......
    @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);
        }
        mContentParent.requestApplyInsets();
        final Callback cb = getCallback();
        if (cb != null && !isDestroyed()) {
            cb.onContentChanged();
        }
    }
}