Android-AppCompatActivity和Activity UI绘制不同的地方

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AppCompatActivity的UI加载和Activity的基本一致，但是有一个地方是有差异的会存在布局替换的操作：

final ContentFrameLayout contentView = (ContentFrameLayout) subDecor.findViewById(
        R.id.action_bar_activity_content);

final ViewGroup windowContentView = (ViewGroup) mWindow.findViewById(android.R.id.content);
if (windowContentView != null) {
    // There might be Views already added to the Window's content view so we need to
    // migrate them to our content view
    while (windowContentView.getChildCount() > 0) {
        final View child = windowContentView.getChildAt(0);
        windowContentView.removeViewAt(0);
        contentView.addView(child);
    }

    // Change our content FrameLayout to use the android.R.id.content id.
    // Useful for fragments.
    windowContentView.setId(View.NO_ID);
    contentView.setId(android.R.id.content);

    // The decorContent may have a foreground drawable set (windowContentOverlay).
    // Remove this as we handle it ourselves
    if (windowContentView instanceof FrameLayout) {
        ((FrameLayout) windowContentView).setForeground(null);
    }
}