Activity启动界面绘制

入口就从这个setContentView方法进入了

public class DemoActivity extends Activity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        super.setContentView(R.layout.view_demo);
    }
}

Activity中的setContentView方法

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

getWindow()获取的是 com.android.internal.policy.impl.PhoneWindow
这个对象是PhoneWindow中的方法。

    @Override
    public void setContentView(int layoutResID) {
        if (mContentParent == null) {
            installDecor();
        } else {
            mContentParent.removeAllViews();
        }
        mLayoutInflater.inflate(layoutResID, mContentParent);
        final Callback cb = getCallback();
        if (cb != null && !isDestroyed()) {
            cb.onContentChanged();
        }
    }

installDecor初始化DecorView，DecorView是什么呢？
布局文件如下

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:id="@+id/view_demo_root"
android:orientation="vertical"              
android:layout_width="match_parent"
android:layout_height="match_parent">
</LinearLayout>

@Override
protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        super.setContentView(R.layout.view_demo);
        View root_view = findViewById(R.id.view_demo_root);
        while (root_view != null) {
            Log.d("ViewDemo", root_view.getClass().toString());
            root_view = (View) root_view.getParent();
        }
}

Log打印：

class android.widget.LinearLayout
class android.widget.FrameLayout
class com.android.internal.widget.ActionBarOverlayLayout
class com.android.internal.policy.impl.PhoneWindow$DecorView

从代码中可以看到DecorView为所有组件的父容器

下面进入mLayoutInflater.inflate(layoutResID, mContentParent)方法中

// 为节省篇幅，删除一些调试代码 
public View inflate(int resource, ViewGroup root) {
    return inflate(resource, root, root != null);
}

public View inflate(int resource, ViewGroup root, boolean attachToRoot) {
XmlResourceParser parser = getContext().getResources().getLayout(resource);
try {
      return inflate(parser, root, attachToRoot);
} finally {
      parser.close();
}
}

public View inflate(XmlPullParser parser, ViewGroup root, boolean attachToRoot) {
        synchronized (mConstructorArgs) {
            final AttributeSet attrs = Xml.asAttributeSet(parser);
            Context lastContext = (Context)mConstructorArgs[0];
            mConstructorArgs[0] = mContext;
            View result = root;
            try {
                // Look for the root node.
                int type;
                while ((type = parser.next()) != XmlPullParser.START_TAG &&
                        type != XmlPullParser.END_DOCUMENT) {
                    // Empty
                }

           if (type != XmlPullParser.START_TAG) {
                      throw new InflateException(parser.getPositionDescription()   + ": No start tag found!");
           }
           final String name = parser.getName();
           // 根标签为merge时，root不能为空
           if (TAG_MERGE.equals(name)) {
                    if (root == null || !attachToRoot) {
                        throw new InflateException("<merge /> can be used only with a       valid "+ "ViewGroup root and attachToRoot=true");
                    }
                    rInflate(parser, root, attrs, false);
                } else {
                    // Temp is the root view that was found in the xml
                    View temp;
                    if (TAG_1995.equals(name)) {
                        temp = new BlinkLayout(mContext, attrs);
                    } else {
                        temp = createViewFromTag(root, name, attrs);
                    }

                    ViewGroup.LayoutParams params = null;

                    if (root != null) {
                        // Create layout params that match root, if supplied
                        params = root.generateLayoutParams(attrs);
                        if (!attachToRoot) {
                            // Set the layout params for temp if we are not attaching. 
                            // (If we are, we use addView, below)
                            temp.setLayoutParams(params);
                        }
                    }

                    // 解析temp的所有子View
                    rInflate(parser, temp, attrs, true);

                    // 将temp添加到root中
                    if (root != null && attachToRoot) {
                        root.addView(temp, params);
                    }

                    // 如果未指定root或者不附加到root，则返回xml所代表的view;
                    if (root == null || !attachToRoot) {
                        result = temp;
                    }
                }
            } catch (XmlPullParserException e) {
                InflateException ex = new InflateException(e.getMessage());
                ex.initCause(e);
                throw ex;
            } catch (IOException e) {
                InflateException ex = new InflateException(parser.getPositionDescription()+ ": " + e.getMessage());
                ex.initCause(e);
                throw ex;
            } finally {
                // Don't retain static reference on context.
                mConstructorArgs[0] = lastContext;
                mConstructorArgs[1] = null;
            }
            return result;
        }
    }

可以看到最终都调用了rInflate

void rInflate(XmlPullParser parser, View parent, final AttributeSet attrs,
boolean finishInflate) throws XmlPullParserException, IOException {
        final int depth = parser.getDepth();
        int type;
        while (((type = parser.next()) != XmlPullParser.END_TAG ||
        parser.getDepth() > depth) && type != XmlPullParser.END_DOCUMENT)             {
            if (type != XmlPullParser.START_TAG) {
                continue;
            }
            final String name = parser.getName();
            if (TAG_REQUEST_FOCUS.equals(name)) {
                parseRequestFocus(parser, parent);
            } else if (TAG_INCLUDE.equals(name)) {
                if (parser.getDepth() == 0) {
                    throw new InflateException("<include /> cannot be the root element");
                }
                parseInclude(parser, parent, attrs);
            } else if (TAG_MERGE.equals(name)) {
                throw new InflateException("<merge /> must be the root element");
            } else if (TAG_1995.equals(name)) {
                final View view = new BlinkLayout(mContext, attrs);
                final ViewGroup viewGroup = (ViewGroup) parent;
                final ViewGroup.LayoutParams params = viewGroup.generateLayoutParams(attrs);
                rInflate(parser, view, attrs, true);
                viewGroup.addView(view, params);                
            } else {
                final View view = createViewFromTag(parent, name, attrs);
                final ViewGroup viewGroup = (ViewGroup) parent;
                final ViewGroup.LayoutParams params = viewGroup.generateLayoutParams(attrs);
                rInflate(parser, view, attrs, true);
                viewGroup.addView(view, params);
            }
        }
        if (finishInflate) parent.onFinishInflate();
    }

rInflate通过XmlPullParser解析出XML里所有的组件，并使用递归的方式逐层去获取子组件。看到这里我们就知道为什么不建议在View中做过多的嵌套了吧，因为递归调用会占用大量的栈帧，导致内存使用飙高，这样程序会栈溢出和内存溢出的风险。
最终 rInflate 调用 createViewFromTag 方法生成了view 并加载到 DecorView中。

最后通知Callback，ContentView发生改变

final Callback cb = getCallback();
if (cb != null && !isDestroyed()) {
            cb.onContentChanged();
}

在Activity方法attach中可以发现setCallback就是Activity本身

 final void attach(...) {
        ...
        mWindow = PolicyManager.makeNewWindow(this);
        mWindow.setCallback(this);
        ...
    }
    public void onContentChanged() {
    }

界面发生变化后会调用Activity重写的onContentChanged，通知界面已经发生了改变。