Drawing mechanism of view (I)

brief introduction

We are learning android When , Directly in xml It is specified in android The label of , Can show a variety of interfaces , But we don't know the drawing process , So as to hold a learning attitude , From the perspective of source code view The drawing mechanism of .

Catalog

1.view Drawing process of （ One ）
2. summary

view Drawing process of

Before the analysis , We can take a look at the following flow chart first :

Every Activity All owned Window The object of ,Android by Window Provides a unique implementation class PhoneWindow.PhoneWindow After all Window, It doesn't have much View Related capabilities , however PhoneWindow Hold one of Android It's a very important part of View object Decor( decorate )View, It's in PhoneWindow The definitions in are as follows ：

public class PhoneWindow extends Window{
    
  //  Top of the line view The node of , Play a window Decorated root component 
  private DecorView mDecor; 
}

see DecorView The inheritance relationship tells us that ,DecorView Inherited from FrameLayout.

public class DecorView extends FrameLayout {
    

}

among DecorView The son of view It's a LinerLayout The layout contains TitleView and ContentView,ContentView The sub layout of is FrameLayout. We are usually in Activity Medium onCreate() In the method ,setContentView(int layoutResID) The incoming layout affects ContentView.

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

among getWindow() Namely PhoneWindow object , Among them setContentView() Method , Source code is as follows ：

@Override
public void setContentView(int layoutResID) {
// mContentParent That is the above mentioned ContentView Parent container of , If NULL, call installDecor() Generate 
  if (mContentParent == null) {   
    installDecor();
  } else if (!hasFeature(FEATURE_CONTENT_TRANSITIONS)) {
    //  have FEATURE_CONTENT_TRANSITIONS The feature indicates that Transition
    // mContentParent Not for null, The remove decorView All son View
    mContentParent.removeAllViews();
  }
  if (hasFeature(FEATURE_CONTENT_TRANSITIONS)) {
  final Scene newScene = Scene.getSceneForLayout(mContentParent, layoutResID,
                    getContext());
            transitionTo(newScene);
  } else {
    //  The general situation will come here , call mLayoutInflater.inflate() Method to fill the layout 
    mLayoutInflater.inflate(layoutResID, mContentParent);
  }
  . . .
  // cb That is Window The associated Activity
  final Callback cb = getCallback();
  if (cb != null && !isDestroyed()) {
    //  call onContentChanged() Callback method notification Activity The contents of the window have changed 
    cb.onContentChanged();
  }

  . . .
}

We mainly look at mLayoutInflater.inflate(layoutResID, mContentParent):

public View inflate(@LayoutRes int resource, @Nullable ViewGroup root) {
  return inflate(resource, root, root != null);
}

public View inflate(@LayoutRes int resource, @Nullable ViewGroup root, boolean attachToRoot) {
  final Resources res = getContext().getResources();
  . . .
  final XmlResourceParser parser = res.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);  
        mConstructorArgs[0] = mContext;  
        View result = root;  
        try {  
            int type;  
            while ((type = parser.next()) != XmlPullParser.START_TAG &&  
                    type != XmlPullParser.END_DOCUMENT) {  
            }  
            if (type != XmlPullParser.START_TAG) {  
                throw new InflateException(parser.getPositionDescription()  
                        + ": No start tag found!");  
            }  
            final String name = parser.getName();  
            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);  
            } else {  
                View temp = createViewFromTag(name, attrs);  
                ViewGroup.LayoutParams params = null;  
                if (root != null) {  
                    params = root.generateLayoutParams(attrs);  
                    if (!attachToRoot) {  
                        temp.setLayoutParams(params);  
                    }  
                }  
                rInflate(parser, temp, attrs);  
                if (root != null && attachToRoot) {  
                    root.addView(temp, params);  
                }  
                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;  
        }  
        return result;  
    }  
}  

From here we can clearly see ,LayoutInflater It's really just using Android Provided pull Parsing method to parse the layout file . We can notice that createViewFromTag() This method , Pass in the node name and parameters . See the method name , We should be able to guess , It is used to create... Based on the node name View Object's . stay createViewFromTag() The inside of the method will call createView() Method , Then use reflection to create View And return .
So let's do that view The root node of is created. Next, let's continue to check rInflate(parser, temp, attrs):

private void rInflate(XmlPullParser parser, View parent, final AttributeSet attrs)  
        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 {  
            final View view = createViewFromTag(name, attrs);  
            final ViewGroup viewGroup = (ViewGroup) parent;  
            final ViewGroup.LayoutParams params = viewGroup.generateLayoutParams(attrs);  
            rInflate(parser, view, attrs);  
            viewGroup.addView(view, params);  
        }  
    }  
    parent.onFinishInflate();  
}  

summary

You can see , The same is createViewFromTag() Method to create View Example , Then recursively call rInflate() Method to find this View Child elements below , After each recursion, this View Add to parent layout . In this way, after the entire layout file is parsed, a complete DOM structure , Finally, the top-level root layout will be returned to , thus inflate() It's all over .

Let's keep on learning view The drawing mechanism of ( Two )