闲谈自定义控件源码-view如何展示在界面上

布局是如何展示在activity上面的？

    直观的看到 在activity 的oncreate(Bundle savedInstanceState)回调中的setContentView(View view)方法中设置布局。

   Oncreate()回调是由谁来调用的呢？setContentView(View view)作用？

    

final void performCreate(Bundle icicle) {
        onCreate(icicle);
        mActivityTransitionState.readState(icicle);
        performCreateCommon();
    }

 由performCreate()去调用，performCreate()在何时被调用不得而知。

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

通过调用成员变量window的setContentView(int resId)方法，知道phoneWindow继承自window 看PhoneWindow的源码

@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)) {
            ...
        } else {
            mLayoutInflater.inflate(layoutResID, mContentParent);
        }
        mContentParent.requestApplyInsets();
        final Callback cb = getCallback();
        if (cb != null && !isDestroyed()) {
            cb.onContentChanged();
        }
    }

mContentParent 是用来放置DecorView或DecorView的child，如果没有装载Decorview就去装载。再看installDecor()源码

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

generateDecor() 创建一个DecorView，gennerateLayout()

  protected ViewGroup generateLayout(DecorView decor) {
        // Apply data from current theme.
        TypedArray a = getWindowStyle();
...
}

应用theme 再看一下setContentView核心的一句代码：mLayoutInflater.inflate(layoutResID, mContentParent);点进去看：

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

继续点进去看

 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();
        }
    }

继续点击inflate(parser, root, attachToRoot);

 public View inflate(XmlPullParser parser, @Nullable ViewGroup root, boolean attachToRoot) {
        synchronized (mConstructorArgs) {
            Trace.traceBegin(Trace.TRACE_TAG_VIEW, "inflate");
            final Context inflaterContext = mContext;
            final AttributeSet attrs = Xml.asAttributeSet(parser);
            Context lastContext = (Context) mConstructorArgs[0];
            mConstructorArgs[0] = inflaterContext;
            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();
                  ...
                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, inflaterContext, attrs, false);
                } else {
                    // Temp is the root view that was found in the xml
                    final View temp = createViewFromTag(root, name, inflaterContext, 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);
                        }
                    }
                   ...
                    // Inflate all children under temp against its context.
                    rInflateChildren(parser, temp, attrs, true);
                    // We are supposed to attach all the views we found (int temp)
                    // to root. Do that now.
                    if (root != null && attachToRoot) {
                        root.addView(temp, params);
                    }
                    // Decide whether to return the root that was passed in or the
                    // top view found in xml.
                    if (root == null || !attachToRoot) {
                        result = temp;
                    }
                }
            } catch (XmlPullParserException e) {
               ...
            } catch (Exception e) {
               ...
            } finally {
               ...
            }
            Trace.traceEnd(Trace.TRACE_TAG_VIEW);
            return result;
        }
    }

看到

1：LayoutInflator 底层是通过XmlPullParser的pull解析来讲xml文件转化为java实例。

2：当遇到<merge/>标签时只会讲标签里面的子节点加入到父节点中。在写item的布局时顶层用<merge/>标签可以减少布局嵌套。

3：如果参数ViewRoot为空，attchToRoot是true和false都是没有意义的。如果ViewRoot不为空，attchToRoot为true则使用ViewRoot的LayoutParams，否则创建一个LayoutParams