引言

通过setContentView()加载布局文件时，有时加载的根部局的宽高属性会失效，如果在根部局下又套一层相同属性的宽高容器布局，则又会起效果，这到底时为啥呢？我们去跟踪加载流程 去瞅瞅看。

布局文件得加载方式

activity的加载视图的方法

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

getWindow()返回的是window抽象类,而phoneWindow是window的唯一实现类，直接去看phoneWindow的setContentView()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();
    }
}

由此可以看出，传进来的布局文件最终还是交给LayoutInflater对象来加载，同时传入一个mContentParent对象（ViewGroup类型）
fragment加载视图的方式

 @Override
    public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
        return inflater.inflate(R.layout.activity_main, container, false);
    }

很显然，也是通过LayoutInfater里的inflate方法来加载视图。

LayoutInflater源码解析

根据上面的解析，无论是activity或者fragment都是通过LayoutInflater对象来加载布局文件，现在去看看inflate()方法做了哪些事。

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

  public View inflate(XmlPullParser parser, @Nullable ViewGroup root) {
        return inflate(parser, root, root != null);
    }

public View inflate(@LayoutRes int resource, @Nullable ViewGroup root, boolean attachToRoot) {
        final Resources res = getContext().getResources();
        if (DEBUG) {
            Log.d(TAG, "INFLATING from resource: \"" + res.getResourceName(resource) + "\" ("
                  + Integer.toHexString(resource) + ")");
        }

        View view = tryInflatePrecompiled(resource, res, root, attachToRoot);
        if (view != null) {
            return view;
        }
        XmlResourceParser parser = res.getLayout(resource);
        try {
            return inflate(parser, root, attachToRoot);
        } finally {
            parser.close();
        }
    }

不难看出，这三个重载方法最后都调用了同一个加载方法

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 {
                advanceToRootNode(parser);
                final String name = parser.getName();

              //merge标签下
                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 {
                    // 根据tag创建根视图
                    final View temp = createViewFromTag(root, name, inflaterContext, attrs);

                    ViewGroup.LayoutParams params = null;

                    if (root != null) {
                       
                        // Create layout params that match root, if supplied
                        //如果传递进来的ViewGroup不为空，则添加layoutParams参数
                        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.
                    //加载子控件
                    rInflateChildren(parser, temp, attrs, true);
                    // We are supposed to attach all the views we found (int temp)
                    // to root. Do that now.
                    //如果传进来的viewgroup不为null，同时第三个参数为true
                    //则添加childView
                    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.
                    // 如果传递进来的ViewGroup为空或者第三个参数为false
                    // 则将从布局文件中加载的View返回
                    if (root == null || !attachToRoot) {
                        result = temp;
                    }
                }

            } catch (XmlPullParserException e) {
                final InflateException ie = new InflateException(e.getMessage(), e);
                ie.setStackTrace(EMPTY_STACK_TRACE);
                throw ie;
            } catch (Exception e) {
                final InflateException ie = new InflateException(
                        getParserStateDescription(inflaterContext, attrs)
                        + ": " + e.getMessage(), e);
                ie.setStackTrace(EMPTY_STACK_TRACE);
                throw ie;
            } finally {
                // Don't retain static reference on context.
                mConstructorArgs[0] = lastContext;
                mConstructorArgs[1] = null;

                Trace.traceEnd(Trace.TRACE_TAG_VIEW);
            }

            return result;
        }
    }

这里是通过pull解析器对xml文件进行解析。
划重点！！只有传递过来的viewGroup不为null时，才会设置layoutParams参数。