LayoutInflater.inflate参数配置

在使用自定义View的过程中经常需要使用LayoutInflater.inflate()方法添加布局文件，如下是我们要添加的布局：layout_item.xml：

activity_main.xml布局如下：

MainActivity.java如下：

public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        LinearLayout mRoot = findViewById(R.id.root);
        View view = LayoutInflater.from(this).inflate(R.layout.layout_item, null);
        mRoot.addView(view);

    }
}

这样就可以把layout_item.xml布局添加到activity_main.xml布局的LinearLayout下面了，下图是运行后的显示效果：

发现layout_item.xml的根布局LinearLayout的layout_width="200dp"和layout_height="200dp"失效了，这是什么原因？

我们进入inflate()方法一探究竟，该方法最终会调用如下3个参数的inflate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 {
                // 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 (DEBUG) {
                    System.out.println("**************************");
                    System.out.println("Creating root view: "
                            + name);
                    System.out.println("**************************");
                }

                if (TAG_MERGE.equals(name)) {
                    if (root == null || !attachToRoot) {
                        throw new InflateException(" 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) {
                        if (DEBUG) {
                            System.out.println("Creating params from root: " +
                                    root);
                        }
                        // 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);
                        }
                    }

                    if (DEBUG) {
                        System.out.println("-----> start inflating children");
                    }

                    // Inflate all children under temp against its context.
                    rInflateChildren(parser, temp, attrs, true);

                    if (DEBUG) {
                        System.out.println("-----> done inflating children");
                    }

                    // 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) {
                final InflateException ie = new InflateException(e.getMessage(), e);
                ie.setStackTrace(EMPTY_STACK_TRACE);
                throw ie;
            } catch (Exception e) {
                final InflateException ie = new InflateException(parser.getPositionDescription()
                        + ": " + 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;
        }
    }

可以看到这里是使用Android提供的pull解析方式来解析布局文件的，其中TAG_MERGE表示标签，createViewFromTag是根据节点名来创建View对象的，里面会先判断mFactory2 != null是否成立，如果成立，调用mFactory2来创建View，否则使用下面这段代码来反射创建View对象:

if (-1 == name.indexOf('.')) {
        view = onCreateView(parent, name, attrs);
} else {
         view = createView(name, null, attrs);
}

我们回到调用createViewFromTag的位置往下，这里会先判断root是否为空，如果不为空，通过下面这段代码给temp设置布局参数，temp即所要添加的布局的根布局：

// 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);
}

继续往下，rInflateChildren(parser, temp, attrs, true)是循环遍历这个根布局下的子元素，继续往下我们可以看到：

                    // 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;
                    }

如果root不为null且attachToRoot为true，则通过root.addView(temp, params)添加布局；如果root为null或attachToRoot为false，直接返回temp。

看到这里大家应该明白为什么layout_item.xml的根布局的大小失效了，原因就是因为root为null，直接返回了temp。如果想要layout_item.xml的根布局的大小不失效，root就不能为null，改为View view = LayoutInflater.from(this).inflate(R.layout.layout_item, mRoot, false）即可。

下面总结一下：
对于inflate(int resource, ViewGroup root, boolean attachToRoot)的参数：
1.如果root为null，不论attachToRoot为true或false，所添加的布局的根布局将失效；
2.如果root不为null且attachToRoot为true，待添加的布局将自动添加到root中；
3.如果root不为null且attachToRoot为false，待添加的布局不会自动添加到root，此时还需要手动addView();