Android LayoutInflater.inflate源码解析

一年多以前看过源码，感觉了解比较透彻了，长时间不经大脑思考，靠曾经总结的经验使用inflate方法，突然发现不知道什么时候忘记其中的原理了，上网查了一些资料，还各有不同，反而把我搞糊涂了，还是自己看源码来的实在。

inflate有多个重载方法，不过殊途同归，最后的归宿都是下面这个家伙：

   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("<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) {
                        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) {
                InflateException ex = new InflateException(e.getMessage());
                ex.initCause(e);
                throw ex;
            } catch (Exception 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;
            }

            Trace.traceEnd(Trace.TRACE_TAG_VIEW);

            return result;
        }
    }

1.首先来看看 root!=null && attachToRoot==true的情况：

    if (root != null && attachToRoot) {
          root.addView(temp, params);
     }

直接将layout添加至root中，此方法相当于将layout的代码直接放置在root布局的根布局下。

此方法的返回值为root。

2.再看看root!=null && attachToRoot==false的情况：

        if (root != null) {
            params = root.generateLayoutParams(attrs);
            if (!attachToRoot) {
                temp.setLayoutParams(params);
            }
        }

此方法返回的值为layout，且layout中设置了自己的layoutParams。

3.最后就是root==null的情况了：

此方法返回的也是layout，与第二种情况不同的是layout中不含layoutParams。

注：与第1中情况不同，第2和第3种情况需自己调用addview方法添加至相应布局中。

总结：

1. root!=null && attachToRoot==true  ----->  将layout完全地添加到root中，返回root。

2. root!=null && attachToRoot==false------>  返回的layout（包含layoutParams）

3. root==nul ------------------------------->  返回layout（不包含layoutParams）