安卓关于inflate方法的总结

android inflate

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前言

android中我们经常用到inflate方法，今天对这个方法进行一个总结，inflate方法调用方式如下：

• View.inflate(Context context, @LayoutRes int resource, ViewGroup root);
• LayoutInflater.from(Context context).inflate(@LayoutRes int resource, ViewGroup root, boolean attachToRoot);

说白了，一种是通过View类调用其静态方法inflate。另一种是获取LayoutInflater对象，调用LayoutInflater类的inflate方法。

分析

那我们就分别看下这两个方法，首先看下View类的inflate方法。

代码很简单，通过LayoutInflater.from(context);生成一个LayoutInflater对象，调用其inflate方法，传递参数resource,root.

那我们接着看下LayoutInflater中的inflate方法。LayoutInflater中有好几个inflate方法，但是最终调用的都是下边这个方法，所以我们主要分析下边这个方法。

/**
     * Inflate a new view hierarchy from the specified XML node. Throws
     * {@link InflateException} if there is an error.
     * 

     * Important   For performance
     * reasons, view inflation relies heavily on pre-processing of XML files
     * that is done at build time. Therefore, it is not currently possible to
     * use LayoutInflater with an XmlPullParser over a plain XML file at runtime.
     *
     * @param parser XML dom node containing the description of the view
     *        hierarchy.
     * @param root Optional view to be the parent of the generated hierarchy (if
     *        attachToRoot is true), or else simply an object that
     *        provides a set of LayoutParams values for root of the returned
     *        hierarchy (if attachToRoot is false.)
     * @param attachToRoot Whether the inflated hierarchy should be attached to
     *        the root parameter? If false, root is only used to create the
     *        correct subclass of LayoutParams for the root view in the XML.
     * @return The root View of the inflated hierarchy. If root was supplied and
     *         attachToRoot is true, this is root; otherwise it is the root of
     *         the inflated XML file.
     */
    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;
        }
    }

我们首先看下这几个参数

• XmlPullParser parser：我们传过来的resourceid被转化成XmlPullParser对象，用于解析布局。
• ViewGroup root：这个传进的是一个ViewGroup对象，当attachToRoot为true的时候，root就是当前inflate方法返回view的父view，当attachToRoot为false的时候，root就是一个普通的view，用来帮助决定inflate方法返回view的LayoutParams，这块不理解的话，可以看下view的绘制流程，子view的大小是由父view的measurespec和子view的宽高值决定的。
• boolean attachToRoot，这个就是上边说的，用来决定inflate返回的view跟root是否存在父子布局关系。true表示存在父子关系，系统会将inflate方法返回的view添加到root的子view里。false不添加。

三个参数都明白了，我们在使用inflate方法时，parser参数是保证要传递的，因为我们要新建一个view保证是要有布局支撑的，root可以为null，但是如果为null就代表inflate返回的view没有容器支撑，那么parser布局你写的宽高等数值可能不会起作用，所以必须传一个root容器，attachToRoot当你想创建的view直接为root的子view就设置为true，如果你想创建的view只是需要一个容器，那么就设置attachToRoot为false。

总结

以上基本把LayoutInflater类的inflate方法分析完了。其实View.inflate调用的还是LayoutInflater中的inflate方法，只是attachToRoot的值被默认设置为false，那么说到头来用哪个方法都是一样的，看我们传参的需求，选择最方便的方式即可。

参考：https://blog.csdn.net/u012702547/article/details/52628453#reply