Android LayoutInflater的用法详解

相信我们在开发过程中肯定接触过LayoutInflater，比如ListView的适配器里的getView方法里通过LayoutInflater.from(Context).inflater来加载xml布局，在Fragment里的onCreateView里面也是一样，加载布局一共三种方法。
1，在Activity里面调用getLayoutInflater()
2, 通过LayoutInflater.from(context).inflater()
3, context.getSystemService(Context.LAYOUT_INFLATER_SERVICE))
以上的三种方式从实现上都是一样的，Activity里面的getLayoutInflater()实际上调用的是PhoneWindow的实现，而PhoneWindow里源码的处理是LayoutInflater.from(context).inflater()，往下查找最终调用context.getSystemService。
context.getSystemService是Android里一个比较重要的api，是Activity的一个方法，根据传入的Name来取得对应的Object，然后转换成相应的服务对象。以下是系统相应的服务。
传入的Name返回的对象说明
WINDOW_SERVICE WindowManager 管理打开的窗口程序
LAYOUT_INFLATER_SERVICE LayoutInflater 取得xml里定义的view
ACTIVITY_SERVICE ActivityManager 管理应用程序的系统状态
POWER_SERVICE PowerManger 电源的服务
ALARM_SERVICE AlarmManager 闹钟的服务
NOTIFICATION_SERVICE NotificationManager 状态栏的服务
KEYGUARD_SERVICE KeyguardManager 键盘锁的服务
LOCATION_SERVICE LocationManager 位置的服务，如GPS
SEARCH_SERVICE SearchManager 搜索的服务
VEBRATOR_SERVICE Vebrator 手机震动的服务
CONNECTIVITY_SERVICE Connectivity 网络连接的服务
WIFI_SERVICE WifiManager Wi-Fi服务
TELEPHONY_SERVICE TeleponyManager 电话服务

但是LayoutInflater.from(context).inflater()的方法这么多，那它们到底是什么样的用法呢？

 /**
341     * Inflate a new view hierarchy from the specified xml resource. Throws
342     * {@link InflateException} if there is an error.
343     *
344     * @param resource ID for an XML layout resource to load (e.g.,
345     *        <code>R.layout.main_page</code>)
346     * @param root Optional view to be the parent of the generated hierarchy.
347     * @return The root View of the inflated hierarchy. If root was supplied,
348     *         this is the root View; otherwise it is the root of the inflated
349     *         XML file.
350     */
351    public View inflate(int resource, ViewGroup root) {
352        return inflate(resource, root, root != null);
353    }
354
355    /**
356     * Inflate a new view hierarchy from the specified xml node. Throws
357     * {@link InflateException} if there is an error. *
358     * <p>
359     * <em><strong>Important</strong></em>&nbsp;&nbsp;&nbsp;For performance
360     * reasons, view inflation relies heavily on pre-processing of XML files
361     * that is done at build time. Therefore, it is not currently possible to
362     * use LayoutInflater with an XmlPullParser over a plain XML file at runtime.
363     *
364     * @param parser XML dom node containing the description of the view
365     *        hierarchy.
366     * @param root Optional view to be the parent of the generated hierarchy.
367     * @return The root View of the inflated hierarchy. If root was supplied,
368     *         this is the root View; otherwise it is the root of the inflated
369     *         XML file.
370     */
371    public View inflate(XmlPullParser parser, ViewGroup root) {
372        return inflate(parser, root, root != null);
373    }
374
375    /**
376     * Inflate a new view hierarchy from the specified xml resource. Throws
377     * {@link InflateException} if there is an error.
378     *
379     * @param resource ID for an XML layout resource to load (e.g.,
380     *        <code>R.layout.main_page</code>)
381     * @param root Optional view to be the parent of the generated hierarchy (if
382     *        <em>attachToRoot</em> is true), or else simply an object that
383     *        provides a set of LayoutParams values for root of the returned
384     *        hierarchy (if <em>attachToRoot</em> is false.)
385     * @param attachToRoot Whether the inflated hierarchy should be attached to
386     *        the root parameter? If false, root is only used to create the
387     *        correct subclass of LayoutParams for the root view in the XML.
388     * @return The root View of the inflated hierarchy. If root was supplied and
389     *         attachToRoot is true, this is root; otherwise it is the root of
390     *         the inflated XML file.
391     */
392    public View inflate(int resource, ViewGroup root, boolean attachToRoot) {
393        if (DEBUG) System.out.println("INFLATING from resource: " + resource);
394        XmlResourceParser parser = getContext().getResources().getLayout(resource);
395        try {
396            return inflate(parser, root, attachToRoot);
397        } finally {
398            parser.close();
399        }
400    }

上面的方法非常清晰，直接看下inflate(parser, root, attachToRoot);

 /**
403     * Inflate a new view hierarchy from the specified XML node. Throws
404     * {@link InflateException} if there is an error.
405     * <p>
406     * <em><strong>Important</strong></em>&nbsp;&nbsp;&nbsp;For performance
407     * reasons, view inflation relies heavily on pre-processing of XML files
408     * that is done at build time. Therefore, it is not currently possible to
409     * use LayoutInflater with an XmlPullParser over a plain XML file at runtime.
410     *
411     * @param parser XML dom node containing the description of the view
412     *        hierarchy.
413     * @param root Optional view to be the parent of the generated hierarchy (if
414     *        <em>attachToRoot</em> is true), or else simply an object that
415     *        provides a set of LayoutParams values for root of the returned
416     *        hierarchy (if <em>attachToRoot</em> is false.)
417     * @param attachToRoot Whether the inflated hierarchy should be attached to
418     *        the root parameter? If false, root is only used to create the
419     *        correct subclass of LayoutParams for the root view in the XML.
420     * @return The root View of the inflated hierarchy. If root was supplied and
421     *         attachToRoot is true, this is root; otherwise it is the root of
422     *         the inflated XML file.
423     */
424    public View inflate(XmlPullParser parser, ViewGroup root, boolean attachToRoot) {
425        synchronized (mConstructorArgs) {
426            final AttributeSet attrs = Xml.asAttributeSet(parser);
427            Context lastContext = (Context)mConstructorArgs[0];
428            mConstructorArgs[0] = mContext;
429            View result = root;
430
431            try {
432                // Look for the root node.
433                int type;
434                while ((type = parser.next()) != XmlPullParser.START_TAG &&
435                        type != XmlPullParser.END_DOCUMENT) {
436                    // Empty
437                }
438
439                if (type != XmlPullParser.START_TAG) {
440                    throw new InflateException(parser.getPositionDescription()
441                            + ": No start tag found!");
442                }
443
444                final String name = parser.getName();
445
446                if (DEBUG) {
447                    System.out.println("**************************");
448                    System.out.println("Creating root view: "
449                            + name);
450                    System.out.println("**************************");
451                }
452
453                if (TAG_MERGE.equals(name)) {
454                    if (root == null || !attachToRoot) {
455                        throw new InflateException("<merge /> can be used only with a valid "
456                                + "ViewGroup root and attachToRoot=true");
457                    }
458
459                    rInflate(parser, root, attrs, false);
460                } else {
461                    // Temp is the root view that was found in the xml
462                    View temp;
463                    if (TAG_1995.equals(name)) {
464                        temp = new BlinkLayout(mContext, attrs);
465                    } else {
466                        temp = createViewFromTag(root, name, attrs);
467                    }
468
469                    ViewGroup.LayoutParams params = null;
470                    // 当父层不为空时
471                    if (root != null) {
472                        if (DEBUG) {
473                            System.out.println("Creating params from root: " +
474                                    root);
475                        }
476                        // Create layout params that match root, if supplied
                              // 获取父层的参数并赋值
477                        params = root.generateLayoutParams(attrs);
                              //  子布局不贴上也会被设置LayoutParams
478                        if (!attachToRoot) {
479                            // Set the layout params for temp if we are not
480                            // attaching. (If we are, we use addView, below)
481                            temp.setLayoutParams(params);
482                        }
483                    }
484
485                    if (DEBUG) {
486                        System.out.println("-----> start inflating children");
487                    }
488                    // Inflate all children under temp
489                    rInflate(parser, temp, attrs, true);
490                    if (DEBUG) {
491                        System.out.println("-----> done inflating children");
492                    }
493
494                    // We are supposed to attach all the views we found (int temp)
495                    // to root. Do that now.
                          // 父布局不为空并且贴上会被add到父层
496                    if (root != null && attachToRoot) {
497                        root.addView(temp, params);
498                    }
499
500                    // Decide whether to return the root that was passed in or the
501                    // top view found in xml.
                          // 父布局为空或者没有贴上，result就是View本身
502                    if (root == null || !attachToRoot) {
503                        result = temp;
504                    }
505                }
506
507            } catch (XmlPullParserException e) {
508                InflateException ex = new InflateException(e.getMessage());
509                ex.initCause(e);
510                throw ex;
511            } catch (IOException e) {
512                InflateException ex = new InflateException(
513                        parser.getPositionDescription()
514                        + ": " + e.getMessage());
515                ex.initCause(e);
516                throw ex;
517            } finally {
518                // Don't retain static reference on context.
519                mConstructorArgs[0] = lastContext;
520                mConstructorArgs[1] = null;
521            }
522          // 最后返回 result
523            return result;
524        }
525    }

由上源码我们可以得出：
inflate(layout, null)返回的是View本身，且View本身所设置的布局参数无效，由父层和子View决定大小。
inflate(layout, null, false)同上一样，当父层为空，第三个值是否为真没有意义
inflate(layout, parent)子布局会被加入到父层并设置布局参数，具体大小由父层和子View决定
inflate(layout, parent, false)同上一样，区别就是false返回的是子View本身。
inflate(layout, parent, true)同第三个方法一样，返回父层View