关于android:targetSdkVersion所导致的问题

最近两天一直在纠结个问题，就是我们新版的软件通过IDEA编译出来运行在4.4的手机上
整个相机UI是完全错乱的，同事几个手机运行都一样，错乱的样子就是整个UI压缩挤压在一起，
完全不是你在布局里面设置的还具有相对位置的样子。 但是通过IDEA的布局文件的design按钮看到
的布局展示demo又是正常的，所以，一直怀疑是编译的问题，或者某些控件的id是否有重复。
于是重新rebuild，重新删除out，gen目录，重新编译，问题依旧。

晕哦，怎么回事？于是，我想仔细去分析下。

于是随意又用meizu的4.1系统的手机去试了下，结果，我擦，居然正常的，怎么回事？

问了同事最近有没有改过相机相关的代码，都说没有。我看了提交记录，的确是没有啊，怎么可能呢？
实在想不明白，后来追踪以前的记录，发现改了manifest里面一个地方就是android：targetSdkVersion这个属性，
晕，这个属相和相机UI展示有毛线关系，哎，还是看看吧。同事从android：targetSdkVersion = 14改成了
android：targetSdkVersion = 19，于是恢复试试，结果，我去，居然运行ok了，再改回19试试，擦，
又不ok了。好了，原因找到了，是android：targetSdkVersion的问题，可是为什么android：targetSdkVersion会
影响到相机UI的展示？八竿子打不着啊！！

android:targetSdkVersion="14",targetSdkVersion属性会告诉系统应用是在api level为14的系统上进行的测试，
应用不允许有向上兼容的行为。当应用运行在版本更高的api level的系统上时，
应用还是按照targetSdkVersion版本的运行，而不需要根据更高版本的系统来显示。但是为什么会出现这个问题？

接下来我看了下4.4系统api的说明，在Build.java类里面
        /**
         * Android 4.4: KitKat, another tasty treat.
         *
         * 
Applications targeting this or a later release will get these
         * new changes in behavior:
         * 

         * 
 The default result of {android.preference.PreferenceActivity#isValidFragment
         * PreferenceActivity.isValueFragment} becomes false instead of true.
         * 
 In {@link android.webkit.WebView}, apps targeting earlier versions will have
         * JS URLs evaluated directly and any result of the evaluation will not replace
         * the current page content.  Apps targetting KITKAT or later that load a JS URL will
         * have the result of that URL replace the content of the current page
         * 
 {@link android.app.AlarmManager#set AlarmManager.set} becomes interpreted as
         * an inexact value, to give the system more flexibility in scheduling alarms.
         * 
 {@link android.content.Context#getSharedPreferences(String, int)
         * Context.getSharedPreferences} no longer allows a null name.
         * 
 {@link android.widget.RelativeLayout} changes to compute wrapped content
         * margins correctly.
         * 
 {@link android.app.ActionBar}'s window content overlay is allowed to be
         * drawn.
         * 
The {@link android.Manifest.permission#READ_EXTERNAL_STORAGE}
         * permission is now always enforced.
         * 
Access to package-specific external storage directories belonging
         * to the calling app no longer requires the
         * {@link android.Manifest.permission#READ_EXTERNAL_STORAGE} or
         * {@link android.Manifest.permission#WRITE_EXTERNAL_STORAGE}
         * permissions.
         * 
         */
        public static final int KITKAT = 19;

里面有句话说了android.widget.RelativeLayout} changes to compute wrapped content margins correctly，说
重新修改了RelativeLayout计算margin的方式，我想，可能是不是和改动RelativeLayout有关呢？接下来又去
看了RelativeLayout的api说明。擦，这下貌似有问题了，

RelativeLayout里面介绍说
Note: In platform version 17 and lower, RelativeLayout was affected by a measurement bug that could cause child views to be measured with incorrect MeasureSpec values. 
(See MeasureSpec.makeMeasureSpec for more details.) 
This was triggered when a RelativeLayout container was placed in a scrolling container, 
such as a ScrollView or HorizontalScrollView. 
If a custom view not equipped to properly measure with the MeasureSpec mode UNSPECIFIED
was placed in a RelativeLayout, this would silently work anyway as RelativeLayout 
would pass a very large AT_MOST MeasureSpec instead.
This behavior has been preserved for apps that set android:targetSdkVersion="17" 
or older in their manifest's uses-sdk tag for compatibility. 
Apps targeting SDK version 18 or newer will receive the correct behavior

他说RelativeLayout里面的MeasureSpec.makeMeasureSpec在17以前实现是有问题的，17以后才改了，哦，
MeasureSpec.makeMeasureSpec这个方法有问题，好了，继续调查。

我们的相机ui是这样写的
    
        
        

        
        
    
   这个CameraLayout是一个自定义的View，继承的是ViewGroup，
   ----public class CameraLayout extends ViewGroup-----
   然后CameraLayout里面重写了onMeasure方法，也用到了MeasureSpec.makeMeasureSpec方法，恩，可能有问题，继续
       @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        super.onMeasure(widthMeasureSpec, heightMeasureSpec);
        int width = MeasureSpec.getSize(widthMeasureSpec);
        int height = MeasureSpec.getSize(heightMeasureSpec);
        calcLayoutRect(width, height);
        final int count = getChildCount();
        for (int i = 0; i < count; i++) {
            final View child = getChildAt(i);
            if (child.getVisibility() != View.GONE) {
                switch (child.getId()) {
		    // 就是这个android:id="@+id/layout_camera_preview"
                    case PREVIEW_ID:
                        int wm2 = MeasureSpec.makeMeasureSpec(MeasureSpec.EXACTLY，preLayRect.right - preLayRect.left);
                        int hm2 = MeasureSpec.makeMeasureSpec(MeasureSpec.EXACTLY，preLayRect.bottom - preLayRect.top);
                        child.measure(wm2, hm2);
                        break;
		    // 就是这个android:id="@+id/layout_camera_bottom_bar"
                    case BOTTOM_ID:
                        int wm3 = MeasureSpec.makeMeasureSpec(MeasureSpec.EXACTLY，botLayRect.right - botLayRect.left);
                        int hm3 = MeasureSpec.makeMeasureSpec(MeasureSpec.EXACTLY，botLayRect.bottom - botLayRect.top);
                        child.measure(wm3, hm3);
                        break;
                    default:
                        break;
                }
            }
        }
    }

仔细一看，发现 MeasureSpec.makeMeasureSpec(preLayRect.right - preLayRect.left,MeasureSpec.EXACTLY);这个方法
参数传反了，public static int makeMeasureSpec(int size, int mode)方法第一个应该是size，第二个是mode，但是我们自己
不小心写错了。那么为什么写错了，一直没出错呢，用户没反馈呢，不应该啊。

继续看源码
当我们把android:targetSdkVersion="14"配置成14的时候，也就是使用4.0的实现，
4.0源码的makeMeasureSpec是这样实现的，

4009        /**
14010         * Creates a measure specification based on the supplied size and mode.
14011         *
14012         * The mode must always be one of the following:
14013         * 

14014         *  
{@link android.view.View.MeasureSpec#UNSPECIFIED}
14015         *  
{@link android.view.View.MeasureSpec#EXACTLY}
14016         *  
{@link android.view.View.MeasureSpec#AT_MOST}
14017         * 
14018         *
14019         * @param size the size of the measure specification
14020         * @param mode the mode of the measure specification
14021         * @return the measure specification based on size and mode
14022         */
14023        public static int makeMeasureSpec(int size, int mode) {
14024            return size + mode;
14025        }

大爷的，这就是上面系统说的那个问题了，这个实现不管你参数传没传反，都是返回一样的结果。
所以，配置成android:targetSdkVersion="14，即使参数传反了，也没问题。

如果配置android:targetSdkVersion="19",那么采用4.4的实现来运行代码。
而4.4修改后的实现是这样的
        /**
         * Creates a measure specification based on the supplied size and mode.
         *
         * The mode must always be one of the following:
         * 

         *  
{@link android.view.View.MeasureSpec#UNSPECIFIED}
         *  
{@link android.view.View.MeasureSpec#EXACTLY}
         *  
{@link android.view.View.MeasureSpec#AT_MOST}
         * 
         *
         * 
Note: On API level 17 and lower, makeMeasureSpec's
         * implementation was such that the order of arguments did not matter
         * and overflow in either value could impact the resulting MeasureSpec.
         * {@link android.widget.RelativeLayout} was affected by this bug.
         * Apps targeting API levels greater than 17 will get the fixed, more strict
         * behavior.
         *
         * @param size the size of the measure specification
         * @param mode the mode of the measure specification
         * @return the measure specification based on size and mode
         */
        public static int makeMeasureSpec(int size, int mode) {
            if (sUseBrokenMakeMeasureSpec) {
                return size + mode;
            } else {
                return (size & ~MODE_MASK) | (mode & MODE_MASK);
            }
        }

sUseBrokenMakeMeasureSpec是什么呢？从源码可知它是用来适配的。
// Older apps may need this compatibility hack for measurement.
sUseBrokenMakeMeasureSpec = targetSdkVersion <= JELLY_BEAN_MR1;

恩，问题找到了，当你配置android:targetSdkVersion="19"后，此时
sUseBrokenMakeMeasureSpec == false，然后走的逻辑就是
return (size & ~MODE_MASK) | (mode & MODE_MASK);因为此时我们方法
参数传发了，所以，就导致UI布局整个出了问题，错乱了。

当配置android:targetSdkVersion="14"后，及时当前运行在4.4的手机上，
sUseBrokenMakeMeasureSpec也为ture，系统走的还是老的错误实现方式布局（虽然是错误的，
但是google能让它正常运行，只是实现不对而已），所以，就还是ok的。

这就是和原因所在。