Android什么时候进行View中Background的加载

对大多数Android的开发者来说，最经常的操作莫过于对界面进行布局，View中背景图片的加载是最经常做的。但是我们很少关注这个过程，这篇文章主要解析view中背景图片加载的流程。了解view中背景图片的加载（资源的加载）可以让我们对资源加载的过程进行一些优化，另外当需要进行整个应用的换肤时，也可以更得心应手。

View图片的加载，我们最常见的就是通过在XML文件当中进行drawable的设置，然后让Android系统帮我们完成，或者手动写代码加载成Bitmap，然后加载到View上。这篇文章主要分析Android在什么时候以及怎么帮我们完成背景图片的加载的，那么我们就从Activity.setContentView还是LayoutInflater.inflate(...)方法开始分析。

不管是从Activity.setContentView(...)还是LayoutInflater.inflate(...)方法进行View的初始化，最终都会到达LayoutInflater.inflate(XmlPullParser parser, ViewGroup root, boolean attachToRoot)这个方法中。在这里我们主要关注View的背景图片加载，对于XML如何解析和加载就放过了。

 public View inflate(XmlPullParser parser, ViewGroup root, boolean attachToRoot) {

 synchronized (mConstructorArgs) {

 final AttributeSet attrs = Xml.asAttributeSet(parser);

 Context lastContext = (Context)mConstructorArgs[0];

 mConstructorArgs[0] = mContext;

 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, attrs, false);

 } else {

 // Temp is the root view that was found in the xml

 View temp;

 if (TAG_1995.equals(name)) {

 temp = new BlinkLayout(mContext, attrs);

 } else {

 temp = createViewFromTag(root, name, 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

 rInflate(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 (IOException e) {

 InflateException ex = new InflateException(

 parser.getPositionDescription()

 + ": " + e.getMessage());

 ex.initCause(e);

 throw ex;

 } finally {

 // Dont retain static reference on context.

 mConstructorArgs[0] = lastContext;

 mConstructorArgs[1] = null;

 return result;

 }

上面这么长一串代码，其实思路很清晰，就是针对XML文件进行解析，然后根据XML解析出的每一个节点进行View的初始化，紧接着将View的Layout参数设置到View上，然后将View添加到它的父控件上。
为了了解View是怎么被加载出来的，我们只需要了解

 temp = createViewFromTag(root, name, attrs);

跟进去看看。

 /*

 * default visibility so the BridgeInflater can override it.

 View createViewFromTag(View parent, String name, AttributeSet attrs) {

 if (name.equals("view")) {

 name = attrs.getAttributeValue(null, "class");

 if (DEBUG) System.out.println("******** Creating view: " + name);

 try {

 View view;

 if (mFactory2 != null) view = mFactory2.onCreateView(parent, name, mContext, attrs);

 else if (mFactory != null) view = mFactory.onCreateView(name, mContext, attrs);

 else view = null;

 if (view == null mPrivateFactory != null) {

 view = mPrivateFactory.onCreateView(parent, name, mContext, attrs);

 if (view == null) {

 if (-1 == name.indexOf(.)) {

 view = onCreateView(parent, name, attrs);

 } else {

 view = createView(name, null, attrs);

 if (DEBUG) System.out.println("Created view is: " + view);

 return view;

 } catch (InflateException e) {

 throw e;

 } catch (ClassNotFoundException e) {

 InflateException ie = new InflateException(attrs.getPositionDescription()

 + ": Error inflating class " + name);

 ie.initCause(e);

 throw ie;

 } catch (Exception e) {

 InflateException ie = new InflateException(attrs.getPositionDescription()

 + ": Error inflating class " + name);

 ie.initCause(e);

 throw ie;

 }

上面代码的重点在于try...Catch里的内容。try包起来的东西就是对View进行初始化，注意到上面代码中有几个Factory，这些Factory可以在View进行初始化，也就是说其实我们可以在这里干预View的初始化。从上面代码我们可以知道，如果我们自定义了一个Factory，那么当前要初始化的View会优先被我们自定义的Factory初始化，而不通过系统默认的Factory初始化。那么如果我们要自定义Factory，应该在哪里定义呢？容易想到，Factory必须要赶在资源加载前自定义完成，所以我们应该在onCreate(...)的this.setContentView(...)之前设置LayoutInflater.Factory。

 getLayoutInflater().setFactory(factory);

接下来我们看到上面函数里面的

 if (-1 == name.indexOf(.)) {

 view = onCreateView(parent, name, attrs);

 } else {

 view = createView(name, null, attrs);

 }

这段函数就是对View进行初始化，有两种情况，一种是系统自带的View，它在

 if (-1 == name.indexOf(.))

这里面进行初始化，因为如果是系统自带的View，传入的那么一般不带系统的前缀"android.view."。另一个分支初始化的是我们自定义的View。我们跟进onCreateView看看。

 protected View onCreateView(String name, AttributeSet attrs)

 throws ClassNotFoundException {

 return createView(name, "android.view.", attrs);

 public final View createView(String name, String prefix, AttributeSet attrs)

 throws ClassNotFoundException, InflateException {

 Constructor ? extends View constructor = sConstructorMap.get(name);

 Class ? extends View clazz = null;

 try {

 if (constructor == null) {

 // Class not found in the cache, see if its real, and try to add it

 clazz = mContext.getClassLoader().loadClass(

 prefix != null ? (prefix + name) : name).asSubclass(View.class);

 if (mFilter != null clazz != null) {

 boolean allowed = mFilter.onLoadClass(clazz);

 if (!allowed) {

 failNotAllowed(name, prefix, attrs);

 constructor = clazz.getConstructor(mConstructorSignature);

 sConstructorMap.put(name, constructor);

 } else {

 // If we have a filter, apply it to cached constructor

 if (mFilter != null) {

 // Have we seen this name before?

 Boolean allowedState = mFilterMap.get(name);

 if (allowedState == null) {

 // New class -- remember whether it is allowed

 clazz = mContext.getClassLoader().loadClass(

 prefix != null ? (prefix + name) : name).asSubclass(View.class);

 boolean allowed = clazz != null mFilter.onLoadClass(clazz);

 mFilterMap.put(name, allowed);

 if (!allowed) {

 failNotAllowed(name, prefix, attrs);

 } else if (allowedState.equals(Boolean.FALSE)) {

 failNotAllowed(name, prefix, attrs);

 Object[] args = mConstructorArgs;

 args[1] = attrs;

 final View view = constructor.newInstance(args);

 if (view instanceof ViewStub) {

 // always use ourselves when inflating ViewStub later

 final ViewStub viewStub = (ViewStub) view;

 viewStub.setLayoutInflater(this);

 return view;

 } catch (NoSuchMethodException e) {

 InflateException ie = new InflateException(attrs.getPositionDescription()

 + ": Error inflating class "

 + (prefix != null ? (prefix + name) : name));

 ie.initCause(e);

 throw ie;

 } catch (ClassCastException e) {

 // If loaded class is not a View subclass

 InflateException ie = new InflateException(attrs.getPositionDescription()

 + ": Class is not a View "

 + (prefix != null ? (prefix + name) : name));

 ie.initCause(e);

 throw ie;

 } catch (ClassNotFoundException e) {

 // If loadClass fails, we should propagate the exception.

 throw e;

 } catch (Exception e) {

 InflateException ie = new InflateException(attrs.getPositionDescription()

 + ": Error inflating class "

 + (clazz == null ? " unknown " : clazz.getName()));

 ie.initCause(e);

 throw ie;

 }

从onCreateView(...)中我们知道，其实createViewFromTag(...)中对View的初始化最终都是通过createView(...)这个函数进行初始化的，不同只在于系统控件需要通过onCreateView(...)加上前缀，以便类加载器(ClassLoader)正确地通过类所在的包初始化这个类。createView(...)这个函数的思路很清晰，不看catch里面的内容，try里面开头的两个分支就是用来将所要用的类构造函数提取出来，Android系统会对使用过的类构造函数进行缓存，因为像TextView这些常用的控件可能会被使用很多次。接下来，就是通过类构造函数对View进行初始化了。我们注意到传入构造函数的mConstructorArgs是一个包含两个元素的数组。

 final Object[] mConstructorArgs = new Object[2];

那么我们就很清楚了，它就是调用系统控件中对应两个参数的构造函数。为了方便，我们就从最基础的View进行分析。

 public View(Context context, AttributeSet attrs) {

 this(context, attrs, 0);

 public View(Context context, AttributeSet attrs, int defStyle) {

 this(context);

 TypedArray a = context.obtainStyledAttributes(attrs, com.android.internal.R.styleable.View,

 defStyle, 0);

 Drawable background = null;

 int leftPadding = -1;

 int topPadding = -1;

 int rightPadding = -1;

 int bottomPadding = -1;

 int startPadding = UNDEFINED_PADDING;

 int endPadding = UNDEFINED_PADDING;

 int padding = -1;

 int viewFlagValues = 0;

 int viewFlagMasks = 0;

 boolean setScrollContainer = false;

 int x = 0;

 int y = 0;

 float tx = 0;

 float ty = 0;

 float rotation = 0;

 float rotationX = 0;

 float rotationY = 0;

 float sx = 1f;

 float sy = 1f;

 boolean transformSet = false;

 int scrollbarStyle = SCROLLBARS_INSIDE_OVERLAY;

 int overScrollMode = mOverScrollMode;

 boolean initializeScrollbars = false;

 boolean leftPaddingDefined = false;

 boolean rightPaddingDefined = false;

 boolean startPaddingDefined = false;

 boolean endPaddingDefined = false;

 final int targetSdkVersion = context.getApplicationInfo().targetSdkVersion;

 final int N = a.getIndexCount();

 for (int i = 0; i i++) {

 int attr = a.getIndex(i);

 switch (attr) {

 case com.android.internal.R.styleable.View_background:

 background = a.getDrawable(attr);

 break;

 case com.android.internal.R.styleable.View_padding:

 padding = a.getDimensionPixelSize(attr, -1);

 mUserPaddingLeftInitial = padding;

 mUserPaddingRightInitial = padding;

 leftPaddingDefined = true;

 rightPaddingDefined = true;

 break;

 //省略一大串无关的函数

 }

由于我们只关注View中的背景图是怎么加载的，注意这个函数其实就是遍历AttributeSet attrs这个东西，然后对View的各个属性进行初始化。我们直接进入

 background = a.getDrawable(attr);

这里看看(TypedArray.getDrawable)。

 public Drawable getDrawable(int index) {

 final TypedValue value = mValue;

 if (getValueAt(index*AssetManager.STYLE_NUM_ENTRIES, value)) {

 if (false) {

 System.out.println("******************************************************************");

 System.out.println("Got drawable resource: type="

 + value.type

 + " str=" + value.string

 + " int=0x" + Integer.toHexString(value.data)

 + " cookie=" + value.assetCookie);

 System.out.println("******************************************************************");

 return mResources.loadDrawable(value, value.resourceId);

 return null;

 }

我们发现它调用mResources.loadDrawable(...)，进去看看。

 /*package*/ Drawable loadDrawable(TypedValue value, int id)

 throws NotFoundException {

 if (TRACE_FOR_PRELOAD) {

 // Log only framework resources

 if ((id 24) == 0x1) {

 final String name = getResourceName(id);

 if (name != null) android.util.Log.d("PreloadDrawable", name);

 boolean isColorDrawable = false;

 if (value.type = TypedValue.TYPE_FIRST_COLOR_INT 

 value.type = TypedValue.TYPE_LAST_COLOR_INT) {

 isColorDrawable = true;

 final long key = isColorDrawable ? value.data :

 (((long) value.assetCookie) 32) | value.data;

 Drawable dr = getCachedDrawable(isColorDrawable ? mColorDrawableCache : mDrawableCache, key);

 if (dr != null) {

 return dr;

 Drawable.ConstantState cs = isColorDrawable

 ? sPreloadedColorDrawables.get(key)

 : (sPreloadedDensity == mConfiguration.densityDpi

 ? sPreloadedDrawables.get(key) : null);

 if (cs != null) {

 dr = cs.newDrawable(this);

 } else {

 if (isColorDrawable) {

 dr = new ColorDrawable(value.data);

 if (dr == null) {

 if (value.string == null) {

 throw new NotFoundException(

 "Resource is not a Drawable (color or path): " + value);

 String file = value.string.toString();

 if (TRACE_FOR_MISS_PRELOAD) {

 // Log only framework resources

 if ((id 24) == 0x1) {

 final String name = getResourceName(id);

 if (name != null) android.util.Log.d(TAG, "Loading framework drawable #"

 + Integer.toHexString(id) + ": " + name

 + " at " + file);

 if (DEBUG_LOAD) Log.v(TAG, "Loading drawable for cookie "

 + value.assetCookie + ": " + file);

 if (file.endsWith(".xml")) {

 try {

 XmlResourceParser rp = loadXmlResourceParser(

 file, id, value.assetCookie, "drawable");

 dr = Drawable.createFromXml(this, rp);

 rp.close();

 } catch (Exception e) {

 NotFoundException rnf = new NotFoundException(

 "File " + file + " from drawable resource ID #0x"

 + Integer.toHexString(id));

 rnf.initCause(e);

 throw rnf;

 } else {

 try {

 InputStream is = mAssets.openNonAsset(

 value.assetCookie, file, AssetManager.ACCESS_STREAMING);

 // System.out.println("Opened file " + file + ": " + is);

 dr = Drawable.createFromResourceStream(this, value, is,

 file, null);

 is.close();

 // System.out.println("Created stream: " + dr);

 } catch (Exception e) {

 NotFoundException rnf = new NotFoundException(

 "File " + file + " from drawable resource ID #0x"

 + Integer.toHexString(id));

 rnf.initCause(e);

 throw rnf;

 if (dr != null) {

 dr.setChangingConfigurations(value.changingConfigurations);

 cs = dr.getConstantState();

 if (cs != null) {

 if (mPreloading) {

 if (verifyPreloadConfig(value, "drawable")) {

 if (isColorDrawable) {

 sPreloadedColorDrawables.put(key, cs);

 } else {

 sPreloadedDrawables.put(key, cs);

 } else {

 synchronized (mTmpValue) {

 //Log.i(TAG, "Saving cached drawable @ #" +

 // Integer.toHexString(key.intValue())

 // + " in " + this + ": " + cs);

 if (isColorDrawable) {

 mColorDrawableCache.put(key, new WeakReference Drawable.ConstantState (cs));

 } else {

 mDrawableCache.put(key, new WeakReference Drawable.ConstantState (cs));

 return dr;

 }

就是这个函数了，所有View的背景的加载都在这里了。这个函数的逻辑就比较复杂了，大体说来就是根据背景的类型（纯颜色、定义在XML文件中的，或者是一张静态的背景），如果缓存里面有，就直接用缓存里的。

总结一下，经过上面的分析，我们知道了，Android就是在Activity.setContentView(...)中为我们进行资源文件的加载，精确到具体的函数的话，资源文件的加载就是在每一个被初始化的View的构造函数中进行加载的。

作者：kissazi2 
出处：http://www.cnblogs.com/kissazi2/ 
本文版权归作者所有，欢迎转载，但未经作者同意必须保留此段声明，且在文章页面明显位置给出原文连接，否则保留追究法律责任的权利。

转载：http://www.cnblogs.com/kissazi2/p/4310055.html

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