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 {    // Don't 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 it's 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 < N; 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。