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。