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Android什么时候进行View中Background的加载

日期：2017-05-12点击：340收藏

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; }