Android加载View中Background详解_安卓软件开发教程-查字典教程网
Android加载View中Background详解
Android加载View中Background详解
发布时间:2016-12-28 来源:查字典编辑
摘要: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...

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。

复制代码 代码如下:

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

复制代码 代码如下:

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。

复制代码 代码如下:

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。

复制代码 代码如下:

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)。

复制代码 代码如下:

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。

复制代码 代码如下:

/*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。

以上就是本文的全部内容了,希望对大家能够有所帮助。

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