动态加载资源原理
通常我们调用getResources()方法获取资源文件
public Resources getResources() {
return mResources;
}mResources = mPackageInfo.getResources(mainThread);public Resources getResources(ActivityThread mainThread) {
if (mResources == null) {
mResources = mainThread.getTopLevelResources(mResDir,
Display.DEFAULT_DISPLAY, null, this);
}
return mResources;
}Resources getTopLevelResources(String resDir, int displayId, Configuration overrideConfiguration, CompatibilityInfo compInfo) {
ResourcesKey key = new ResourcesKey(resDir, displayId, overrideConfiguration, compInfo.applicationScale, compInfo.isThemeable);
Resources r;
synchronized (mPackages) {
// ...
WeakReference wr = mActiveResources.get(key);
r = wr != null ? wr.get() : null;
if (r != null && r.getAssets().isUpToDate()) {
if (false) {
Slog.w(TAG, "Returning cached resources " + r + " " + resDir
+ ": appScale=" + r.getCompatibilityInfo().applicationScale);
}
return r;
}
}
AssetManager assets = new AssetManager();
assets.setThemeSupport(compInfo.isThemeable);
if (assets.addAssetPath(resDir) == 0) {
return null;
}
// ...
r = new Resources(assets, dm, config, compInfo);
if (false) {
Slog.i(TAG, "Created app resources " + resDir + " " + r + ": "
+ r.getConfiguration() + " appScale="
+ r.getCompatibilityInfo().applicationScale);
}
synchronized (mPackages) {
WeakReference wr = mActiveResources.get(key);
Resources existing = wr != null ? wr.get() : null;
if (existing != null && existing.getAssets().isUpToDate()) {
// Someone else already created the resources while we were
// unlocked; go ahead and use theirs.
r.getAssets().close();
return existing;
}
// XXX need to remove entries when weak references go away
mActiveResources.put(key, new WeakReference(r));
return r;
}
} ResourcesKey key = new ResourcesKey(resDir, displayId, overrideConfiguration, compInfo.applicationScale, compInfo.isThemeable);因此只要这个Map中包含多个指向不同资源路径的Resources对象或者说我们有指向不同路径的资源的Resources对象，就可以访问多个路径的资源，即有实现访问其他APK文件中的资源的可能。
创建Resources对象的主要逻辑为
AssetManager assets = new AssetManager();
assets.setThemeSupport(compInfo.isThemeable);
if (assets.addAssetPath(resDir) == 0) {
return null;
}
r = new Resources(assets, dm, config, compInfo);AssetManager的构造函数：
public AssetManager() {
synchronized (this) {
if (DEBUG_REFS) {
mNumRefs = 0;
incRefsLocked(this.hashCode());
}
init();
if (localLOGV) Log.v(TAG, "New asset manager: " + this);
ensureSystemAssets();
}
}static void android_content_AssetManager_init(JNIEnv* env, jobject clazz)
{
AssetManager* am = new AssetManager();
if (am == NULL) {
jniThrowException(env, "java/lang/OutOfMemoryError", "");
return;
}
// 将Framework的资源文件添加到AssertManager对象的路径中。
am->addDefaultAssets();
ALOGV("Created AssetManager %p for Java object %p\n", am, clazz);
env->SetIntField(clazz, gAssetManagerOffsets.mObject, (jint)am);
}
bool AssetManager::addDefaultAssets()
{
// /system
const char* root = getenv("ANDROID_ROOT");
LOG_ALWAYS_FATAL_IF(root == NULL, "ANDROID_ROOT not set");
String8 path(root);
// kSystemAssets定义为static const char* kSystemAssets = "framework/framework-res.apk";
// 因此，path为/system/framework/framework-res.apk,framework对应的资源文件
path.appendPath(kSystemAssets);
return addAssetPath(path, NULL);
}/**
* Add an additional set of assets to the asset manager. This can be
* either a directory or ZIP file. Not for use by applications. Returns
* the cookie of the added asset, or 0 on failure.
* {@hide}
*/
public native final int addAssetPath(String path);static jint android_content_AssetManager_addAssetPath(JNIEnv* env, jobject clazz, jstring path)
{
ScopedUtfChars path8(env, path);
if (path8.c_str() == NULL) {
return 0;
}
AssetManager* am = assetManagerForJavaObject(env, clazz);
if (am == NULL) {
return 0;
}
void* cookie;
// 在native代码中完成添加资源路径的工作
bool res = am->addAssetPath(String8(path8.c_str()), &cookie);
return (res) ? (jint)cookie : 0;
}受此过程的提醒，我们是不是可以自己创建一个Resources对象，让它的包含我们指定路径的资源，就可以实现访问其他的资源了呢？答案是肯定的，利用这个思想可以实现资源的动态加载，换肤、换主题等功能都可以利用这种方法实现。
于是，主要思想就是创建一个AssetManager对象，利用addAssetPath函数添加指定的路径，用其创建一个Resources对象，使用该Resources对象获取该路径下的资源。
需要注意的是addAssetPath函数是hide的，可以使用反射调用。
public void loadRes(String path){
try {
assetManager = AssetManager.class.newInstance();
Method addAssetPath = AssetManager.class.getMethod("addAssetPath", String.class);
addAssetPath.invoke(assetManager, path);
} catch (Exception e) {
}
resources = new Resources(assetManager, super.getResources().getDisplayMetrics(), super.getResources().getConfiguration());
// 也可以根据资源获取主题
}
getPackageManager().getApplicationInfo("xxx", 0).sourceDir;@Override
public Resources getResources() {
return resources == null ? super.getResources() : resources;
}
@Override
public AssetManager getAssets() {
return assetManager == null ? super.getAssets() : assetManager;
}动态加载资源
两种不同风格的按钮，默认的是本应用提供的资源，还有一种作为另一个单独的插件APK程序存放在手机的其他路径中，当选择不同的风格时加载不同的图片资源。
插件APK仅仅包含了一些资源文件。
宿主程序的代码具体如下
private AssetManager assetManager;
private Resources resources;
private RadioGroup rg;
private ImageView iv;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
iv = (ImageView) findViewById(R.id.iv);
rg = (RadioGroup) findViewById(R.id.rg);
rg.setOnCheckedChangeListener(new OnCheckedChangeListener() {
@Override
public void onCheckedChanged(RadioGroup group, int checkedId) {
switch (checkedId) {
case R.id.default_skin:
assetManager = null;
resources = null;
iv.setImageDrawable(getResources().getDrawable(R.drawable.ic_launcher));
break;
case R.id.skin1:
String dexPath = "";
try {
dexPath = getPackageManager().getApplicationInfo("com.example.plugin", 0).sourceDir;
} catch (NameNotFoundException e) {
e.printStackTrace();
}
loadRes(dexPath);
// 由于重写了getResources方法，因此这时返回的是我们自己维护的Resources对象，因此可以访问到他的编号id的资源
iv.setImageDrawable(getResources().getDrawable(0x7f020000));
break;
}
}
});
}
public void loadRes(String path){
try {
assetManager = AssetManager.class.newInstance();
Method addAssetPath = AssetManager.class.getMethod("addAssetPath", String.class);
addAssetPath.invoke(assetManager, path);
} catch (Exception e) {
}
resources = new Resources(assetManager, super.getResources().getDisplayMetrics(), super.getResources().getConfiguration());
}
@Override
public Resources getResources() {
return resources == null ? super.getResources() : resources;
}
@Override
public AssetManager getAssets() {
return assetManager == null ? super.getAssets() : assetManager;
}public static final int ic_launcher=0x7f020000;插件APK提供getImageId函数获取图片资源的id
public class Plugin {
public static int getImageId() {
return R.drawable.ic_launcher;
}
}private void setImage(String dexPath) {
DexClassLoader loader = new DexClassLoader(dexPath, getApplicationInfo().dataDir, null, this.getClass().getClassLoader());
try {
Class clazz = loader.loadClass("com.example.plugin.Plugin");
Method getImageId = clazz.getMethod("getImageId");
int ic_launcher = (int) getImageId.invoke(clazz);
iv.setImageDrawable(getResources().getDrawable(ic_launcher));
} catch (Exception e) {
e.printStackTrace();
}
}插件管理的一种方式
对于每个插件，在AndroidManifest.xml中声明一个空的Activity，并添加他的action，比如：
PackageManager pm = getPackageManager();
List resolveinfos = pm.queryIntentActivities(intent, 0);
ActivityInfo activityInfo = resolveinfos.get(i).activityInfo;
dexPaths.add(activityInfo.applicationInfo.sourceDir); 
private AssetManager assetManager;
private Resources resources;
private LinearLayout ll;
private ImageView iv;
private Button btn;
private List dexPaths = new ArrayList();
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
iv = (ImageView) findViewById(R.id.iv);
ll = (LinearLayout) findViewById(R.id.ll);
btn = (Button) findViewById(R.id.btn);
btn.setOnClickListener(new OnClickListener() {
@Override
public void onClick(View v) {
resources = null;
iv.setImageDrawable(getResources().getDrawable(R.drawable.ic_launcher));
}
});
Intent intent = new Intent("android.intent.plugin");
PackageManager pm = getPackageManager();
final List resolveinfos = pm.queryIntentActivities(intent, 0);
for (int i = 0; i < resolveinfos.size(); i++) {
final ActivityInfo activityInfo = resolveinfos.get(i).activityInfo;
dexPaths.add(activityInfo.applicationInfo.sourceDir);
// 根据查询到的插件数添加按钮
final Button btn = new Button(this);
btn.setText("风格" +(i+1));
btn.setTag(i);
ll.addView(btn, new LinearLayout.LayoutParams(LayoutParams.WRAP_CONTENT,LayoutParams.WRAP_CONTENT));
btn.setOnClickListener(new OnClickListener() {
@Override
public void onClick(View v) {
int index = (Integer)btn.getTag();
String dexPath = dexPaths.get(index);
loadRes(dexPath);
setImage(resolveinfos.get(index).activityInfo);
}
});
}
}
private void setImage(ActivityInfo activityInfo) {
DexClassLoader loader = new DexClassLoader(activityInfo.applicationInfo.sourceDir, getApplicationInfo().dataDir, null, this.getClass().getClassLoader());
try {
Class clazz = loader.loadClass(activityInfo.packageName + ".Plugin");
Method getImageId = clazz.getMethod("getImageId");
int ic_launcher = (int) getImageId.invoke(clazz);
iv.setImageDrawable(getResources().getDrawable(ic_launcher));
} catch (Exception e) {
e.printStackTrace();
}
}
public void loadRes(String path) {
try {
assetManager = AssetManager.class.newInstance();
Method addAssetPath = AssetManager.class.getMethod("addAssetPath",
String.class);
addAssetPath.invoke(assetManager, path);
} catch (Exception e) {
e.printStackTrace();
}
resources = new Resources(assetManager, super.getResources()
.getDisplayMetrics(), super.getResources().getConfiguration());
}
@Override
public Resources getResources() {
return resources == null ? super.getResources() : resources;
}
@Override
public AssetManager getAssets() {
return assetManager == null ? super.getAssets() : assetManager;
} com.example.plugin
|-- Plugin.java
com.example.plugin2
|-- Plugin.java
Plugin类的内容一样，为提供给宿主程序反射调用的类
注册空的activity
代码点此下载