最近一直在读安卓安装过程的源代码，也在博客中分析了整个安装流程，但是只是一步步流水账的形式，对代码整体设计架构没涉及，但是看下来安卓framework的设计运用了大量设计模式思想，这种设计模式利于代码架构整体管理以及拓展，但是对于阅读学习分析的人尤其是没有设计模式基础的人来阅读带来了难度，今天针对其中运用的一些设计模式做个简单分析，其中不对之处还望大神指出，本文只是个人理解。
今天刚好看到在进行apk的安装的过程中第一步就是INIT_COPY中确认了一个HandlerParams实例，整个安装流程中这个类起到了重要作用，他用到一个重要的设计模式就是模板方法模式。那么什么是模板方法模式呢？它可以这样定义，他是一个算法的骨架，但是它将一些步骤延迟到子类中。模板方法使得子类在不改变算法结构的情况下，重新定义算法中的某些步骤，这样整个算法变得更灵活，一个算法架构可以完成更多的工作而不是固定的流程，就比如我们买了一个锅来炒菜，整体的炒菜流程是一个算法骨架，就是将锅放在火上，然后开火，然后炒菜，然后在灭火。所有的炒菜都会遵循这个流程，这个流程就是一个算法骨架，但是具体你要炒什么菜由你具体的做法和用料决定，这就交给不同的子类去完成，这样不同的子类可以做出形形色色的菜。炒菜到此为止，让我们来看看安卓怎么定义HandlerParams的。
private abstract class HandlerParams {
private static final int MAX_RETRIES = 4; /**
* Number of times startCopy() has been attempted and had a non-fatal
* error.
*/
private int mRetries = 0; /** User handle for the user requesting the information or installation. */
private final UserHandle mUser;
String traceMethod;
int traceCookie; HandlerParams(UserHandle user) {
mUser = user;
} UserHandle getUser() {
return mUser;
} HandlerParams setTraceMethod(String traceMethod) {
this.traceMethod = traceMethod;
return this;
} HandlerParams setTraceCookie(int traceCookie) {
this.traceCookie = traceCookie;
return this;
} final boolean startCopy() {
boolean res;
try {
if (DEBUG_INSTALL) Slog.i(TAG, "startCopy " + mUser + ": " + this); if (++mRetries > MAX_RETRIES) {
Slog.w(TAG, "Failed to invoke remote methods on default container service. Giving up");
mHandler.sendEmptyMessage(MCS_GIVE_UP);
handleServiceError();
return false;
} else {
handleStartCopy();
res = true;
}
} catch (RemoteException e) {
if (DEBUG_INSTALL) Slog.i(TAG, "Posting install MCS_RECONNECT");
mHandler.sendEmptyMessage(MCS_RECONNECT);
res = false;
}
handleReturnCode();
return res;
} final void serviceError() {
if (DEBUG_INSTALL) Slog.i(TAG, "serviceError");
handleServiceError();
handleReturnCode();
} abstract void handleStartCopy() throws RemoteException;
abstract void handleServiceError();
abstract void handleReturnCode();
}从这段代码我们看到里面定义了两个算法架构流程，一个是处理拷贝的动作就是正常的工作流程，也就是startCopy这个函数，还有一个处理错误的算法流程serviceError（异常处理）。两个流程里都有固定的架构，以startCopy为例，会先判断重试的次数，如果重试次数达到上限则抛出错误返回false，但是这里会首先发送一个MCS_GIVE_UP的message，然后调用抽象函数handlerServiceError，这里就是我们设计模式里提到的延迟到子类处理的函数，这样子类可以根据自己的实际情况来处理错误。
刚才是判断重试的错误次数达到上限，如果没有达到上限来到handleStartCopy函数，这个同样是抽象函数，交由子类去实现，这样子类根据自己的实际情况去做，然后再处理返回码handleReturnCode，这个也是由子类根据自己具体的情况返回。后面的错误处理流程不做详解。
接下来看看子类是如何利用这个算法架构的，安卓使用这个父类最主要的是两个，一个是正常安装时对apk的拷贝过程，就是将其考到/data/app/的过程，还有一个测量apk的过程，这里我们以正常拷贝过程为例，代码如下：
/*
* Invoke remote method to get package information and install
* location values. Override install location based on default
* policy if needed and then create install arguments based
* on the install location.
*/
public void handleStartCopy() throws RemoteException {
int ret = PackageManager.INSTALL_SUCCEEDED; // If we're already staged, we've firmly committed to an install location
if (origin.staged) {
if (origin.file != null) {
installFlags |= PackageManager.INSTALL_INTERNAL;
installFlags &= ~PackageManager.INSTALL_EXTERNAL;
} else if (origin.cid != null) {
installFlags |= PackageManager.INSTALL_EXTERNAL;
installFlags &= ~PackageManager.INSTALL_INTERNAL;
} else {
throw new IllegalStateException("Invalid stage location");
}
} final boolean onSd = (installFlags & PackageManager.INSTALL_EXTERNAL) != 0;
final boolean onInt = (installFlags & PackageManager.INSTALL_INTERNAL) != 0;
final boolean ephemeral = (installFlags & PackageManager.INSTALL_EPHEMERAL) != 0;
PackageInfoLite pkgLite = null; if (onInt && onSd) {
// Check if both bits are set.
Slog.w(TAG, "Conflicting flags specified for installing on both internal and external");
ret = PackageManager.INSTALL_FAILED_INVALID_INSTALL_LOCATION;
} else if (onSd && ephemeral) {
Slog.w(TAG, "Conflicting flags specified for installing ephemeral on external");
ret = PackageManager.INSTALL_FAILED_INVALID_INSTALL_LOCATION;
} else {
pkgLite = mContainerService.getMinimalPackageInfo(origin.resolvedPath, installFlags,
packageAbiOverride); if (DEBUG_EPHEMERAL && ephemeral) {
Slog.v(TAG, "pkgLite for install: " + pkgLite);
} /*
* If we have too little free space, try to free cache
* before giving up.
*/
if (!origin.staged && pkgLite.recommendedInstallLocation
== PackageHelper.RECOMMEND_FAILED_INSUFFICIENT_STORAGE) {
// TODO: focus freeing disk space on the target device
final StorageManager storage = StorageManager.from(mContext);
final long lowThreshold = storage.getStorageLowBytes(
Environment.getDataDirectory()); final long sizeBytes = mContainerService.calculateInstalledSize(
origin.resolvedPath, isForwardLocked(), packageAbiOverride); try {
mInstaller.freeCache(null, sizeBytes + lowThreshold);
pkgLite = mContainerService.getMinimalPackageInfo(origin.resolvedPath,
installFlags, packageAbiOverride);
} catch (InstallerException e) {
Slog.w(TAG, "Failed to free cache", e);
} /*
* The cache free must have deleted the file we
* downloaded to install.
*
* TODO: fix the "freeCache" call to not delete
* the file we care about.
*/
if (pkgLite.recommendedInstallLocation
== PackageHelper.RECOMMEND_FAILED_INVALID_URI) {
pkgLite.recommendedInstallLocation
= PackageHelper.RECOMMEND_FAILED_INSUFFICIENT_STORAGE;
}
}
} if (ret == PackageManager.INSTALL_SUCCEEDED) {
int loc = pkgLite.recommendedInstallLocation;
if (loc == PackageHelper.RECOMMEND_FAILED_INVALID_LOCATION) {
ret = PackageManager.INSTALL_FAILED_INVALID_INSTALL_LOCATION;
} else if (loc == PackageHelper.RECOMMEND_FAILED_ALREADY_EXISTS) {
ret = PackageManager.INSTALL_FAILED_ALREADY_EXISTS;
} else if (loc == PackageHelper.RECOMMEND_FAILED_INSUFFICIENT_STORAGE) {
ret = PackageManager.INSTALL_FAILED_INSUFFICIENT_STORAGE;
} else if (loc == PackageHelper.RECOMMEND_FAILED_INVALID_APK) {
ret = PackageManager.INSTALL_FAILED_INVALID_APK;
} else if (loc == PackageHelper.RECOMMEND_FAILED_INVALID_URI) {
ret = PackageManager.INSTALL_FAILED_INVALID_URI;
} else if (loc == PackageHelper.RECOMMEND_MEDIA_UNAVAILABLE) {
ret = PackageManager.INSTALL_FAILED_MEDIA_UNAVAILABLE;
} else {
// Override with defaults if needed.
loc = installLocationPolicy(pkgLite);
if (loc == PackageHelper.RECOMMEND_FAILED_VERSION_DOWNGRADE) {
ret = PackageManager.INSTALL_FAILED_VERSION_DOWNGRADE;
} else if (!onSd && !onInt) {
// Override install location with flags
if (loc == PackageHelper.RECOMMEND_INSTALL_EXTERNAL) {
// Set the flag to install on external media.
installFlags |= PackageManager.INSTALL_EXTERNAL;
installFlags &= ~PackageManager.INSTALL_INTERNAL;
} else if (loc == PackageHelper.RECOMMEND_INSTALL_EPHEMERAL) {
if (DEBUG_EPHEMERAL) {
Slog.v(TAG, "...setting INSTALL_EPHEMERAL install flag");
}
installFlags |= PackageManager.INSTALL_EPHEMERAL;
installFlags &= ~(PackageManager.INSTALL_EXTERNAL
|PackageManager.INSTALL_INTERNAL);
} else {
// Make sure the flag for installing on external
// media is unset
installFlags |= PackageManager.INSTALL_INTERNAL;
installFlags &= ~PackageManager.INSTALL_EXTERNAL;
}
}
}
} final InstallArgs args = createInstallArgs(this);
mArgs = args; if (ret == PackageManager.INSTALL_SUCCEEDED) {
// TODO: http://b/22976637
// Apps installed for "all" users use the device owner to verify the app
UserHandle verifierUser = getUser();
if (verifierUser == UserHandle.ALL) {
verifierUser = UserHandle.SYSTEM;
} /*
* Determine if we have any installed package verifiers. If we
* do, then we'll defer to them to verify the packages.
*/
final int requiredUid = mRequiredVerifierPackage == null ? -1
: getPackageUid(mRequiredVerifierPackage, MATCH_DEBUG_TRIAGED_MISSING,
verifierUser.getIdentifier());
if (!origin.existing && requiredUid != -1
&& isVerificationEnabled(verifierUser.getIdentifier(), installFlags)) {
final Intent verification = new Intent(
Intent.ACTION_PACKAGE_NEEDS_VERIFICATION);
verification.addFlags(Intent.FLAG_RECEIVER_FOREGROUND);
verification.setDataAndType(Uri.fromFile(new File(origin.resolvedPath)),
PACKAGE_MIME_TYPE);
verification.addFlags(Intent.FLAG_GRANT_READ_URI_PERMISSION); // Query all live verifiers based on current user state
final List<ResolveInfo> receivers = queryIntentReceiversInternal(verification,
PACKAGE_MIME_TYPE, 0, verifierUser.getIdentifier()); if (DEBUG_VERIFY) {
Slog.d(TAG, "Found " + receivers.size() + " verifiers for intent "
+ verification.toString() + " with " + pkgLite.verifiers.length
+ " optional verifiers");
} final int verificationId = mPendingVerificationToken++;
verification.putExtra(PackageManager.EXTRA_VERIFICATION_ID, verificationId);
verification.putExtra(PackageManager.EXTRA_VERIFICATION_INSTALLER_PACKAGE,
installerPackageName); verification.putExtra(PackageManager.EXTRA_VERIFICATION_INSTALL_FLAGS,
installFlags); verification.putExtra(PackageManager.EXTRA_VERIFICATION_PACKAGE_NAME,
pkgLite.packageName); verification.putExtra(PackageManager.EXTRA_VERIFICATION_VERSION_CODE,
pkgLite.versionCode); if (verificationInfo != null) {
if (verificationInfo.originatingUri != null) {
verification.putExtra(Intent.EXTRA_ORIGINATING_URI,
verificationInfo.originatingUri);
}
if (verificationInfo.referrer != null) {
verification.putExtra(Intent.EXTRA_REFERRER,
verificationInfo.referrer);
}
if (verificationInfo.originatingUid >= 0) {
verification.putExtra(Intent.EXTRA_ORIGINATING_UID,
verificationInfo.originatingUid);
}
if (verificationInfo.installerUid >= 0) {
verification.putExtra(PackageManager.EXTRA_VERIFICATION_INSTALLER_UID,
verificationInfo.installerUid);
}
} final PackageVerificationState verificationState = new PackageVerificationState(
requiredUid, args); mPendingVerification.append(verificationId, verificationState);
final List<ComponentName> sufficientVerifiers = matchVerifiers(pkgLite,
receivers, verificationState); /*
* If any sufficient verifiers were listed in the package
* manifest, attempt to ask them.
*/
if (sufficientVerifiers != null) {
final int N = sufficientVerifiers.size();
if (N == 0) {
Slog.i(TAG, "Additional verifiers required, but none installed.");
ret = PackageManager.INSTALL_FAILED_VERIFICATION_FAILURE;
} else {
for (int i = 0; i < N; i++) {
final ComponentName verifierComponent = sufficientVerifiers.get(i); final Intent sufficientIntent = new Intent(verification);
sufficientIntent.setComponent(verifierComponent);
mContext.sendBroadcastAsUser(sufficientIntent, verifierUser);
}
}
} final ComponentName requiredVerifierComponent = matchComponentForVerifier(
mRequiredVerifierPackage, receivers);
if (ret == PackageManager.INSTALL_SUCCEEDED
&& mRequiredVerifierPackage != null) {
Trace.asyncTraceBegin(
TRACE_TAG_PACKAGE_MANAGER, "verification", verificationId);
/*
* Send the intent to the required verification agent,
* but only start the verification timeout after the
* target BroadcastReceivers have run.
*/
verification.setComponent(requiredVerifierComponent);
mContext.sendOrderedBroadcastAsUser(verification, verifierUser,
android.Manifest.permission.PACKAGE_VERIFICATION_AGENT,
new BroadcastReceiver() {
@Override
public void onReceive(Context context, Intent intent) {
final Message msg = mHandler
.obtainMessage(CHECK_PENDING_VERIFICATION);
msg.arg1 = verificationId;
mHandler.sendMessageDelayed(msg, getVerificationTimeout());
}
}, null, 0, null, null); /*
* We don't want the copy to proceed until verification
* succeeds, so null out this field.
*/
mArgs = null;
}
} else {
/*
* No package verification is enabled, so immediately start
* the remote call to initiate copy using temporary file.
*/
ret = args.copyApk(mContainerService, true);
}
} mRet = ret;
} @Override
void handleReturnCode() {
// If mArgs is null, then MCS couldn't be reached. When it
// reconnects, it will try again to install. At that point, this
// will succeed.
if (mArgs != null) {
processPendingInstall(mArgs, mRet);
}
} @Override
void handleServiceError() {
mArgs = createInstallArgs(this);
mRet = PackageManager.INSTALL_FAILED_INTERNAL_ERROR;
} public boolean isForwardLocked() {
return (installFlags & PackageManager.INSTALL_FORWARD_LOCK) != 0;
}
}这段代码很长，主要是判断安装位置和空间，并创建了一个InstallArgs实例，对apk进行了一系列的校验，然后交由InstallArgs去做下一步动作，由InstallParams父类我们知道当前面的动作执行完最终还要执行handleReturnCode,在这里也被实现了
void handleReturnCode() {
// If mArgs is null, then MCS couldn't be reached. When it
// reconnects, it will try again to install. At that point, this
// will succeed.
if (mArgs != null) {
processPendingInstall(mArgs, mRet);
}
}如果InstallArgs不为空就会去调用最后的一个步骤processPendingInstall(mArgs, mRet);当我们知道这个设计模式以后这段代码的层次分析已经很清晰了，理解起来是不是更加简单了，感觉只需三步就可完成apk的安装，其实这只是将步骤分层次剥离，InstallArgs承担了另外一大部分工作。InstallArgs这个类同样是模板方法这种设计模式，以便于实现不同的安装方式，而且apk的搬家功能也用到这个父类，我们正常安装的过程是实现了FileInstallArgs这个子类，这里又包含了apk的真正拷贝以及库的搬移动作，而FileInstallArgs这个类又把任务交由了IMediaContainerService来完成，这个服务是由一个apk来完成的，没想到吧，核心的安装竟然还要调用一个apk（DefaultContainerService.apk）。这个先不详述了，后续再做代码追踪
这就是具体的算法的实现过程，如果时测量又重新继承一次父类的算法架构，按照自己的流程去定制，这就是模板方法的魅力所在，架构设计人员不用去管具体的实现，只需要定义出算法骨架，将算法与架构剥离。遵循此类架构的算法流程都可以继承这个父类来完成自己的任务。
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