安装app结束回调的returnCode的解读

• 如代码所示：

PackageInstallObserver observer = new PackageInstallObserver();
pm.installPackage(uri, observer, installFlags, null);

调用系统函数 installPackage 进行apk安装的时候，
第二个参数会传入一个观察者实例：observer，
PackageInstallObserver 的声明和实现的代码：

class PackageInstallObserver extends IPackageInstallObserver.Stub
{
    @Override
    public void packageInstalled(String packagename, int returnCode) {
            。。。。。。
    }
}

当安装结束时，就会回调这个观察者 observer 的回调函数：
packageInstalled，其中参数 returnCode 表示着不同的安装结果。
这些不同的结果标识，定义在：
/frameworks/base/core/java/android/content/pm/PackageManager.java
例如：

/**
     * Installation return code: this is passed to the
     * {@link IPackageInstallObserver} on success.
     *
     * @hide
     */
    @SystemApi
    public static final int INSTALL_SUCCEEDED = 1;

    /**
     * Installation return code: this is passed to the
     * {@link IPackageInstallObserver} if the package is already installed.
     *
     * @hide
     */
    @SystemApi
    public static final int INSTALL_FAILED_ALREADY_EXISTS = -1;
    ......
/**
     * Installation parse return code: this is passed to the
     * {@link IPackageInstallObserver} if the parser did not find any
     * certificates in the .apk.
     *
     * @hide
     */
    @SystemApi
    public static final int INSTALL_PARSE_FAILED_NO_CERTIFICATES = -103;

更详细的解释可以参考：
packageInstalled函数的参数 returnCode 的源码解读

在返回 结果标识的同时，PackageManager.java 还提供了解析结果标识的函数：
调用：

PackageManager pm = mContext.getPackageManager();
pm.installStatusToString(returnCode);

installStatusToString 函数定义在：
/frameworks/base/core/java/android/content/pm/PackageManager.java

public static String installStatusToString(int status, String msg) {
        final String str = installStatusToString(status);
        if (msg != null) {
            return str + ": " + msg;
        } else {
            return str;
        }
    }

public static String installStatusToString(int status) {
        switch (status) {
            case INSTALL_SUCCEEDED: return "INSTALL_SUCCEEDED";
            case INSTALL_FAILED_ALREADY_EXISTS: return "INSTALL_FAILED_ALREADY_EXISTS";
        ......