[Android 学习笔记] instant-run 源码学习

instant-run 源码地址
仔细过一遍 README, 对 instant-run 就会有一个大体的了解;
instant-run 的目的是方便 Android 开发者, 减少 build, install, debug 的耗时, 做到修改代码后在调试设备上的 APP 不重启就可以直接看到修改的结果(虽然 instant-run 没有完全实现);

instant-run 工作模式:

1. Hot Swap: 不需要重启 Actitvity 就可以看到修改结果
2. Cold Swap: 需要重启 Actitvity 才能看到修改结果
3. Reinstall: 需要重启 APP 才能看到修改结果

只修改了源代码时可以 Hot Swap,
修改了资源文件时需要 Cold Swap,
修改了AndroidManifest.xml 时就需要重启 APP 了

instant-run 实现方案

Host Swap 的实现方案是 "代理模式", 在 APP 首次运行时, 在编译期间添加了 instant-install Gradle Task, 当打开了 Android Studio 的 instant-run 功能时, 执行 assembleDebug 时依赖执行了下面的 Task:

Executing tasks: [:app:assembleDebug]

Configuration on demand is an incubating feature.
:app:buildInfoDebugLoader
:app:preBuild UP-TO-DATE
:app:preDebugBuild UP-TO-DATE
:app:checkDebugManifest
:app:prepareComAndroidSupportAnimatedVectorDrawable2600Alpha1Library
:app:prepareComAndroidSupportAppcompatV72600Alpha1Library
:app:prepareComAndroidSupportConstraintConstraintLayout102Library
:app:prepareComAndroidSupportSupportCompat2600Alpha1Library
:app:prepareComAndroidSupportSupportCoreUi2600Alpha1Library
:app:prepareComAndroidSupportSupportCoreUtils2600Alpha1Library
:app:prepareComAndroidSupportSupportFragment2600Alpha1Library
:app:prepareComAndroidSupportSupportMediaCompat2600Alpha1Library
:app:prepareComAndroidSupportSupportV42600Alpha1Library
:app:prepareComAndroidSupportSupportVectorDrawable2600Alpha1Library
:app:prepareDebugDependencies
:app:compileDebugAidl UP-TO-DATE
:app:compileDebugRenderscript UP-TO-DATE
:app:generateDebugBuildConfig UP-TO-DATE
:app:generateDebugResValues UP-TO-DATE
:app:generateDebugResources UP-TO-DATE
:app:mergeDebugResources UP-TO-DATE
:app:processDebugManifest UP-TO-DATE
:app:processDebugResources UP-TO-DATE
:app:generateDebugSources
:app:incrementalDebugJavaCompilationSafeguard UP-TO-DATE
:app:javaPreCompileDebug
:app:compileDebugJavaWithJavac
:app:compileDebugJavaWithJavac - is not incremental (e.g. outputs have changed, no previous execution, etc.).
:app:mergeDebugShaders UP-TO-DATE
:app:compileDebugShaders UP-TO-DATE
:app:generateDebugAssets UP-TO-DATE
:app:mergeDebugAssets UP-TO-DATE
:app:processDebugJavaRes NO-SOURCE
:app:transformResourcesWithMergeJavaResForDebug UP-TO-DATE
:app:packageInstantRunResourcesDebug UP-TO-DATE
:app:fastDeployDebugExtractor UP-TO-DATE
:app:generateDebugInstantRunAppInfo UP-TO-DATE
:app:checkManifestChangesDebug
:app:transformClassesWithExtractJarsForDebug UP-TO-DATE
:app:transformClassesWithInstantRunVerifierForDebug
:app:transformClassesWithDependencyCheckerForDebug UP-TO-DATE
:app:compileDebugNdk NO-SOURCE
:app:mergeDebugJniLibFolders UP-TO-DATE
:app:transformNativeLibsWithMergeJniLibsForDebug UP-TO-DATE
:app:transformNativeLibsAndResourcesWithJavaResourcesVerifierForDebug UP-TO-DATE
:app:transformClassesWithInstantRunForDebug
:app:transformClassesEnhancedWithInstantReloadDexForDebug
:app:incrementalDebugTasks
:app:preColdswapDebug
:app:transformClassesWithInstantRunSlicerForDebug
:app:transformClassesWithDexForDebug
:app:validateSigningDebug
:app:transformDexWithInstantRunDependenciesApkForDebug
:app:transformDexWithInstantRunSlicesApkForDebug
:app:transformNativeLibsWithStripDebugSymbolForDebug UP-TO-DATE
:app:packageDebug UP-TO-DATE
:app:buildInfoGeneratorDebug
:app:compileDebugSources
:app:assembleDebug

BUILD SUCCESSFUL

Total time: 2.828 secs

可以看到有一些关于 instant-run 的 task

:app:incrementalDebugJavaCompilationSafeguard UP-TO-DATE
:app:packageInstantRunResourcesDebug UP-TO-DATE
:app:generateDebugInstantRunAppInfo UP-TO-DATE
:app:transformClassesWithInstantRunVerifierForDebug
:app:transformClassesWithInstantRunForDebug
:app:transformClassesEnhancedWithInstantReloadDexForDebug
:app:incrementalDebugTasks
:app:transformClassesWithInstantRunSlicerForDebug
:app:transformDexWithInstantRunDependenciesApkForDebug
:app:transformDexWithInstantRunSlicesApkForDebug

那这些额外的 task 在 APP 编译期间做了哪些工作?
举个例子, APP 源码中有一个 class A:

package com.google.android.foo;

public class A {
    int intField;  
    public int someMethod() {
        return intField;
    }
}

instant-run 在首次编译期间在 class A 中添加了一个静态变量, 并在方法实现中先判断新加的变量是否为空, 不为空就执行 access$dispatch 方法, 并且不执行原有的语句, 这就是 "代理模式":

package com.google.android.foo;

public class A {
    static IncrementalChange $change = null;
    int intField;
    public int someMethod() {
         if ($change != null) {
            return $change.access$dispatch("someMethod_()I", this)
           }
      return intField;
    }
}

查看 instant-run 源码, 发现 IncrementalChange 只是一个简单的接口

package com.android.tools.fd.runtime;

public interface IncrementalChange {
    Object access$dispatch(String id, Object... args);
}

接着当开发者在 Android Studio 中变更了class A 时:

package com.google.android.foo;

public class A {
    int intField;
    public int someMethod() {
        return intField*2;  // 改了方法实现
    }
}

重新 instant-run 后会产生一个补丁 APK, 其中变更后的 class A 编译成了

package com.google.android.foo;

public class A$Override implements IncrementalChange {

    public static int someMethod(A sender) {
        return sender.intField*2;
    }
    public static Object dispatch(A sender, String methodName, String signature) {
        if (methodName.equals("someMethod")) {
            return someMethod(sender);
        }
    }
}

instant-run 生成补丁文件后将补丁发给已经在测试设备上运行的 APP, 这时 APP 已经启动一个 SocketServer , SocketServer 接收到补丁后基于 DexClassLoader 和反射机制将补丁中的 A$Override 赋值给已经实例化的 Class A, 这样再调用 Class A 的 someMethod 方法时就会执行补丁中同名的静态someMethod方法, 这样不需要重新启动 APP 就能将对源代码的变更同步到测试设备上, 极大的方便了开发者调试代码.