为什么我仍然可以使用 support-v4 库而不在 gradle 上导入它

Question

我很困惑，在我的gradle中我没有导入support-v4，但我似乎可以使用该库，默认情况下support-v4是否作为依赖项包含在内？见下文了吗？同样在我的项目文件夹中，每次创建项目和各个模块时，我都可以看到 jars 支持 v4？

格拉德尔项目：

// Top-level build file where you can add configuration options common to all sub-projects/modules.
buildscript {
    repositories {
        jcenter()
    }
    dependencies {
        classpath 'com.android.tools.build:gradle:2.3.3'
        // NOTE: Do not place your application dependencies here; they belong
        // in the individual module build.gradle files
    }
}
allprojects {
    repositories {
        jcenter()
        maven{
            url "https://maven.google.com"
        }
    }
}
task clean(type: Delete) {
    delete rootProject.buildDir
}

我的应用格拉德尔：

apply plugin: 'com.android.application'
android {
    compileSdkVersion 26
    buildToolsVersion "26.0.1"
    defaultConfig {
        applicationId "com.example.ok.applicationtest_v4lib"
        minSdkVersion 19
        targetSdkVersion 26
        versionCode 1
        versionName "1.0"
        testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"
    }
    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
        }
    }
}
dependencies {
    compile fileTree(include: ['*.jar'], dir: 'libs')
    androidTestCompile('com.android.support.test.espresso:espresso-core:2.2.2', {
        exclude group: 'com.android.support', module: 'support-annotations'
    })
    compile 'com.android.support:appcompat-v7:26.0.1'
    compile 'com.android.support.constraint:constraint-layout:1.0.2'
    compile 'com.android.support:design:26.0.1'
    testCompile 'junit:junit:4.12'
}

-

下面的活动能够导入支持-v4，而无需在 gradle 中导入支持-v4：

package com.example.ok.applicationtest_v4lib;
import android.os.Bundle;
import android.support.design.widget.FloatingActionButton;
import android.support.design.widget.Snackbar;
import android.support.v4.app.ActivityCompat;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.Toolbar;
import android.view.View;
import android.view.Menu;
import android.view.MenuItem;
public class MainActivity extends AppCompatActivity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);
        FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab);
        fab.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Snackbar.make(view, "Replace with your own action", Snackbar.LENGTH_LONG)
                        .setAction("Action", null).show();
            }
        });
        //I am able to use below class yet I have not imported v4 library.
        ActivityCompat
    }
    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.menu_main, menu);
        return true;
    }
    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();
        //noinspection SimplifiableIfStatement
        if (id == R.id.action_settings) {
            return true;
        }
        return super.onOptionsItemSelected(item);
    }
}

Answer 1

如果你有appcompat-v7，你就有appcompat-v4。

运行./gradlew app:dependencies，您将看到

+--- com.android.support:appcompat-v7:25.4.0
|    +--- com.android.support:support-annotations:25.4.0
|    +--- com.android.support:support-v4:25.4.0 (*)
|    +--- com.android.support:support-vector-drawable:25.4.0
|    |    +--- com.android.support:support-annotations:25.4.0
|    |    --- com.android.support:support-compat:25.4.0 (*)
|    --- com.android.support:animated-vector-drawable:25.4.0
|         +--- com.android.support:support-vector-drawable:25.4.0 (*)
|         --- com.android.support:support-core-ui:25.4.0 (*)