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理解Android中不同的Context

编程语言2024-10-20 17:11:28

作者：两日的blog

Context是什么，有什么用

在Android开发中，Context是一个抽象类，它是Android应用程序环境的一部分。它提供了访问应用程序资源和执行各种操作的接口。可以说，Context是Android应用程序与系统环境进行交互的桥梁。

Context的作用包括：

  1. 访问应用程序资源：通过Context，可以获取应用程序的资源，如字符串、布局文件、图像等。这些资源可以在应用程序的各个组件中使用，例如Activity、Service、BroadcastReceiver等。
  2. 启动组件：通过Context，可以启动其他组件，如启动Activity、启动Service、发送广播等。它提供了访问系统服务的能力，如启动其他应用程序、发送系统广播等。
  3. 获取应用程序的上下文：通过Context，可以获取应用程序的上下文，如获取ApplicationContext，用于在整个应用程序中共享数据或执行全局操作。
  4. 访问系统服务：通过Context，可以访问各种系统服务，如获取系统级的服务（如传感器服务、位置服务）、访问设备功能（如摄像头、存储器）、执行网络操作等。
  5. 访问应用程序的文件：通过Context对象，可以获取应用程序的文件目录，创建、读取、写入和删除文件等操作。
  6. 处理资源生命周期：通过Context，可以管理应用程序资源的生命周期，如创建、销毁对象、注册和注销监听器等。它提供了一种机制，确保资源的正确使用和释放，避免内存泄漏等问题。
public abstract AssetManager getAssets();

/**
 * Returns a Resources instance for the application's package.
 * <p>
 * <strong>Note:</strong> Implementations of this method should return
 * a Resources instance that is consistent with the AssetManager instance
 * returned by {@link #getAssets()}. For example, they should share the
 * same {@link Configuration} object.
 *
 * @return a Resources instance for the application's package
 * @see #getAssets()
 */
public abstract Resources getResources();

/** Return PackageManager instance to find global package information. */
public abstract PackageManager getPackageManager();

/** Return a ContentResolver instance for your application's package. */
public abstract ContentResolver getContentResolver();

/**
 * Return the Looper for the main thread of the current process.  This is
 * the thread used to dispatch calls to application components (activities,
 * services, etc).
 * <p>
 * By definition, this method returns the same result as would be obtained
 * by calling {@link Looper#getMainLooper() Looper.getMainLooper()}.
 * </p>
 *
 * @return The main looper.
 */
public abstract Looper getMainLooper();

/**
 * Return an {@link Executor} that will run enqueued tasks on the main
 * thread associated with this context. This is the thread used to dispatch
 * calls to application components (activities, services, etc).
 */
public Executor getMainExecutor() {
    // This is pretty inefficient, which is why ContextImpl overrides it
    return new HandlerExecutor(new Handler(getMainLooper()));
}

 
public abstract Context getApplicationContext();

public final CharSequence getText(@StringRes int resId) {
    return getResources().getText(resId);
}

/**
 * Returns a localized string from the application's package's
 * default string table.
 *
 * @param resId Resource id for the string
 * @return The string data associated with the resource, stripped of styled
 *         text information.
 */
@NonNull
public final String getString(@StringRes int resId) {
    return getResources().getString(resId);
}

/**
 * Returns a localized formatted string from the application's package's
 * default string table, substituting the format arguments as defined in
 * {@link java.util.Formatter} and {@link java.lang.String#format}.
 *
 * @param resId Resource id for the format string
 * @param formatArgs The format arguments that will be used for
 *                   substitution.
 * @return The string data associated with the resource, formatted and
 *         stripped of styled text information.
 */
@NonNull
public final String getString(@StringRes int resId, Object... formatArgs) {
    return getResources().getString(resId, formatArgs);
}

/**
 * Returns a color associated with a particular resource ID and styled for
 * the current theme.
 *
 * @param id The desired resource identifier, as generated by the aapt
 *           tool. This integer encodes the package, type, and resource
 *           entry. The value 0 is an invalid identifier.
 * @return A single color value in the form 0xAARRGGBB.
 * @throws android.content.res.Resources.NotFoundException if the given ID
 *         does not exist.
 */
@ColorInt
public final int getColor(@ColorRes int id) {
    return getResources().getColor(id, getTheme());
}

/**
 * Returns a drawable object associated with a particular resource ID and
 * styled for the current theme.
 *
 * @param id The desired resource identifier, as generated by the aapt
 *           tool. This integer encodes the package, type, and resource
 *           entry. The value 0 is an invalid identifier.
 * @return An object that can be used to draw this resource.
 * @throws android.content.res.Resources.NotFoundException if the given ID
 *         does not exist.
 */
@Nullable
public final Drawable getDrawable(@DrawableRes int id) {
    return getResources().getDrawable(id, getTheme());
}

/**
 * Returns a color state list associated with a particular resource ID and
 * styled for the current theme.
 *
 * @param id The desired resource identifier, as generated by the aapt
 *           tool. This integer encodes the package, type, and resource
 *           entry. The value 0 is an invalid identifier.
 * @return A color state list.
 * @throws android.content.res.Resources.NotFoundException if the given ID
 *         does not exist.
 */
@NonNull
public final ColorStateList getColorStateList(@ColorRes int id) {
    return getResources().getColorStateList(id, getTheme());
}

 /**
 * Set the base theme for this context.  Note that this should be called
 * before any views are instantiated in the Context (for example before
 * calling {@link android.app.Activity#setContentView} or
 * {@link android.view.LayoutInflater#inflate}).
 *
 * @param resid The style resource describing the theme.
 */
public abstract void setTheme(@StyleRes int resid);

/** @hide Needed for some internal implementation...  not public because
 * you can't assume this actually means anything. */
@UnsupportedAppUsage
public int getThemeResId() {
    return 0;
}

/**
 * Return the Theme object associated with this Context.
 */
@ViewDebug.ExportedProperty(deepExport = true)
public abstract Resources.Theme getTheme();

/**
 * Retrieve styled attribute information in this Context's theme.  See
 * {@link android.content.res.Resources.Theme#obtainStyledAttributes(int[])}
 * for more information.
 *
 * @see android.content.res.Resources.Theme#obtainStyledAttributes(int[])
 */
@NonNull
public final TypedArray obtainStyledAttributes(@NonNull @StyleableRes int[] attrs) {
    return getTheme().obtainStyledAttributes(attrs);
}

/**
 * Retrieve styled attribute information in this Context's theme.  See
 * {@link android.content.res.Resources.Theme#obtainStyledAttributes(int, int[])}
 * for more information.
 *
 * @see android.content.res.Resources.Theme#obtainStyledAttributes(int, int[])
 */
@NonNull
public final TypedArray obtainStyledAttributes(@StyleRes int resid,
        @NonNull @StyleableRes int[] attrs) throws Resources.NotFoundException {
    return getTheme().obtainStyledAttributes(resid, attrs);
}

/**
 * Retrieve styled attribute information in this Context's theme.  See
 * {@link android.content.res.Resources.Theme#obtainStyledAttributes(AttributeSet, int[], int, int)}
 * for more information.
 *
 * @see android.content.res.Resources.Theme#obtainStyledAttributes(AttributeSet, int[], int, int)
 */
@NonNull
public final TypedArray obtainStyledAttributes(
        @Nullable AttributeSet set, @NonNull @StyleableRes int[] attrs) {
    return getTheme().obtainStyledAttributes(set, attrs, 0, 0);
}

/**
 * Retrieve styled attribute information in this Context's theme.  See
 * {@link android.content.res.Resources.Theme#obtainStyledAttributes(AttributeSet, int[], int, int)}
 * for more information.
 *
 * @see android.content.res.Resources.Theme#obtainStyledAttributes(AttributeSet, int[], int, int)
 */
@NonNull
public final TypedArray obtainStyledAttributes(@Nullable AttributeSet set,
        @NonNull @StyleableRes int[] attrs, @AttrRes int defStyleAttr,
        @StyleRes int defStyleRes) {
    return getTheme().obtainStyledAttributes(
        set, attrs, defStyleAttr, defStyleRes);
}

总之，Context在Android开发中具有重要的作用，它提供了访问应用程序资源、启动组件、访问系统服务以及处理资源生命周期的能力。开发者可以使用Context来实现各种应用程序功能和与系统环境的交互。

Context有哪些

Context 本身是一个抽象类，主要实现类为 ContextImpl，另外有子类 ContextWrapper 和 ContextThemeWrapper，另外还有其他由上述三个类引申出来的Context类，Application/Service/Activity，他们的继承关系如下：

理解Android中不同的Context,第1张

ContextImpl/ContextWrapper/ContextThemeWrapper的区别

ContextImpl ContextWrapper ContextThemeWrapper
ContextImpl是Context的主要实现类，它提供了大部分Context的基本功能和行为。它是Android框架中真正的上下文实现类，用于处理应用程序的资源访问、组件启动、文件操作和系统服务等操作。 ContextWrapper是一个包装类，用于对现有的Context对象进行包装或修改其功能。它是Context的一个间接子类，可以通过继承ContextWrapper类来扩展Context的功能，例如添加自定义的行为或修改Context的行为。 ContextThemeWrapper：ContextThemeWrapper是Context的另一个包装类，它继承自ContextWrapper类。与ContextWrapper类似，ContextThemeWrapper也是用于包装现有的Context对象，但它还提供了自己的主题资源。通过ContextThemeWrapper，可以为特定的上下文设置不同的主题，以实现界面的样式和外观的变化。

ContextImpl

上文说到，Context本身是一个抽象类，主要的实现类就是ContextImpl，即Context的那些功能都是在ContexImpl中实现的，即ContextImpl实际承担着提供应用程序资源访问、组件启动和系统服务等功能的责任。

public class ContextImpl extends Context {
    private Resources mResources;
    private Theme mTheme;
   
    void setResources(Resources r) {
        if (r instanceof CompatResources) {
            ((CompatResources) r).setContext(this);
        }
        mResources = r;
    }
    
    @Override
    public Resources getResources() {
        return mResources;
    }
    
    @Override
    public void setTheme(int resId) {
        synchronized (mSync) {
            if (mThemeResource != resId) {
                mThemeResource = resId;
                initializeTheme();
            }
        }
    }
    
    public Resources.Theme getTheme() {
        synchronized (mSync) {
            if (mTheme != null) {
                return mTheme;
            }
    
            mThemeResource = Resources.selectDefaultTheme(mThemeResource,
                    getOuterContext().getApplicationInfo().targetSdkVersion);
            initializeTheme();
    
            return mTheme;
        }
    }
    
    private void initializeTheme() {
        if (mTheme == null) {
            mTheme = mResources.newTheme();
        }
        mTheme.applyStyle(mThemeResource, true);
    }
    
    // 其他方法的实现省略...
}

在ContextImpl，我们重点关注一下Resource及Theme的相关实现，ContextImpl中提供了getResources/setResources方法，用于获取Resources以及设置Resources，以提供资源的访问。

在getTheme/setTheme用于获取Theme以及设置Theme，以提供对主题的访问.

重点看一下getTheme()方法，该方法，会首先获取mThemeResource，这里直接选择的系统默认主题，系统会根据不同的sdk版本选择不同的默认主题。

@UnsupportedAppUsage
public static int selectDefaultTheme(int curTheme, int targetSdkVersion) {
    return selectSystemTheme(curTheme, targetSdkVersion,
            com.android.internal.R.style.Theme,
            com.android.internal.R.style.Theme_Holo,
            com.android.internal.R.style.Theme_DeviceDefault,
            com.android.internal.R.style.Theme_DeviceDefault_Light_DarkActionBar);
}

/** @hide */
public static int selectSystemTheme(int curTheme, int targetSdkVersion, int orig, int holo,
        int dark, int deviceDefault) {
    if (curTheme != ID_NULL) {
        return curTheme;
    }
    if (targetSdkVersion < Build.VERSION_CODES.HONEYCOMB) {
        return orig;
    }
    if (targetSdkVersion < Build.VERSION_CODES.ICE_CREAM_SANDWICH) {
        return holo;
    }
    if (targetSdkVersion < Build.VERSION_CODES.N) {
        return dark;
    }
    return deviceDefault;
}

通过ContextImpl的实例，应用程序可以获取到Resources对象和Theme对象，从而实现对资源和主题的访问和处理。需要注意的是，这是一个简化的，实际的ContextImpl源码非常复杂，还涉及到处理上下文的生命周期、系统服务的获取等方面的逻辑。

ContextWrapper

ContextWrapper是一个包装类，内部包含一个mBase成员变量，所有的实现都是调用mBase的方法。

public class ContextWrapper extends Context {
    @UnsupportedAppUsage
    Context mBase;

    public ContextWrapper(Context base) {
        mBase = base;
    }
    
    @Override
    public void setTheme(int resid) {
        mBase.setTheme(resid);
    }
    
    /** @hide */
    @Override
    @UnsupportedAppUsage
    public int getThemeResId() {
        return mBase.getThemeResId();
    }
    
    @Override
    public Resources.Theme getTheme() {
        return mBase.getTheme();
    }
    
    @Override
    public ClassLoader getClassLoader() {
        return mBase.getClassLoader();
    }
    
    @Override
    public String getPackageName() {
        return mBase.getPackageName();
    }
    
 }

ContextThemeWrapper

ContextThemeWrapper继承自ContextWrapper，从名字中可以看出，该类主要是跟主题相关的包装类：

public class ContextThemeWrapper extends ContextWrapper {
    ...
    
    @Override
    public Resources getResources() {
        return getResourcesInternal();
    }
    
    private Resources getResourcesInternal() {
        if (mResources == null) {
            if (mOverrideConfiguration == null) {
                mResources = super.getResources();
            } else {
                final Context resContext = createConfigurationContext(mOverrideConfiguration);
                mResources = resContext.getResources();
            }
        }
        return mResources;
    }
    
    @Override
    public void setTheme(int resid) {
        if (mThemeResource != resid) {
            mThemeResource = resid;
            initializeTheme();
        }
    }
    
    @Override
    public Resources.Theme getTheme() {
        if (mTheme != null) {
            return mTheme;
        }
    
        mThemeResource = Resources.selectDefaultTheme(mThemeResource,
                getApplicationInfo().targetSdkVersion);
        initializeTheme();
    
        return mTheme;
    }
    
    @UnsupportedAppUsage
    private void initializeTheme() {
        final boolean first = mTheme == null;
        if (first) {
            mTheme = getResources().newTheme();
            final Resources.Theme theme = getBaseContext().getTheme();
            if (theme != null) {
                mTheme.setTo(theme);
            }
        }
        onApplyThemeResource(mTheme, mThemeResource, first);
    }
    
    ...

}

和ContextImpl相比较，ContextThemeWrapper中获取资源以及主题的代码有所不同，多了一个Configuration，其他行为大致一致。

另外在AppCompat中，默认的主题为Theme_AppCompat_Light，

package androidx.appcompat.view;

public class ContextThemeWrapper extends ContextWrapper {
    ... 
    @Override
    public Resources.Theme getTheme() {
        if (mTheme != null) {
            return mTheme;
        }
    
        if (mThemeResource == 0) {
            mThemeResource = R.style.Theme_AppCompat_Light;
        }
        initializeTheme();
    
        return mTheme;
    }
}

App中不同Context对象的Theme

我们在开发中，经常会用到各种Context，常用的有activity/application/applicationContext/baseContext，为了测试不同Context中Theme对象，我们编写如下代码：

class MainActivity : AppCompatActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        printLog("baseContext is ${baseContext.themeResId} baseContext is $baseContext")
        printLog("application is ${application.themeResId} application is $application")
        printLog("applicationContext is ${applicationContext.themeResId} applicationContext is $applicationContext")
        printLog("activity is ${this.themeResId}")

    }

    private fun printLog(msg: String) {
        println("MainActivity themeResId in $msg")
    }
}

我们分别获取每个Context对应的themeResId，即每个Context中Theme对应的resId：

理解Android中不同的Context,第2张

对代码运行结果我们有如下结论：

  1. getApplication和getApplicationContext得到的是同一个Application实例对象；
  2. Application对象中的themeResId 为0 ，Application其实也有主题的应用，毕竟主题样式都是针对UI元素的；
  3. Activity****中的主题和****getBaseContext****中的主题是不一样的，具体对应什么主题下文将进行探究
  4. getBaseContext中得到的是ContextThemeWrapper，这点让我有点意外，之前的理解都是Activity启动时，会新建一个ContextImpl对象，在attachBaseContext中赋予Activity中的mBase，于是仔细研究一下发现，其实是AppCompatActivity做了替换：
//androidx.appcompat.app.AppCompatActivity
// AppCompatActivity重写了Activity中的attachBaseContext方法
@Override
protected void attachBaseContext(Context newBase) {
    super.attachBaseContext(getDelegate().attachBaseContext2(newBase));
}

我们看一下代理类AppCompatDelegateImpl中attachBaseContext2的实现：

//androidx.appcompat.app.AppCompatDelegateImpl

@NonNull
@Override
@CallSuper
public Context attachBaseContext2(@NonNull final Context baseContext) {
    mBaseContextAttached = true;

    final int modeToApply = mapNightMode(baseContext, calculateNightMode());

    // If the base context is a ContextThemeWrapper (thus not an Application context)
    // and nobody's touched its Resources yet, we can shortcut and directly apply our
    // override configuration.
    if (sCanApplyOverrideConfiguration
            && baseContext instanceof android.view.ContextThemeWrapper) {
        final Configuration config = createOverrideConfigurationForDayNight(
                baseContext, modeToApply, null);

        ContextThemeWrapperCompatApi17Impl.applyOverrideConfiguration(
                (android.view.ContextThemeWrapper) baseContext, config);
        return baseContext;
        
    }

    // Again, but using the AppCompat version of ContextThemeWrapper.
    if (baseContext instanceof ContextThemeWrapper) {
        final Configuration config = createOverrideConfigurationForDayNight(
                baseContext, modeToApply, null);
        
        ((ContextThemeWrapper) baseContext).applyOverrideConfiguration(config);
        return baseContext;
        
    }

    // We can't apply the configuration directly to the existing base context, so we need to
    // wrap it. We can't create a new configuration context since the app may rely on method
    // overrides or a specific theme -- neither of which are preserved when creating a
    // configuration context. Instead, we'll make a best-effort at wrapping the context and
    // rebasing the original theme.
    if (!sCanReturnDifferentContext) {
        return super.attachBaseContext2(baseContext);
    }

    Configuration configOverlay = null;

    final Configuration config = createOverrideConfigurationForDayNight(
            baseContext, modeToApply, configOverlay);
  
      //重点1：新建ContextThemeWrapper对象将传入的baseContext赋值给ContextWrapper中的mBase，
      // 并且ContextThemeWrapper中的主题为Theme_AppCompat_Empty
    // Next, we'll wrap the base context to ensure any method overrides or themes are left
    // intact. Since ThemeOverlay.AppCompat theme is empty, we'll get the base context's theme.
    final ContextThemeWrapper wrappedContext = new ContextThemeWrapper(baseContext,
            R.style.Theme_AppCompat_Empty);
    wrappedContext.applyOverrideConfiguration(config);

    // Check whether the base context has an explicit theme or is able to obtain one
    // from its outer context. If it throws an NPE because we're at an invalid point in app
    // initialization, we don't need to worry about rebasing under the new configuration.
    boolean needsThemeRebase;
    try {
        needsThemeRebase = baseContext.getTheme() != null;
    } catch (NullPointerException e) {
        needsThemeRebase = false;
    }

    if (needsThemeRebase) {
        // Attempt to rebase the old theme within the new configuration. This will only
        // work on SDK 23 and up, but it's unlikely that we're keeping the base theme
        // anyway so maybe nobody will notice. Note that calling getTheme() will clone
        // the base context's theme into the wrapped context's theme.
        ResourcesCompat.ThemeCompat.rebase(wrappedContext.getTheme());
    }

    
    return super.attachBaseContext2(wrappedContext);
}

//androidx.appcompat.app.AppCompatDelegate
@NonNull
@CallSuper
public Context attachBaseContext2(@NonNull Context context) {
// 重点2，将上一步包装了baseContext的ContextThemeWrapper对象进一步赋值给Activity的mBase
    attachBaseContext(context);
    return context;
}

最终AppCompatActivity中的mBase是包装了ContextImpl的ContextThemeWrapper对象，并且其主题为Theme_AppCompat_Empty

关于第三点，getBaseActivity和Activity中的主题到底是哪一个，我们可以根据resId和resources索引表resource.arsc（直接将apk文件拖到AndroidStudio中就可以看到该文件）找到：

2131755410和2131755474对应16进制为0x7f100192与0x7f1001d2

理解Android中不同的Context,第3张
理解Android中不同的Context,第4张

可以看到，getBaseActivity和Activity中的主题分别对应Theme_AppCompat_Empty 与我们在AndroidManifest.xml中设置的应用主题Theme.ThemeTest

总结

Context是Android应用程序与系统环境进行交互的桥梁，主要实现类是ContextImpl, 可以访问应用程序资源/启动组件/访问系统服务/访问应用程序的文件等，而Context可以分为三种：ContextImpl/ContextWrapper/ContextThemeWrapper，不同ContextImpl 是Context的主要实现类，ContextWrapper是简单的包装类，所有的实现都由其内部的mBase成员完成，ContextThemeWrapper继承自ContextWrapper ，它的主要继承者是Activity，和其他两个Context不同的是，他内部对应用资源和主题有不同的行为，在应用中使用跟主题相关的Context时，最好使用activity，而不要使用getBaseContext或者applictaion.


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