面试题:Android 中 Intent 采用了什么设计模式?
答案是采用了原型模式。原型模式的好处在于方便地拷贝某个实例的属性进行使用、又不会对原实例造成影响,其逻辑在于对 Cloneable
接口的实现。
话不多说看下 Intent
的关键源码:
// frameworks/base/core/java/android/content/Intent.java
public class Intent implements Parcelable, Cloneable {...private static final int COPY_MODE_ALL = 0;private static final int COPY_MODE_FILTER = 1;private static final int COPY_MODE_HISTORY = 2;@Overridepublic Object clone() {return new Intent(this);}public Intent(Intent o) {this(o, COPY_MODE_ALL);}private Intent(Intent o, @CopyMode int copyMode) {this.mAction = o.mAction;this.mData = o.mData;this.mType = o.mType;this.mIdentifier = o.mIdentifier;this.mPackage = o.mPackage;this.mComponent = o.mComponent;this.mOriginalIntent = o.mOriginalIntent;...if (copyMode != COPY_MODE_FILTER) {...if (copyMode != COPY_MODE_HISTORY) {...}}}...
}
可以看到 Intent
实现的 clone()
逻辑是直接调用了 new 并传入了自身实例,而非调用 super.clone() 进行拷贝。
默认的拷贝策略是 COPY_MODE_ALL
,顾名思义,将完整拷贝源实例的所有属性进行构造。其他的拷贝策略是 COPY_MODE_FILTER
指的是只拷贝跟 Intent-filter 相关的属性,即用来判断启动目标组件的 action、data、type、component、category 等必备信息。无视启动 flag、bundle 等数据。
// frameworks/base/core/java/android/content/Intent.java
public class Intent implements Parcelable, Cloneable {...public @NonNull Intent cloneFilter() {return new Intent(this, COPY_MODE_FILTER);}private Intent(Intent o, @CopyMode int copyMode) {this.mAction = o.mAction;...if (copyMode != COPY_MODE_FILTER) {this.mFlags = o.mFlags;this.mContentUserHint = o.mContentUserHint;this.mLaunchToken = o.mLaunchToken;...}}
}
还有中拷贝策略是 COPY_MODE_HISTORY
,不需要 bundle 等历史数据,保留 action 等基本信息和启动 flag 等数据。
// frameworks/base/core/java/android/content/Intent.java
public class Intent implements Parcelable, Cloneable {...public Intent maybeStripForHistory() {if (!canStripForHistory()) {return this;}return new Intent(this, COPY_MODE_HISTORY);}private Intent(Intent o, @CopyMode int copyMode) {this.mAction = o.mAction;...if (copyMode != COPY_MODE_FILTER) {...if (copyMode != COPY_MODE_HISTORY) {if (o.mExtras != null) {this.mExtras = new Bundle(o.mExtras);}if (o.mClipData != null) {this.mClipData = new ClipData(o.mClipData);}} else {if (o.mExtras != null && !o.mExtras.isDefinitelyEmpty()) {this.mExtras = Bundle.STRIPPED;}}}}
}
总结起来:
Copy Mode | action 等数据 | flags 等数据 | bundle 等历史 |
---|---|---|---|
COPY_MODE_ALL | YES | YES | YES |
COPY_MODE_FILTER | YES | NO | NO |
COPY_MODE_HISTORY | YES | YES | NO |
除了 Intent
,Android 源码中还有很多地方采用了原型模式。
-
Bundle
也实现了 clone(),提供了 new Bundle(this) 的处理:public final class Bundle extends BaseBundle implements Cloneable, Parcelable {...@Overridepublic Object clone() {return new Bundle(this);} }
-
组件信息类
ComponentName
也在 clone() 中提供了类似的实现:public final class ComponentName implements Parcelable, Cloneable, Comparable<ComponentName> {...public ComponentName clone() {return new ComponentName(mPackage, mClass);} }
-
工具类
IntArray
亦是如此:public class IntArray implements Cloneable {...@Overridepublic IntArray clone() {return new IntArray(mValues.clone(), mSize);} }
原型模式也不一定非得实现 Cloneable,提供了类似的实现即可。比如:
-
Bitmap
没有实现该接口但提供了copy()
,内部将传递原始 Bitmap 在 native 中的对象指针并伴随目标配置进行新实例的创建:public final class ComponentName implements Parcelable, Cloneable, Comparable<ComponentName> {...public Bitmap copy(Config config, boolean isMutable) {...noteHardwareBitmapSlowCall();Bitmap b = nativeCopy(mNativePtr, config.nativeInt, isMutable);if (b != null) {b.setPremultiplied(mRequestPremultiplied);b.mDensity = mDensity;}return b;} }