Android Parcleable接口的调用源码层分析

目录

• android Parcelable 源码解析
• 一、writeToParcel
• 二、createFromParcel(Parcel source)
• 三、private Bundle mExtras
• 四、parcelledData.readArrayMapInternal(map, count, mClassLoader)
• 五、readParcelable(loader)

Android Parcelable 源码解析

大家都知道，要想在Intent里面传递一些非基本类型的数据，有两种方式，一种实现Parcelable，另一种是实现Serializable接口。今天先不说Serializable 接口，只说Parcelable。我们知道，Parcelable 只是一个接口，里面有几个关键方法：

一、writeToParcel

   /** 
     * Flatten this object in to a Parcel. 
     * 
     * @param dest The Parcel in which the object should be written. 
     * @param flags Additional flags about how the object should be written. 
     * May be 0 or {@link #PARCELABLE_WRITE_RETURN_VALUE}. 
     */ 
    public void writeToParcel(Parcel dest, @WriteFlags int flags); 

这个方法会让你把当前你需要保存的数据，写进Parcel 里。flags 可以写0 ，也可以写PARCELABLE_WRITE_RETURN_VALUE。这两个什么区别呢？后面再说。

这个里面，你需要调用传给你的Parcel 对象dest，把你需要的数据传递进去。Such as:

     public void writeToParcel(Parcel out, int flags) { 
         out.writeInt(mData); 
     } 

同时需要实现一个Creator, 用来恢复对象，如果没有实现这个Creator,那么恢复的时候，会报错。

  /** 
     * Interface that must be implemented and provided as a public CREATOR 
     * field that generates instances of your Parcelable class from a Parcel. 
     */ 
    public interface Creator<T&javascriptgt; { 
        /** 
         * Create a new instance of the Parcelable class, instantiating it 
         * from the given Parcel whose data had previously been written by 
         * {@link Parcelable#writeToParcel Parcelable.writeToParcel()}. 
         * 
         * @param source The Parcel to read the object's data from. 
         * @return Returns a new instance of the Parcelable class. 
         */ 
        public T createFromParcel(Parcel source); 
        /** 
         * Create a new array of the Parcelable class. 
         * 
         * @param size Size of the array. 
         * @return Returns an array of the Parcelable class, with every entry 
         * initialized to null. 
         */ 
        public T[] newArray(int size); 
    } 

二、createFromParcel(Parcel source)

这个方法是，当你恢复对象的时候，会把source 传递给你，让你去读取。

官方给的例子：

  public static final Parcelable.Creator MyParcelable> CREATOR 
             = new Parcelable.Creator<MyParcelable>() { 
         public MyParce开发者_JS开发lable createFromParcel(Parcel in) { 
             return new MyParcelable(in); 
         } 
         public MyParcelable[] newArray(int size) { 
             return new MyParcelable[size]; 
         } 
     }; 
     private MyParcelable(Parcel in) { 
         mData = in.readInt(); 
     } 

那么为什么这几个方法就可以把一个对象放到intent 里面呢？然后还可以取出来？

我们看下源码：

 php /** 
     * Add extended data to the intent.  The name must include a package 
     * prefix, for example the app com.android.contacts would use names 
     * like "com.android.contacts.ShowAll". 
     * 
     * @param name The name of the extra data, with package prefix. 
     * @param value The Parcelable data value. 
     * 
     * @return Returns the same Intent object, for chaining multiple calls 
     * into a single statement. 
     * 
     * @see #putExtras 
     * @see #removeExtra 
     * @see #getParcelableExtra(String) 
     */ 
    public @NonNull Intent putExtra(String name, Parcelable value) { 
        if (mExtras == null) { 
            mExtras = new Bundle(); 
        } 
        mExtr编程as.putParcelable(name, value); 
        return this; 
    } 

我们可以看到，其实是放到了mExtras 里面。

三、private Bundle mExtras

他其实是个Bundle，Bundle 其实也是实现了Parcelable 接口

public final class Bundle extends BaseBundle implements Cloneable, Parcelable { 

我们看下Bundle putParcelable 的实现：

/** 
 * Inserts a Parcelable value into the mapping of this Bundle, replacing 
 * any existing value for the given key.  Either key or value may be null. 
 * 
 * @param key a String, or null 
 * @param value a Parcelable object, or null 
 */ 
public void putParcelable(@Nullable String key, @Nullable Parcelable value) { 
    unparcel(); 
    mMap.put(key, value); 
    mFlags &= ~FLAG_HAS_FDS_KNOWN; 
} 

进入unparcel();

/** 
 * If the underlying data are stored as a Parcel, unparcel them 
 * using the currently assigned class loader. 
 */ 
/* package */ void unparcel() { 
    synchronized (this) { 
        final Parcel source = mParcelledData; 
        if (source != null) { 
            initializeFromParcelLocked(source, /*recycleParcel=*/ true); 
        } else { 
            if (DEBUG) { 
                Log.d(TAG, "unparcel " 
                        + Integer.toHexString(System.identityHashCode(this)) 
                        + ": no parcelled data"); 
            } 
        } 
    } 
} 

正常的情况下，mParcelledData是null 的。我们可以看到，其实这里面只是简单的put 进去。

ok ，传递数据的时候，Bundle 是要传递过去的，肯定会调用writeToParcel。

  /** 
     * Writes the Bundle contents to a Parcel, typically in order for 
     * it to be passed through an IBinder connection. 
     * @param parcel The parcel to copy this bundle to. 
     */ 
    @Override 
    public void writeToParcel(Parcel parcel, int flags) { 
        final boolean oldAllowFds = parcel.pushAllowFds((mFlags & FLAG_ALLOW_FDS) != 0); 
        try { 
            super.writeToParcelInner(parcel, flags); 
        } finally { 
            parcel.restoreAllowFds(oldAllowFds); 
        } 
    } 

调用了 super.writeToParcelInner(parcel, flags);

我们看下BaseBundle 的 writeToParcelInner(parcel, flags);：

/** 
 * Writandroides the Bundle contents to a Parcel, typically in order for 
 * it to be passed through an IBinder connection. 
 * @param parcel The parcel to copy this bundle to. 
 */ 
void writeToParcelInner(Parcel parcel, int flags) { 
    // If the parcel has a read-write helper, we can't just copy the blob, so unparcel it first. 
    if (parcel.hasReadwriteHelper()) { 
        unparcel(); 
    } 
    // Keep implementation in sync with writeToParcel() in 
    // frameworks/native/libs/binder/PersistableBundle.cpp. 
    final ArrayMap<String, Object> map; 
    synchronized (this) { 
        // unparcel() can race with this method and cause the parcel to recycle 
        // at the wrong time. So synchronize Access the mParcelledData's content. 
        if (mParcelledData != null) { 
            if (mParcelledData == NoImagePreloadHolder.EMPTY_PARCEL) { 
                parcel.writeInt(0); 
            } else { 
                int length = mParcelledData.dataSize(); 
                parcel.writeInt(length); 
                parcel.writeInt(BUNDLE_MAGIC); 
                parcel.appendFrom(mParcelledData, 0, length); 
            } 
            return; 
        } 
        map = mMap; 
    } 
    // Special case for empty bundles. 
    if (map == null || map.size() <= 0) { 
        parcel.writeInt(0); 
        return; 
    } 
    int lengthPos = parcel.dataPosition(); 
    parcel.writeInt(-1); // dummy, will hold length 
    parcel.writeInt(BUNDLE_MAGIC); 
    int startPos = parcel.dataPosition(); 
    parcel.writeArrayMapInternal(map); 
    int endPos = parcel.dataPosition(); 
    // Backpatch length 
    parcel.setDataPosition(lengthPos); 
    int length = endPos - startPos; 
    parcel.writeInt(length); 
    parcel.setDataPosition(endPos); 
} 

里面写了一堆，关键是 parcel.writeArrayMapInternal(map); 这句把map 写到了parcel 里面。

我们看下Parcel 的writeArrayMapInternal方法：

  /** 
     * Flatten an ArrayMap into the parcel at the current dataPosition(), 
     * growing dataCapacity() if needed.  The Map keys must be String objects. 
     */ 
    /* package */ void writeArrayMapInternal(ArrayMap<String, Object> val) { 
        if (val == null) { 
            writeInt(-1); 
            return; 
        } 
        // Keep the format of this Parcel in sync with writeToParcelInner() in 
        // frameworks/native/libs/binder/PersistableBundle.cpp. 
        final int N = val.size(); 
        writeInt(N); 
        if (DEBUG_ARRAY_MAP) { 
            RuntimeException here =  new RuntimeException("here"); 
            here.fillInStackTrace(); 
            Log.d(TAG, "Writing " + N + " ArrayMap entries", here); 
        } 
        int startPos; 
        for (int i=0; i<N; i++) { 
            if (DEBUG_ARRAY_MAP) startPos = dataPosition(); 
            writeString(val.keyAt(i)); 
            writeValue(val.valueAt(i)); 
            if (DEBUG_ARRAY_MAP) Log.d(TAG, "  Write #" + i + " " 
                    + (dataPosition()-startPos) + " bytes: key=0x" 
                    + Integer.toHexString(val.keyAt(i) != null ? val.keyAt(i).hashCode() : 0) 
                    + " " + val.keyAt(i)); 
        } 
    } 

首先写了长度，然后写k，写 value。我们看下这里的writeValue方法

   public final void writeValue(Object v) { 
        if (v == null) { 
            writeInt(VAL_NULL); 
        } else if (v instanceof String) { 
            writeInt(VAL_STRING); 
            writeString((String) v); 
        } else if (v instanceof Integer) { 
            writeInt(VAL_INTEGER); 
            writeInt((Integer) v); 
        } else if (v instanceof Map) { 
            writeInt(VAL_MAP); 
            writeMap((Map) v); 
        } else if (v instanceof Bundle) { 
            // Must be before Parcelable 
            writeInt(VAL_BUNDLE); 
            writeBundle((Bundle) v); 
        } else if (v instanceof PersistableBundle) { 
            writeInt(VAL_PERSISTABLEBUNDLE); 
            writePersistableBundle((PersistableBundle) v); 
        } else if (v instanceof Parcelable) { 
            // IMPOTANT: cases for classes that implement Parcelable must 
            // come before the Parcelable case, so that their specific VAL_* 
            // types will be written. 
            writeInt(VAL_PARCELABLE); 
            writeParcelable((Parcelable) v, 0); 
        } else if (v instanceof Short) { 
            writeInt(VAL_SHORT); 
            writeInt(((Short) v).intValue()); 
        } 
.... 
    } 

如果发现写的是Parcelable 的话，就writeParcelable

public final void writeParcelable(Parcelable p, int parcelableFlags) { 
    if (p == null) { 
        writeString(null); 
        return; 
    } 
    writeParcelableCreator(p); 
    p.writeToParcel(this, parcelableFlags); 
} 
public final void writeParcelableCreator(Parcelable p) { 
    String name = p.getClass().getName(); 
    writeString(name); 
} 

这里首先会写一下Parcelable 对象的类名字，然后调用了Parcelable 对象的writeToParcel。也就是自己实现的方法，就会把我们想要传递的数据写到Parcel 里面去。

OK ，这样，Parcelable 接口的writeToParcel 方法就被调用了。

我们再看下Parcel 的readFromParcel

    /** 
     * Reads the Parcel contents into this Bundle, typically in order for 
     * it to be passed through an IBinder connection. 
     * @param parcel The parcel to overwrite this bundle from. 
     */ 
    public void readFromParcel(Parcel parcel) { 
        super.readFromParcelInner(parcel); 
        mFlags = FLAG_ALLOW_FDS; 
        maybePrefillHasFds()javascript; 
    } 
super.readFromParcelInner(parcel); 
    private void readFromParcelInner(Parcel parcel, int length) { 
        if (length < 0) { 
            throw new RuntimeException("Bad length in parcel: " + length); 
        } else if (length == 0) { 
            // Empty Bundle or end of data. 
            mParcelledData = NoImagePreloadHolder.EMPTY_PARCEL; 
            return; 
        } 
        final int magic = parcel.readInt(); 
        if (magic != BUNDLE_MAGIC) { 
            throw new IllegalStateException("Bad magic number for Bundle: 0x" 
                    + Integer.toHexString(magic)); 
        } 
        if (parcel.hasReadWriteHelper()) { 
            // If the parcel has a read-write helper, then we can't lazily-unparcel it, so just 
            // unparcel right away. 
            synchronized (this) { 
                initializeFromParcelLocked(parcel, /*recycleParcel=*/ false); 
            } 
            return; 
        } 
        // Advance within this Parcel 
        int offset = parcel.dataPosition(); 
        parcel.setDataPosition(MathUtils.addOrThrow(offset, length)); 
        Parcel p = Parcel.obtain(); 
        p.setDataPosition(0); 
        p.appendFrom(parcel, offset, length); 
        p.adoptClassCookies(parcel); 
        if (DEBUG) Log.d(TAG, "Retrieving "  + Integer.toHexString(System.identityHashCode(this)) 
                + ": " + length + " bundle bytes starting at " + offset); 
        p.setDataPosition(0); 
        mParcelledData = p; 
    } 

很简单，把当前的mParcelledData 赋了值。

我们调用getParcelable 的时候，会首先 unparcel();

    public <T extends Parcelable> T getParcelable(@Nullable String key) { 
        unparcel(); 
        Object o = mMap.get(key); 
        if (o == null) { 
            return null; 
        } 
        try { 
            return (T) o; 
        } catch (ClassCastException e) { 
            typeWarning(key, o, "Parcelable", e); 
            return null; 
        } 
    } 
    /* package */ void unparcel() { 
        synchronized (this) { 
            final Parcel source = mParcelledData; 
            if (source != null) { 
                initializeFromParcelLocked(source, /*recycleParcel=*/ true); 
            } else { 
                if (DEBUG) { 
                    Log.d(TAG, "unparcel " 
                            + Integer.toHexString(System.identityHashCode(this)) 
                            + ": no parcelled data"); 
                } 
            } 
        } 
    } 
private void initializeFromParcelLocked(@NonNull Parcel parcelledData, boolean recycleParcel) { 
        if (LOG_DEFUSABLE && sShouldDefuse && (mFlags & FLAG_DEFUSABLE) == 0) { 
            Slog.wtf(TAG, "Attempting to unparcel a Bundle while in transit; this may " 
                    + "clobber all data inside!", new Throwable()); 
        } 
        if (isEmptyParcel(parcelledData)) { 
            if (DEBUG) { 
                Log.d(TAG, "unparcel " 
                        + Integer.toHexString(System.identityHashCode(this)) + ": empty"); 
            } 
            if (mMap == null) { 
                mMap = new ArrayMap<>(1); 
            } else { 
                mMap.erase(); 
            } 
            mParcelledData = null; 
            return; 
        } 
        final int count = parcelledData.readInt(); 
        if (DEBUG) { 
            Log.d(TAG, "unparcel " + Integer.toHexString(System.identityHashCode(this)) 
                    + ": reading " + count + " maps"); 
        } 
        if (count < 0) { 
            return; 
        } 
        ArrayMap<String, Object> map = mMap; 
        if (map == null) { 
            map = new ArrayMap<>(count); 
        } else { 
            map.erase(); 
            map.ensureCapacity(count); 
        } 
        try { 
            parcelledData.readArrayMapInternal(map, count, mClassLoader); 
        } catch (BadParcelableException e) { 
            if (sShouldDefuse) { 
                Log.w(TAG, "Failed to parse Bundle, but defusing quietly", e); 
                map.erase(); 
            } else { 
                throw e; 
            } 
        } finally { 
            mMap = map; 
            if (recycleParcel) { 
                recycleParcel(parcelledData); 
            } 
            mParcelledData = null; 
        } 
        if (DEBUG) { 
            Log.d(TAG, "unparcel " + Integer.toHexString(System.identityHashCode(this)) 
                    + " final map: " + mMap); 
        } 
    }

四、parcelledData.readArrayMapInternal(map, count, mClassLoader)

最终调用了Parcel 类的readArrayMapInternal

/* package */ void readArrayMapInternal(ArrayMap outVal, int N, 
    ClassLoader loader) { 
    if (DEBUG_ARRAY_MAP) { 
        RuntimeException here =  new RuntimeException("here"); 
        here.fillInStackTrace(); 
        Log.d(TAG, "Reading " + N + " ArrayMap entries", here); 
    } 
    int startPos; 
    while (N > 0) { 
        if (DEBUG_ARRAY_MAP) startPos = dataPosition(); 
        String key = readString(); 
        Object value = readValue(loader); 
        if (DEBUG_ARRAY_MAP) Log.d(TAG, "  Read #" + (N-1) + " " 
                + (dataPosition()-startPos) + " bytes: key=0x" 
                + Integer.toHexString((key != null ? key.hashCode() : 0)) + " " + key); 
        outVal.append(key, value); 
        N--; 
    } 
    outVal.validate(); 
} 

调用了readValue

public final Object readValue(ClassLoader loader) { 
        int type = readInt(); 
        switch (type) { 
        case VAL_NULL: 
            return null; 
        case VAL_STRING: 
            return readString(); 
        case VAL_INTEGER: 
            return readInt(); 
        case VAL_MAP: 
            return readHashMap(loader); 
        case VAL_PARCELABLE: 
            return readParcelable(loader); 
    ....... 
    } 

五、readParcelable(loader)

    public final <T extends Parcelable> T readParcelable(ClassLoader loader) { 
        Parcelable.Creator<?> creator = readParcelableCreator(loader); 
        if (creator == null) { 
            return null; 
        } 
        if (creator instanceof Parcelable.ClassLoaderCreator<?>) { 
          Parcelable.ClassLoaderCreator<?> classLoaderCreator = 
              (Parcelable.ClassLoaderCreator<?>) creator; 
          return (T) classLoaderCreator.createFromParcel(this, loader); 
        } 
        return (T) creator.createFromParcel(this); 
    } 
public final Parcelable.Creator<?> readParcelableCreator(ClassLoader loader) { 
        String name = readString(); 
        if (name == null) { 
            return null; 
        } 
        Parcelable.Creator<?> creator; 
        synchronized (mCreators) { 
            HashMap<String,Parcelable.Creator<?>> map = mCreators.get(loader); 
            if (map == null) { 
                map = new HashMap<>(); 
                mCreators.put(loader, map); 
            } 
            creator = map.get(name); 
            if (creator == null) { 
                try { 
                    // If loader == null, explicitly emulate Class.forName(String) "caller 
                    // classloader" behavior. 
                    ClassLoader parcelableClassLoader = 
                            (loader == null ? getClass().getClassLoader() : loader); 
                    // Avoid initializing the Parcelable class until we know it implements 
                    // Parcelable and has the necessary CREATOR field. http://b/1171613. 
                    Class<?> parcelableClass = Class.forName(name, false /* initialize */, 
                            parcelableClassLoader); 
                    if (!Parcelable.class.isAssignableFrom(parcelableClass)) { 
                        throw new BadParcelableException("Parcelable protocol requires that the " 
                                + "class implements Parcelable"); 
                    } 
                    Field f = parcelableClass.getField("CREATOR"); 
                    if ((f.getModifiers() & Modifier.STATIC) == 0) { 
                        throw new BadParcelableException("Parcelable protocol requires " 
                                + "the CREATOR object to be static on class " + name); 
                    } 
                    Class<?> creatorType = f.getType(); 
                    if (!Parcelable.Creator.class.isAssignableFrom(creatorType)) { 
                        // Fail before calling Field.get(), not after, to avoid initializing 
                        // parcelableClass unnecessarily. 
                        throw new BadParcelableException("Parcelable protocol requires a " 
                                + "Parcelable.Creator object called " 
                                + "CREATOR on class " + name); 
                    } 
                    creator = (Parcelable.Creator<?>) f.get(null); 
                } 
                catch (IllegalAccessException e) { 
                    Log.e(TAG, "Illegal access when unmarshalling: " + name, e); 
                    throw new BadParcelableException( 
                            "IllegalAccessException when unmarshalling: " + name); 
                } 
                catch (ClassNotFoundException e) { 
                    Log.e(TAG, "Class not found when unmarshalling: " + name, e); 
                    throw new BadParcelableException( 
                            "ClassNotFoundException when unmarshalling: " + name); 
                } 
                catch (NoSuchFieldException e) { 
                    throw new BadParcelableException("Parcelable protocol requires a " 
                            + "Parcelable.Creator object called " 
                            + "CREATOR on class " + name); 
                } 
                if (creator == null) { 
                    throw new BadParcelableException("Parcelable protocol requires a " 
                            + "non-null Parcelable.Creator object called " 
                            + "CREATOR on class " + name); 
                } 
                map.put(name, creator); 
            } 
        } 
        return creator; 
    } 

里面会加载你的Parcel 类，如果发现没有creator 就会抛异常。等等，最终调用了你的类的createFromParcel。

到此这篇关于Android Parcleable接口的调用源码层分析的文章就介绍到这了,更多相关Android Parcleable接口内容请搜索我们以前的文章或继续浏览下面的相关文章希望大家以后多多支持我们！