2024-03-13发表2025-04-15更新Android28 分钟读完 (大约4152个字)
00-面试查漏补缺

2024.3.13

Handler

sendMessage
post
- 有callback的message
- looper循环执行
sendMessage
- 发送自定义的message
- 可以没有callback
- 可以有what、obj去和接收者传递消息
Message源码（仅public）

Message的获取
Handler包装的runnable
new Message()
obtain() 推荐，Message中的recycle方法会回收不用的Message，obtain的时候如果消息池不空，可以服用消息池
public final class Message implements Parcelable {
    /**
     * User-defined message code so that the recipient can identify
     * what this message is about. Each {@link Handler} has its own name-space
     * for message codes, so you do not need to worry about yours conflicting
     * with other handlers.
     */
    public int what;

    /**
     * arg1 and arg2 are lower-cost alternatives to using
     * {@link #setData(Bundle) setData()} if you only need to store a
     * few integer values.
     */
    public int arg1;

    /**
     * arg1 and arg2 are lower-cost alternatives to using
     * {@link #setData(Bundle) setData()} if you only need to store a
     * few integer values.
     */
    public int arg2;

    /**
     * An arbitrary object to send to the recipient.  When using
     * {@link Messenger} to send the message across processes this can only
     * be non-null if it contains a Parcelable of a framework class (not one
     * implemented by the application).   For other data transfer use
     * {@link #setData}.
     *
     * <p>Note that Parcelable objects here are not supported prior to
     * the {@link android.os.Build.VERSION_CODES#FROYO} release.
     */
    public Object obj;
    @UnsupportedAppUsage
    /*package*/ int flags;

    /**
     * The targeted delivery time of this message. The time-base is
     * {@link SystemClock#uptimeMillis}.
     * @hide Only for use within the tests.
     */
    @UnsupportedAppUsage
    @VisibleForTesting(visibility = VisibleForTesting.Visibility.PACKAGE)
    public long when;


    /** @hide */
    public static final Object sPoolSync = new Object();

    /**
     * Return a new Message instance from the global pool. Allows us to
     * avoid allocating new objects in many cases.
     */
    public static Message obtain() {
        synchronized (sPoolSync) {
            if (sPool != null) {
                Message m = sPool;
                sPool = m.next;
                m.next = null;
                m.flags = 0; // clear in-use flag
                sPoolSize--;
                return m;
            }
        }
        return new Message();
    }

    /**
     * Same as {@link #obtain()}, but copies the values of an existing
     * message (including its target) into the new one.
     * @param orig Original message to copy.
     * @return A Message object from the global pool.
     */
    public static Message obtain(Message orig) {
        Message m = obtain();
        m.what = orig.what;
        m.arg1 = orig.arg1;
        m.arg2 = orig.arg2;
        m.obj = orig.obj;
        m.replyTo = orig.replyTo;
        m.sendingUid = orig.sendingUid;
        m.workSourceUid = orig.workSourceUid;
        if (orig.data != null) {
            m.data = new Bundle(orig.data);
        }
        m.target = orig.target;
        m.callback = orig.callback;

        return m;
    }

    /**
     * Same as {@link #obtain()}, but sets the value for the <em>target</em> member on the Message returned.
     * @param h  Handler to assign to the returned Message object's <em>target</em> member.
     * @return A Message object from the global pool.
     */
    public static Message obtain(Handler h) {
        Message m = obtain();
        m.target = h;

        return m;
    }

    /**
     * Same as {@link #obtain(Handler)}, but assigns a callback Runnable on
     * the Message that is returned.
     * @param h  Handler to assign to the returned Message object's <em>target</em> member.
     * @param callback Runnable that will execute when the message is handled.
     * @return A Message object from the global pool.
     */
    public static Message obtain(Handler h, Runnable callback) {
        Message m = obtain();
        m.target = h;
        m.callback = callback;

        return m;
    }

    /**
     * Same as {@link #obtain()}, but sets the values for both <em>target</em> and
     * <em>what</em> members on the Message.
     * @param h  Value to assign to the <em>target</em> member.
     * @param what  Value to assign to the <em>what</em> member.
     * @return A Message object from the global pool.
     */
    public static Message obtain(Handler h, int what) {
        Message m = obtain();
        m.target = h;
        m.what = what;

        return m;
    }

    /**
     * Same as {@link #obtain()}, but sets the values of the <em>target</em>, <em>what</em>, and <em>obj</em>
     * members.
     * @param h  The <em>target</em> value to set.
     * @param what  The <em>what</em> value to set.
     * @param obj  The <em>object</em> method to set.
     * @return  A Message object from the global pool.
     */
    public static Message obtain(Handler h, int what, Object obj) {
        Message m = obtain();
        m.target = h;
        m.what = what;
        m.obj = obj;

        return m;
    }

    /**
     * Same as {@link #obtain()}, but sets the values of the <em>target</em>, <em>what</em>,
     * <em>arg1</em>, and <em>arg2</em> members.
     *
     * @param h  The <em>target</em> value to set.
     * @param what  The <em>what</em> value to set.
     * @param arg1  The <em>arg1</em> value to set.
     * @param arg2  The <em>arg2</em> value to set.
     * @return  A Message object from the global pool.
     */
    public static Message obtain(Handler h, int what, int arg1, int arg2) {
        Message m = obtain();
        m.target = h;
        m.what = what;
        m.arg1 = arg1;
        m.arg2 = arg2;

        return m;
    }

    /**
     * Same as {@link #obtain()}, but sets the values of the <em>target</em>, <em>what</em>,
     * <em>arg1</em>, <em>arg2</em>, and <em>obj</em> members.
     *
     * @param h  The <em>target</em> value to set.
     * @param what  The <em>what</em> value to set.
     * @param arg1  The <em>arg1</em> value to set.
     * @param arg2  The <em>arg2</em> value to set.
     * @param obj  The <em>obj</em> value to set.
     * @return  A Message object from the global pool.
     */
    public static Message obtain(Handler h, int what,
            int arg1, int arg2, Object obj) {
        Message m = obtain();
        m.target = h;
        m.what = what;
        m.arg1 = arg1;
        m.arg2 = arg2;
        m.obj = obj;

        return m;
    }

    /** @hide */
    public static void updateCheckRecycle(int targetSdkVersion) {
        if (targetSdkVersion < Build.VERSION_CODES.LOLLIPOP) {
            gCheckRecycle = false;
        }
    }

    /**
     * Return a Message instance to the global pool.
     * <p>
     * You MUST NOT touch the Message after calling this function because it has
     * effectively been freed.  It is an error to recycle a message that is currently
     * enqueued or that is in the process of being delivered to a Handler.
     * </p>
     */
    public void recycle() {
        if (isInUse()) {
            if (gCheckRecycle) {
                throw new IllegalStateException("This message cannot be recycled because it "
                        + "is still in use.");
            }
            return;
        }
        recycleUnchecked();
    }
    /**
     * Return the targeted delivery time of this message, in milliseconds.
     */
    public long getWhen() {
        return when;
    }

    public void setTarget(Handler target) {
        this.target = target;
    }

    /**
     * Retrieve the {@link android.os.Handler Handler} implementation that
     * will receive this message. The object must implement
     * {@link android.os.Handler#handleMessage(android.os.Message)
     * Handler.handleMessage()}. Each Handler has its own name-space for
     * message codes, so you do not need to
     * worry about yours conflicting with other handlers.
     */
    public Handler getTarget() {
        return target;
    }

    /**
     * Retrieve callback object that will execute when this message is handled.
     * This object must implement Runnable. This is called by
     * the <em>target</em> {@link Handler} that is receiving this Message to
     * dispatch it.  If
     * not set, the message will be dispatched to the receiving Handler's
     * {@link Handler#handleMessage(Message)}.
     */
    public Runnable getCallback() {
        return callback;
    }

    /** @hide */
    @UnsupportedAppUsage
    public Message setCallback(Runnable r) {
        callback = r;
        return this;
    }

    /**
     * Obtains a Bundle of arbitrary data associated with this
     * event, lazily creating it if necessary. Set this value by calling
     * {@link #setData(Bundle)}.  Note that when transferring data across
     * processes via {@link Messenger}, you will need to set your ClassLoader
     * on the Bundle via {@link Bundle#setClassLoader(ClassLoader)
     * Bundle.setClassLoader()} so that it can instantiate your objects when
     * you retrieve them.
     * @see #peekData()
     * @see #setData(Bundle)
     */
    public Bundle getData() {
        if (data == null) {
            data = new Bundle();
        }

        return data;
    }

    /**
     * Like getData(), but does not lazily create the Bundle.  A null
     * is returned if the Bundle does not already exist.  See
     * {@link #getData} for further information on this.
     * @see #getData()
     * @see #setData(Bundle)
     */
    public Bundle peekData() {
        return data;
    }

    /**
     * Sets a Bundle of arbitrary data values. Use arg1 and arg2 members
     * as a lower cost way to send a few simple integer values, if you can.
     * @see #getData()
     * @see #peekData()
     */
    public void setData(Bundle data) {
        this.data = data;
    }

    /**
     * Chainable setter for {@link #what}
     *
     * @hide
     */
    public Message setWhat(int what) {
        this.what = what;
        return this;
    }

    /**
     * Sends this Message to the Handler specified by {@link #getTarget}.
     * Throws a null pointer exception if this field has not been set.
     */
    public void sendToTarget() {
        target.sendMessage(this);
    }

    /**
     * Returns true if the message is asynchronous, meaning that it is not
     * subject to {@link Looper} synchronization barriers.
     *
     * @return True if the message is asynchronous.
     *
     * @see #setAsynchronous(boolean)
     */
    public boolean isAsynchronous() {
        return (flags & FLAG_ASYNCHRONOUS) != 0;
    }

    /**
     * Sets whether the message is asynchronous, meaning that it is not
     * subject to {@link Looper} synchronization barriers.
     * <p>
     * Certain operations, such as view invalidation, may introduce synchronization
     * barriers into the {@link Looper}'s message queue to prevent subsequent messages
     * from being delivered until some condition is met.  In the case of view invalidation,
     * messages which are posted after a call to {@link android.view.View#invalidate}
     * are suspended by means of a synchronization barrier until the next frame is
     * ready to be drawn.  The synchronization barrier ensures that the invalidation
     * request is completely handled before resuming.
     * </p><p>
     * Asynchronous messages are exempt from synchronization barriers.  They typically
     * represent interrupts, input events, and other signals that must be handled independently
     * even while other work has been suspended.
     * </p><p>
     * Note that asynchronous messages may be delivered out of order with respect to
     * synchronous messages although they are always delivered in order among themselves.
     * If the relative order of these messages matters then they probably should not be
     * asynchronous in the first place.  Use with caution.
     * </p>
     *
     * @param async True if the message is asynchronous.
     *
     * @see #isAsynchronous()
     */
    public void setAsynchronous(boolean async) {
        if (async) {
            flags |= FLAG_ASYNCHRONOUS;
        } else {
            flags &= ~FLAG_ASYNCHRONOUS;
        }
    }

    /** Constructor (but the preferred way to get a Message is to call {@link #obtain() Message.obtain()}).
    */
    public Message() {
    }
}
Looper每次循环在做什么

00-面试查漏补缺

View绘制流程图

起点：ViewRootImpl

00-面试查漏补缺

2024.3.13

Handler

Message源码（仅public）

Looper每次循环在做什么

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