1 /*
2  * Copyright (C) 2016 The Android Open Source Project
3  *
4  * Licensed under the Apache License, Version 2.0 (the "License");
5  * you may not use this file except in compliance with the License.
6  * You may obtain a copy of the License at
7  *
8  *      http://www.apache.org/licenses/LICENSE-2.0
9  *
10  * Unless required by applicable law or agreed to in writing, software
11  * distributed under the License is distributed on an "AS IS" BASIS,
12  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13  * See the License for the specific language governing permissions and
14  * limitations under the License.
15  */
16 package com.android.internal.view;
17 
18 import android.view.View;
19 import android.view.ViewTreeObserver;
20 
21 /**
22  * An OnPreDrawListener that will remove itself after one OnPreDraw call. Typical
23  * usage is:
24  * <pre><code>
25  *     OneShotPreDrawListener.add(view, () -> { view.doSomething(); })
26  * </code></pre>
27  * <p>
28  * The listener will also remove itself from the ViewTreeObserver when the view
29  * is detached from the view hierarchy. In that case, the Runnable will never be
30  * executed.
31  */
32 public class OneShotPreDrawListener implements ViewTreeObserver.OnPreDrawListener,
33         View.OnAttachStateChangeListener {
34     private final View mView;
35     private ViewTreeObserver mViewTreeObserver;
36     private final Runnable mRunnable;
37     private final boolean mReturnValue;
38 
OneShotPreDrawListener(View view, boolean returnValue, Runnable runnable)39     private OneShotPreDrawListener(View view, boolean returnValue, Runnable runnable) {
40         mView = view;
41         mViewTreeObserver = view.getViewTreeObserver();
42         mRunnable = runnable;
43         mReturnValue = returnValue;
44     }
45 
46     /**
47      * Creates a OneShotPreDrawListener and adds it to view's ViewTreeObserver. The
48      * return value from the OnPreDrawListener is {@code true}.
49      *
50      * @param view The view whose ViewTreeObserver the OnPreDrawListener should listen.
51      * @param runnable The Runnable to execute in the OnPreDraw (once)
52      * @return The added OneShotPreDrawListener. It can be removed prior to
53      * the onPreDraw by calling {@link #removeListener()}.
54      */
add(View view, Runnable runnable)55     public static OneShotPreDrawListener add(View view, Runnable runnable) {
56         return add(view, true, runnable);
57     }
58 
59     /**
60      * Creates a OneShotPreDrawListener and adds it to view's ViewTreeObserver.
61      *
62      * @param view The view whose ViewTreeObserver the OnPreDrawListener should listen.
63      * @param returnValue The value to be returned from the OnPreDrawListener.
64      * @param runnable The Runnable to execute in the OnPreDraw (once)
65      * @return The added OneShotPreDrawListener. It can be removed prior to
66      * the onPreDraw by calling {@link #removeListener()}.
67      */
add(View view, boolean returnValue, Runnable runnable)68     public static OneShotPreDrawListener add(View view, boolean returnValue, Runnable runnable) {
69         OneShotPreDrawListener listener = new OneShotPreDrawListener(view, returnValue, runnable);
70         view.getViewTreeObserver().addOnPreDrawListener(listener);
71         view.addOnAttachStateChangeListener(listener);
72         return listener;
73     }
74 
75     @Override
onPreDraw()76     public boolean onPreDraw() {
77         removeListener();
78         mRunnable.run();
79         return mReturnValue;
80     }
81 
82     /**
83      * Removes the listener from the ViewTreeObserver. This is useful to call if the
84      * callback should be removed prior to {@link #onPreDraw()}.
85      */
removeListener()86     public void removeListener() {
87         if (mViewTreeObserver.isAlive()) {
88             mViewTreeObserver.removeOnPreDrawListener(this);
89         } else {
90             mView.getViewTreeObserver().removeOnPreDrawListener(this);
91         }
92         mView.removeOnAttachStateChangeListener(this);
93     }
94 
95     @Override
onViewAttachedToWindow(View v)96     public void onViewAttachedToWindow(View v) {
97         mViewTreeObserver = v.getViewTreeObserver();
98     }
99 
100     @Override
onViewDetachedFromWindow(View v)101     public void onViewDetachedFromWindow(View v) {
102         removeListener();
103     }
104 }
105