1 /* 2 * Copyright 2021 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 17 package androidx.compose.runtime 18 19 /** 20 * The [Composable] function is declared to expect an applier with the name [applier]. The [applier] 21 * name can be an arbitrary string but is expected to be a fully qualified name of a class that is 22 * annotated by [ComposableTargetMarker] containing a descriptive name to be used in diagnostic 23 * messages. 24 * 25 * The [applier] name is used in diagnostic messages but, if it refers to a marked annotation, 26 * [ComposableTargetMarker.description] is used instead of the class name. 27 * 28 * The Compose compiler plugin can, in most cases, infer this or an equivalent 29 * [ComposableInferredTarget], for composable functions. For example, if a composable function calls 30 * another composable function then both must be of the same group of composable functions (that is, 31 * have declared or inferred the same [applier] value). This means that, if the function called is 32 * already determined to be in a group, them the function that calls it must also be in the same 33 * group. If two functions are called of different groups then the Compose compiler plugin will 34 * generate an diagnostic message describing which group was received and which group was expected. 35 * 36 * The grouping of composable functions corresponds to the instance of [Applier] that is required to 37 * be used by the [Composer] to apply changes to the composition. The [Applier] is checked at 38 * runtime to ensure the [Applier] that is expected by a composable function is the one supplied at 39 * runtime. This annotation, and the corresponding validation performed by the Compose compiler 40 * plugin, can detect mismatches at compile time, and issue a diagnostic message when calling a 41 * [Composable] function will result in the [Applier] check failing. 42 * 43 * In most cases this annotation can be inferred. However, this annotation is required for 44 * [Composable] functions that call [ComposeNode] directly, for abstract methods, such as interfaces 45 * functions (which do not contain a body from which the plugin can infer the annotation), when 46 * using a composable lambda in sub-composition, or when a composable lambda is stored in a class 47 * field or global variable. 48 * 49 * Leaving the annotation off in such cases will result in the compiler ignoring the function and it 50 * will not emit the diagnostic when the function is called incorrectly. 51 * 52 * @param applier The applier name used during composable call checking. This is usually inferred by 53 * the compiler. This can be an arbitrary string value but is expected to be a fully qualified 54 * name of a class that is marked with [ComposableTargetMarker]. 55 */ 56 @Retention(AnnotationRetention.BINARY) 57 @Target( 58 AnnotationTarget.FILE, 59 AnnotationTarget.CLASS, 60 AnnotationTarget.FUNCTION, 61 AnnotationTarget.PROPERTY_GETTER, 62 AnnotationTarget.TYPE, 63 AnnotationTarget.TYPE_PARAMETER, 64 ) 65 annotation class ComposableTarget(val applier: String) 66